Mid-Cretaceous Magmatic Evolution and
Intrusion-Related Metallogeny
of the Tintina Gold Province,
Yukon and Alaska
Craig J.R. Hart
M.Sc.
University of British Columbia 1995
B.Sc.
McMaster University 1986
This thesis is presented for the degree of
Doctor of Philosophy
Centre for Global Metallogeny
School of Earth and Geographical Sciences
The University of Western Australia
October 2004
1
Supervisors
David I. Groves and Richard J. Goldfarb
Collaborators
John L. Mair, Lara L. Lewis, Moira Smith, Dan T. McCoy, Roger Hulstein, Paul
Roberts, Tom Bundtzen Mike E. Villeneuve, Arne A. Bakke, David Selby, Chris Wijns
2
All work presented in this thesis is original and that of the candidate, unless otherwise
specified, acknowledged or referenced.
__________________________________
Craig J.R. Hart
18 October 2004
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“There is nothing so sobering as an outcrop.”
Francis Pettijohn (undated)
The apical portion of the Tuna Stock as exposed in eastern Yukon. This slightly peraluminous pluton is a member of the mid-Cretaceous Tungsten Plutonic Suite.
4
Abstract
The Tintina Gold Province (TGP) comprises numerous (>15) gold belts and districts
throughout interior Alaska and Yukon that are associated with Cretaceous plutonic rocks. With a
gold endowment of ~70Moz, most districts are defined by their placer gold contributions, which
comprise greater than 30 Moz, but four districts have experience significant increases in gold
exploration with notable discoveries at Fort Knox (5.4 Moz), Donlin Creek (12.3 Moz), Pogo (5.8
Moz), True North (0.79 Moz), and Brewery Creek (0.85 Moz). All significant TGP gold deposits
are spatially and temporally related to reduced (ilmenite-series) and radiogenic Cretaceous
intrusive rocks that intrude (meta-) sedimentary strata. The similar characteristics that these
deposits share are the foundation for the development of a reduced intrusion-related gold deposit
model. Associated gold deposits have a wide variety of geological and geochemical features and
are categorized as intrusion-centered (includes intrusion-hosted, skarns and replacements), shearrelated, and epizonal. The TGP is characterized by vast, remote under-explored areas, unglaciated
regions with variable oxidation depths and discontinuous permafrost, which, in combination with
a still-evolving geological model, create significant exploration challenges.
Twenty-five Early and mid-Cretaceous (145-90 Ma) plutonic suites and belts are defined
across Alaska and Yukon on the basis of lithological, geochemical, isotopic, and geochronometric
similarities.
These features, when combined with aeromagnetic characteristics, magnetic
susceptibility measurements, and whole-rock ferric:ferrous ratios define the distribution of
magnetite- and ilmenite-series plutonic belts. Magnetite-series plutonic belts are dominantly
associated with the older parts of the plutonic episode and comprise subduction-generated
metaluminous plutons that are distributed preferentially in the more seaward localities dominated
by primitive tectonic elements. Ilmenite-series plutonic belts comprise slightly-younger, slightlyperaluminous plutons in more landward localities in pericratonic to continental margin settings.
They were likely initiated in response to crustal thickening following terrane collision. The
youngest plutonic belt forms a small, but significant, magnetite-series belt in the farthest inboard
position, associated with alkalic plutons that were emplaced during weak extension.
Intrusion-related metallogenic provinces with distinctive metal associations are distributed,
largely in accord with classical redox-sensitive granite-series. Copper, Au and Fe mineralisation
are associated with magnetite-series plutons and tungsten mineralisation associated with
ilmenite-series plutons. However, there are some notable deviations from expected associations,
as intrusion-related Ag-Pb-Zn deposits are few, and significant tin mineralisation is rare. Most
significantly, many gold deposits and occurrences are associated with ilmenite-series plutons
which form the basis for the reduced intrusion-related gold deposit model.
Among the mid-Cretaceous TGP plutonic suites, the Tombstone, Mayo and Tungsten suites are
the most metallogenically prolific. The Tombstone suite is alkalic, variably fractionated, slightly
late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The
Tungsten suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite
i
dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The
Tombstone suite was derived from an enriched, previously-depleted lithospheric mantle, the
Tungsten suite intrusions from continental crust including, but not dominated by, carbonaceous
pelitic rocks, and the Mayo suite from similar sedimentary crustal source, but is mixed with a
distinct mafic component from an enriched mantle source.
Each of these three suites has a distinctive metallogeny that is related to the source and
redox characteristics of the magma. The Tombstone suite has a Au-Cu-Bi association that is
characteristic of most oxidized and alkalic magmas, but also has associated, and enigmatic, UTh-F mineralisation. The reduced Tungsten suite intrusions are characterized by world-class
tungsten skarn deposits with less significant Cu, Zn, Sn, and Mo anomalies. The Mayo suite
intrusions are characteristically gold-enriched, with associated As, Bi, Te, and W associations.
All suites also have associated, but distal and lower temperature, Ag-Pb- and Sb-rich mineral
occurrences.
Although processes such as fractionation, volatile enrichment, and phase
separation are ultimately required to produce economic concentrations of ore elements from
crystallizing magmas, the nature of the source materials and their redox state play an important
role in determining which elements are effectively concentrated by magmatic processes.
Intrusion-related gold systems have an implied genetic relationship with a cooling pluton,
but confident links are difficult to establish. U-Pb and Ar-Ar geochronology in the TGP has
established main mineralizing events at circa 90 and 70 Ma, but precise temporal links between
magmatism and mineralization have been difficult to confirm. An integrated geochronological
approach to understand the magmatic-hydrothermal evolution of the Fort Knox, Clear Creek,
Scheelite Dome, Mactung and Cantung intrusion-related deposits using SHRIMP zircon U-Pb,
mica Ar-Ar, molybdenite Re-Os, and TIMS U-Pb data. TIMS U-Pb data, and SEM imagery of
plutonic zircons indicate considerable inheritance and Pb-loss in the zircons. SHRIMP analyses
avoid Pb-loss and inheritance and result in dates that are up to 4 m.y. older than the TIMS dates.
Ar-Ar dates for magmatic biotite and hornblende indicate that most plutons cooled quickly, but
Ar-Ar dates on hydrothermal micas from mineralization are 2-3 million years younger than the
magmatic mica dates suggesting that Ar retention in hydrothermal micas may differ from that
in magmatic micas. Re-Os molybdenite dates are in best agreement, and within the uncertainty
of the SHRIMP dates for the host plutons. Direct comparisons of the absolute ages indicate
magmatic-hydrothermal durations from 1.1 to 3.3 million years, which expand from 2.2 to 4.2
m.y. with the extremes of the uncertainties. These durations contrast thermal modeling which
indicates rapid cooling to below ~250°C within 0.03 to 0.3 million years. Long-lived magmatichydrothermal systems may result from episodic replentishment of assembling magmatic plutons.
Alternatively, the discrepancy may result from within-system uncertainties of geochronological
decay constants and the compounding effects of decay constant errors across isotopic systems
which conspire to increase the uncertainties of comparative dates.
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Contents
Abstract ......................................................................................................................... i
Contents ..................................................................................................................... iii
List of Appendices ...................................................................................................... vii
Acknowledgements ....................................................................................................viii
Chapter One
INTRODUCTION
Preamble ..................................................................................................................... 1
Characteristics of IRGS ............................................................................................. 2
Concerns of Classif cation .......................................................................................... 2
Thesis Aims and Objectives ......................................................................................... 3
Thesis Organisation ..................................................................................................... 3
Supportive and Associated Contributions .................................................................... 4
References .................................................................................................................. 7
Chapter Two
GEOLOGY, EXPLORATION and DISCOVERY in the TINTINA GOLD PROVINCE,
ALASKA and YUKON
Abstract ...................................................................................................................... 11
Introduction ................................................................................................................ 11
Historical Exploration Activity ..................................................................................... 12
The Early Years ....................................................................................................... 13
The Modern Era ....................................................................................................... 15
Contemporary Efforts .............................................................................................. 15
Regional Geology of the TGP Belts and Associated Gold Mineralization .................. 18
Deposits of the Kuskokwim Mineral Belt, southwestern Alaska .............................. 18
Fairbanks district, eastern Alaska ............................................................................ 20
Goodpaster district, eastern Alaska ......................................................................... 22
Tombstone gold belt, central Yukon ......................................................................... 24
Other Districts .......................................................................................................... 25
Summary ................................................................................................................... 28
Styles of Gold Mineralization ..................................................................................... 30
Intrusion-centered .................................................................................................... 30
Sheeted Veins .................................................................................................... 31
Skarns ................................................................................................................. 33
Replacements, Disseminations & Breccias ........................................................ 34
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Shear-related ores ................................................................................................... 35
Epizonal Mineralization ............................................................................................ 37
Summary and Implications for Ore Genesis .............................................................. 38
Exploration and Discovery Case Histories ................................................................. 41
Fort Knox ................................................................................................................. 41
Dublin Gulch ............................................................................................................ 42
Brewery Creek ......................................................................................................... 43
True North ................................................................................................................ 44
Donlin Creek ............................................................................................................ 45
Pogo ........................................................................................................................ 46
Exploration Methods & Targeting in the TGP.............................................................. 48
Pluton Distribution, Character and Lithology ........................................................... 48
Elemental Associations ............................................................................................ 49
Placer-Related Targets ............................................................................................ 49
Structural Targeting ................................................................................................. 49
Geochemical Methods for TGP Exploration ............................................................ 50
Geophysical Methods for TGP Exploration ............................................................. 52
Drilling as an Exploration Tool ................................................................................. 53
Additional Exploration Considerations ....................................................................... 53
Conclusion ................................................................................................................. 56
References ................................................................................................................ 57
Chapter Three
The NORTHERN CORDILLERAN MID-CRETACEOUS PLUTONIC PROVINCE:
ILMENITE/MAGNETITE-SERIES GRANITOIDS
and INTRUSION-RELATED MINERALISATION
Abstract ......................................................................................................................67
Introduction ................................................................................................................ 67
Tectonic Framework ................................................................................................... 70
Plutonic Sites ............................................................................................................. 71
Early and Mid-Cretaceous Plutonic Belts and Suites ................................................ 72
Tosina-St. Elias Belt ................................................................................................. 72
Nutzotin-Kluane Belt ................................................................................................ 75
St. Lawrence-Seward-Koyukuk Belt ........................................................................ 76
Tanacross-Dawson Range-Whitehorse Belt ............................................................ 79
Ruby-Kaiyuh-Nyac Belt ........................................................................................... 79
Yukon-Tanana Uplands Plutons .............................................................................. 80
Anvil-Hyland-Cassiar Belt ........................................................................................ 81
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Southern Alaska Range ........................................................................................... 82
Tok-Tetlin Belt .......................................................................................................... 83
Tungsten Plutonic Suite ........................................................................................... 83
Mayo Plutonic Suite ................................................................................................. 84
Fairbanks-Salcha Plutonic Suite .............................................................................. 84
Livengood - Tombstone Belt .................................................................................... 85
Redox State Characteristics ...................................................................................... 86
Aeromagnetic Character .......................................................................................... 86
Magnetic Susceptibility ............................................................................................ 86
Ferric:Ferrous Ratios ............................................................................................... 89
Space, Time & Metallogenic Distribution of Redox-Series Plutons ............................ 90
Distribution Patterns ................................................................................................ 90
Temporal Variations ................................................................................................. 93
Metallogeny ............................................................................................................. 95
Tectonic Setting of Redox-Series Plutons .................................................................. 98
Conclusions ............................................................................................................. 100
References .............................................................................................................. 102
Chapter Four
SOURCE and REDOX CONTROLS on METALLOGENIC VARIATIONS in
INTRUSION-RELATED ORE SYSTEMS, TOMBSTONE-TUNGSTEN BELT, YUKON
TERRITORY, CANADA
Abstract .................................................................................................................... 119
Introduction .............................................................................................................. 119
Tectonic Setting ....................................................................................................... 120
Characteristics of the Tombstone-Tungsten Belt ..................................................... 120
Plutonic Suites ......................................................................................................... 122
Nomenclature ........................................................................................................ 124
Tombstone Plutonic Suite ...................................................................................... 124
Mayo Plutonic Suite ............................................................................................... 126
Tungsten Plutonic Suite ......................................................................................... 127
Intrusion-Related Metallogeny ................................................................................. 127
Geochemistry ........................................................................................................... 129
Isotopic Data ............................................................................................................ 133
Redox State ............................................................................................................. 134
Zircon Character ...................................................................................................... 136
Discussion ............................................................................................................... 136
Source Regions of Magmas .................................................................................. 136
Redox Characteristics of Source Materials ........................................................... 139
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Relationships Between Redox State of Magmas and Metallogeny ....................... 139
Conclusions ............................................................................................................. 141
References .............................................................................................................. 143
Chapter Five
INTEGRATED U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar GEOCHRONOLOGY of
MINERALIZING PLUTONS in YUKON and ALASKA: DURATION of MAGMATICHYDROTHERMAL SYSTEMS
Abstract .................................................................................................................... 153
Introduction .............................................................................................................. 154
Regional Geology - Tintina Gold Province ............................................................... 154
Geology of the Plutons ............................................................................................ 156
Fort Knox ............................................................................................................... 156
Clear Creek ........................................................................................................... 156
Scheelite Dome ..................................................................................................... 158
Dublin Gulch .......................................................................................................... 158
Mactung ................................................................................................................. 158
Cantung ................................................................................................................. 159
Geochronology ........................................................................................................ 159
Fort Knox ............................................................................................................... 160
Clear Creek ........................................................................................................... 163
Dublin Gulch .......................................................................................................... 164
Scheelite Dome ..................................................................................................... 165
Mactung ................................................................................................................. 165
Cantung ................................................................................................................. 166
Summary ...................................................................................................................... 167
Discussion ....................................................................................................................169
Comparison of Data ................................................................................................ 169
Duration of the Magmatic-Hydrothermal Systems .................................................. 171
Modeled Durations of Cooling Plutons ................................................................... 173
Durations of Cooling Plutons from Empirical Data ................................................. 174
Limitations of Data Integration ................................................................................ 174
The Road Ahead ..................................................................................................... 176
References ................................................................................................................... 177
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Chapter Six
CONCLUSIONS
Conclusions ............................................................................................................. 183
Tintina Gold Province ............................................................................................ 183
Northern Cordillera Mid-Cretaceous Plutonic Province ......................................... 184
Source and Redox Controls on Tombstone -Tungsten Belt Intrusions .................. 185
Integrated Geochronology and Duration of TTB Systems ..................................... 185
Final Remarks .......................................................................................................... 186
APPENDICES
1 Sample Locations .................................................................................................189
2 SHRIMP.data.........................................................................................................190
2 Ar-Ar data and plots .............................................................................................192
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Acknowledgements
This thesis is the result of encouragement and support from Richard Goldfarb and David
Groves. Both have played so many roles to get me to this point that they cannot all be listed,
but all are appreciated. Keep on challenging me! I am grateful for the opportunity to be part
of the Centre for Global Metallogeny and The University of Western Australia.
Sincere thanks are owed to Grant Abbott and the Yukon Geological Survey for their
continued encouragement and support of this endeavour. As well, the encouragement of my
YGS colleagues Maurice Colpron, Mike Burke, Don Murphy and Julie Hunt, and the smooth
administrative support of Rod Hill and Monique Raitchey are really appreciated. As well, Lara
Lewis is thanked for field assistance, GIS-support, and for filling-in-the-gaps.
A special thanks to John Mair who has always challenged me with new information, insight
and ideas, and made this thesis better as a result of them.
I am grateful for the collaborations and easy exchange of information afforded me by my
Tintina Gold Province colleagues and collaborators over the years: Erin Marsh, David Selby,
Julian Stephens, Dan McCoy, Tim Baker, Jim Lang, John Thompson, Moira Smith, Jim
Mortensen, Paul Jensen, Rainer Newberry, Al Doherty, Roger Hulstein, Gerry Carlson, Arne
Bakke, Tom Bundtzen, Cam Rombach and Mark Lindsey.
Thanks to the UWA SHRIMP crew, Neal McNaughton, Ian Fletcher, Matt Baggott, Matt
Godfrey, Natalie Kositicin, April Pickard for SHRIMP support and insight.
Cheers to the CGM post-grad crew particularly office-mates John Pigois, Amit Eliyahu as
well as Louis Bucci, Brock Salier, Chris Grainger, Steve Garwin, Erik van Noort for keeping
it sweeet. Janet Thicket provided assistance when needed, kept me out of trouble with
administrators, and amazingly keeps the Centre running.
Reviews and comments of this thesis by Shunso Ishihara, Edward Spooner and Bernd
Lehmann are sincerely appreciated.
Finally, thanks to my wife Kelly for her ongoing support, for shouldering a greater share of
parenting responsibilities, and for enduring the tasks and challenges of being married to a field
geologist. Also, heaps of thanks to Cooper and Anika for putting up with a part-time Dad over
much of the past three years, and for enduring numerous flights across the planet,
..... from -40°C to +40°C.
viii
Chapter 1
Introduction
Chapter One
Introduction
Preamble
A connection between granitic (sensu lato) rocks and gold ores has long been recognized
(Agricola, 1556). Early models were developed early in the last century in response to
observations made in the European mineral districts (deLaunay, 1900; Berg 1927) or those
championed by American economic geologists (Lindgren, 1913; Spurr, 1923; Emmons, 1933).
Since then, links between magmatically-derived hydrothermal fluids and gold deposits have
been broadly speculated, intensively studied and hotly debated, but a geochemical or isotopic
“silver bullet” that can unequivocally link the two remains elusive.
The development of field-based deposit models, such as the porphyry copper model of
Lowell and Guilbert (1970), reinforced by theoretical, fluid inclusion, geochemical and
isotopic data, has led to almost universal acceptance that porphyry copper±gold±molybdenum
deposits are genetically related to a causative, normally hosting or proximal, intrusion.
Similarly, associated high-sulfidation silver-gold epithermal deposits (e.g. Hedenquist and
Lowenstern, 1994) and gold-bearing skarns (e.g. Meinert, 1992) are also considered to be the
products of fluids derived from cooling intrusions. Other gold-bearing hydrothermal deposits
that do not show such a close and consistent relationship to granitiod intrusions are more
controversial. Orogenic gold deposits are a classic example in which there is a broad spatial
and temporal connection to granitoid magmatism (Groves et al., 1998; Goldfarb et al., 2001),
but direct genetic relationships to specific plutons or suites of intrusions are not supported by
the bulk of evidence.
Recently, a new ore deposit class that encompasses a wide range of gold mineralization
styles has been introduced. This classification, known as intrusion-related gold systems
(IRGS), which lack proximal base-metal associations, emphasises a genetic relationship with
intrusions having a low primary oxidation state (Thompson et al., 1999; Lang et al., 2000).
As such, this deposit model contrasts markedly with the wide range of intrusion-related
gold deposits (e.g. porphyry systems) that are characterized by associated economic copper
mineralization and that are related to oxidized plutons (Sillitoe, 1991).
The IRGS model was largely developed from observations made on gold deposits and
occurrences throughout central Alaska and Yukon, which collectively comprise the Tintina
Gold Province (Hart et al., 2002). Development of the IRGS classification was based on the
wide-array of gold deposit styles, occurrence types and granitoids that occur throughout the
Tintina Gold Province (Newberry 1995; McCoy et al. 1997; Thompson et al., 1999; Lang
et al., 2000; Thompson and Newberry, 2000), as well as some global examples (Sillitoe and
Thompson, 1998). The IRGS classification has been used as a template to identify other
regions in the world where like-deposits may exist.
1
Chapter 1
Introduction
The model has been broadly adopted, is the topic of special volumes (Tucker and Smith,
2000; Lang and Baker, 2001) and has been used as a basis for exploration of gold targets
throughout the world. Despite the relative infancy of IRGS, only having been characterised
circa 2000, a search on the internet search engine Google (Oct. 2004) for “intrusion-related
gold”, returns approximately 600 hits.
Characteristics of IRGS
Based on observations from numerous authors as well as information presented herein, the
following characteristics of IRGS are as follows.
1) Tectonic setting is best characterized as the inboard portion of a continental margin, well
cratonward from the continental margin arc.
2) Associated regional metallogeny is best characterized as a tungsten, or less commonly
tin, district.
3) Associated plutonic rocks comprise small plutons composed of metaluminous to slightly
peraluminous, intermediate to felsic granitoid compositions.
4) The plutons mostly belong to the ilmenite-series (Ishihara, 1977), suggesting a
dominantly reduced primary oxidation state, although there are local variations in redox
state.
5) There is a wide-range of styles of mineralization, typically centred around a causative
pluton.
6) Mineralisation has low-total sulphide contents and have limited hydrothermal alteration
interpreted to result from low fluid volumes.
7) Metal assemblages are gold-dominant with anomalous Bi, W, As, Te and/or Sb, and
typically have non-economic base-metal concentrations.
8) Fluid systems are dominated by carbonic hydrothermal fluids.
Concerns of Classif cation
Many of the features listed above for IRGS are non-diagnostic or can be subjectively
interpreted (e.g. levels of Bi or other elements), and granitoids are common in most
epigenetic mineral districts. This presents a problem in unequivocal classification.
Further, the classification has been broadened by some authors with additions and
modifications (e.g. Rowins, 2000; Robert, 2001), or adapted to suit other diverse
geological situations where several of the factors listed above are not met (Hall et al.,
2001; McCuaig et al., 2001). The classification has even been applied-to deposits in
geological settings lacking granitoids because of their possible and presumed occurrence at
depth.
Deficiencies in classification are expected of immature and evolving deposit models,
and these have been noted by many authors (Thompson and Newberry, 2000; Goldfarb et
al., 2000; Lang and Baker, 2001; Groves et al. 2003). In order to establish more robust
criteria for this class of deposits, these authors note the need for: 1) improved basic
2
Chapter 1
Introduction
descriptions of the styles of mineralization, 2) better constraints on the timing and nature
of events at all scales, 3) improved appreciation for the tectonic and magmatic settings, 4)
research on associated magmatic-hydrothermal processes, and 5) increased understanding
of structural controls on these systems.
It is within this framework of understanding and mis-understanding that this thesis is
founded. Thesis aims presented below are mainly directed towards points 2, 3 and 4 as
listed above.
Thesis Aims and Objectives
The principal aims of this thesis are to: 1) develop a better understanding of the gold
metallogeny of the Tintina Gold Province and the intrusion-related gold system model; 2)
establish the mid-Cretaceous plutonic framework, particularly magmatic redox conditions,
of the Tintina Gold Province; 3) establish controls on metallogenic variations within the
Tombstone-Tungsten Belt of the Tintina Gold Province; and 4) establish a chronology for
magmatic-hydrothermal systems within the Tombstone-Tungsten Belt.
Collectively, these aims are designed to: 1) provide a better foundation upon which
to understand intrusion-related gold systems; 2) highlight the role of the mid-Cretaceous
magmatic history and tectonics in controlling intrusion-related metallogeny; 3) determine
the effect that magma source and redox plays in controlling intrusion-related metallogeny;
4) improve our understanding of the timeframes of magmatic-hydrothermal systems; and 5)
develop an improved basis for exploration targeting.
Thesis Organisation
In order to achieve the four principal aims, the research herein is presented as a series
of four papers which each form a chapter. Each paper deals with a specific component of
the project that contributes towards achieving the aims and objectives of the research. The
first paper establishes a foundation for understanding the Tintina Gold Province (TGP) and
its associated gold deposits. The second paper establishes the nature of the mid-Cretaceous
plutonic episode within the TGP. The third paper establishes the petrogenetic controls and
associated metallogenic variations of the three most-prolific plutonic suites that comprise
the Tombstone-Tungsten Belt (TTB). The fourth paper documents the geochronology of
magmatic-hydrothermal systems across the TTB, and evaluates the durations of such systems.
At the time of submission (October 2004), the papers are at variable stages within the
publication process. The first two papers are published, the third is “in press” and set to be
published shortly, and the fourth is “in review”. Contributions from co-authors are defined at
the beginning of each chapter. Cited references are presented at the end of each chapter.
Paper 1 Tintina Gold Province
This paper resulted from a presentation on the Tintina Gold Province invited by the Society
of Economic Geologists, for their first stand alone meeting in Denver, USA, in May 2002. The
associated paper, “Geology, Exploration and Discovery in the Tintina Gold Province, Alaska
and Yukon” was published in the Society of Economic Geologists Special Publication 9, p.
241-274, in 2002.
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Chapter 1
Introduction
Paper 2 Northern Cordillera Mid-Cretaceous Plutonic Province
This paper resulted from an invitation from Shunso Ishihara who was the editor of a
Special Issue of the journal of Resource Geology. This special issue was to commemorate
Dr. Ishihara’s 70th birthday and his lifetime contribution to understanding intrusion-related ore
systems. The paper, “The Northern Cordillera Mid-Cretaceous Plutonic Province: Ilmenite/
Magnetite-Series Granitoids and Intrusion-Related Mineralisation” was published in Resource
Geology, v. 54, no. 3, p. 253-280, in 2004.
Paper 3 Source and Redox of Tombstone -Tungsten Belt Intrusions
This paper is based on to a presentation made at the Fifth Hutton Symposium on the Origin
of Granites and Related Rocks in Toyohashi, Japan in May 2003. The paper “Source and
Redox Controls of Intrusion-Related Metallogeny, Tombstone-Tungsten Belt, Yukon, Canada”
will be published shortly in the Fifth Hutton Symposium Volume to be published in the
Transactions of the Royal Society of Edinburgh: Earth Sciences, and jointly distributed as a
Special Volume by the Geological Society of America.
Paper 4 Comparative Geochronology and Duration of TTB Deposits
This paper is based on to a presentation made at the SEG 2004 Meeting in Perth, Australia,
under the theme “Cutting Edge Developments in Mineral Exploration”. The presented paper,
“Integrated U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar Geochronology of Mineralising Plutons
in Yukon and Alaska; Duration of Magmatic-Hydrothermal Systems” has subsequently been
submitted to a special issue of Mineralium Deposita, and is currently in the review process.
Supporting and Associated Contributions
Research directed towards developing improved understandings in intrusion-related gold
systems in Alaska, Yukon and China has been particularly fruitful as a result of collaborations
with many colleagues. Many additional unabstracted oral and poster presentations have not
been listed.
Papers
1. Goldfarb, R.J., Baker, T., Dube, B. Groves, D.I., Hart, C.J.R., Robert, F., in review.
World distribution, productivity, character, and genesis of gold deposits in metamorphic
terranes. Society of Economic Geologists, 100th Anniversary Volume.
2. Mair, J.L., Hart, C.J.R., and Stephens, J.R., in review. Deformation history of the western
Selwyn Basin, Yukon, Canada: Implications for orogen evolution and mid-Cretaceous
magmatism. Geological Society of America.
3. Stephens, J.R., Mair, J.L., Oliver, N.H.S., Hart, C.J.R., Baker, T., 2004. Structural and
mechanical controls on intrusion-related deposits of the Tombstone Gold Belt, Yukon,
Canada, with comparisons to other vein-hosted ore-deposit types. Journal of Structural
Geology, v. 26, p. 1025-1041.
4. Selby, D., Creaser, R.A., Heaman, L.M. and Hart, C.J.R., 2003. Re-Os and U-Pb
geochronology of the Clear Creek, Dublin Guch and Mactung deposits, Tombstone Gold
Belt, Yukon Canada: Absolute timing relationships between plutonism and mineralization.
Canadian Journal of Earth Sciences, v. 40, p. 1839-1852.
4
Chapter 1
Introduction
5. Groves, D.I., Goldfarb, R.J., Robert, F., and Hart, C.J.R., 2003. Gold deposits in
metamorphic belts: Overview of current understanding, outstanding problems, future
research, and exploration significance. Economic Geology, v. 98, p. 1-29.
6. Marsh, E.E., Goldfarb, R.J., Hart, C.J.R. and Johnson, C.A., 2003. Geology and
geochemistry of the Clear Creek intrusion-related gold occurrences, Tintina Gold
Province, Yukon, Canada. Canadian Journal of Earth Sciences, v. 40, p. 681-699.
7. Hart, C.J.R., Wang, Y., Goldfarb, R.J., Begg, G., Mao, J. and Dong, L., 2003. Axi and
associated epithermal gold deposits in the western Tianshan, Xinjiang, PR China. In:
Tectonic Evolution and Metallogeny of the Chinese Altay and Tianshan, Mao J, Goldfarb
RJ, Seltmann R, Wang D, Xiao W, and Hart C.J.R. (eds). International Association on the
Genesis of Ore Deposits, London, p. 209-226.
8. Goldfarb, R..J., Mao, J., Hart, C.J.R., Wang, D., Anderson, E., and Wang, Z., 2003.
Tectonic and metallogenic evolution of the Altay Shan, Northern Xinjiang Uygur
Autonomous Region, northwestern China. In: Tectonic Evolution and Metallogeny of the
Chinese Altay and Tianshan, Mao J, Goldfarb RJ, Seltmann R, Wang D, Xiao W, and Hart
C. International Association on the Genesis of Ore Deposits, London, p. 209-226.
9. Harris, M.J., Symons, D.T.A., Blackburn, W.H., Hart, C.J.R., and Villeneuve, M., 2003.
Travels of the Cache Creek Terrane: a paleomagnetic, geobarometric and Ar/Ar study of
the Jurassic Fourth of July Batholith, Canadian Cordillera. Tectonophysics, v. 262, p. 137159.
10. Selby, D., Creaser, R.A, Hart, C.J.R., Rombach C., Thompson J.F.H., Smith M.T., Bakke
A.A., and Goldfarb R.J., 2002. Application of Re-Os molybdenite dating for determining
distinct episodes of mineralization: absolute timing of sulfide and gold mineralization at
the Fort Knox and Pogo gold deposits, Alaska. Geology, v. 30, p. 791-794.
11. Hart, C.J.R., Goldfarb, R.J., Qiu, Y., Snee, L., Miller, L.D., and Miller, M.L., 2002.
Gold deposits of the northern margin of the North China Craton: multiple late PaleozoicMesozoic mineralizing events. Mineralium Deposita, v. 37, p. 326-351.
Extended Abstracts/Short Course Notes
1. Mair, J.L., Hart, C.J.R., Groves, D.I. and Goldfarb, R.J., 2003. The nature of Tombstone
Plutonic Suite rocks at Scheelite Dome, Tintina Gold Province: Evidence for an enriched
mantle contribution. In: The Ishihara Symposium: Granites and Associated Metallogenesis,
GEMOC Macquarie University, July 2003, p. 222-234.
2. Mair, J.L., and Hart, C.J.R., 2003. Reduced intrusion-related gold deposits: Examples
from the Tintina Gold Province. In: Orogenic and Intrusion-related Gold Deposits, Short
Course notes, Centre for Global Metallogeny, Perth, February 2003
3. Stephens, J.R., Mair, J.L., Hart, C.J.R., Oliver, N.H.S., and Baker, T., 2002. Structural
controls on intrusion-related gold mineralization, central Yukon Territory, Canada:
comparison to other deposit styles and potential for giant deposits. In: Giant Ore Deposits
Workshop, Poster Abstracts, University of Tasmania, Hobart, Australia, p. 49-53.
4. Stephens, J.R., Mair, J.L., Hart C.J.R., Oliver N.H.S. and Baker, T. 2002. A structural
exploration model for intrusion-related gold mineralization in the Tombstone Gold Belt,
central Yukon. In: Regional Geologic Framework and Deposit Specific Exploration
Models for Intrusion-Related Gold Mineralization, Yukon and Alaska: Notes from the
third annual technical meeting, S. Ebert (ed) . Mineral Deposit Research Unit, University
of British Columbia p. 92-108.
5. Stephens, J.R., Oliver N.H.S., Mair, J.L, Hart C.J.R., and Baker, T. 2002. A mechanical
analysis of intrusion-related gold systems, central Yukon, Canada. In. Applied Structural
Geology for Mineral Exploration and Mining. International Symposium Abstract Volume,
Australian Institute of Geoscientists Bulletin 36.
5
Chapter 1
Introduction
6. Hart, C.J.R., McCoy, D.T., Goldfarb, R.J., Smith, M., Roberts, P., Hulstein, R., Bakke,
A.A., Bundtzen, T.K., 2002. Geology, exploration and discovery in the Tintina Gold
Province, Alaska and Yukon. Global Exploration 2002: Integrated Methods for Discovery,
E.E. Marsh, R.J. Goldfarb and W.C. Day (eds). Society of Economic Geologists, p. 25-26.
7. McCoy, D., Newberry, R., Hart, C.J.R., Bundtzen, T., Farnstrom, H., Werdon, M.,
Szumigala, D., 2002. Geology, Exploration, and Discovery in the Tintina Gold Province,
Alaska and Yukon. AAPG Cordilleran Section Meeting, Anchorage
8. Mair, J.L., Hart, C.J.R., and Groves, D.I., 2002. Tectonic setting of intrusion-related
gold mineralisation of the Tombstone Gold Belt, Yukon, Canada. In: Giant Ore Deposits
Workshop, Poster Abstracts, University of Tasmania, Hobart, Australia, p. 31-35
9. Hart, C.J.R., Selby, D., and Creaser, R.A., 2001. Timing relationships between
plutonism and gold mineralization in the Tintina Gold Belt (Yukon and Alaska) using ReOs molybdenite dating. In: 2001: A Hydrothermal Odyssey, Conference Abstracts, P.J.
Williams (ed), Economic Geology Research Unit Contribution 59, p. 72-73.
10. Lindsay M.J., Baker, T., Hart, C.J.R., and Oliver, N.H.S., 2001. The structural history
and mineral paragenesis of the Brewery Creek gold deposit, Yukon Territory, Canada.
In: 2001: A Hydrothermal Odyssey, Conference Abstracts, P.J. Williams (ed), Economic
Geology Research Unit, Contribution 59, p. 118-119.
11. Stephens, J.R., Mair, J.L., Oliver, N.H.S., Baker, T., and Hart, C.J.R., 2001. Structural
control on intrusion-related gold deposits in the central Yukon Territory, Tintina Gold Belt,
Canada: Implications for exploration. In: 2001: A Hydrothermal Odyssey, Conference
Abstracts, P.J. Williams (ed), Economic Geology Research Unit Contribution 59, p. 191192.
Abstracts
1. Lewis, L.L., Hart, C.J.R., and Garrett, R.G., 2004. Compatible behaviour of beryllium
in fractionating granitic magmas, Selwyn Magmatic Province. Abstracts with Program,
Geological Association of Canada, SS10-P08.
2. Goldfarb, R.J., Groves, D.I., and Hart, C.J.R., 2004. Overview on gold deposits in
metamorphic belts – orogenic and intrusion-related deposits. Ishihara Symposium
Abstract Volume, Tokyo.
3. Hart, C.J.R., Villeneuve, M., Mair J.L., D. Selby, D., and Creaser, R., 2003. Integrated
geochronology of intrusion-related gold systems: Examples from Yukon & Alaska. 13th
Goldschmidt Conference, Kurashiki, Japan, Geochimica et Cosmochimica Acta, v. 67, p.
A136.
4. Selby, D., Creaser, R.A., Heaman, L.M., and Hart, C.J.R., 2003. Re-Os and U-Pb
geochronology of the Clear Creek, Dublin Gulch and Mactung deposits, Tombstone Gold
Belt, Yukon, Canada: Absolute timing relationships between plutonism and mineralization.
Abstracts with Program, Geological Association of Canada, Vancouver.
5. Hart, C.J.R., Mair, J.L., Groves, D.I. and Goldfarb, R.J., 2003. The influence of source
variations on redox state and metallogeny of a cross-orogen plutonic province, Yukon,
Canada. The Origin of Granits and Related Rocks, Fifth Hutton Symposium, Toyohashi
Japan. Geological Survey of Japan, Interim-Report 29, p. 42.
6. Marsh, E., Goldfarb, R., Hart, C.J.R., and Farmer, G.L., 2001. The Clear Creek
intrusion-related gold deposit, Tintina Gold Belt, Yukon, Canada. Abstracts with Program,
Geological Society of America, Boston, Mass.
7. McCausland, P.J.A., Symons, D.T.A., Hart, C.J.R., and Blackburn, W.H., 2001.
Preliminary paleomagnetism and geothermobarometry of the Granite Mountain Batholith,
Yukon: Uplift remanence implies minimal tectonic motion for the Yukon-Tanana Terrane.
Geological Association of Canada, Program with Abstracts, St. Johns.
6
Chapter 1
Introduction
8. Selby, D., Creaser, R.A., and Hart, C.J.R., 2001. Timing relationship between plutonism
and gold mineralization: Re-Os molybdenite study of the reduced intrusion-related gold
deposits of the Tombstone plutonic suite, Yukon and Alaska. GAC MAC Annual Meeting,
St. Johns, Newfoundland, p. 134.
9. Hart, C.J.R., Baker, T., Lindsay, M.J., Oliver, N.H.S., Stephens, J.R., and Mair, J.L.,
2000. Structural controls on the Tombstone Plutonic suite gold deposits, Tintina Gold
Belt, Yukon. Geological Society of America, Cordilleran Section Abstracts with Programs,
v. 32, p. A-18.
10. Farmer, G.L., Mueller, S., Marsh, E., Goldfarb, R.J., and Hart, C.J.R., 2000. Isotopic
evidence on sources of Au-related mid-Cretaceous Tombstone Plutonic Suite granitic
rocks, Clear Creek district, Yukon. Cordilleran Section Abstracts with Programs,
Geological Society of America, v. 32 p. A-13.
References
Agricola, Georgius, 1556: De Re Metallica: Froben Press, Basel; translated into English by
Hoover, Herbert Clark and Lou. Henry Hoover; Mining Magazine, London, 1912;
(reprinted by Dover Publications, Inc., New York, 1950).
Berg, Georg, 1927. Zonal distribution of ore deposits in Central Europe; Economic Geology,
v. 22, p. 113-132.
deLaunay, L., 1900. Les variations de felons métallifères en profondeur. Reviews Géneral de
Science, v. 11, p. 575-580.
Emmons, W.H., 1926. Relations of metalliferous lode systems to igneous intrusions,
Transactions of the American Institute of Mining, Metallurgy and Engineering, v. 74,
p. 29-70.
Goldfarb, R.J., Groves, D.I., and Gardoll, S., 2001. Orogenic gold and geologic time: A global
synthesis: Ore Geology Reviews, v. 18, p. 1–75.
Goldfarb, R.J., Hart, C.J.R., Miller, M., Miller, L., Farmer, G.L., and Groves, D.I., 2000. The
Tintina Gold Belt: A global perspective. British Columbia and Yukon Chamber of
Mines, Special Volume 2, p. 5-34.
Groves, D.I., Goldfab, R.J., Gebre-Mariam, M., Hagemann, S.G., and Robert, F., 1998.
Orogenic gold deposits: A proposed classification in the context of their crustal
distribution and relationshipis to other deposit types. Ore Geology Reviews, v. 13, p.
7-27.
Hedenquist, J.W. and Lowenstern, J.B., 1994. The role of magmas in the formation of
hydrothermal ore deposits. Nature, . 370, p. 519-527.
Hall, G.A., Wall, V.J., and Massey, S., 2001. Archean pluton-related (thermal aureole) gold:
The Kalgoorlie exploration model. In:2001: A Hydrothermal Odyssey, P.J. Williams
(ed), Townsville, Economic Geology Research Unit Contribution 59, p. 66-67.
Ishihara, S., 1977. The magnetite-series and ilmenite-series granitic rocks. Mining Geology,
v. 27, p. 293-305.
Lang, J.R., and Baker, T., 2001. Intrusion-related gold systems: the present level of
understanding. Mineralium Deposita, v. 36, p. 477-489.
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Introduction
Lang, J.R., Baker, T., Hart, C.J.R. and Mortensen, J.K., 2000 - An exploration model for
intrusion-related gold systems. Society of Economic Geologists Newsletter 40, 1-15.
Lindgren, W., 1933. Mineral Deposits. McGraw Hill, New York and London, 930 p.
Lowell, J.D., and Guilbert, J.M., 1970. Lateral and vertical alteration-mineralization zoning in
porphyry ore deposits. Economic Geology, v. 65, p. 373-408. .
McCuaig, T.C., Behn, M., Stein, H., Hagemann, S.G., McNaughton, N.J., Cassidy, K.F.,
Champion, D. and Wyborn L., 2001. The Boddington gold mine: A new style of
Archaean Au-Cu deposit. In 4th International Archean Symposium, Extended
Abstracts. AGSO-Geoscience Australia Record 2001/37, p. 453-455.
Meinert, L.D., 1992. Skarns and skarn deposits. Geoscience Canada, v. 19, p. 145-162.
Newberry, R. J., McCoy, D.T., and Brew, D.A., 1995. Plutonic-hosted gold ores in Alaska:
Igneous vs. metamorphic origins. Resource Geology Special Issue 18, p. 57-100.
Robert, F., 2001. Syenite-associated disseminated gold deposits in the Abitibi greenstone belt,
Canada. Mineralium Deposita, v. 36, p. 503-516.
Rowins, S., 2000. Reduced porphyry copper-gold deposits; a new variation on an old theme.
Geology, v. 28, p. 491-494.
Sillitoe, R.H., 1991. Intrusion-related gold deposits. In: Gold Metallogeny and Exploration,
R.P. Foster (ed), Blackie, Glasgow, p. 165-209.
Sillitoe, R.H., and Thompson, J.F.H., 1998. Intrusion-related vein gold deposits: types,
tectono-magmatic settings and difficulties of distinction from orogenic gold deposits.
Resource Geology, v. 48, p. 237-250.
Spurr, J.E., 1923. The Ore Magmas: A Series of Essays on Ore Deposition. McGraw-Hill, New
York, 915 p.
Thompson, J.F.H. and Newberry, R.J., 2000. Gold deposits related to reduced granitic
intrusions. Reviews in Economic Geology, v. 13, p. 377-400.
Thompson, J.F.H., Sillitoe, R.H., Baker, T., Lang, J.R., and Mortensen, J.K., 1999. IntrusionRelated Gold Deposits Associated with Tungsten-Tin Provinces. Mineralium Deposita,
v. 34, p. 323-334.
Tucker, T.L. and Smith, M.T. (chairs), 2000. The Tintina Gold Belt: Concepts, exploration
and discoveries. British Columbia Chamber of Mines, Cordilleran Roundup, Special
Volume 2, 225 p.
8
Chapter 2
Tintina Gold Province
Chapter Two
Geology, Exploration and Discovery in the
Tintina Gold Province,
Alaska and Yukon
Craig J.R. Hart1,2, Dan T. McCoy3, Richard J. Goldfarb1,4, Moira Smith5,
Paul Roberts5, Roger Hulstein6, Arne A. Bakke7, and Thomas K. Bundtzen8
1
Centre for Global Metallogeny
School of Earth and Geographical Sciences
University of Western Australia
Crawley, Western Australia, Australia, 6009
2
Yukon Geological Survey
Box 2703 (K-10)
Whitehorse, Yukon, Canada, Y1A 2C6
3
Placer Dome Exploration
P.O. Box 110249
Fairbanks, Alaska, 99511
4
U.S. Geological Survey
Box 25046, MS 964, Denver Federal Center,
Denver, Colorado, USA, 80225
5
Teck-Cominco Ltd.
600-200 Burrard Street
Vancouver, British Columbia, Canada, V6C 3L9
6
281 Alsek Road
Whitehorse, Yukon Territory, Canada, Y1A 4T1
7
Kinross Gold USA
Box 73726, Fairbanks, Alaska, USA, 99707
8
Pacific Rim Geological Consulting
Box 81906, Fairbanks, Alaska, USA, 99708
9
Chapter 2
Tintina Gold Province
Preface to Chapter Two
This paper represents an invited presentation on the Tintina Gold Province by the Society of
Economic Geologists for their first stand alone meeting in Denver, USA, in May 2002. This
associated paper, “Geology, Exploration and Discovery in the Tintina Gold Province, Alaska
and Yukon” was published in 2002 in Society of Economic Geologists Special Publication 9,
p. 241-274. The spelling and format for citations and references follow those that are used in
publications of the Society of Economic Geologists, a USA-based organisation.
Justif cation of Authorship: This manuscript is the culmination of several field seasons and
numerous site visits throughout the Yukon and east-central Alaska. The manuscript, produced
in the early stages of the Ph.D. project, is co-authored by many colleagues to bring together a
breadth of exploration expertise about this vast region, in particular to include knowledge of
developments in Alaska, as well as Yukon, where my experience is greatest. Dr. Dan McCoy, Dr.
Richard Goldfarb, Mr. Arne Bakke and Dr. Thomas Bundtzen are all experts on Alaskan geology
and mineralisation and have contributed to the exploration section on Alaska. Specifically, Dr.
McCoy provided information about the Ester Dome region of the Fairbanks district, Dr. Goldfarb
provided information about regional geochemical surveys and general Alaskan geology, Mr.
Bakke was responsible for most of the exploration at Fort Knox and provided expertise about
exploration efforts there, and Dr. Bundtzen provided information about the Kuskokwin region.
Dr. Moira Smith and Paul Roberts are geologists who had worked at the Pogo deposit and
they provided information regarding its discovery and geology. Mr. Roger Hulstein provided
information regarding exploration techniques and applications. The candidate planned and
produced the whole of the manuscript, initiated and developed the models presented herein, and
compiled and constructed all of the figures and tables. The co-authors, particularly Dr. Goldfarb,
provided editorial comments.
10
Chapter 2
Tintina Gold Province
Geology, Exploration and Discovery in the
Tintina Gold Province,
Alaska and Yukon
Abstract
The Tintina Gold Province (TGP) comprises numerous (>15) individual gold belts and
districts throughout interior Alaska and Yukon with a gold endowment of ~68Moz. Most
districts are defined by their placer gold contributions, which comprise greater than 30 Moz,
but four districts have experience significant increases in gold exploration with notable new
discoveries at Fort Knox (5.4 Moz), Donlin Creek (12.3 Moz), Pogo (5.8 Moz), True North
(0.79 Moz), and Brewery Creek (0.85 Moz). All significant TGP gold deposits are spatially and
temporally related to reduced and radiogenic Cretaceous intrusive rocks that intrude (meta)sedimentary strata. These like characteristics are the foundation for the development of an
intrusion-related gold system model. Associated gold deposits have a wide variety of geological
and geochemical features, but can be categorized as intrusion-centered (includes intrusionhosted, skarns and replacements), shear-related and epizonal. Although intrusion-related gold
systems have an implied genetic relationship with a granitoid, confident links are difficult to
establish for many shear-related and epizonal deposits. U-Pb and Ar-Ar geochronology has
established main mineralizing events at circa 90 and 70 Ma, but precise links temporal links
between magmatism and mineralization have been difficult to confirm with Ar-Ar methods,
but new Re-Os molybdenite dating confirms that Fort Knox is 91 Ma and that Pogo’s Liese
veins are probably 104 Ma. Notable characteristics of the TGP pertinent to exploration, such
as vast, remote under-explored areas, unglaciated regions with variable oxidation depths and
discontinuous permafrost, in combination with an evolving geological model, create exploration
challenges that can be overcome with the pragmatic application of selected exploration
techniques and strategies. Regional stream geochemistry followed-up with soil geochemistry
has been most effective although the application of the intrusion-related model to areas with
known mineral occurrences has also been successful.
Introduction
The “Tintina Gold Belt” is a vast, gold-rich region of interior Alaska and central Yukon that
has a 100-year-long history and encouraging potential. The concept of a “Tintina Gold Belt”
was put forward in 1997 by junior exploration company personnel in an effort to highlight the
region’s 68 million ounce (Moz) gold endowment (C. Freeman and G. Carlson, oral. commun.,
2001). The Tintina Gold Belt’s underlying geology is complex, and the nature of the gold
deposits so variable, that no specific geologic elements precisely define it (Smith, 2000).
Although gold deposits allied with middle and Late Cretaceous intrusions are a key element,
some workers include more marginal elements such as Tertiary epithermal deposits, Triassic CuAu deposits, and the Klondike placer deposits. These considerable variations in deposit styles
and ages have resulted in a poor understanding of the relationships among the deposit types.
The Tintina Gold Belt is composed of numerous distinct gold belts and districts. We
therefore follow the lead of Bundtzen et al. (2000), and define the region instead, as a gold
11
Chapter 2
Tintina Gold Province
Figure 1. Tintina Gold Province is composed of numerous gold belts and districts throughout
central Alaska and Yukon. Most of the districts are mainly defined by placer gold production, but
many regions, notably the Kuskokwim, Fairbanks and Tombstone regions have seen high levels
of exploration and concomitant discovery throughout the 1990s. KK-Kuskokwim, RP-RubyPoorboy, FB-Fairbanks, CL-Circle, CH-Chulitna, KT-Kantishna, Bo-Bonniville, RS-Richardson,
GP-Goodpaster, EG-Eagle, 40-Forty-mile, 60-Sixymile, KD-Klondike, DR-Dawson Range, TBTombstone, TG-Tungsten, HY-Hyland River.
province (Fig. 1). Recent exploration efforts in the Tintina Gold Province (TGP) have focussed
on a new class of intrusion-related gold deposits that are spatially, temporally and perhaps
genetically associated with Cretaceous intrusions. These deposits account for approximately 35
Moz of in-ground resources and a considerable, but uncertain part of the placer production. The
wide-ranging styles of mineralization allied with these intrusions have been incorporated within
an intrusion-related metallogenic model (McCoy et al., 1997; Thompson et al., 1999; Hart
et al., 2000; Lang et al., 2000; Thompson and Newberry, 2000; Lang and Baker, 2001). The
main tenet of the model is that variations in deposit styles are accounted for, according to their
position with respect to a central, presumably causative, pluton, and the depth of formation.
This zoned model forms the basis for regional and property-scale exploration, but as some gold
deposits lack defining features of the model, their genetic associations with a central pluton are
ambiguous.
In this paper, the geology of the gold belts and districts that comprise the TGP are described,
with particular emphasis on the nature of mid-Cretaceous intrusion-related gold mineralization
in the Fairbanks district, Tombstone gold belt and Goodpaster district, and Late Cretaceous
intrusion-related ores of the Kuskokwim region. The various styles of mineralization found
within these districts are described, and the intrusion-related model used to account for their
formation is evaluated in this context. Exploration and discovery case histories of significant
deposits are presented, and followed by exploration strategies and methods that have proved
effective in dealing with the unique exploration challenges of the region.
Historical Exploration Activity
Considerable evolution in exploration trends and concepts, in response to changing metal
prices and advances in exploration and mining technology (Fig. 2) have characterized the 100
years of mining activity in the TGP. Most exploration in the TGP since ~1990 was focussed
on mineralization that is spatially associated with mid- and Late Cretaceous plutonic rocks. In
12
Chapter 2
Tintina Gold Province
Figure 2. a) Relative lode gold exploration and mining activity, and placer gold production
trends for the Tintina Gold Province. Note that the timescale is not linear. b) Total exploration
expenditures in Alaska and Yukon, with Alaskan gold exploration expenditures. Data from
Buntdzen et al., 1990, Burke, 2000, and other sources. C) Average monthly price of gold.
particular, exploration in the Fairbanks and Goodpaster districts of east-central Alaska (Fig. 3),
the Tombstone gold belt of central Yukon (Fig. 4), and the Kuskokwim region of southwestern
Alaska (Fig. 5) are emphasized.
The Early Years
Russian prospectors explored the southern Kuskokwim region of Alaska for gold as early
as the 1830s. Tens of thousands of would-be miners infiltrated the north during the Klondike
gold rush of 1898, and a series of gold rushes in the region continued until 1916. American,
Canadian and European prospectors and miners panned for gold and searched for lode sources
in central Yukon beginning in 1885. Tens of thousands of would-be miners infiltrated westcentral Yukon during the Klondike gold rush of 1898, and branched out to interior Alaska in
pursuit of the next big rush. This occurred a few years later in the Fairbanks district (Fig.
3), where placer gold was discovered in 1901 and eventually led to a series of discoveries
throughout the district such that it became Alaska’s principal gold producing region during
13
Chapter 2
Tintina Gold Province
Figure 3. Distribution of gold districts (ruled pattern) and significant gold deposits in east-central
Alaska in relation to the distribution of Cretaceous granitoids (grey fill). Much of the region
between the Tintina and Denali faults is underlain by Yukon-Tanana Terrane. Also shown is the
Eocene Circle Hot-Springs pluton in the Circle district.
the early 1900s. Gold production peaked in 1909 (Parker, 1929) and was followed by the
introduction of dredges in the 1920, such that 12 were in operation by 1930 within the Fairbanks
district. Fairbanks district placer production remained significant even after World War II, but
most dredges were inactive by the end of the 1950s.
The first hardrock mines started on metasedimentary rock-hosted, gold-bearing quartz veins
in 1910 and, by 1913, ten small mills were in operation (Brooks, 1915) in the Cleary Summit
and Ester Dome areas of the Fairbanks district (Fig. 3). Unlike placer mining, lode mining
declined significantly during WWI. By 1930, placer gold production (3.9 Moz) far outweighed
hardrock production (100,000 oz) in the district. Lode activity increased during the Great
Depression of the early 1930s, and with an adjustment of the gold price from $20 to ~$35/oz
in 1934. As of 1960, total Fairbanks lode gold production was 239,247 oz (Cobb, 1973).
Other small interior Alaska gold lodes produced intermittently from 1911 to 1960, but were
economically insignificant.
In the Yukon, early prospecting in what is now the Tombstone gold belt was concentrated
in the Mayo area, with placer discoveries between 1884 and 1901 on creeks draining midCretaceous plutons at Dublin Gulch, Scheelite Dome and Clear Creek (Fig. 4). Initial lode
staking in 1901 was followed up by underground exploration on numerous auriferous gold veins
14
Chapter 2
Tintina Gold Province
Figure 4. Distribution of gold deposits, belts and mid-Cretaceous plutons across central and
southern Yukon Territory. Note that most of the mineralization is preferentially associated with
plutons that form the leading edge of magmatism. Not all of the deposits shown are gold, some are
tungsten (W) and silver (Pb). Dawson Range deposits are Late Cretaceous.
from 1912 to 1916. All early gold exploration focussed on high gold-grade sulfide-bearing
quartz veins, although lodes of tungsten and silver were also explored. Total early lode gold
production in the Yukon was minimal.
The Modern Era
Modern exploration efforts in the 1960s and 1970s focussed on base metals, and gold
exploration languished. This was due, in part, to exploration success through the development
of the porphyry copper model, the advent of helicopter-supported regional geochemical surveys,
and the low price of gold as fixed by the gold standard. Granitoids throughout central Alaska
were evaluated for their copper and molybdenum porphyry potential. In the Yukon, the focus
was mainly on tungsten following the recognition of the Yukon’s high potential (Cathro, 1969)
and the subsequent discovery of the Cantung skarn adjacent to a mid-Cretaceous pluton (Fig. 4)
in 1971. Fuelled by high tungsten prices, continued exploration resulted in significant scheeliterich skarn discoveries at Mactung, Bailey, Lened, Clea and Ray Gulch, all on the margins of
mid-Cretaceous plutons. Follow-up of placer cassiterite occurrences in the Mayo area resulted
in the discovery of a few sub-economic lode tin occurrences.
Contemporary Efforts
Contemporary exploration for intrusion-related gold deposits in the TGP followed the
boom in the price of gold in the early 1980s. In Yukon, large crystal-filled vugs hosting native
15
Chapter 2
Tintina Gold Province
gold were discovered at Emerald Lake, a copper skarn was determined to be gold-rich at the
Marn deposit, and exploration focussed on large sulfide-rich auriferous veins adjacent to the
Antimony Mountain pluton (Fig. 4). Using sedimentary rock-hosted gold deposits in Nevada’s
Roberts Mountains allochthon as an analogy, exploration for Carlin-type occurrences in
analogous Selwyn Basin strata resulted in follow-ups of geochemical anomalies and properties
with As, Sb and Hg signatures (e.g., Brick, Ida/Oro). Most of this field work was in variably
calcareous country rocks within the thermal aureole of mid-Cretaceous plutons. In addition,
gold exploration throughout all of Canada accelerated during the “flow-through” tax-incentive
financing era of 1987 to 1990, during which time the Selwyn Basin-hosted Brewery Creek
deposit was discovered in the TGP.
Significant changes in exploration targeting, resulted from the recognition of a new bulktonnage resource at the historic Fort Knox deposit near Fairbanks (Fig. 3), and the subsequent
recognition that TGP plutons could host “porphyry-style” gold mineralization (Hollister, 1992).
Exploration focussed towards finding additional large-tonnage, intrusion-related gold deposits
in Alaska led to the discoveries in the early 1990s at Ryan Lode in the Fairbanks district, and
Vinasale (DiMarchi, 1993) and Donlin Creek (Retherford and McAtee, 1994) in southwestern
Alaska (Figs. 3 & 5). Exploration was spurred by technological advances that allowed
profitable recovery of gram-level gold, either through conventional milling (Fort Knox) or heap
leaching operations (Brewery Creek). Exploration staff involved with the Fort Knox discovery
acquired and explored similar targets in Yukon (Scheelite Dome, Clear Creek; Fig. 4), and
developed a resource at Dublin Gulch’s Eagle Zone in 1994.
Exploration and discovery of sedimentary rock-hosted gold ores at True North (Fig. 3),
Brewery Creek and Scheelite Dome (Fig. 4), focussed additional exploration interest out of
the granitoids and into the adjacent country rocks of the TGP. The discovery of the high-grade
Liese veins at Pogo (Fig. 3) in 1996 further encouraged this shift in exploration focus, and
perhaps more importantly, opened a large area of east-central Alaska and west-central Yukon not
previously considered prospective. Exploration in the Tungsten/Hyland River belt of eastern
Yukon during 1997-2000 focussed on thermal aureoles resulted in the discovery of the Sprogge
and Fer occurrences (Fig. 4).
In Alaska, expenditures on precious metals exploration increased from a low of $US 6M
in 1986, to $57M in 1990 and $42M in 1998. Total exploration expenditures for gold in
Alaska and Yukon since 1990 are approximately $US 500M. Considerable exploration has
been undertaken by Canadian mining companies with investment capital raised through share
offerings on the stock market.
Of the region’s ~68 Moz Au endowment, approximately 31 Moz were recovered from placer
production (Table 1). The current bedrock resource of approximately 35 Moz has mostly been
discovered since 1990. Historical lode gold production through to 1995 accounts for only 1
Moz, but more than 2 Moz have since been produced, mainly from Fort Knox and to lesser
extent, Brewery Creek. Current production and resource figures for the TGP districts and belts
are given in Table 1.
16
Chapter 2
Tintina Gold Province
Table 1. Resource data and placer gold production for Tintina Gold Province occurrences and districts.
DISTRICT, Deposit
FAIRBANKS
Fort Knox*
True North*
Ryan Lode*
Dolphin
Gil*
Cleary Summit
Placer
District total
GOODPASTER
Pogo, reserve
Richardson placers
PRE-MINING RESOURCE
Mt
gpt
total
Moz
169.0
0.93
5.4
14.6
3.6
27.7
9.2
1.4
1.69
3.04
0.7
1.37
34
0.79
0.347
0.67
0.43
1.59
9.25
9.0
18.86
2.91
12.3
Shotgun
Nixon Fork
32.8
0.100
0.93
42
1.1
0.17
Vinasale
Other, lode
Placer
District total
10.3
2.4
0.8
TOMBSTONE BELT
Marn
Horn
Brewery Creek*
Dublin Gulch
GRAND TOTALS
0.03
This paper
0.301
8.1
8.1
Bundtzen 1996, ref ned ounces
2.1
This paper, Smith et al., 2000
Bundtzen and Reger, 1977
0.183
0.066
14.3
1.0
12.1
6.2
0.25
3.5
3.25
2.2
30
3.16
3.16
0.125
1.2
0.6
0.25
2.18
0.3
0.01
17.2
99
8.6
30
1.44
1.19
Nerco Press Release, 1.7 gpt cut-off
.0016
0.06
0.1
0.20
0.5
0.45
2.84
0.10
0.10
0.85
4.1
5.1
36
36.63
M,I&I, 1.5gpt cutoff; Ebert et al., 2000,
minable reserve of 57 Mt
Inferred, 0.55 gpt cutoff, Rombach
95-99 prod’n (C. Puchner, pers. comm.,
2001), pre-65 (Cutler, 1994)
Bundtzen and Miller, 1997
Bundtzen and Miller, 1997
Bundtzen and Miller, 1997
0.2
1.00
0.05
0.54
0.1
Placer
District total
OTHER YUKON
Moosehorn/Longline
Klondike
Placer 40-mile
Placer 60-mile
Dawson Range
District totals
Bakke 2000, this paper
This paper
This paper
0.10
122.1
Note/Reference
1.8
5.8
KUSKOKWIM
Donlin Creek,
OTHER ALASKA
Liberty Bell
Golden Zone
Illinois Creek
Honker
Placer Circle
Placer Eagle
Placer 40-mile
Placer Bonif eld &
Kantishna
Placer Rampart
Placer Hot Springs
Placer Ruby-Poorman
Sub-total
PRODUCTION
Placer
Lode
Moz
Moz
0.06
0.27
0.38
0.38
0.05
16
0.3
0.325
0.05
16.73
31.30
Flanigan, 1998
Flanigan, 1998
Bundtzen et al, 1984
Cobb, 1973
Cobb, 1973
Cobb, 1973
Brown and Nesbitt 1987
estimate
Oxide, global resource of 40 Mt
Smit et al., 1996, minable
resource of 50.4Mt of 0.93gpt for
estimate
0.27
0.004
0.05
0.06
0.114
estimate
estimate
estimate
estimate
estimate
2.82
Notes: Total resource plus placer production equals 67.94 million ounces (Moz). Tonnages are resources
calculated using a variety of cut-offs and represent various levels of uncertainty that variably include
measured, inferred, and indicated estimates. Unreferenced data from various unpublished sources, press
releases and web sites. *indicates minable totals of producing mines. Some totals may appear incorrect
due to rounding.
17
Chapter 2
Tintina Gold Province
Regional Geology of the TGP Belts and Associated
Gold Mineralization
The TGP consists of several gold belts and gold-rich mineral districts that primarily underlie
the region between the Tintina/Kaltag and Denali/Farewell fault systems in Alaska and Yukon,
as well as the region northeast of the Tintina fault in Yukon, an area of ~500,000 km² (Fig. 1).
The TGP is underlain by three main tectonic elements:
1) Polymetamorphosed Neoproterozoic and Paleozoic metasedimentary and metaigneous
assemblages of the pericratonic Yukon-Tanana terrane (YTT), which is divisible into a number
of sub-terranes and underlies much of interior Alaska and west-central Yukon;
2) East-central Yukon, north of the Tintina fault, is underlain by basinal clastic rocks and
limestone that were deposited as a continental margin assemblage known as the Selwyn Basin;
3) Southwestern Alaska’s amalgamated basement assemblages are overlain by great thicknesses
of weakly deformed Cretaceous strata deposited in the Kuskokwim and related flysch basins.
Gold mineralization in most belts and districts is spatially and temporally related to mid or
Late Cretaceous plutonic rocks that intruded across the regions’ diverse tectonic elements. The
youngest, most inboard (cratonward) portions of the gold-associated mid-Cretaceous plutonic
suites intruded YTT in interior east-central Alaska and Selwyn Basin strata in central Yukon.
Late Cretaceous plutonic rocks that intruded the Kuskokwim basinal strata in southwestern
Alaska have associated gold mineralization.
Post mid-Cretaceous dextral transcurrent motion along the Tintina-Kaltag fault system
displaced southern YTT ~450 km to the northwest, likely in response to its collision with
the northern and landward edge of the Wrangellia Terrane (Plafker and Berg, 1994). This
resulted in the significant amount of offset of mid-Cretaceous, gold-related plutons between
the Tombstone gold belt in Yukon, and the Fairbanks district in Alaska (Fig. 1). Similarly, the
Denali–Farewell fault system, with a total of ~350 km of dextral displacement, cut Kuskokwim
basin strata and offset many of the ca. 70 Ma plutons and gold deposits (e.g. Miller et al., 2002).
About fifteen distinct gold districts (or belts, as used for the Yukon) define the TGP in
east-central Alaska and Yukon, with a westerly continuation of the province to the numerous
districts that lie within the Kuskokwim region. Most of these districts are more notable for their
placer production, as their hardrock sources are insignificant, or have not yet been discovered
or exploited. District descriptions below emphasize those districts with significant lode
exploration activity directed towards Cretaceous intrusion-related gold occurrences.
Deposits of the Kuskokwim Mineral Belt, southwestern Alaska
Numerous gold deposits associated with Late Cretaceous intrusive rocks comprise the 550km-long, northeast-trending Kuskokwim mineral belt in southwestern Alaska (Fig. 5). The belt
includes a ~12 Moz Au resource at Donlin Creek, with less significant resources at the Shotgun,
Vinasale and Nixon Fork deposits. Approximately 3.1 Moz of placer gold has been produced
from the region, as well as 70,000 oz from lodes (Bundtzen and Miller, 1997).
The mineral belt is underlain by a thick succession of Upper Cretaceous clastic strata of
the Kuskokwim Group, which was deposited as an overlap assemblage upon various accreted
terranes (Fig. 5). Clastic sedimentation was initiated at about 95 Ma and continued until
18
Chapter 2
Tintina Gold Province
Figure 5. Generalized regional geology of the Kuskokwim mineral belt, modified
from Bundtzen and Miller (1997).
near the end of the Cretaceous (Bundtzen and Gilbert, 1993; Miller et al., 2000). The strata
were deformed by broad regional folding throughout the Late Cretaceous. Dextral strike-slip
motion along the Denali–Farewell and adjacent parallel fault systems, has further modified the
architecture of the original basin(s) and associated basement terranes (Miller et al., 2002).
Magmatism within southwestern Alaska began at about 75 Ma and continued for
approximately 20 m.y., likely in response to a period of NNE–SSW compression (Miller et al.,
2002). Two main suites of Late Cretaceous to early Tertiary magmatic rocks are associated
with gold mineralization (Bundtzen and Miller, 1997; Miller and Bundtzen, 1994). Alkalic,
metaluminous, intermediate to mafic, volcanic-plutonic complexes, with equivalent, subaerial
volcanic rocks, are associated with many deposits and prospects in the Kuskokwim region.
These include the granite-hosted gold veins at Golden Horn and the gold-rich skarns at Nixon
Fork. Felsic to intermediate, peraluminous porphyry dikes, sills, and stocks, dated mainly
between 71 and 67 Ma, are spatially associated with some of the other gold deposits in the
region, including Donlin Creek.
The alkalic volcanic-plutonic-hosted gold systems, many also enriched in copper, tungsten,
and boron, have been described by Bundtzen and Miller (1997). They favor classification of
19
Chapter 2
Tintina Gold Province
these gold deposits into an alkalic porphyry Cu-Au model. Golden Horn, the most important
of these systems, occurs as auriferous stockworks within a monzonite body with chlorite and
dolomite as the main alteration minerals. Muscovite-biotite-quartz and sericite-ankerite-quartz
alteration assemblages were interpreted to pre-date the main period of gold and sulfide mineral
deposition. Where the ~70 Ma granitoids are intrude Ordovician carbonates of the Farewell
terrane, copper- and gold-rich calcic skarns developed (e.g. Nixon Fork, Newberry et al., 1997).
Silver-tin-tourmaline veins, stockworks, and replacements in the Kuskokwim basin range in
age from about 70-60 Ma and may be related to volcanic-plutonic-hosted gold lodes and/or to
slightly younger magmatism (Bundtzen and Miller, 1997).
Most of the peraluminous-intrusion-associated, gold-bearing deposits occur as quartzcarbonate sulfide veinlet swarms and stockwork systems developed in small stocks, dikes, sills
and adjacent sedimentary rocks. Arsenopyrite and pyrite are the dominant sulfide minerals,
although stibnite and cinnabar are not uncommon. At some locations, gold and sulfide minerals,
occur as disseminations and segregations in the igneous rocks (e.g., Vinasale, DiMarchi, 1993).
At Donlin Creek and Vinasale, most of the gold is chemically bound within the arsenopyrite
lattice (Dodd, 1998; McCoy, 2000). In addition, numerous small epithermal Hg- and Sb-rich
veins and veinlets formed throughout the Kuskokwim region at about 70 Ma (Gray et al., 1997),
mainly distal to the igneous rocks.
Limited isotopic dating suggests that gold occurrences in the Kuskokwim Group are
relatively restricted in age. Dating of hydrothermal micas at Donlin Creek (Gray et al., 1997;
Szumigala et al., 1999) Vinasale Mountain (DiMarchi, 1993), and Shotgun (Rombach and
Newberry, 2001) consistently indicate deposition of gold at about 70 Ma, which is co-eval with
the initial and most voluminous period of magmatism. Fluid inclusion data from the deposits
suggest deposition of gold occurred at temperatures of approximately 200-250 ºC in an epizonal
environment (perhaps ≤1 km), except at Shotgun which was a much hotter deposit (350-650 °C;
Rombach, 2000).
Fairbanks district, eastern Alaska
The most economically important gold deposits in the TGP are in the Fairbanks district of
interior Alaska (Fig. 6) and include the Fort Knox, Ryan Lode and True North deposits, all of
which have contemporary production. The total gold resource of these and other deposits in the
district exceeds 9 Moz, with historical placer production exceeding 8 Moz.
The gold deposits of the Fairbanks district are hosted within four YTT assemblages that vary
between medium and high metamorphic grades - the Fairbanks Schist and Muskox assemblage
are amphibolite facies, the Birch Hill is greenschist facies, and Chatanika terrane rocks are
eclogite facies. Protoliths are dominantly clastic rocks with minor volcanic and carbonate
rock components. Protolith ages are predominantly early to middle Paleozoic, except for the
Fairbanks Schist which may be as old as Proterozoic. Sedimentary rocks of the Chatanika
terrane reached peak eclogite facies at ~210 to 180 Ma, and were exhumed and thrust over
YTT rocks during the Early Cretaceous (~135 Ma) (Douglas, 1997; McCoy, 2000). Dates on
metamorphic minerals yield consistent cooling ages of 110 to 102 Ma (Wilson et al., 1985;
McCoy et al., 1997; Douglas and Layer, 1999). Many of the rocks are altered to chlorite and
secondary albite, a feature likely related to mid-Cretaceous plutonism (K. Clautice, written
comm., 1997).
20
Chapter 2
Tintina Gold Province
Figure 6. General geology and mineral deposits of the Fairbanks district. Placer workings are from
Robinson et al., 1990. Faults are from Newberry et al. (1996).
Felsic to intermediate intrusions were emplaced throughout the district, and are best exposed
at Gilmore, Pedro, and Ester domes (Fig. 6), where plutons and their hornfels zones form
resistant topographic highs, drainages contain gold placers, and most of the economic lode gold
mineralization is recognized. These late- to post-tectonic granites and granodiorites, emplaced
at 2-5 km, are characterized as I-type, ilmenite-series, calc-alkaline plutons and considered to be
arc-related in origin despite their elevated initial strontium ratios (Newberry et al., 1995; McCoy
et al., 1997; Aleinikoff et al., 1999). The most reliable age dates indicate that the plutons
were emplaced at about 91 Ma (McCoy et al., 1997; McCoy, 2000). An Eocene thermal event
associated with subaerial volcanism is recognized by low-temperature hydrothermal alteration
associated with Eocene fission track dates (Murphy and Bakke, 1993) and variably reset Ar-Ar
dates throughout the district.
Gold mineralization in the district is hosted in moderately- to steeply-dipping, NW-striking
(earlier) and NE-striking (later) faults that are thought to have had predominantly strike-slip
movement (Robinson et al., 1990; LeLacheur, 1991; McCoy et al., 1997). Subsequent motion
along the Tintina-Kaltag and Denali-Farewell fault systems resulted in new and reactivated NEtrending sinistral faults that cut much of central Alaska (LeLacheur, 1991). Oblique dip-slip
movement along these faults, which continues today, has resulted in the current exposure of
varying levels of the plutonic systems as evidenced by in both the geology and geobarometric
data (Newberry and Burns, 1999; McCoy et al., 1997).
Gold mineralization was emplaced within the plutons (e.g., Fort Knox, Dolphin, part of the
Ryan Lode), proximal to the plutons (within 100s to 1000’s of meters of outer contacts; e.g.,
most of the Ryan Lode, most skarns) and as far as 10 km from large igneous bodies (e.g., Hi-Yu,
Gil). The nature of gold mineralization varies considerably within the district from discreet,
21
Chapter 2
Tintina Gold Province
high-grade, shear-hosted, sulfide-rich veins (Ryan Lode), to fracture-hosted, sulfide-poor highgrade veins (Hi-Yu), to diffuse veins or disseminated ores (True North), to widespread, lowgrade sheeted vein arrays (Fort Knox), to skarn-related ores (Gil). Mineralization other than
gold-rich ores includes numerous, but sub-economic tungsten skarns, and small silver- and basemetal-rich replacements and veins (Metz, 1991). Alteration associated with gold mineralization
varies from common early potassic and/or albitic, to more local late quartz-sericitic alteration.
Gold deposition in the district is approximately coeval with mid-Cretaceous plutonism.
Most Ar-Ar dates on micas from the intrusions give ages between 92 and 86 Ma, with the spread
of data resulting from cooling or resetting (McCoy et al., 1997; McCoy, 2000) as they are
younger than an unpublished, widely quoted ~92 Ma U-Pb crystallization age of the granitoids
(in Bakke, 1995). Ar-Ar dates on white mica from the Fairbanks district lodes are typically
at the younger end of this spread and considered to be up to 4 m.y. younger than the pluton.
Recent Re-Os dating of molybdenites from a gold-bearing sheeted quartz veins, of the type that
compose the Fort Knox ore body, gave a 3 point isochron age of 92.4 Ma (Selby et al., 2001;
Hart et al; 2001) indicating that mineralization was probably coeval with pluton crystallization.
Zoned metal assemblages typically vary from Au-Bi,Te±W, in and adjacent to, igneous
rocks, to more As-and Sb-rich where the lodes cut metasedimentary rocks. Flanigan et al.
(2000) show a systematic decrease in Bi:Au ratios and Au-Bi correlation coefficents for goldbearing veins with distance from plutons in the Fairbanks district. McCoy et al. (1997) show
that this relationship holds true even within a single deposit as the intrusion-hosted ore at the
Ryan Lode deposit has a higher Bi:Au ratio and a better Bi-Au correlation than the schist-hosted
veins. Some workers have made efforts to include all gold mineralization in the Fairbanks
district in all-encompassing magmatic models (McCoy et al., 1997; McCoy, 2000), whereas
others have invoked models in which intrusion and schist-hosted mineralization had a distinctly
different genesis (Metz, 1991; Goldfarb et al., 1997; 2000).
Goodpaster district, eastern Alaska
The Goodpaster district (Fig. 7), about 150 km east-southeast of Fairbanks, has seen
increased exploration activity following the 1996 discovery of the Liese ore body on the Pogo
property which is the only significant resource in the district. The Blue Lead deposit, 40 km
east of Pogo, has had minor gold production. Placer gold production from the region is limited
to a few thousand ounces.
The Goodpaster district is mainly underlain by silliminite-grade Neoproterozoic to midPaleozoic paragneiss, and Mississippian(?) and Early Cretaceous (128 Ma, U-Pb) felsic
orthogneiss (Smith et al., 1999) of the YTT. These rocks were uplifted from high-P and
moderate-T conditions, relatively slowly throughout the Early Cretaceous (Hansen et al.,
1991) in response to an extensive Early to middle Cretaceous extensional event (Pavlis et al.,
1993; Rubin et al., 1995). The region’s structural history is complex and poorly understood,
with at least two periods of metamorphism and ductile deformation, the latest extending from
approximately 130-110 Ma. Rocks were imbricated along low-angle faults, and subsequently
dismembered along a series of numerous NE-trending fault zones that parallel the Shaw Creek
fault (Fig. 7). These faults may have controlled some of the region’s gold mineralization, and
influenced the present shape and level of unroofing of mineralized zones.
22
Chapter 2
Tintina Gold Province
Figure 7. Regional geological setting of the Goodpaster district showing relation of gold
occurrences (stars) with batholiths of mid-Cretaceous (red fill) and of Late Cretaceous or Tertiary
(pink fill) age.
Intrusive rocks are voluminous, lithologically variable and largely mid-Cretaceous in age
(Newberry, 2000). They include dikes, sills, small stocks and batholiths of granite, quartz
monzonite, granodiorite, quartz diorite and diorite, and are largely calc-alkaline, I-type and
moderately reduced. The largest single body is the Goodpaster batholith, dominated by an
equigranular, medium-grained biotite granite interior and a relatively mafic border phase, with
associated pegmatitic, dioritic and granitic dikes. Mid-Cretaceous intrusive rocks at Pogo are as
old as ~107 Ma (U-Pb monazite), with U-Pb dates of approximately 94.5 Ma on a diorite body
and ~93 Ma on the Goodpaster batholith also recorded (Smith et al., 1999).
Gold mineralization at Pogo occurs in a 2- to 3-km-wide, 15-km-long WNW-trending
zone a few kilometers south of the southern margin of the Goodpaster batholith. This “Pogo
Trend” includes thick, shallowly dipping, auriferous quartz veins, stockwork and sheeted vein
zones, steeply-dipping, shear-hosted quartz veins and quartz-stibnite veins and breccias. The
most significant mineralization is the Liese zone which comprises two or more tabular, gentlydipping, shear-hosted quartz veins (L1 and L2) averaging 7 m in thickness, has an estimated
gold resource of 8.96 tonnes @ 18.86 g/t (Smith et al., 2000).
The Liese veins are partially hosted in a zone of 107 Ma granitic dikes, and alteration
associated with the Liese zone is overprinted by alteration related to a diorite dyke dated at 94.5
Ma (U-Pb zircon). 40Ar/39Ar cooling ages on hydrothermal biotite and sericite typically return
ages of ~91 Ma or younger, thus suggesting a protracted or multi-phase thermal history (Smith
et al., 1999). Recent direct dating of molybdenite in the quartz veins by Re-Os methods (D.
Selby, pers. commun.2001) suggests an age of mineralization of 104 Ma. Re-Os molybdenite
ages of two other occurrences in the “Pogo Trend” are ~95 Ma.
23
Chapter 2
Tintina Gold Province
The Blue Lead and Grey Lead mines, ~40 km ESE of the Liese Zone at the eastern end
of the “Pogo Trend”, had limited historical gold production from small, discontinuous, highgrade gold-quartz veins and stockworks along a gneiss and granodiorite contact. Auriferous
veins have massive to vuggy white quartz with arsenopyrite, pyrite, stibnite, boulangerite
and jamesonite, and have an association with Pb and Sb which is unlike the Liese zone
mineralization.
Tombstone Gold Belt, central Yukon
Gold deposits and occurrences in the Tombstone gold belt of Yukon include the Brewery
Creek deposit with about 270,000 oz of past production, the Dublin Gulch deposit with a
geological resource of ~ 4 Moz, and various placer deposits with about 500,000 oz of gold
production. The mineral occurrences are coincident with the broad arc-like trend of the 550km-long Tombstone plutonic suite that spans central Yukon (Fig. 4). Plutons, and associated
gold mineralization, are mainly in Neoproterozoic to Mesozoic continental margin strata of the
Selwyn Basin. Middle Jurassic and younger northerly-vergent folds and thrusts deformed all
units into the current ENE-trending fold belt geometry of the northern Selwyn fold belt (Gordey
and Anderson, 1993). This includes the thrust-bounded assemblages marked by the Dawson,
Tombstone, and Robert Service faults. A broad east-trending, shallow-dipping zone of highlystrained lower greenschist-grade psammitic rocks, known as the Tombstone strain zone, cuts the
thrust faults (Murphy, 1997).
Several suites of mid-Cretaceous granitoids, known collectively as the Selwyn Plutonic
Suite, were emplaced throughout the deformed Selwyn Basin strata (Anderson, 1983, 1987,
1988; Gordey and Anderson, 1993) and are divisible into the (from oldest to youngest) Anvil,
Hyland River, Tungsten, south Lansing, and Tombstone suites (Mortensen et al., 2000). The
gold-related Tombstone plutonic suite (TPS) consists of more than 110 stocks and plutons which
are typically small (~5 km²) to medium (~100 km²). The TPS granitoids are generally medium
-grained, to porphyritic, hornblende-biotite granodiorite and quartz monzonite, although they
vary from quartz-poor alkalic rocks (tinguaite, syenite) to mafic-rich (pyroxenite, diorite) to
quartz-rich, two-mica granitoids. Aplite and pegmatite occur locally. Geochemically, most of
the TPS rocks are metaluminous, calc-alkaline, I-type with highly fractionated peraluminous
muscovite- or tourmaline-bearing phases. Most TPS plutons have a low primary oxidation state,
high initial Sr ratios (> 0.71; Armstrong, 1988) and heavy δ18O values (14-16 ‰; Marsh et al.,
2001) that indicate a significant crustal component.
Gold occurrences are in the plutons, in the hornfels zones, and in structural zones or
reactive lithologies as far as 20 km from major (exposed) plutons (Hart et al., 2000). Gold
mineralization includes intrusion-hosted, sulfide-poor sheeted quartz veins (Dublin Gulch),
sulfidized epizonal fracture networks and breccias in sills (Brewery Creek), sulfide replacements
of variably calcareous strata (Scheelite Dome), well-developed, high-grade, sulfide-rich skarn
(Marn, Horn), weakly-developed retrograde skarns (McQuesten, Mike Lake), and fault-hosted
sulfide-rich Au-As-Sb veins (AJ, Hawthorne). Tungsten±Au skarns are common, and placer
gold commonly occurs in creeks draining TPS plutons in unglaciated regions. Other significant
deposit types include tungsten skarns, Sn-Ag greisens, and Ag-Pb veins.
The TPS intrusions are well-dated and consistently yield U-Pb ages of 92±2 Ma (Murphy,
1997). Although there are few published dates on the gold mineralization, the available
24
Chapter 2
Tintina Gold Province
analyses are mainly coincident with magma crystallization. At Clear Creek, 40Ar-39Ar dates of
~ 91 Ma have been reported on micas in gold-bearing veins (Marsh et al., 2001) and U-Pb dates
at Emerald Lake overlap both 40Ar-39Ar cooling dates on the magmatic body and on gangue
minerals in quartz veins at about 93 Ma (Coulson et al., in press).
Factor analysis of mineralized rock samples from the Clear Creek and Scheelite Dome each
indicate that there are two geochemically distinct, but paragenetically-related, metal suites
associated with gold. The dominant association is characterized by Au-Te-Bi ± W, As, which is
also characteristic of the Fairbanks district (McCoy et al., 1997). The weaker association of AgPb-Bi ±-Sb, Cu, Au, is more characteristic of Ag-Pb-Zn mineralization of the Keno Hill district
located a few tens of kilometers to the east. Bismuth and Te correlate with gold in the more
deeply-formed deposits, such as Clear Creek and Dublin Gulch (Marsh et al., 1999; Hitchens
and Orssich 1995; Smit et al., 1996; Flanigan et al., 2000; Maloof et al., 2001), whereas Sb and
Hg do so in the more shallowly-developed deposits such as Brewery Creek (Diment and Craig,
1999).
Other Districts
The Ruby-Poorman district (Fig. 1), in west-central Alaska about 500 km west of
Fairbanks, has produced about 500,000 oz of placer gold. Geology is dominated by the Ruby
batholith, a 115-110 Ma, coarse-grained, composite, peraluminous body with trace element
and radiogenic isotope compositions suggestive of crustal melt derivation (Arth et al., 1989).
South of the Kaltag fault and west of the Ruby batholith, several Au-Ag-As-Sn-Zn prospects
and small deposits are associated with plutonic rocks of the same age and composition as the
Ruby batholith. Deposits in the Illinois Creek area are the only lode deposits that have reported
resource and production. These deposits include gold- bearing quartz veins, Zn-Pb-Ag-Au-Snrich replacement zones in carbonate rocks, and the highly-oxidized auriferous Illinois Creek
shear zone that has produced ~56,000 oz Au and 220,000 oz Ag. All of the deposits have
elevated concentrations of Bi and Te, but have significantly higher Ag/Au and Sn/Au ratios than
do the other mid-Cretaceous intrusion-related deposits of interior Alaska (Flanigan, 1998).
Located between the Fairbanks and Goodpaster districts, the Richardson district (Fig. 3)
consists of scattered gold placers with a total production of about 100,000 oz, and one past lode
producer, the Democrat deposit, which produced ~2,000 oz Au (Bundtzen and Reger, 1977).
The district is underlain by rocks of the YTT that are similar to those in the Goodpaster district.
The Democrat deposit occurs in an argillically-altered and densely-veined rhyolite porphyry
dike that is dated at ~90 Ma by 40Ar/39Ar on white mica (McCoy et al., 1997). Mineralization
in the district appears to be at least partly controlled by the northwest-trending Richardson
lineament (Bundtzen and Reger, 1977).
Better known for its VMS potential, the Bonnif eld district located 100 km south
of Fairbanks (Fig. 3), is underlain by Devonian metamorphic rocks of the YTT, which
include significant felsic metatuff as well as carbonaceous and variably calcareous phyllite.
Metamorphism is greenschist facies. The district has historic placer production of about 75,000
oz, and only 7,000 oz of lode gold, all from the Liberty Bell gold mine , which is a replacementstyle deposit adjacent to a mid-Cretaceous intrusion (~ 92 Ma K-Ar; J. Dilles, written comm.,
1993). Mineralization consists of pyrite, arsenopyrite, pyrrhotite, native Bi and Bi±Pb±Sb
25
Chapter 2
Tintina Gold Province
sulfosalts, although the pluton itself also contains anomalous gold associated with an ankeritetourmaline stockworks and fault-hosted veins (Suleyman, 1994).
The Kantishna district, about 170 km southwest of Fairbanks, has a total placer production
of approximately 90,000 oz Au derived from creeks that drained small lode deposits.
Approximately 8,000 oz of gold and 33,000 oz of silver were mined from lodes prior to 1980
(Bundtzen, 1981). Most lodes are within a 60 km-long, northeast-trending fault zone. The
district is underlain by greenschist facies metapelite and amphibolite facies metavolcanic and
metasedimentary rocks of the Spruce Creek sequence that correlate with the Paleozoic schists of
the Fairbanks district (Newberry et al., 1996). Intrusive rocks in the district include hypabyssal
plutonic rocks of intermediate composition, felsic quartz-feldspar porphyry of unknown age
and affinity, and Eocene mafic dikes and plugs. Bundtzen (1981) identified three types of shear
zone-hosted gold veins: 1) arsenopyrite- (scheelite)-Au quartz veins (i.e., Banjo, Spruce Creek);
2) galena-sphalerite-tetrahedrite-siderite veins (i.e., Quigley Ridge); and 3) massive stibnitepyrite-quartz±Au veins (i.e., Slate Creek). Two gold-bearing arsenopyrite-scheelite-quartz veins
in the schist yielded 40Ar/39Ar dates of ~90 Ma whereas hydrothermal mica associated with a
galena-sphalerite-tetrahedrite-siderite vein was~68 Ma (McCoy, 2000).
The Chulitna district, located approximately halfway between Fairbanks and Anchorage
in south-central Alaska (Fig. 1), is underlain by Permian to Jurassic volcanic and sedimentary
rocks (Wrangellia terrane) and Jurassic-Cretaceous flysch of the southern Alaska Range. Gold
deposits in the Chulitna district are primarily associated with hypabyssal, intermediate to felsic,
75-65 Ma reduced porphyritic plutons (Hawley and Clark, 1974; Swainbank et al., 1977). The
Golden Zone is the district’s only significant deposit, and produced minor amounts of Ag, Au,
Cu, and Pb. Recent drilling, however, has delineated a resource of approximately 600,000 oz
Au, in a near-vertical quartz monzodiorite-hosted breccia pipe consisting of altered igneous
rock fragments cemented by quartz-carbonate-sericite-sulfide. Potassium-Ar ages of ~70
Ma for primary biotite and secondary white mica indicate that Golden Zone mineralization
was essentially contemporaneous with plutonism. Gold mineralization is directly associated
with Bi-Te minerals, predominantly occurring as inclusions in arsenopyrite. The Au-As-BiTe metallogeny is similar to that seen in deposits throughout southwestern and east-central
Alaska and the Yukon. However the presence of economically significant copper and latest
Cretaceous dates suggest a closer association with deposits of the Kuskokwim region. (Gage
and Newberry, 2001).
The Circle district, 90 km northeast of Fairbanks (Fig. 3), produced more than 1 Moz of
placer gold since the original discovery in 1893 (Masterman, 1991). To date, the Circle district
has had no reported lode production but recent exploration activity has discovered several
occurrences, including Table Mountain, Joker and Portage Creek. The district is underlain
by polymetamorphic schists of the YTT, that are intruded by two plutonic suites. The first
is composed of intermediate to felsic calc-alkaline intrusions, similar to other gold-related
plutons of the TGP. The largest of these, the Two-Bit pluton, forms the uplands above many
of the highly productive placer creeks in the area and yields maximum 40Ar/39Ar ages of 89 Ma
(McCoy et al., 1997). The predominant Au-As±Bi±Te±Zn occurrences, however are either
within or peripheral to smaller plutons and dikes.
The second plutonic suite consists of highly fractionated, variably peraluminous, alkalic to
26
Chapter 2
Tintina Gold Province
very felsic syenomonzonite and granite plutons with accessory garnet and fluorite. This 58-50
Ma suite hosts sub-economic Sn-Ag±Bi±U mineral occurrences (Masterman, 1991; McCoy et
al., 1997). The Eocene magmatic event, particularly the emplacement of the large Circle Hot
Springs intrusion, has mainly reset many dates from earlier calc-alkaline plutons and associated
occurrences resulting in complicated interpretations and an impression that the district is
principally early Tertiary in age (McCoy et al., 1997). Despite Early Tertiary dates from the
various gold occurrences, the geological attributes of the Table Mountain, Hope-Homestake,
Joker and Portage Creek occurrences are more similar to those with a mid-Cretaceous plutonic
association.
The Klondike, Forty-mile, Sixty-mile, Eagle districts, which occur along and near the
Yukon-Alaska border, are principally placer gold producers, with subordinate lode production
and no current reserves. Placer production from the Klondike district has been more than 13
million crude ounces of gold, but only about 50,000 oz of lode production is known. Forty-mile
(combined Yukon and Alaska sides), Sixty-mile and Eagle placer production is about 800,000,
330,000 and 50,000 oz respectively. Each of these three districts is underlain by juxtaposed
sub-terranes of the YTT and obducted ophiolite sequences of the Slide Mountain/Seventy-mile
terrane. Although the Klondike region is cut by Late Cretaceous and Eocene dikes and small
plugs of felsic magmatic rocks, Mesozoic plutonic rocks are otherwise lacking. The gold lodes,
which have unequivocal characteristics of orogenic gold deposits, have few available dates,
notably 134 and 140 Ma K-Ar dates on muscovite from one of the Klondike vein occurrences
(Rushton et al., 1993).
The Forty-mile district contains numerous Late Triassic to Early Jurassic plutons and
the nature of the gold mineralization is variable, possibly indicating several hydrothermal
episodes (Newberry, 2000). The nature of gold mineralization in the Sixty-mile district is also
variable, although some gold is probably related to Late Cretaceous intrusions (Glasmacher
and Friedrich, 1992). The Eagle district has no significant lode resource, but has several small
lode prospects and has had minor placer production. Several occurrences throughout the Eagle,
Sixty-mile and Forty-mile districts were interpreted to suggest the presence of: 1) 184 Ma
intrusion-related Au; 2) mid-Cretaceous intrusion-related Au and/or W; 3) Late Cretaceous CuMo-Au, and 4) Eocene epithermal Au-Hg mineralization (Newberry et al., 1997).
The Dawson Range belt, in west-central Yukon (Fig. 4) hosts mainly porphyry copper
and associated epithermal gold occurrences, and numerous small placer deposits. Episodic
lode production has generated about 50,000 oz Au, whereas the placer production exceeds
200,000 oz. The Dawson Range is mainly underlain by YTT metamorphic rocks intruded by
metaluminous I-type, mid-Cretaceous batholiths, and variably alkalic Late Cretaceous plugs.
Both suites have a high primary oxidation state. Notable gold occurrences are at Mount
Freegold, Mount Nansen, and the large, low-grade Casino Cu-Au-Mo porphyry. In westernmost
Dawson Range near the Alaskan border, the Moosehorn Ranges area host numerous gold veins,
such as the Longline deposit, and associated residual placers. Unlike most Dawson Range
occurrences, the Longline veins occur in a structural array and have characteristics similar to
veins associated with orogenic gold deposits such as banding textures in the veins, high Au:
Ag ratios, low base-metal grades, and coarse gold. The veins contain arsenopyrite, but are not
anomalous in bismuth.
27
Chapter 2
Tintina Gold Province
Like the Tombstone belt, the Tungsten and Hyland River belts of eastern Yukon overlap
with the areas containing mid-Cretaceous plutonic rocks that intrude Paleozoic and older
Selwyn Basin strata. The Tungsten suite is exposed across a 220-km-long region between
the Mactung deposit to south of the Cantung deposit (Fig. 4). The plutons of this belt have
lithological and geochemical similarities with the TPS, but are typically larger (to 250 km²),
are more differentiated with a greater proportion of peraluminous two-mica granites, and are
slightly older (e.g. ~97 to 95 Ma, Mortensen et al., 2000). Mineralization is dominated by the
large Mactung, Cantung and Clea tungsten skarns with only a few small gold occurrences.
The Hyland River belt of plutons (Tay River of Heffernan and Mortensen, 2000) include large
intrusive masses that occupy ~ 40 % of the area (Fig. 4). The batholiths are dominated by
magnetite-bearing, peraluminous, 98-96 Ma, two-mica monzongranites but the region also
contains plutons of the older Anvil suite in the west, and younger Tungsten suite in the east
(Heffernan and Mortensen, 2000). Associated gold occurrences include the Hyland, Fer, HY,
Hit, Sun and Sprogge. All consist of structurally controlled zones of breccias, stockworks
and veins with associated sulfide replacement bodies in adjacent calcareous strata. Pyrite and
lesser arsenopyrite are the dominant sulfide minerals, but anomalous bismuth is reported at the
Hyland deposit, which has a Main Zone oxide resource of 350,000 Moz Au. Mineralization is
interpreted to lie above associated plutons as intrusive rocks are not exposed at surface.
Summary
The numerous districts have a range of characteristics (Table 2), but most are dominated by
placer production as their hardrock sources are insignificant. Four districts have experienced
significant increases in lode exploration, specifically in areas with Cretaceous intrusive rocks,
with ensuing discoveries and resource definition - Fairbanks district (17 Moz), Kuskokwim
mineral belt (17 Moz), Goodpaster district (6 Moz), and Tombstone gold belt (5 Moz).
Metallogenically, the Fairbanks and Tombstone gold districts are also characterized by the
numerous tungsten and lesser tin occurrences, whereas tin and mercury-antimony deposits
characterize the Kuskokwim region. The Goodpaster district lacks a recognized metallogenic
association with gold. Exploration for gold ores associated with Cretaceous intrusions is also
notable in the Ruby (Illinois Creek), Kantishna (Quigley Ridge), Chulitna (Golden Zone),
Bonnefield (Liberty Bell) and Circle (Table Mountain) districts in Alaska, and in the Dawson
Range (Longline) and Sixty-Mile districts in the Yukon. Although most TGP districts are
underlain by rocks of the YTT, the Kuskokwim region is an overlap sedimentary assemblage
that is partly south of the Denali-Farewell fault system, the Chulitna district is underlain by
rocks of the Wrangellia terrane to the south of the Denali fault system, and the Tombstone,
Tungsten and Hyland River districts are on the ancient North American continental margin.
The tectonic setting(s) associated with emplacement of the Cretaceous plutonic suites, and
formation of the gold lodes, are not well defined. Geochemical data on the intrusive rocks
indicate variable settings, with tectonic discriminant plots (i.e., Pearce et al., 1984) indicating:
1) Plutons of the Fairbanks district are arc-related; 2) Tombstone gold belt plutons overlap the
volcanic arc, within-plate and syn-collisional fields; and 3) the Late Cretaceous Kuskokwim
intrusive rocks fall in the within-plate, syn-collisional and arc fields, although these plutons
were notable effected by hydrothermal alteration (Newberry, 2000; Flanigan et al., 2000).
The variability in geochemical data may partly reflect widespread alteration of many of the
28
Chapter 2
Tintina Gold Province
Table 2. Districts and Deposits of the Tintina Gold Province divided by approximate ages.
Age
Late Cretaceous
71-67 Ma
75-65 Ma
Mid-Cretaceous
92-90 Ma
District
Deposits
Kuskokwim
Donlin Creek
Nixon Fork
Vinasale
Shotgun
Golden Horn
Golden Zone
Chulitna
Fairbanks
90 Ma
Richardson
Kantishna
92 Ma
89 Ma
Bonnif eld
Circle
94-92 Ma
96-92 Ma
107-104 Ma
~111 Ma
Jurassic and older?
140 Ma and older?
Tombstone
Tungsten/Hyland
Goodpaster
Ruby-Poorman
Klondike
Forty-Mile
Sixty-Mile
Eagle
Fort Knox
Ryan Lode
Gil
Cleary Summit
Ester Dome
True North?
Democrat
Banjo
Slate Creek
Liberty Bell
Table Mountain
Joker
Portage Creek
Brewery Creek
Dublin Gulch
Scheelite Dome
Clear Creek
Marn
Hyland
Pogo
Blue Lead
Illinois Creek
Sheba, Lone Star
intrusions, but most likely indicates that the ca. 90-70 Ma calc-alkaline intrusions of interior
Alaska and Yukon were emplaced into a variety of tectonic settings along a complexly
deforming Cretaceous continental margin.
Fundamental features of gold deposits throughout most of the TGP are their associations
with Cretaceous felsic intrusions that are reduced and radiogenic, and marine sedimentary or
meta-sedimentary country rocks. Deposits allied with mid-Cretaceous intrusions are as old
as 111 Ma (Illinois Creek deposit), or 104 Ma (Pogo deposit), but are dominantly 96 to 90 Ma
(Fort Knox deposit, Tombstone gold belt). Late Cretaceous intrusive rocks and associated
gold mineralization are mainly 71-65 Ma within the Kuskokwim mineral belt and Chulitna
district. Most gold-associated plutons in the aforementioned districts and belts have low
primary oxidation states, as indicated by low ferric/ferrous ratios (0.2 to 0.5), low magmatic
susceptibilities, and ilmenite>magnetite (Newberry et al., 1995; McCoy et al., 1997; Lang and
Baker, 2001). This feature appears to influence the magma’s ability to become enriched in gold
during crystallization but the mechanism is poorly constrained (Thompson et al., 1999). Plutons
of these districts also have elevated initial Sr and Nd ratios, radiogenic lead ratios and elevated
δ18O values (Aleinikoff et al., 1987, 1999; Armstrong, 1988; Newberry et al., 1995; Newberry
and Solie, 1994, 1995; Farmer et al., 2000; Marsh et al., 2001). These signatures indicate
a component of crustal contamination that may play a role in their dominantly lithophile
metallogeny (Thompson and Newberry, 2000). Most TGP associated Cretaceous plutons have
intruded sedimentary or meta-sedimentary strata with variably reductive character and it is not
known if the reduced signature of the magmas is primary or acquired feature.
29
Chapter 2
Tintina Gold Province
Styles of Gold Mineralization
The TGP is characterized by a wide range of mineralization styles that have spatial and
temporal relations with reduced and radiogenic Cretaceous felsic intrusions rocks. As a result of
exploration and research efforts, mostly in the Fairbanks district and Tombstone gold belt, many
of these styles of mineralization have been combined into an inclusive model for what have
become known as intrusion-related gold systems (Thompson et al., 1999; Lang et al., 2000;
Thompson and Newberry, 2000). Such deposits have the following features: 1) significant
gold but uneconomic base metals; 2) characterized by lowfS2 ore assemblage; 3) association
with reduced, ilmenite-bearing, I-type intrusions; 4) spatially-associated tungsten and tin
occurrences; 5) a Bi-Te±W geochemical association for intrusion-hosted, and intrusion-proximal
deposits, and an As-Sb±Hg signature for more distal country rock-hosted mineralization;
Despite their considerable variation in mineralization style, many intrusion-related deposits
occur in a predictably zoned fashion with respect to a central, causative intrusion. Also
commonly included within this model are epizonal styles of gold of mineralization, such as
those that characterize the Donlin Creek deposit, and shear-related veins including Pogo’s Liese
deposit. Descriptions of TGP mineralization are presented below in terms of these three main
styles: 1) intrusion-centered; 2) shear-related; and 3) epizonal (Fig. 8).
Figure 8.
Schematic
representation
the
three
main styles of mineralization
associated with Cretaceous
intrusive rocks in the Tintina
Gold Province. The genetic
relationships that the various
styles have with magmatism,
as well as the nature of the
relationships between the
styles, are the focus of current
research efforts.
Intrusion-centered
Intrusion-centered mineralization includes the range of deposit styles developed within
plutons and in the surrounding thermal aureoles, including sheeted vein arrays, skarns, and
replacements, and breccias. They occur in outwardly zoned patterns with predictable variations
in style, mineralogy and geochemical associations. As such, these various styles are the defining
components for the intrusion-related gold system model, particularly defining characteristics of
gold deposits that seemingly must be genetically associated with the central intrusion. These
patterns have been fully recognized mainly with respect to the mid-Cretaceous intrusions of
the TGP, although Late Cretaceous intrusions have associated skarn mineralization (i.e., Nixon
Fork; see Cutler, 1994; Newberry et al., 1997). .Dublin Gulch (Fig. 9) and Fort Knox (Fig. 10)
are excellent examples of intrusion-centered mineralizing systems as they have several styles of
gold mineralization.
30
Chapter 2
Tintina Gold Province
Figure 9. Distribution of varying styles of mineralization, placer gold and placer
scheelite at Dublin Gulch. Bismuthinite and scheelite has also been found in the
placer concentrates. Data from Boyle (1965).
Figure 10. General geology of the Fort Knox pluton (from Bakke (1997). Sheeted
veins occur throughout the pluton. The Stepovich W-Au skarn is southwest of the
pluton, beyond the limits of the figure.
Sheeted Veins
Sheeted vein arrays are characteristic of intrusion-related gold systems and are widely
developed at Fort Knox and Dublin Gulch, but occur to some extent in many of the midCretaceous plutons of the Fairbanks district and Tombstone gold belt (see Bakke, 1995; Hitchins
and Orssich, 1995; McCoy et al., 1997, Maloof et al., 2001). The steeply-dipping parallel
veins are millimeters to centimeters thick, continuous for several tens of meters (Fig. 11), and
typically form arrays in the apical parts of small plutons or in adjacent wall-rocks (Fig. 12).
31
Chapter 2
Tintina Gold Province
Figure 11. Typical steep-dipping, intrusion-hosted, sheeted auriferous quartz vein array.
This example is from Clear Creek area, Yukon. Marker pen is 14 cm.
The veins are dominated by massive, translucent to clear gray, and less often white quartz, with
subordinate coarse-grained, white alkali feldspar and mica commonly on the margins. Total
sulfide mineral content is low, varying from 0.1 to 3%, and includes pyrite>pyrrhotite>arsenop
yrite with less common native bismuth, tetradymite, bismuthinite and molybdenite. Scheelite
is locally common (to 1%). Many vein arrays have parallel and proximal pegmatite, aplite or
lamprophyre dikes, and locally, pegmatite dikes show upward and along-strike transitions to
sheeted quartz veins (Bakke, 1995). Vein density ranges from 1 to 20 per meter. Individual
veins typically have grades of 3-50 gpt Au. Alteration varies from visually absent to zones
2-3 times wider than vein thickness and can include albitic or potassium feldspar selvages,
carbonatization, and/or sericitization. Gold typically occurs as free-gold adjacent to or within
bismuth minerals. Locally maldonite (Au2Bi) occurs, or more commonly, wormy intergrowths
of Au and Bi indicate maldonite exsolution.
Variations of this style include interconnected bifurcating, white quartz veins that cut most of
the sheeted vein ore at the Fort Knox deposit, but these are considerably less common at other
deposits. The have a similar dominant orientation to the sheeted veins, range from hairline
to several centimeters in width, and average about 15 per meter at Fort Knox (Bakke, 1995;
McCoy et al., 1997). The veins also contain coarse-grained white mica, have sericitic selvages
and more sulfide minerals and are characterized by quartz-sericite alteration of adjacent wallrock. Their grades are erratic although locally very high. Bismuthinite and native gold grains
often occur together whereas native bismuth and maldonite are absent (McCoy, 2000). Another
variation is apparent at the Emerald Lake occurrence where parallel vein arrays link large
miarolitic cavities (Duncan et al., 1997). None of the variants includes true multi-directional
stockwork veins such as those that are typical of porphyry deposits.
32
Chapter 2
Tintina Gold Province
Figure 12. Generalized cross-section of the Dublin Gulch Eagle Zone. Distribution
of sheeted veins are schematic but emphasize their position near the apex of the pluton
adjacent to, and locally beyond, the wall-rock contracts. Modified from Smit et al., (1996)
with drill-intersection from Hitchins and Orrsich (1997)
Skarns
Gold and tungsten skarns occur near mid-Cretaceous plutons throughout the Fairbanks
district (Pedro, Spruce Hen, Stepovich) and Tombstone gold belt (Horn, Marn, Scheelite Dome),
and with Late Cretaceous stocks in the Kuskokwim mineral belt (Nixon Fork). However, the
TGP’s largest skarns are the scheelite-dominated Mactung, Cantung and Ray Gulch deposits
in Yukon. Skarns specifically characterized as Au±W skarns (Au> 2g/t) occur adjacent to
metaluminous calc-alkaline plutons that host sheeted vein ores. The Stepovich skarn, for
example, is immediately south, and ~700 m above the Fort Knox intrusion (Fig. 6). Skarns are
dominated by fine-grained aggregates of pyroxene, biotite and less commonly garnet. Ores
occur as sulfide-rich assemblages characterized by pyrrhotite ± chalcopyrite ± bismuthinite ±
scheelite that overprints a retrograde silicate assemblage (Allegro, 1987; Newberry et al., 1997).
Some Au±W skarns contain significant arsenopyrite, such as the Table Mountain skarn (McCoy
et al., 1997). The Ray Gulch tungsten skarn is adjacent to the Dublin Gulch pluton (Fig. 9), but
lacks retrograde alteration and gold (Brown, 2001).
Gold±Cu skarns, such as the Horn, Marn (Brown and Nesbitt, 1987) and Mike Lake
occurrences (Lilly, 1999) are often adjacent to alkalic plutons of the western Tombstone gold
belt (Fig. 4). Skarn assemblages are dominated by coarse-grained hedenbergitic pyroxene,
almandine garnet and wollastonite. The overprinting sulfide mineral assemblage is coarsegrained and dominated by massive pyrrhotite with ~2% chalcopyrite, 0.5% bismuthinite and
free gold with fluorite, and locally scapolite. Gold grades are typically high (8-40 gpt Au). On
the other side of the TGP, the Au±Cu skarn zones at Nixon Fork appear as vertical pipes that
are zoned from garnet cores, through garnet-pyroxene zones, to wollastonite-idocrase-scapolite
rims. They show a spatial association with granite porphyry dikes that cut the main Late
Cretaceous quartz monzonite to quartz monzodiorite stock (Newberry et al., 1997).
Skarns with weakly developed, fine-grained calc-silicate assemblages are typically distal to
plutonic contacts. The Gil deposit, for example, located ~ 9 km east of the Fort Knox deposit
(Fig. 6), is dominated by variably retrograded pyroxene, amphibole, chlorite, calcite and epidote
(Bakke et al., 2000). Gold mineralization hosts unevenly distributed amounts of pyrrhotite and
33
Chapter 2
Tintina Gold Province
Figure 13. General geology of the Scheelite Dome area indicating the extensive gold
mineralization outside of the pluton in, and beyond the thermal aureole (from Mair et al., 2000).
arsenopyrite, lesser chalcopyrite and variable scheelite. Gold associated trace elements include
Bi, Te±Mo±W±As (Bakke et al., 2000). However, much of the Gil skarn assemblage contains
quartz veins similar to those in the Fort Knox, and they may carry the gold. The McQuesten
and Aurex skarn occurrences in the Tombstone gold belt, consist of gold-enriched horizons in
weakly developed skarn assemblages about 12 km from the nearest exposed pluton. Positive
correlations of gold with bismuth and tungsten are evident, and arsenic anomalies are also
common.
Replacements, Disseminations & Breccias
Non-skarn-bearing mineralization styles within the hornfelsed aureole, either above or
adjacent to a mid-Cretaceous pluton, include variable replacements of and disseminations in
calcareous and non-calcareous hornfels, and hydrothermal breccias. Polymetallic replacement
bodies with elevated gold values have been recognized at the Cheechako and Nordale
occurrences in the Fairbanks district. Disseminated to semi-massive sulfide replacements of
calc-phyllite in the hornfels zone at Scheelite Dome are dominated by pyrrhotite and lesser
auriferous arsenopyrite are widespread over several kilometers and cut by several generations
of thin quartz-sulfide veins (Fig. 13; Mair et al., 2000; O’Dea et al., 2000). Replacementstyle mineralization at the enigmatic Liberty Bell deposit in the Bonnifield district consists of
arsenopyrite, pyrrhotite and bismuthinite with associated tourmaline-K-feldspar alteration near
mid-Cretaceous granite dikes (Suleyman, 1994). The occurrence of rare garnet and epidote has
encouraged this deposit to be classified as a skarn (Newberry et al., 1997). Decalcification and
silica replacement of sandstone was reported in Brewery Creek’s North Slope zone (Diment and
Craig, 1999).
Breccia-hosted mineralization is recognized in thermal aureoles in the Tombstone gold
belt. Structural breccias occur in faults and at fault intersections at several locations, most
notably at the Scheelite Dome (O’Dea et al., 2000) and Clear Creek (Stephens et al., 2000)
occurrences. Magmatic-hydrothermal breccias are not common. The best such example may
be the Bear Paw zone at Clear Creek. This is a clast-supported breccia dominated by country
rock schist fragments with lesser granite fragments in a dark matrix of milled rock fragments
34
Chapter 2
Tintina Gold Province
Figure 14. General geology
and distribution of veins,
placers and faults on Ester
Dome. Veins from Robinson
et al., 1990 and unpublished
data. Faults from Newberry
et al., 1996. 1-#2 Grant; 2
Stanford; 3-Alder; 4-Elmes; 5Kevin’s Dream; 6-Dorothy; 7Yellow Eagle; 8-Silver Dollar;
9-Lincoln;
10-Killarney;
11-Farmer;
12-McQueen;
13-St. Jude; 14-St. Paul; 15Clipper; 16-Wandering Jew;
17- Mohawk; 19-McDonald;
20-Borovich; 21-Hudson; 23Maloney; 24-Lookout. Dark
yellow areas represent mined
placer deposits as shown on
Robinson et al. (1990).
and fine-grained tourmaline(?) and chlorite(?) that contains up to a few percent pyrrhotite, pyrite
and chalcopyrite (Stephens and Weekes, 2000). A quartz matrix is locally present. Fractured,
veined, brecciated and tourmaline-altered hornfels with quartz veining and sporadic gold values
and anomalous Au, Bi, As and Sb outcrop at the Red Mountain and Black Hill occurrences near
the town of Mayo (Murphy, 1997). In the Chulitna district, the Golden Zone deposit appears
as a copper- and gold-rich breccia pipe, which has been suggested to be the top of a gold-rich
porphyry system (Swainbank et al., 1977).
Shear-related ores
The most-common style of mineralization throughout the TGP ranges from large quartz
bodies in ductile shear zones, to brittle, gouge-filled sulfide-rich anastamosing shears, to
disseminated ores along and adjacent to shears. Most of these ore-types are simply veins hosted
in country rocks at some variable distances from, or locally within, Cretaceous intrusions.
The most significant vein resources are in the Ester Dome and Cleary Summit regions in the
Fairbanks district, and at the Pogo property in the Goodpaster district. The genesis of these ores
is less clear than the above intrusion-centered mineralization systems as their origins may relate
to a specific plutonic body or, in contrast, to a more regional process.
The approximately two dozen or so significant veins on Ester Dome are mainly within a
40 km² area and include the Ryan Lode, Grant, McQueen, Mohawk and Ready Bullion which
are small past-producers, as well as the more recently discovered Silver Dollar and Rhyolite
occurrences (Fig. 14). All but the southern Ryan Lode are hosted in schist. The veins vary from
NNE- to ENE –trending, with the Ryan Lode and Ready Bullion in parts of more regional,
NE-trending shear zones, whereas the other veins are mainly in secondary faults related to leftlateral shears (LeLacheur, 1991; Cameron, 2000). The shear zones generally exhibit brittle
features with associated clay-rich fault-gouge, fault breccias or cataclasite. Associated sulfide
35
Chapter 2
Tintina Gold Province
Figure 15. Distribution of veins in the Cleary Summit area. 1-Eagan; 2-Hi-Yu; 3-Basham;
4-Whitehorse; 5-Mizpah; 5-Too Much Gold; 7-McNeil; 8-Ohio-Mayflower; 9-Stringer; 10Pennsylvanian; 11-Antimiony; 12-American Eagle; 13-Henry Ford; 14-McCarty; 15-Nordale; 16Ebberts; 17-Pioneer; 18-Spirit; 19-Chatham; 20-Christina; 21-Quemboe; 22- Foster Hungerford;
23-Jupiter-Mars; 24-Blue Moon; 25-Rex; 26-Anna Mary; 27-BP; 28-Wyoming; 29-Cleary Hill;
30-Colorado; 31-Wackwitz. Modified from Robinson et al., 1990. Position of cross-cutting faults
approximated from Newberry et al., 1996.
minerals include arsenopyrite and pyrite, with late stibnite. Some veins, such as at the Ryan
Lode, have associated zones of diffuse fracture-controlled, low-grade mineralization.
Past-producing deposits in the Cleary Summit area include those developed on the Hi-Yu,
Christina, Cleary Hill, McCarty and Chatham veins. These veins combine to form an 8-kmlong array dominated by NW-trending and lesser NE-trending veins in schists of the YTT,
which follow along the northern limb of the ENE-trending Cleary anticline (Fig. 15). The veins,
mostly occurring as open-space fractures, are dominated by massive white and ribbon quartz,
have high gold grades (~10 g/t) and have a variable sulfide mineral content. Gold-related
phases include arsenopyrite, stibnite, jamesonite, and lesser tetrahedrite, boulangerite, pyrite,
galena and rare bismuth-bearing minerals (Metz, 1991; Walsh and Rao; 1991; McCoy et al.,
1997).
The Liese zone veins at Pogo occur as two or more, gently-dipping, parallel and tabular
polyphase quartz bodies in gneiss that range from 1- to 20-m-thick (Smith et al., 1999). The
earliest stage of mineralization is characterized by narrow shear veins that are largely ductile
in nature, having formed during reverse motion along low-angle mylonite zones. Where the
vein margins are not extensively sheared, extensional sheeted veins are present in the hangingwall. Distinct zones of early quartz are granular with interstitial sericite or microcline (Moore,
2000). These are exploited by the much more voluminous main stage veins, which consist
of white massive to banded quartz veins with pyrite-arsenopyrite bands that were emplaced
during a change to brittle deformation and normal motion along the host structures (D. Rhys
pers. comm., 2000). These veins are cut by narrow, steeply-dipping, sulfide-rich breccias. The
Liese veins contain ~3% ore minerals, including pyrite, pyrrhotite, löllingite, arsenopyrite,
chalcopyrite, bismuthinite, molybdenite, galena, sphalerite, various Ag-Pb-Bi±S minerals,
maldonite, native bismuth, and native gold. Early biotite and later quartz-sericite stockwork and
sericite-Fe-dolomite alteration are spatially associated with the Liese zone.
Shear-hosted veins at the Longline occurrence in western Dawson Range district in
westernmost Yukon, occur within a structurally-controlled array hosted by mid-Cretaceous
granodiorite (Ritcey et al., 2000). Veins are as thick as 1 m and dominated by massive
and multiphase, coarse-grained white quartz. Many veins are locally vuggy, banded, have
ribbon textures or rarely, sheared. Sulfide minerals, totaling less than a few percent, include
36
Chapter 2
Tintina Gold Province
arsenopyrite, galena, sphalerite, stibnite and boulangerite. Gold occurs as free grains to 2 mm
or as infillings in sulfide minerals, mainly arsenopyrite.
In the Tombstone gold belt, arsenopyrite- and stibnite-rich veins, such as the AJ (Antimony
Mountain) and Hawthorne (Scheelite Dome) veins, are country-rock hosted and within a
few hundred meters of the intrusion, and are probably not unlike veins on Ester Dome in the
Fairbanks district. However, silver-rich galena veins, such as the Peso and Rex near the Dublin
Gulch deposit, are typically several, to several 10s of kilometers from the nearest exposed
pluton. The silver veins of the Keno Hill district, which have produced > 200 Moz Ag, are 10
to 40 km from the nearest exposed pluton. Stibnite forms pods in fault zone-hosted quartz veins
at the Scrafford, Frederich and Gilmer deposits in the Fairbanks district, and continuous goldrich veins with documented antimony resources at the Stampede and Slate Creek deposits in the
Kantishna district.
Quartz-filled shears in the Fort Knox intrusion form moderately-dipping zones of brecciated
and granulated white quartz that range in thickness from 0.3 to 1.5m and are continuous
for hundreds of meters (Fig. 10). The shears contain micron-sized gold associated with
bismuthinite or bismite that add considerably to the total resource of the Fort Knox deposit.
Intense phyllic alteration also overprints argillic alteration on vein margins and clay gouge, but
may be related to post-gold hydrothermal alteration or supergene oxidation. The crushed nature
of the quartz results from small (< 3m) displacements along the shears.
Epizonal Mineralization
Some of the most important TGP deposits, with considerable gold resources, have features
compatible with emplacement at low temperatures and pressures. Notable examples of these
high-level, perhaps best termed epizonal deposits include the True North (Harris and Gorton
1998; Bakke et al., 2000), Brewery Creek (Diment and Craig, 1999; Lindsay et al., 2000) and
Donlin Creek (Ebert et al., 2000) deposits. All are characterized by structurally-controlled
fracture networks of thin quartz-sulfide veinlets and matrix-filling breccias. The deposits are
hosted in carbonaceous strata or dike/sill systems that have intruded the sedimentary rocks. The
veins are locally drusy or composed of fine-grained quartz, and the sulfide mineral assemblage
is dominated by arsenopyrite and pyrite with late stibnite. In rocks susceptible to alteration,
it is most often characterized as weak phyllic, dominated by the formation of illite, weak
silicification and disseminated pyrite. The deposits are also characterized by a similar metal
assemblage of Au-As-Sb-Hg (Flanigan et al., 2000). In contrast to the more deeply-formed
intrusion-centered and shear related ores, concentrations of Bi and W in the epizonal ores are
typically below analytical detection limits.
Structurally, Brewery Creek (Fig. 16) and True North (Fig. 17) consist of several ore-bodies
controlled by secondary structures (and their intersections) in the hanging-wall of a regionally
significant thrust faults. Both Brewery Creek and Donlin Creek appear to have experienced a
degree of extension following compression. Most Donlin Creek ores are structurally-controlled,
as they are hosted in rigid intrusive rocks with contrasting rheology to the adjacent shales (L.
Miller, oral commun., 2000). High-grade zones occur in more densely fractured rocks. Where
unoxidized, gold is mostly in solid solution with sulfide minerals (even in extremely high-grade
samples), either in arsenopyrite or in arsenian rims surrounding pyrite (Diment and Craig, 1999;
37
Chapter 2
Tintina Gold Province
Figure 16. General geology of the Brewery Creek area.
Figure 17. General geology of the True North area, Alaska. Modified from
Bakke et al. (2000).
Ebert et al., 2000; McCoy, 2000). Oxidation has liberated the gold in near surface zones at True
North and Brewery Creek to depths of about 30m, leaving untested sulfide resources at depth.
Summary and implications for ore genesis
Each of the three broad mineralizing styles has been included in the intrusion-related
gold system model. The differences between them reflect different conditions of formation
(Thompson and Newberry, 2000). However, the nature of each style’s genetic links with
magmatism are not always evident, particularly for the shear-related and epizonal examples.
Links are typically characterized by numerous geological features, but where ambiguous,
38
Chapter 2
Tintina Gold Province
geochronological or geochemical constraints are highlighted.
Intrusion-centered mineralizing systems typically have intrusion-hosted sheeted vein arrays
as a central tenet. These systems have adjacent gold or tungsten skarns where suitable hostrocks are available. Within the thermal aureole, gold-bearing replacements, disseminations,
more distal skarns and breccias occur. The variability of these occurrences reflect lateral
variations in temperatures and changes in nature of the fluids away from a central pluton,
with additional complexity supplied by variations in structural controls and reactive host-rock
lithologies. These various styles of gold mineralization that are associated with mid-Cretaceous
plutons in the TGP are the basis for the intrusion-related gold model (Fig. 18).
Sheeted vein arrays have clear geological links with magmatism, as identified by the
transitional nature of the auriferous sheeted veins with pegmatite and aplite dikes (Bakke,
1995; McCoy et al., 1997). K-spar, mica, and tourmaline as gangue minerals, as well as
sodic and potassic alteration likely indicate a magmatic fluid source. Temporal links between
mineralization and magmatism as provided by Ar-Ar dates, indicated a gap of a few million
years (McCoy et al., 1997) but new Re-Os dates on ore-stage molybdenite at Fort Knox
indicate that ore formation may have been co-eval with granitoid crystallization (Hart et al.,
2001). Skarn mineralization has a clear spatial and temporal association with the intrusive
rocks, however as gold deposition in skarns typically results from a later, overprinting fluid
(Meinert, 1998), the direct link between the mineralizing fluid and a particular magma is
generally not-straightforward. The diverse styles of gold-bearing sulfide replacements,
disseminations, breccias and distal skarns are developed within the thermal aureoles and as
such have a consistent spatial relationship to magmatism. Limited data suggest that these
replacement deposits at the most distal parts of the intrusion-centered model occurred at still
relatively high temperatures (380-450°C; McCoy et al., 1997; Suleyman, 1994; J. Mair pers.
comm., 2001) or in association with K-feldspar alteration. Importantly, intrusive host rocks are
not the mineralizing magmas as they have cooled enough to be brittley fractured. More likely,
mineralizing fluids were sourced from distal, or underlying plutons.
Shear-related gold veins occur at varying distances from a pluton or thermal aureoles, but
are locally within the intrusions. As such, their genetic relationships with the nearest, or roughly
coeval, pluton is difficult to ascertain (Sillitoe and Thompson, 1998). The gangue and sulfide
mineralogy of shear-related veins purported to be intrusion-related are highly variable and differ
significantly from intrusion-hosted sheeted vein arrays. In general, shear-hosted veins within
the country rocks have higher percentages of sulfide minerals, lack bismuthoids and scheelite,
as well as gangue K-feldspar and micas. This may relate to differences in wall-rock chemistry
such that some elements (e.g. antimony, arsenic, mercury) are locally derived from the country
rock sequences. Zoning from intrusion-proximal As to distal Sb to more distal Pb-Ag veins,
although not well documented throughout the TGP, is consistent with expected metallogenic
variations with decreasing fluid temperatures, particularly if these veins are considered part of
the intrusion-centered model described above. Many vein sets however, such as those at Ester
Dome or Cleary Summit, lack notable zoning. Liese veins have been interpreted to be a deep
variety of reduced intrusion-related gold mineralization where the lack of zoning may result
from the lack of significant thermal gradients between the causative intrusion and the site ore
deposition.
39
Chapter 2
Tintina Gold Province
Figure 18. A geological model of intrusion-hosted, proximal and distal styles of mineralization
that develop around mid-Cretaceous plutons. Modified from Hart et al. (2000); Lang et al.
(2000); and Lang and Baker (2001).
Pogo’s Liese veins are a notable exception to many of the aforementioned shear vein
generalizations as it’s gangue and sulfide mineralogy are similar to sheeted veins. The L1 and
L2 shear veins host bismuthoids and scheelite, and have mica and K-feldspar in the gangue.
Elevated bismuth content or Bi:Au correlations of shear-related veins, has been suggested as
an indicator of a magmatic fluid source (McCoy et al., 1997; Flanigan et al., 2000) however,
alternative fluid sources may obtain such a signature through interaction with granitic rocks or
through enriched country rocks (Goldfarb et al., 2000).
Confident and precise timing constraints of shear vein formation have been difficult to
obtain as argon dates on fine-grained alteration minerals such as hydrothermal white mica give
inconsistent results, due in part to partial resetting, and absolute ages of pluton crystallization
are not well established by U-Pb methods. Two Ryan Lode Ar-Ar plateau ages of alteration
white micas are discordant by one-and-a half million years at 87.6 and 89.1 Ma, and are variably
younger than the ~90.4±0.6 Ma dates from the adjacent pluton (McCoy et al., 1997) for which
U-Pb data is not available. Circa 91 Ma Ar-Ar dates on alteration minerals from Pogo’s Liese
veins are approximately coeval with the nearby 92 Ma Goodpaster batholith (in Smith et al.,
1999), but significantly younger than recently reported Re-Os molybdenum dates of ~104 Ma
(Selby et al., in review). The uncertain reliability of the Ar-Ar dates for most shear-related veins
makes it difficult to determine temporal relationships between mineralization and magmatism,
and thereby assert a temporal association.
Many shear-related veins in the TGP (e.g. Liese zone, Longline, many at Cleary Summit)
have features, such as banded sulfide minerals, ribbon quartz, tensile vein arrays, ductile shears,
lack of zoning etc.. - that are characteristic of orogenic gold deposits (i.e., Groves et al., 1998).
Additionally, many veins lack clear spatial or temporal associations, or zoning relations with a
40
Chapter 2
Tintina Gold Province
central mineralizing pluton and are therefor difficult to accommodate within an intrusion-related
model. However, since some of these veins also have features or spatial-temporal associations
with magmatism, (e.g. Liese, Ryan Lode) a magmatic source cannot be entirely discounted. The
Liese veins for example, may simply reflect a deeper level manifestation of the intrusion-related
model (Thompson and Newberry, 2000).
The Brewery Creek, True North and Donlin Creek deposits have characteristics consistent
with mineralization formed at low temperatures and pressures. These epizonal deposits
are considered to represent higher-level manifestations of the intrusion-related gold model,
in response to lower temperatures, and potentially greater wall-rock influence (Thompson
and Newberry, 2001; Lang and Baker, 2001). Evidence linking epizonal mineralization to a
magmatic source is incomplete. Potentially causative plutons for the True North deposits are
more than 3 km away and igneous rocks within the deposit are limited to a few sparse, but
altered, dikes. Although the Brewery Creek and Donlin Creek deposits are hosted mainly
in sills and dikes, they are passive structural hosts to mineralization with potential source
intrusions for the fluids yet to be identified. Brewery Creek ores are however, within a few
kilometers of an intrusion-hosted sheeted vein complex (Classic zone) and may provide a link
between the two styles, but this hasn’t been proven. Magmatism and mineralization, however,
are essentially coeval at Donlin Creek (Buntdzen and Miller, 1997; Szumigala et al., 1999).
Age dates for Brewery Creek and True North mineralization are not available.
Exploration and Discovery Case Histories
Exploration and discovery case histories are presented for the most significant intrusionrelated gold deposits of the TGP, to emphasize the effectiveness of various exploration
techniques. They are presented according to similarities of deposit type in order to assist the
reader in recognizing trends and themes between them.
Fort Knox
Gold mineralization was known at the current Fort Knox deposit (Fig. 10) for almost 100
years before large-scale mining. This most recent discovery represents the development of a
historic prospect into a bulk-tonnage deposit. Placer gold was discovered in 1901 at the mouth
of Fish Creek, downstream from what is now the Fort Knox mine, but was considered too
meager to be economically worked. Subsequent headwater prospecting for lode deposits in the
and tributaries of Fish Creek resulted in the discovery of tungsten- and gold-bearing skarns.
Gold-bearing quartz veins were discovered in 1913 over what is now the Fort Knox mine, and
a small mill was assembled and three short shafts were sunk. U.S. Geological Survey (USGS)
geologists then noted the occurrence of bismuthinite, and determined the presence of anomalous
tellurium and a genetic connection with a body of porphyritic biotite granite (Prindle, 1913).
Modern work wasn’t undertaken until 1980, when the high gold price encouraged placer
mining in the region, and bismuthinite nuggets containing abundant gold were discovered.
Follow-up prospecting indicated that gold mineralization was not limited to a high-grade
source, but was widespread. There are no published regional geochemical surveys from the
area surrounding the deposit prior to initiation of a resource evaluation in 1987. Sporadic
exploration efforts by various joint ventures continued until 1992, when Amax Gold, Inc.
41
Chapter 2
Tintina Gold Province
obtained the property. By the end of 1992, 427 holes had been drilled (317 RVC, 110 core) for
a total meterage of nearly 80 km, with a 183 m average depth by the end of 1992. RC chips
were panned and heavy mineral assemblages were recorded. Construction and pre-strip mining
began in 1995 with production initiated in November 1996. A 1998 year-end calculation,
incorporating the data for the past production, gave a reserve of 169 Mt averaging 0.93 g/t.
Total gold production to the end of 2001 is approximately 1.8 Moz.
Soil sampling successfully delineated the ore-body in the early stages of exploration. The Chorizon soils, obtained using augers, provided the best geochemical response and defined a zone
of >100 ppb Au that is approximately coincident with the ore-hosting pluton (Bakke, 1995).
Hollister (1991) noted that bismuth in soils over the deposit was an obvious pathfinder, but
that arsenic was not useful. This is due partly to the deposit’s low arsenopyrite concentration,
and that the resulting low level arsenic anomalies are too diffuse to provide meaningful
information. Correlations of Au with Bi, Te and frequently Mo and W were recognized but
are too inconsistent to be dependable pathfinder elements (Bakke, 1995; McCoy et al., 1997).
Bismuth and tellurium show identical distribution patterns to gold, when all three are abundant.
Subsequent bulldozer trenching of anomalous zones confirmed the effectiveness of soil
geochemistry and facilitated geological mapping. The Fort Knox orebody was not responsive
to a ground magnetometer survey as a uniform low magnetic response exists over the intrusion,
host-rocks and mineralized area. However certain units within the country-rock schists,
particularly the amphibolite and some skarn horizons, show a strong magnetic response and the
data are useful in reconnaissance exploration and mapping. Because of the success of the soil
geochemistry, other geophysical techniques have not been widely applied.
Dublin Gulch
The discovery of the Eagle Zone deposit at Dublin Gulch resulted from of the application
of the Fort Knox geological model to a region of analogous geology (Fig. 9). Similar to
Fort Knox, the Dublin Gulch area consists of a small barely-exposed mid-Cretaceous pluton,
associated historical lode gold prospects, a tungsten skarn, and placer gold deposits with
considerable scheelite. In addition, native bismuth, galena and arsenopyrite were reported as
heavy minerals in placer concentrates (Boyle, 1965). Episodic lode exploration since 1901
focussed on the thick (0.2 –1.5 m) sulfide mineral-bearing quartz veins. The Ray Gulch skarn
deposit (5.4 Mt of 0.82 % WO3) attracted exploration interest beginning in 1978.
Seven stream silt samples collected by the Geological Survey of Canada (GSC, 1990) from
a 5.5 x 1.5 km area that surrounds the Dublin Gulch pluton were variably anomalous in gold,
arsenic, and tungsten (Fig. 19). Gold values were as high as 299 ppb, but are typically ~ 30
ppb with arsenic mostly between 300 and 1,300 ppm. Tungsten values are between 40 and 250
ppm, and are most likely derived from the scattered skarns. Half of the samples had anomalous
antimony values from 6to 50 ppm. There were no analyses for bismuth or tellurium, but
bismuth-bearing minerals were reported from placer concentrates, as was cassiterite.
Recognizing the similarities with Fort Knox, Ivanhoe Golfields Ltd., optioned the Dublin
Gulch property in 1991. They targeted a region of anomalous surface soil samples (>50 ppb
Au) that had been identified but not explored by previous exploration programs. Six excavator
trenches over soil anomalies exposed quartz-arsenopyrite-rich quartz veins on both sides of an
42
Chapter 2
Tintina Gold Province
Figure 19. Regional stream
silt geochemical values for
the Dublin Gulch area. Listed
values are Au/As/W, all in ppm
except gold in ppb. All sample
are anomalous in both gold,
arsenic and tungsten. Values
on other locations are for gold
only. Bd=below detection.
intrusive contact and weak gold mineralization within the intrusion. The region was recognized
as a cupola, and a 1991 HQ drill hole in the intrusion intersected 105.2 m of 1.06 g/t Au
(Hitchins and Orrsich, 1995). Subsequent core and RC drilling discovered the newly-defined
Eagle Zone. Gold values in resulting core samples correlate well with bismuth, but not other
metals (Hitchins and Orrsich, 1995). The Eagle Zone deposit has a flat aeromagnetic response.
Brewery Creek
The Brewery Creek deposit was a geochemical discovery. There were no known placer or
lode occurrences in this area, and plutonic rocks had not been previously mapped in the area.
While exploring for Carlin-like deposits in the region in the fall of 1987, Noranda Exploration
Co. Ltd., followed-up a weak mercury geochemical anomaly generated from the National
Geochemical Reconnaissance stream silt geochemical survey data (Geological Survey of
Canada, 1978). Using follow-up soil geochemistry, with subsequent trenching and reverse
circulation drilling, the original discovery was made, and the positions of additional deposits
in eight zones extending over a strike length of 12 km resulted from a similar methodology.
Following changing ownership, development work and permitting, Viceroy Resource
Corporation poured their first gold bar in November, 1996 and reached full production by May,
1997. Production from 1997 to 2001 was approximately 260,000 oz.
Samples from the GSC regional stream silt survey were analyzed for only a few metals in
1987, but indicated that the Brewery Creek area was regionally anomalous in mercury. Most
of the ten closest samples to the deposit were characterized by values of >200 ppb Hg (Fig.
20). This led to a followed-up company silt sampling survey in the Brewery Creek region and
new samples generated anomalous gold values. Analysis of the Geological Survey of Canada
samples for additional metals (GSC, 1990) indicated that the single sample draining what would
become the Brewery Creek deposit, contained 38 ppb Au, 27 ppm As, and 16ppm Sb. The gold
value was the highest concentration within a 1,000 km2 area. In addition, many of the nearest
stream sediments samples were also anomalous in antimony (>7 ppm). Regionally, the 27 ppm
As value is not anomalous, but is elevated with respect to those within a 25 km radius.
43
Chapter 2
Tintina Gold Province
Figure 20. Regional geochemistry of the Brewery Creek area, undertaken prior to the discovery
of the deposits, which are approximated in black. Values are Au/As/Sb/Hg, all in ppm except Au
and Hg in ppb. Values from GSC, 1990. Sample responsible for discovery underlined. Larger
dotes denote anomalous antimony. Although gold anomalies are scarce, the region has an
elevated Sb values. The high Hg values reflect the anomalously high background values of the
Earn Group. Patterned region denotes intrusive rocks. Region of Tintina Trench and Klondike
River valley is underlain by thick alluvium (white). Dark shaded region is underlain by Earn
Group black shales. All other areas are underlain by assorted Selwyn Basin clastic strata.
Soil samples taken along a ridge above the anomalous silt sample, and returned values
>2,000 ppb Au. Subsequent work outlined a 3.3-km-long, east-trending gold-in-soil anomaly,
with 25-2,000 ppb Au and a uniform As:Au ratio of ~1000:1. Anomalous antimony and widely
dispersed mercury anomalies were also uncovered. Trenching uncovered a south-dipping body
of altered and brecciated intrusive rock with one chip sample averaging 3.1 g/t Au over 40 m.
With limited outcrop on the property, soil geochemistry continued to be the primary exploration
method for much of the life of the mine. Soil geochemistry was particularly effective as an
exploration tool, because the region is mainly unglaciated and thus there are no overlying
materials, except for local loess accumulations. Gold-in-soil anomalies were evaluated by
excavator trenching, chip sampling and grid reverse-circulation drilling. Ground magnetic and
induced polarization-resistivity surveys were undertaken with variable success, but the deeply
weathered oxide ores are generally not responsive to most geophysical methods.
True North
True North is an example of a historical discovery that has relied heavily on soil
geochemistry. Trenching has also been valuable, as few natural outcrops occur on the property,
and the nature of the oxidized mineralization remained ambiguous until recently. Placer gold
was first discovered on the south side of Pedro Dome in 1902 and placer mining of Dome and
Eldorado Creeks continued through much of the first part of the century. Prospecting of hardrock sources led to the mining of small amounts of gold and antimony from quartz veins at
the Soo and Hindenburg deposits, as well as development of numerous small shafts on other
prospects. Exploration of the northern part of what is now the True North deposit, in the late
1960s and 1970s, resulted in the discovery of five mineralized zones. Contemporary exploration
was initiated in 1990 when a four-hole, 1000’ program by Amax Gold Inc. yielded encouraging
44
Chapter 2
Tintina Gold Province
results, property expansions and acquisitions. La Teko Resources Inc. acquired the property in
1993 and carried out exploration including adding 241 drill holes. In 1995, Newmont became
a joint-venture partner, and aggressive integrated exploration resulted in the doubling of the
resource to 42 t Au (Harris and Gorton, 1998).
The most successful exploration method consisted of grid-based, hand-held power auger soil
sampling, which was used to acquire material from the bedrock interface (Harris and Gorton,
1998). Thicker, or frozen cover materials, required the use of ATV-mounted auger drills to
obtain reliable samples. Samples were typically screened to–80-mesh, fire-assayed for gold,
and analyzed for pathfinder elements by ICP. Soil geochemistry was followed by excavator or
bulldozer trenching and chip sampling. Follow-up included short RC drilling. Chip samples
were logged for lithology and integrated with high-resolution aeromagnetic data to construct
bedrock geological maps. This resulted in the recognition of stratigraphic and structural
controls on gold mineralization.
Aggressive drilling resulted in the discovery of several additional zones. For example,
five RC holes drilled northwest of the Central Zone extended shallow, gently dipping, oxide
mineralization ~120m beyond the previous limit. A ~1000-m-long, linear, but discontinuous,
gold-arsenic soil anomaly within the Dome Creek Zone anomaly was explored with three RCholes intersected partially oxidized mineralization, with gold values as much as ~5 g/t Au over 6
m.
Donlin Creek
The discovery of the Donlin Creek deposit resulted from follow-up work above historic
placer gold fields in southwestern Alaska. Placer gold was first discovered in the area circa
1909 and has been worked episodically until recent times. Lode mineralization was discovered
in the 1940s, but exploration wasn’t initiated until 1974, when quartz vein samples yielded
anomalous gold and were followed-up with bulldozer trenching. A rock and soil sampling
program, undertaken by Calista Corp. from 1984 to 1987, was followed by bulldozer trenching,
and channel, rock and soil sampling. RC and core drilling outlined several mineralized regions
and identified a resource of ~230,000 oz Au. Greater than 100,000 m of core and RC drilling
by Placer Dome, has resulted in the definition of a 12.3 Moz resource, although that figure will
change following recent drilling by Novagold.
Despite the region’s poor exposures, prospecting of the rubble crop along ridge crests
has been effective in identifying regions of altered intrusive rocks and mineralization (Ebert
et al., 2000). Soil sampling, supplemented with auger sampling to obtain near-bedrock
samples, is effective with values >250 ppb Au indicative of mineralization (Ebert et al., 2000).
Aeromagnetic data show a subtle magnetic depression that defines the region of rhyodacite
dikes, and associated alteration, which host the ores. The low magnetic contrasts between rock
units (6 nT) was enhanced by removing the polar gradient to develop a residual signature, which
results in better detail (Ebert et al., 2000). The IP chargeability yields broad anomalies, but aids
in geological interpretations, whereas resistivity data are less interpretable. Coincident magnetic
lows and gold-in-soil anomalies are the best near-surface targets.
Regional geochemical sampling prior to extensive trenching, was undertaken by the U.S.G.S.
Minus-80-mesh stream sediment and panned, heavy-mineral concentrate samples were collected
45
Chapter 2
Tintina Gold Province
in small-order tributaries above their junctions with the placer mines on Donlin Creek, but only
a few hundred meters downstream from the current Donlin Creek orebody (Gray et al., 1988).
Analyses by semi-quantitative emission spectrography, with relatively high lower determination
limits (e.g. 0.5 ppm Ag, 200 ppm As, 10 ppm Au, and 100 ppm Sb) yielded few favorable
results. But, subsequent analysis of the same stream sediments by ICP by Motooka et al. (1988)
gave values of 110 ppm As on American Creek, 100 ppm As on Queen Gulch, and 41 ppm As
on Snow Gulch. These ICP data are effective at targeting Donlin Creek-style mineralization.
There were no ICP analyses for Au, Te, and Hg, and no anomalies for Ag, Bi, Sb and W,
which had high lower detection limits (0.3, 6.3, 10, and 9.1 ppm, respectively). Re-analysis
using atomic absorption techniques (Hopkins et al., 1991) indicated that the two samples with
highest arsenic by ICP, also contained anomalous gold at 11 and 35 ppb – this latter sample also
contained 1.6 ppm Hg. However, tellurium was less than the 50 ppb lower limit in all eight
sediment samples from the Donlin Creek area. In summary, stream sediments with >40 ppm As
and >10 ppb Au, with less consistently >0.2 ppm Hg, may best target favorable regions in the
Kuskokwim flysch sequence for Donlin Creek-like mineralization.
Surprisingly, heavy-mineral concentrate samples were unsuccessful indicators of potential
targets at Donlin Creek. Two such panned samples contained 2,000 ppm Sn, which could
reflect a high background in the dikes that host the Donlin Creek deposit, or some form of
contamination. Given the dominance of gold-bearing arsenopyrite and stibnite in the deposit,
the lack of anomalous gold, arsenic and antimony in the concentrate samples is surprising. The
fine-grained nature of these minerals may have resulted in their being washed away during the
panning process.
Pogo
Despite a telegraph line along the Goodpaster River valley since the early 1900’s, surface
exposures of large (2 m diameter) quartz boulders with visible gold lay undiscovered less than
1 km away. Results from a late 1970s U.S. Geological Survey regional geochemical survey of
the Big Delta quadrangle also indicated the presence of the “concealed” Pogo deposit. About
two dozen sites surrounding the subsequently discovered mineralization at the head of Pogo
and Liese Creeks and on Shawnee Peak were sampled. Materials included minus-80-mesh and
heavy-mineral-concentrates of stream sediment. For anomaly enhancement, the non-magnetic
fraction of the heavy-mineral-concentrates were analyzed and, in addition to the raw stream
sediments, oxalic acid-leached residues of the sediments were studied. The creek draining
the northwestern side of the Liese veins (sample #668; O’Leary et al., 1978) is characterized
by some of the most anomalous values from the entire regional survey with 0.1 ppm Au in
minus-80-mesh sediments, 500 ppm As in the leachates, and 10,000 ppm W and 2,000 ppm As
in panned concentrates. The arsenic and gold values are most significant, as numerous other
concentrate samples from the quadrangle also contained as much as 1% W with no Au. The
lack of anomalous bismuth in the concentrates is surprising considering its enrichment in the
ores, but this may partly reflect the high lower-determination limit for the element using the
emission spectrographic analytical techniques.
A geological resource of the two gently-dipping tabular L1 and L2 veins was estimated
at > 4 Moz Au by late 1997. Mineralization at Pogo was originally discovered in 1981 by
WGM, Inc. by follow-up prospecting of multi-element Au-W-As stream sediment anomalies
46
Chapter 2
Tintina Gold Province
Figure 21. Surface projections
of the Liese 1 and 2 zones with
locations of drill hole collars,
and
current
underground
workings. Modified from Smith
et al. (2000) and TeckCominco
website. Stream sediment values
of 35 and 200 ppb Au were
determined for Liese and Pogo
creeks in 1981. Discovery holes
drilled in 1994 are plotted with
their significant intersections.
Data from Roberts et al. (2001).
that led to the discovery of high-grade quartz float near Pogo and Liese creeks. A decade later,
exploration funding was secured through a Teck and Sumitomo Metal Mining Co. joint venture
and subsequent soil sampling and prospecting, resulting in the identification of a several-km²
Au-in-soil anomalies. Three drill holes were drilled in 1994, two of them hit what would later
be recognized as the L1 vein (Fig. 21; Roberts et al., 2001). Originally interpreted as a series of
steeply-dipping quartz veins, by late 1996 the intersections were re-interpreted as a shallowlydipping body that was confirmed in 1997, along with the discovery of a second zone at greater
depth. A geological resource for the two zones (L1 and L2) was estimated at >4 million oz of
gold by late 1997, and an underground exploration program commenced after the 1999 season.
Subsequent property-scale exploration has relied on traditional exploration methods.
Helicopter-assisted stream sediment sampling is the most effective “first pass” sampling
technique (Roberts et al., 2001). Analysis of the fine sand fraction, collected from relatively
high-energy environments, was more effective than a standard silt sample. Bulk-leach
extractable gold (BLEG) and panned, heavy-mineral concentrate samples have also been
effective. Stream sediment samples exhibiting multi-element anomalies, particularly As,
W, and Bi, were followed up with prospecting and contour soil sampling. Soil sampling is
effective, but as B-horizon soils are poorly developed, C-horizon samples are taken, sometimes
using augers, particularly in regions with overlying loess (Roberts et al, 2001). Samples were
analyzed for gold by fire assay, and a full suite of other elements, in particular, Bi, Te, As,
Ag, but also Mo, Sb, Cu and Pb, which are locally useful pathfinder elements. Use of a low
detection limit package that includes Bi and Te is considered critical. Geological mapping
employed geophysical, air photo, field traverses, soil pit logging. Alteration is difficult to map
in the field due to the weathered rocks, but sericite-dolomite alteration is recognized as the best
indicator of proximity to mineralization. Diamond drill targets are selected using a number of
indicators, including the presence of extensive, >100 ppb gold-in-soil anomalies, multi-element
soil anomalies, and surface exposures of large, gold-bearing quartz boulders.
47
Chapter 2
Tintina Gold Province
Exploration Methods & Targeting in the TGP
Pluton Distribution, Character and Lithology
Regionally, gold occurrences that comprise the Tombstone gold belt are associated with the
most inboard (cratonward) limit of mid-Cretaceous plutons (Fig. 4). Plutons of the Fairbanks
district, which are obviously prospective, likely represent the western continuation of the
inboard plutons of the Tombstone belt. Regional delimiting criteria, however, cannot be
applied to the vast plutonic masses of Cretaceous intrusions in east-central Alaska (including
the Goodpaster district (Fig. 3) and Late cretaceous intrusions of southwestern Alaska), partly
because mapping and age dating are not sufficiently detailed.
The Fort Knox and Dublin Gulch deposits are in small (~1 km²), barely unroofed plutons,
where their cupolas are not entirely eroded away (Figs. 9 & 10). As such, the apical portions
of partly unroofed, or even totally unroofed small plutons (up to a few km²) appear to be better
targets for intrusion-related gold systems than larger plutons (e.g. Mustard, 2001). Elongate
plutons, such as those at Fort Knox and Dublin Gulch, with associated sub-parallel dikes, may
be indicative of structural control on both emplacement and focussing of hydrothermal fluids.
Lithological features and compositions of granitoids may indicate gold favorability.
McCoy et al. (1997) identified porphyritic granodiorite and quartz monzonite plutons as being
empirically more prospective. Prospective plutons may show signs of magmatic volatile
production such as aplites and pegmatites, miarolitic cavities, tourmaline-bearing phases or
veins, pneumatolitic textures and greisens, and unidirectional solidification textures. Similarly
lithogeochemical characteristics could differentiate productive from barren systems, but existing
data on mineralized systems indicate considerable variability. Fairbanks district plutons allied
with gold mineralization (Newberry et al., 1990; McCoy et al., 1997) typically follow a calcalkaline differentiation trend and have normative granodiorite and granite compositions. They
are metaluminous to weakly peraluminous and plot dominantly as I-type. Plutons of the
Tombstone gold belt have large variations in compositions with clinopyroxene, hornblende,
biotite or biotite-muscovite as dominant mafic phases. Some plutonic phases are as mafic as
pyroxenite and gabbro. However, gold-associated plutons in the western Tombstone gold belt
are variably alkalic, as are some gold-associated plutons in the Kuskokwim district. Though
highly altered, Late Cretaceous igneous rocks at Donlin Creek plot as granites and granodiorites
have S-type characteristics, are peraluminous and have elevated radiogenic signatures (compiled
data of Newberry, 2000). However, in all cases and in contrast to classic porphyry deposits,
the mineralized plutons result from fluids generated elsewhere, and as such, any particular rock
features or geochemical compositions of a specific granite suite may not be controlling factors
in mineralization, but that gold enrichments may be controlled by other magmatic factors.
Using primary oxidation state and alkalinity indices, Leveille et al. (1988) suggest that
reduced plutons are more favorable gold targets, and favorable plutons in eastern interior Alaska
were identified (Burns et al., 1991). However, elsewhere plutons that are classified as part of the
intrusion-related gold deposit group, of both Phanerozoic (e.g. Mustard, 2001) and Precambrian
(e.g. Robert, 2001) age, are clearly oxidized. Thus, the oxidation state of the intrusion is a
critical local controlling parameter on gold favorability within the TGP
48
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Tintina Gold Province
Elemental Associations
Trace element abundances, and correlations with gold, reflect the temperature of formation
and the nature of the host rocks. Deposits with elevated Bi,Te±W,Mo,Sn concentrations formed
at high temperatures and are characterized by feldspathic alteration, low sulfidation state
(indicated by pyrrhotite-native bismuth-loellingite) mineralization as seen in intrusion-hosted
early sheeted veins (Dublin Gulch), bifurcated vein networks (Fort Knox) and stockworks
(Shotgun). At Pogo, geochemical data indicate strong correlations between Au, Bi and Te,
and weaker correlations among Au, Ag, and As. The presence of As may reflect scavenging
from sedimentary host rocks. Deposits with high As, Sb±Hg concentrations are hosted in
carbonaceous sedimentary rocks and have features characteristic of lower temperature epizonal
deposits (Brewery Creek, Donlin Creek).
Placer-Related Targets
Localities upstream from placer gold occurrences are obvious targets. In the Tombstone gold
belt, the placer deposits have spatial relationships with intrusion-related gold systems at Dublin
Gulch, Scheelite Dome and Clear Creek. However, in the Fairbanks area, despite >8 Moz of
placer gold from the region, placers associated with the largest deposits (Fort Knox and True
North) are minor, suggesting that the majority of gold may be coming from other deposit types
(e.g. shear-related gold deposits), or from deposits that were completely eroded. Evaluating
placer concentrates, or sampling heavy mineral concentrates for Bi-bearing minerals or scheelite
may be beneficial, as was the case in placers draining both Fort Knox (Bakke et al., 1995)
and Dublin Gulch (Fig 9; Boyle, 1965). However, placers containing scheelite, or cassiterite
are typically too widespread to be useful deposit-scale pathfinders, and may be useful only as
regional indicators.
Placers containing stibnite, cinnabar and arsenopyrite may be indicative of a proximal
epizonal occurrence, as all are found in placers draining the Donlin Creek deposit (Ebert et al.,
2000). The True North and Brewery Creek deposits did not generate placers as their gold is
chemically bonded to sulfide minerals and even when liberated by oxidation, is too fine-grained.
Gold from intrusion-hosted deposits like Fort Knox and Dublin Gulch is typically fine-grained,
and of higher fineness than schist-hosted deposits in the same regions. Petrographic and trace
element analysis of placer gold may aid in determining deposit style or proximity to source
(Newberry, written comm., 1998; Knight et al., 1999). Placers draining intrusions, but lacking
magnetite, may indicate a reduced source pluton. However, the lack of placer gold should not
be a deterrent to lode exploration as Pogo and Fort Knox have only a limited placers, and much
of the eastern Yukon has been glaciated and stripped of whatever placer gold occurrences may
have existed.
Structural Targeting
Regional structural targeting has not yet proven to be an effective exploration criterion, and
only limited success has been achieved at a property scale. In the Fairbanks area, the dominant
regional faults are terrane bounding thrust faults, and a series of ENE-trending faults that locally
contain shear-zone hosted mineralization (Fig. 6, 14; Newberry et al., 1996). Many of these
structures are post-ore faults that juxtapose different structural levels. Discriminating between
the syn-mineral and post-mineral faults is challenging, but has resulted in exploration successes
49
Chapter 2
Tintina Gold Province
on Ester Dome (Cameron, 2001). As a whole, NE-, NW-, N-, and E-trending faults are common
throughout gold districts in the east-central Alaskan part of the TGP. However, much more
detail is needed before aspects of timing, abundance and relative displacement can be assessed
to adequately target ores (e.g. Newberry and Burns, 1999).
Low-angle faults and associated structures controlled the emplacement of the several lodes
that comprise each of the Brewery Creek and True North deposits. Despite characterization as
disseminated, both of these deposits have strong structural controls with linear ore zones (Figs.
16 & 17). As well, they lack any evidence of specific stratigraphic control, which may be an
important distinction from many of the Carlin-type disseminated gold deposits in Nevada.
Property-scale structural controls are important in most TGP deposits. Moderately-dipping
reverse fault arrays were important in focussing auriferous fluids at the Scheelite Dome deposit
(Fig. 13; Mair et al., 2000; O’Dea et al., 2000). All intrusion-hosted sheeted vein arrays occur
as tensile features resulting from far-field stresses. Throughout the Tombstone gold belt, many
mineralized tensional zones trend easterly, indicating that north-south extension was dominant
during gold deposition (Poulsen et al., 1997). The sheeted vein sets locally continue outside
of the intrusion, where they are paralleled by pegmatite, aplite, and lamprophyre dikes, and by
sulfide-bearing quartz veins (e.g. Fort Knox, Dublin Gulch). Such extensional zones at Clear
Creek are anomalous in gold and continue for several kilometers along strike (Marsh et al.,
1999). These extensional zones, which may have facilitated magma emplacement, may be
related to early, north-trending strike-slip faults (Stephens et al., 2000).
Geochemical Methods for TGP Exploration
Regional geochemical surveys are extremely effective in outlining gold favorable terrain
for follow-up efforts, as noted in the Pogo and Brewery Creek discovery case-histories. The
U.S.G.S. regional geochemical survey of the Big Delta 1o x 3o quadrangle used minus-80-mesh
stream sediments, heavy-mineral-concentrate of stream sediments, and willow leaf and twig
samples collected from low-order streams (Foster et al., 1979). Sample and data treatment are
as described previously for the Pogo Case History. In addition to indicating the presence of the
Liese veins at Pogo, other anomalies from the U.S.G.S. survey indicate the regional extent of
potential mineralization within the Goodpaster River region. Anomalies in concentrate samples
in the order of 200-300 ppm Bi characterized streams on the northern side of the Goodpaster
River and a number of stream sediments from this area contained 0.05 ppm Au, suggesting
continuation of gold mineralization ~5-10 km north of where it is currently known at Pogo.
About 15-20 km southeast of Pogo, near the historic Blue Lead lode and placer occurrences,
many oxide residue samples contained 500-1,500 ppm As, and a heavy-mineral-concentrate
sample contained 300 ppm Bi, confirming the As-Au-Bi signatures as most favorable for
exploration throughout the district.
Regional stream geochemical surveys in Yukon, undertaken by the GSC, included analyses
of silt and water samples, with a ~13 km² sample density. Analytical techniques and elements
analyzed vary between surveys – typically the minus 80 mesh fraction was analyzed. The data
readily identify broadly anomalous regions that are mainly coincident with the distribution of
the mid-Cretaceous granitoids, and locally identify known deposits and occurrences (Fig. 22).
Most unexposed bedrock the TGP is overlain by variably oxidized regolith since, despite the
50
Chapter 2
Tintina Gold Province
Figure 22. Regional stream silt geochemical data for arsenic from central Yukon as
compiled from Geological Survey of Canada open files, showing the distribution of arsenic
anomalies. The anomalous regions are mostly correlative with exposure and contact
aureoles of mid-Cretaceous granitoids, although not all plutons have associated arsenic
anomalies (e.g. Brewery Creek). Background values are < 50 ppm As, whereas most of the
peaks have values >500 ppm As.
region’s far-northern location, much of central Yukon and Alaska were beyond the limits of most
glacial events during the past 2.5 million years. This effects exploration sampling strategies and
methodologies. Positive effects are that the surficial materials are mainly of local derivation and
have not been dispersed by glacial transport. Additionally, soil and stream sediment samples
are neither diluted nor contaminated with transported glacial materials. Negative factors
include locally thick loess accumulations, and metal-leached B-horizon soils. These and other
sampling challenges, such as the presence of discontinuous permafrost, have encouraged the
use of handheld and ATV-mounted auger drills to collect C-horizon soils. Deep oxidation and
periglacial solifluction may give diffuse geochemical anomalies with low anomaly contrast or
irregular anomaly patterns. Most deposits give surface soil anomalies of 40 to 100 ppb Au, and
C-horizon anomalies of 100 to 250 ppb Au, but soil samples >1 ppm Au are not uncommon
over many mineralized zones. Confident sampling of oxidized rocks is difficult as gold may
be concentrated in friable limonitic or goethitic coatings that are easily removed by rainfall or
during trenching.
Soil sampling is perhaps the single most useful tool for exploration on a property scale,
particularly in light of the lack of exposed rocks. Arsenic, Te, Bi, Sb and base-metals
have proved to be useful pathfinder elements for gold at some properties (e.g. Pogo) using
commercially available multi-element analytical packages with low detection limits for Bi and
Te.
51
Chapter 2
Tintina Gold Province
Figure 23. Total field aeromagnetics for the Clear Creek area west of Mayo. Most plutons have
a low primary oxidation state (i.e. reduced) and co-commitment low magnetic response. Plutons
intruding the carbonaceous Paleozoic sediments give rise to magnetic hornfels thought to result
from pyrrhotite formation. Plutons intruding the more siliciclastic Proterozoic strata yield flat
aeromagnetic responses with no response from the aureole. Several locations with high anomalies
may indicate roof zones above unroofed or partly roofed plutons in carbonaceous strata.
Geophysical Methods for TGP Exploration
The usefulness of geophysical methods in interpreting the nature of the underlying geology
in regions of poor outcrop cannot be overstated - in particular aeromagnetics in combination
with resistivity methods has proven useful in the TGP (e.g., Scheelite Dome: O’Dea et al.,
2000). The most successful application may be for the discovery of unexposed or unmapped
plutons, where indicated by the magnetic anomalies generated from thermal aureoles that
developed where these bodies intruded reducing sedimentary rocks (Fig. 23). For example,
intrusive rocks at the Brewery Creek mine weren’t known prior its discovery, but their aureoles
have a pronounced magnetic signature compared to the country rocks and are easily discerned
from an airborne geophysical survey of the region (Hart et al., 2000). However, the usefulness
of geophysical methods in delimiting the many types of gold ore bodies in the TGP is limited.
The magnetic expressions of the intrusions themselves are otherwise low and flat. They
are difficult or impossible to differentiate from the siliciclastic metasedimentary rocks that are
the hosts for most systems. This is particularly notable because intrusions with low primary
oxidation states have been considered more favorable causative agents of gold mineralization
(Leveille et al., 1988) and these plutons have low magnetic susceptibilities. However, more
detailed ground surveys have shown that mineralized zones at Scheelite Dome are coincident
with magnetic and resistivity lows (Hulstein et al., 1999) which are thought to represent
structures. Post-acquisition processing can also enhance the data as shown at Donlin Creek
(Ebert et al., 2000). Radiometric surveys have proven useful in locating unexposed plutons,
more highly fractionated regions of exposed plutons, or regions with potassic alteration. HighK, biotite-rich alteration in thermal aureoles was discovered using gamma-rays at the Bear Paw
occurrence (Clear Creek deposit) where it is coincident with a magnetic low (Stephens and
52
Chapter 2
Tintina Gold Province
Weekes, 2001). Gravity lows may indicate the presence of larger plutonic bodies at depth, but
the regional quality data is locally coarse (10 km grid), and not useful for discerning smaller
features.
Drilling as an Exploration Tool
Although core drilling is preferred during the initial exploration stages to maximize
geological information, the choice/decision between drilling core vs. reverse circulation (RC)
is usually made on factors other than geological ones. These often include, time (RC is quicker
and the exploration season is short), access (core rigs are helicopter portable and result in less
environmental damage), total cost per foot (related to time, i.e. camp costs), and accessibility
to water, which can be difficult in many unglaciated regions. However, there are reports of
diamond drill holes in oxidized ground with almost 30 m of poor recovery.
Where large, quality samples are desired from a target with fractured and oxidized ground
conditions, RVC will generally provide a better sample and ultimately, more reliable assay
data - particularly above the water table. This was apparent at Dublin Gulch where twinning
of RC and core (HQ) holes showed significant differences in oxidized rock but comparable
results in unoxidized rock (Hitchins and Orssich, 1995). Additionally, as much of the gold
may be in oxidized sulfide minerals as goethite or in gouge in fault zones, it may be washed
away with sludges during core drilling. However, in wet holes, RC sample quality diminishes
considerably with increasing water. Data from wet RC holes drilled for the Fort Knox ore
reserve calculations were considered unreliable compared to twinned core holes and was
disregarded. Previous problems with down-hole contamination using RC rigs have been largely
eliminated with the new center face sampling bits and suitably-sized compressors. Routine
drilling at Fort Knox uses large PQ diameter core with triple-tube core barrels to reduce downhole contamination and ensure large sample sizes.
Additional Exploration Considerations
Grades and tonnages of the different deposit types of the TGP vary considerably (Fig. 24)
between the high-grade, low-tonnage skarns, to the bulk-tonnage intrusion-hosted deposits
that have grades of ~1 g/t Au. Veins in metasedimentary rocks are also typically higher grade
and have a range of tonnages. Epizonal deposits, typically with grades of ~1.4 to 3 g/t, are
characterized by refractory ores. Numerous “replacement-style” deposits with ambiguous
origins lie in the middle ground of both tonnage and grade. Intrusion-hosted, epizonal and vein
deposits all have the potential to host resources of greater than 100 t Au.
Real and estimated capital and operating costs of existing and potential gold deposits in the
TGP are impacted by wide variances in interrelated factors including available infrastructure,
beneficiation costs, and political/environmental factors concerning land usage, access and
potential permitting. The two larger gold operations in the TGP (Fort Knox and Brewery
Creek) are supported by work forces that are housed in nearby towns. Development of both
deposits required relatively short spur roads, and at Fort Knox a short power line. Development
of more remote deposits (Donlin Creek, Vinasale, Dublin Gulch) would require roads and
transmission lines that would be hundreds of kilometers long, or on-site power generation. Onsite diesel generation is particularly expensive for projects whose ores have a high work index
53
Chapter 2
Tintina Gold Province
Figure 24. Grade and tonnage plot for deposits of the Tintina Gold Province. Data
from Table 1.
– specifically weakly altered or unoxidized granitic ores. The enormity of Fort Knox mine and
mill complex within the Fairbanks area, likely requires that very large or very rich deposits need
to be discovered there to initiate a new development. However, a considerable amount of gold
from otherwise sub-economic ores will likely be mined, and processed in the Fort Knox mill.
Remote areas are difficult and expensive to access and as such, are less likely to be wellexplored. Potential mines in remote regions, like the Kuskokwim region, would likely require
the provision of supplies along seasonally open rivers, which would increase both operating cost
and risk. Mining in remote and unspoiled parts of the northern Cordillera require environmental
sensitivity. Conversely, development of deposits in the Fairbanks district may be impeded
by their proximity to suburban development. Because environmental considerations are
paramount, operations designed with zero discharge, such as Fort Knox and Brewery Creek, are
likely to be permitted more quickly.
Mine valuation, on both capital and operating costs, is effected significantly by its
beneficiation method. The nature of the ore and the amount of oxidation are two critical
factors. The Fort Knox, Dublin Gulch, Pogo, Shotgun and the Golden Zone deposits have gold
amenable to cyanide leach regardless of surface oxidation. Some deposits with refractory gold
may be economically viable due to relatively deep oxidation (Brewery Creek, Ryan Lode, and
True North). Additionally, oxidized rocks have a low work index such that at Brewery Creek,
run-of-mine ores were taken to the leach pad without crushing. Oxidation depth is important,
as the vertical extent the of Brewery Creek and True North oxide orebodies are limited and
overlie untested sulfide resources. Deeply oxidized deposits only occur in areas that escaped
recent glaciations such as interior Alaska and west-central Yukon. Glaciation through most of
the elevated ranges of the Kuskokwim region has removed most deeply oxidized ores. The
capital costs associated with constructing roasting or bio-leach facilities for such deposits in
relatively remote regions will impact their development. Alternatively, cyanide extractive heapleach operations of oxidized ores is viable, despite the extremes of a northern climate. Burying
drip lines and exothermic reactions from within the heap are sufficient to keep the fluids from
54
Chapter 2
Tintina Gold Province
Figure 25. Monthly gold production figures from the Brewery Creek heap leach
gold mine, Yukon. From company Annual Reports.
freezing. Gold recovery is year-round, but significantly lower during the winter months (Fig.
25).
In general, support for exploration and development of gold resources in the TGP by the
Alaskan and Yukon governments is positive. In Alaska, exploration expenditures can be written
off against future taxes on gold production, state secured loans may assist and provide new
infrastructure, and regulatory and taxation policy is more positive than in most other places
in the USA. Land tenure and taxation policy is stable and non-prohibitive and native land
tenure was resolved decades ago in a mutually positive manner. Conversely, the high value
of United States currency, remoteness, and lack of “flow-through” financing make exploration
very expensive. In Yukon, exploration benefits from government-funded incentive programs,
exploration tax credits of 25%, federal “flow-through” tax-credits of 15% and a cheap
Canadian dollar. However, uncertainty over the on-going native land-claims process and the
establishment of protected areas have hindered aggressive exploration activity.
Obstacles impeding increased exploration expenditure within the TGP are those that have
effected gold exploration elsewhere – notably the decreasing value of gold and decreased
availability of venture capital for exploration. However, discoveries, such as the Pogo
deposit, can catalyze and facilitate the financing of numerous grassroots exploration programs
throughout the TGP. Compared to most of the world, or elsewhere in North America, the
TGP has excellent mineral potential, security of tenure, excellent geoscience databases, and
is still mainly under-explored. Furthermore, geological and exploration models are still in an
evolutionary state and new deposit types are yet to found – most likely through creative and
aggressive exploration approaches. Continued pragmatic exploration and ensuing discoveries
will ensure that the Tintina Gold Province’s golden tradition continues to mature.
55
Chapter 2
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Conclusions
The TGP is defined by a wide range of gold deposit styles that are spatially and temporally
associated with reduced and radiogenic mid- and Late Cretaceous intrusions that are hosted
in sedimentary or metasedimentary strata across Alaska and Yukon in the North American
Cordillera. They account for an in-ground resource of approximately 37 Moz Au and an
undefined portion of the region’s 30 Moz of placer gold production. Deposits in the TGP form
the basis for the development of the intrusion-related gold system model. The wide range of
deposit styles within the model can be catagorized as intrusion-centered (includes intrusionhosted, skarn, thermal aureole-hosted replacements, breccias, distal skarns etc..), shear-related
veins, and epizonal. The variations in deposit styles result from differences in emplacement
depth and the nature of the host rocks. A genetic association with intrusive rocks is implied, but
difficult to confidently establish for many shear-related and epizonal deposits. Even intrusionhosted deposits are mineralized by granitoids that are generally not exposed making links to a
specific magmatic phase difficult to document.
U-Pb and Ar-Ar geochronology has constrained the TGP’s important magmatic and
mineralizing events at circa 90 and 70 Ma, precise deposit-scale temporal links between
magmatism and mineralization using Ar-Ar have been difficult to confidently establish. ReOs molybdenite dating has proven useful in yielding precise determinations and indicate a
previously unconfirmed ~104 Ma mineralizing event at Pogo’s Liese veins.
Most TGP deposits were discovered through the application of traditional exploration
methods – notably by following-up multi-element regional geochemical anomalies with soil
sampling. Government-sponsored regional geochemical data effectively discern mineral-rich
regions. Anomalous soil values are best obtained using augers and sampling the C-horizon.
Trenching is required to obtain meaningful samples as much of the TGP is unglaciated and
the rocks are deeply oxidized. Some TGP deposits were found through the application of the
intrusion-related model to areas with known gold occurrences.
Acknowledgements
Conversations about various aspects of the Tintina Gold Province with Grant Abbott, Tim
Baker, Mike Burke, Al Doherty, Brian Flanigan, Curt Freeman, Gerry Carlson, Jim Lang, Bill
Lebarge, Mark Lindsay, John Mair, Erin Marsh, Marti Miller, Rainer Newberry, Cam Rombach,
and Julian Stephens, are appreciated. Reviews of an earlier manuscript by Dick Nielson are
Shane Ebert appreciated, and valuable comments and suggestions from John Thompson helped
to construct the present one.
56
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Chapter 3
Mid-Cretaceous Plutonic Suites
Chapter Three
The Northern Cordilleran Mid-Cretaceous
Plutonic Province:
Ilmenite/Magnetite-Series Granitoids
and Intrusion-Related Mineralisation
Craig J.R. Hart1,2, Richard J. Goldfarb3, Lara L. Lewis2 and John L. Mair1
Centre for Global Metallogeny
School of Earth and Geographical Sciences
University of Western Australia
Crawley, Western Australia, Australia 6009
1
Yukon Geological Survey
Box 2703 (K-10)
Whitehorse, Yukon, Canada Y1A 2C6
2
United States Geological Survey
Denver Federal Centre Box 25046 (MS964)
Denver, Colorado, USA 80225
3
65
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Preface to Chapter Three
This paper results from an invitation by Shunso Ishihara (Editor) to contribute to a Special
Issue of Resource Geology as published by the Society of Resource Geology based in Japan.
This Special Issue was initiated to commemorate Dr. Ishihara’s 70th birthday and his lifetime
contribution to understanding intrusion-related ore systems. This paper “The Northern
Cordillera Mid-Cretaceous Plutonic Province: Ilmenite/Magnetite-Series Granitoids and
Intrusion-Related Mineralisation” was published in 2004 in Resource Geology, v. 54, no. 3,
p. 253-280. The spelling and format for citations and references follow those that are used in
Resource Geology.
Justif cation of authorship: This manuscript resulted from a large GIS compilation undertaken
by the candidate as part of the Ph.D. study involving integration of varied datasets from Alaska
and Yukon. Dr. Richard Goldfarb contributed expertise about Alaskan mineral deposits and
provided editorial expertise. Ms. Lara Lewis compiled the magnetic susceptibility data that were
acquired through the candidate’s field work, and provided GIS-support for some of the figures.
Mr. John Mair provided insight about redox controls and petrogenesis of magmatic systems,
and has been involved in ongoing regional igneous petrologic studies with the candidate. The
candidate defined and researched the plutonic suites, compiled and interpreted the redox data,
and made all of the interpretations with respect to metallogeny.
66
Chapter 3
Mid-Cretaceous Plutonic Suites
The Northern Cordilleran mid-Cretaceous
Plutonic Province: Ilmenite/magnetite-series
granitoids and intrusion-related mineralisation
Abstract
Twenty-five Early and mid-Cretaceous (145-90 Ma) plutonic suites and belts are defined
across Alaska and Yukon, in the northern North American Cordillera, on the basis of
lithological, geochemical, isotopic, and geochronometric similarities. These features are
combined with aeromagnetic characteristics, magnetic susceptibility measurements, and
whole-rock ferric:ferrous ratios to ascertain the distribution of magnetite- and ilmenite-series
plutonic belts. Magnetite-series plutonic belts are dominantly associated with the older
parts of the plutonic episode and comprise subduction-generated metaluminous plutons that
are distributed preferentially in the more seaward localities dominated by primitive tectonic
elements. Ilmenite-series plutonic belts comprise slightly younger, slightly peraluminous
plutons in more landward localities in pericratonic to continental margin settings. They were
likely initiated in response to crustal thickening following terrane collision. The youngest
plutonic belt forms a small, but significant, magnetite-series belts in the farthest inboard
position, associated with alkalic plutons that were emplaced during weak extension.
Intrusion-related metallogenic provinces with distinctive metal associations are
distributed, largely in accord with classical redox-sensitive granite-series. Copper, Au and
Fe mineralisation are associated with magnetite-series plutons and tungsten mineralisation
associated with ilmenite-series plutons. However, there are some notable deviations from
expected associations, as intrusion-related Ag-Pb-Zn deposits are few, and significant tin
mineralisation is rare. Most significantly, many gold deposits and occurrences are associated
with ilmenite-series plutons: these form the basis for the newly recognized reduced intrusionrelated gold deposit model.
Introduction
Regional metallogenic characteristics of intrusion-related mineral deposits are mainly
governed by the nature of their associated granitoids. Among the variables that affect
intrusion-related metallogeny, the most important is probably the primary redox state
of the magma. This recognition led Ishihara (1977, 1981) to develop a classification of
magnetite- and ilmenite-series granitoids and to note that each series had specific metallogenic
associations. Generally, magnetite-series granitoids, or magmas with high-oxidizing potential,
form magnetite-bearing plutons that generate sulphide-rich Cu, Au, Mo, Pb, and (or) Zn
deposits, whereas those with low-oxidizing potential are ilmenite-series and preferentially
generate oxide deposits rich in tungsten and tin. This relationship between magmatic redox
state and metallogeny is one of the most powerful first-order discriminators of intrusion-related
metallogeny and a critical factor in regional mineral exploration targeting.
The oxidation state of a pluton is mostly a response to the nature of the source materials
from which it was derived (Carmichael, 1991). Magnetite-series plutons are typically derived
67
Chapter 3
Mid-Cretaceous Plutonic Province Suites
from the partial melting of mafic components in the mantle wedge that are hydrated and
oxidized in response to dehydration of components in the subducting slab. The resultant
magmas are most often emplaced as oxidized, arc-building, calc-alkaline plutons. Ilmeniteseries magmas are mostly derived from or mixed with the partial melts of metasedimentary
rock sequences that contain reducing carbonaceous strata (Ishihara 1981). These magmas
typically result from melting of metasedimentary continental crust in a continental back-arc
position, possibly in response to crustal thickening, or, less- commonly, from near-trench
subduction-related melting of a flysch in an anomalously hot accretionary wedge. These
differing tectonic settings responsible for the generation of magnetite- and ilmenite-series
granitoids make it possible to ascertain, or even re-assemble, the tectonic settings in which
plutonic belts may have formed.
Ilmenite- and magnetite-series plutonic belts typically lie parallel to the continental margins
and their circum-Pacific distributions have been compiled by many workers (Takahashi et al.,
1980; Ishihara 1984, 1998; Takagi and Tsukimura, 1997). However, the northeastern Pacific
Ocean margin (the northern Cordillera of northwestern North America) is a region where
distributions of these belts are particularly poorly known (Ishihara 1998), and a regional
comprehensive description of such has been lacking from the literature. This region of North
America, mainly underlain by the State of Alaska, USA, and the Yukon Territory, Canada
(Fig. 1), hosts intrusions emplaced during multiple stages of magmatism. These include
pre-accretionary oceanic arc plutons as old as Devonian, and syn- and post-accretionary
continental margin Mesozoic-Cenozoic magmatic rocks. Together, these variably overlapping
assemblages have imparted a complex distribution of plutons and batholiths throughout
the region. In particular, overprinting Jurassic, Early Cretaceous, mid-Cretaceous, Late
Cretaceous and early Tertiary magmatic events have combined to generate a complex pattern
of plutons of differing origins, redox states, and, therefore, related metallogeny.
The most regionally extensive and metallogenically important magmatic episode in the
northern Cordillera was that of the mid-Cretaceous, which resulted in the deposition of worldclass intrusion-related tungsten, gold, silver, and copper deposits, and significant resources of
uranium, molybdenum, and tin. The regional distribution of mid-Cretaceous plutonic suites
and belts of differing oxidation states permits characterization of the nature of intrusion-related
metallogeny, and forms the basis for developing an understanding of the tectonic settings in
which various plutonic belts and deposit types form. This is particularly important because: 1)
there are notable differences in the distribution of magnetite- and ilmenite-series plutonic belts
between orogens along the western and eastern portion of the Pacific rim (Ishihara, 1998); and
2) a new class of gold deposits associated with reduced, ilmenite-series intrusions has recently
Figure 1 (following page). Generalized tectonic elements in Yukon and Alaska with significant faults,
features and locations cited in text. Note the distribution of the mid-Cretaceous plutons which extend
from near the Pacific Ocean to several hundred kilometers inland. Insular, Intermontane, and Koyukuk
belts are mostly physiographic distinctions. Distances on faults refer to amount of displacement. Terranes
are denoted by labels: SD-Seward Terrane; KO-Koyukuk Terrane; RB-Ruby Terrane; WR-Wrangellia;
CG-Chugach Terrane; YTT-Yukon-Tanana Terrane; ST-Stikinia; CT-Cassiar Terrane. NWT-Northwest
Territories. Faults: PRF-Pacific Range Fault; DT-Dawson Thrust; SCF-Shaw Creek Fault; VF-Volkmar
Fault.
68
Chapter 3
Mid-Cretaceous Plutonic Suites
69
Chapter 3
Mid-Cretaceous Plutonic Province Suites
been recognized (Lang et al., 2000; Thompson and Newberry, 2000). These differences
highlight varying mechanisms of magma and metal generation across orogenic belts and may
help to develop a better understanding of regional metallogenic variations and controls.
Herein, a framework of Early and mid-Cretaceous plutonic belts and suites for Alaska and
Yukon is established. This is used as a framework upon which the redox state of plutonic belts
and the intrusion-related metallogeny is constructed.
Tectonic Framework
The northern North American Cordillera is composed of variable amounts of exotic and
pericratonic crustal fragments, or terranes, which were accreted to the ancient continental
margin during the Jurassic to early Tertiary (Gabrielse and Yorath, 1991; Monger et al., 1982;
Plafker and Berg, 1994). Many of the accreted terranes are composed of primitive island arcs,
oceanic crust, and flysch, whereas others represent fragments of variably metamorphosed
continental margin assemblages of uncertain origin (Fig. 1). The ancient continental margin
lacks exposed crystalline rocks because it is overlain by >15 km of deformed passive margin
sedimentary strata that variably accumulated between the Mesoproterozoic and the Mesozoic.
Notable within these otherwise carbonate-dominated platforms in Yukon and western
Northwest Territories (NWT) is the Selwyn Basin, a late Neoproterozoic to Carboniferous
clastic basin with important sequences of carbonaceous shales (Abbott et al., 1986). The
Cassiar Terrane represents a block of the ancient continental platform that has been displaced
by Mesozoic strike-slip movement along the Tintina Fault system (Fig. 1).
The Yukon-Tanana, Ruby, and Seward terranes, as well as several other smaller crustal
fragments, represent polydeformed metasedimentary rock terranes that have continental,
or pericratonic, affinities and mostly occupy a position between the ancient continental
margin and outboard accreted terranes (Mortensen, 1992; Plafker and Berg, 1994). The
accreted terranes are dominated by primitive Mesozoic island arcs, such as those that are
characteristic of the Wrangellia, Stikinia, and Koyukuk terranes (Fig. 1), but these may also
include older basement assemblages. Obducted oceanic rocks, such as those of the Cache
Creek, Angayucham, Slide Mountain, and Seventy-Mile terranes, are mostly Paleozoic. They
typically have a thin, elongate distribution, locally with eclogite and blueschist, which forms
an interface at the leading edge of the accreted terranes with the ancient continental margin.
Overlap basinal assemblages, mostly in the form of Jurassic to present-day flysch basins, occur
along the relatively young accretionary margin of southern Alaska, as well as overlapping
previously accreted terranes in the interior.
The nature and timing of many of the accretionary events remain controversial, but most
evidence in Alaska and Yukon suggests a period of assembly involving “Intermontane”
components in the Early Jurassic, and another, involving “Insular” components, in the Early to
mid-Cretaceous (Monger et al., 1982; Monger 1989; Mihalynuk et al., 1994; Plafker and Berg,
1994) The farthest outboard components are still involved in accretion and margin-parallel
translation, with continental to oceanic arcs still growing in far southwestern Alaska (Pavlis et
al., 2004).
70
Chapter 3
Mid-Cretaceous Plutonic Suites
Numerous suites of Jurassic, Cretaceous, and Tertiary plutonic and volcanic rocks
were emplaced into this complex array of terranes and deformed continental margin strata
(Armstrong, 1988; Miller, 1994). Most voluminous are Cretaceous rocks that formed first
in response to rapid convergence between the North American and Pacific plates during the
Cretaceous normal superchron (124-83 Ma) and secondly during the final subduction of the
Kula plate that started at ca. 83 Ma (Engebretson et al.,1985). This latter rapid subduction,
beneath what is now southern Alaska, caused rapid dextral oblique transpression in terranes
that were previously coupled with the North American margin. Ensuing dextral strike-slip
faulting along the Tintina-Kaltag (450 km) and Denali-Fairweather (350 km) fault systems
(Fig. 1), combined with modern day deformation, have added further complexity to the terrane
architecture (Plafker and Berg, 1994; Sisson et al., 2003 and papers therein).
For the purposes of this paper, it is convenient to use the terms “Insular” to refer to
accreted island-arc terranes in the region south of the Denali-Fairweather fault system and
“Intermontane” for the region of pericratonic and arc terranes between the Denali-Fairweather
and Tintina-Kaltag fault systems . The North America margin refers to autochthonous North
America and displaced equivalents.
Plutonic Suites
Early and mid-Cretaceous magmatism was widespread throughout Alaska and Yukon,
with related plutons cutting across boundaries of tectonic elements and occurring from the
near-trench environment along the current continental margin to 700 km inboard from the
current plate margin (Fig. 2). Robust dating methods (U-Pb zircon) have aided significantly
in defining the temporal division of the plutonic elements into distinct intervals. However,
as many parts of the region lack these accurate analyses, particularly in Alaska, there are
large tracts where less robust and relatively imprecise dates (K-Ar, Rb-Sr) are believed to
be skewed towards erroneously young ages due to partial resetting. Nonetheless, several
Cretaceous plutonic events within the region can be defined (Fig. 2). Early Cretaceous (145135 Ma) plutonism was sparse and localized in the Insular terranes, and may define the final
stages of Late Jurassic subduction-related arc magmatism that occurred on the seaward side
of the evolving orogen. For the most part, there was an earliest Cretaceous Cordilleran-wide
magmatic lull (Armstrong, 1988) that continued to ca. 115 Ma. Increased convergence rates
between oceanic plates and North America resulted in a dramatic flare-up in magmatism
at ca. 115-109 Ma that generated several plutonic suites across the breadth of the northern
Cordilleran orogen. Magmatic activity continued through ca. 99 Ma and then migrated further
inland until it terminated dramatically at 90 Ma. The 115 to 90 Ma magmatism was notably
the most widespread, voluminous, diverse, and metallogenically important in both Alaska and
Yukon. Latest Cretaceous magmatism, initiated at ca. 74 Ma, was widespread, but concentrated
in southern Alaska, with less voluminous events in western and eastern Alaska, and central
Yukon (Moll-Stalcup, 1994).
Early to mid-Cretaceous plutonism post-dates deformation of the host terranes, with only
the earliest Cretaceous plutons that intrude the Insular terranes (Hudson, 1979), and the ca.
122-115 Ma plutons in the Yukon-Tanana Uplands of the Intermontane region (Day et al.,
71
Chapter 3
Mid-Cretaceous Plutonic Province Suites
2003), showing evidence of internal deformation. These bodies also typically cut the youngest
regional deformational structures and the major terrane-bounding faults. Most plutons
were intruded after the accretion of the more landward of the Intermontane terranes to the
continental margin, and syn- to post-accretion of the Insular terranes. As such, most of this
plutonic epoch represents transitions from syn- to late- or post-orogenic intrusions.
Early and mid-Cretaceous plutons form several distinct belts and, where plutons within
a belt share similar lithological characteristics and age ranges, they are characterized as
plutonic suites (Fig. 2). More than 20 plutonic suites and belts are identified herein and are
based, in part, on terminologies and distributions used by Anderson (1988), Woodsworth et
al. (1991), Miller (1994), Mortensen et al. (1995, 2000), and Newberry (2000), but many
names and distributions have been modified or added. As well, available digital databases of
geology (USGS, 1997; Gordey and Makepeace, 2003), geochronology (Breitsprecher et al.,
2003, USGS 1999, 2002), aeromagnetics (Saltus, 1997; Saltus and Simmons, 1997; Saltus
et al., 1999; Lowe et al., 2003), and mineral deposits (USGS, 1998; Deklerk, 2003) were
integrated on a GIS platform and used extensively in making the compilation and associated
interpretations presented herein.
Early and Mid-Cretaceous Plutonic Belts and Suites
Herein, the Early and mid-Cretaceous plutons of Alaska and Yukon are divided into 25
separate plutonic belts and suites. Their distributions are shown in Figure 2 and a summary of
their characteristics are given in Table 1. As many of these plutonic belts are along-strike with
belts of similar geologic settings, lithologies or age ranges, they are grouped to form 13 larger
plutonic belts which are described below, largely in order of decreasing age and increasingly
landward position.
Tosina-St. Elias Belt (145-135 Ma)
Earliest Cretaceous plutonism is represented by a >600-km-long belt that is cored by
plutons in the southern Wrangell Mountains in Alaska and the St. Elias plutonic suite in
southwestern Yukon, but may also include plutons in the western Chugach Mountains in
southern Alaska (Figs. 1 and 2) and the Chichagof plutonic suite in the southeastern Alaska
panhandle (Hudson, 1979; Miller, 1994). These suites mainly intrude rocks of the Paleozoic
to Triassic Wrangell and Alexander arc terranes, and to a lesser extent of the Chugach terrane,
and are exposed mostly between the Denali and Border Ranges fault systems. Plutons vary
from large elongate batholiths to small individual stocks. The plutonic suites are dominated
by tonalitic series, variably foliated biotite-hornblende quartz diorite, tonalite, diorite, and
granodiorite, with lesser trondhjemitic rocks, that form the Tonsina-Chichagof belt (Hudson
1983) and may include plutons ascribed to the Chitina arc (Plafker et al., 1989). PotassiumFigure 2. Distribution and age ranges of mid-Cretaceous plutonic suites and belts across Alaska and
Yukon. Age ranges given here may differ from those in the text or literature as the older ages are
emphasized. Many suites, particularly those in Alaska, have a wide range of age dates that typically result
from K-Ar determinations that are susceptible to partial resetting and give dates that are too young. YT
Uplands-Yukon-Tanana Uplands, NWT-Northwest Territories
72
Chapter 3
Mid-Cretaceous Plutonic Suites
73
Mag Susc
x10-3 SI
Werdon et al. (2004)
Day et al. (2003); Dilworth (2003); Werdon
et al. (2004)
Pigage and Anderson (1985)
Driver et al. (2000), Hart and Lewis
(unpublished)
Hart and Lewis (unpublished)
Patton and Moll-Stalcup (2000); Patton et
al. (1984)
Amato et al. (2002)
Miller (1989)
Hudson (1979); Dodds and Campbell
(1988); Pavlis et al. (1988)
Richter et al. (1975a), Hudson (1979);
Dodds and Campbell (1988)
Morrison et al. (1979); Hart (1995, 1997,
unpublished)
Miller (1989)
Best References
Table 1. Characteristics of Early to mid-Cretaceous plutonic suites and belts in Alaska and Yukon with data pertinent to redox-series. Listings are in approximate
decreasing age. Magnetite-series plutonic suites are shaded, those that are weakly magnetite-series are lightly shaded. Ilmenite-series remain unshaded. Ages
in brackets are minimum ranges likely to result from variably reset determinations. Magnetic susceptibility values are averages, ranges for some are given in
brackets. alk=alkaline; calc-alk=calc-alkaline; ilm=ilmenite; mag=magnetite; na=data not available; trondhj-trondhjemitic; var=variable.
Fe2O3/FeO
Series
Aff nity
na
Aeromag
Character
Age
(Ma)
na
Fe-Ti
Mineralogy
Suite/Belt
trondhj-tonalitic
na
Ilm?
Kaiyuh
Mag
145-135
0.6-0.9
12.9
na
low, mod-high
in north
low
Ilm?
Ilm
prob magnetite
Tosina-St. Elias
calc-alkaline>trondhj
0.5-4.0
na
magnetite,
titanite
magnetite,
titanite
titanite,
magnetite
magnetite
ilmenite>titanite
low
low
Mag
118-108
calc-alkaline
0.4-3.0
na
na
na
low, mixed
moderate
106-96
aluminous
0.06-0.3
1.5-5.5
Nyac
Yukon-Tanana
Uplands
Anvil
Cassiar
Hyland
106, 94
calc-alk
calc-alkaline
high,
Chugach low
high
Nutzotin-Kluane
115-109
alk>calc-alk>peralk
na
0.1-1.1
na
na
ilmenite
moderate
Ilm
Weak
Mag
Weak
Mag
Ilm
Salcha
101-93
104-99 (93)
na
Mag
110-99
alk>calc-alk
aluminous>calc-alk
0.3
na
0.15 (0.02-0.38)
low, mixed
Ilm
Mag
Tok-Tetlin
Dawson Range
na
na
Armstrong et al. (1986)
Richter et al. (1975a)
Tempelman-Kluit and Wanless (1975);
Selby et al. (1999); Hart (unpublished)
Bouley et al. (1995)
calc-alkaline
na
high
Whitehorse
108-100
112-108 (99)
aluminous>calc-alk
na
0.1-1.0
na
0.77
na
titanite>magnet
ite>ilmenite
na
low
moderate-high
Mag
100-88
alk-calc-alk?
mixed
West Koyukuk
112
aluminous?
aluminous
na
0.3-1.4
2.3
ilmenite
??
100-96 (90)
Miller (1989)
Southwest Alaska
Range
East Seward
Mag,
var.
Mag
Ilm
111-108
109-102 (93)
aluminous>calc-alk
aluminous>calc-alk
0.4-1.5
na
magnetite
high
St Lawrence Island
Ruby
109-95
109-99 (96)
aluminous
0.2-1.0, up to 6
0.08
(0.01-0.23)
na
5.1
probably
magnetite
na
Mag,
var
Mag
na
high
0.5-5.0
Ilm
Gordey and Anderson (1993); Hart et al.
(2005)
Murphy (1997); Hart et al. (2005)
Blum (1983, 1985)
calc-alkaline
low
Ilm
Ilm
98-83
East Koyukuk
titanite,
magnetite
ilmenite
low
low
Amato et al. (1994); Armstrong et al. (1986)
Richter et al. (1976)
Anderson (1987); Hart et al. (2005)
Reifenstuhl et al. (1997a)
0.16
titanite
titanite
Ilm?
Mag
Mag
Mag
0.1-0.3
98-94
aluminous>calc-alk
aluminous>calc-alk
na
low
moderate
high, mixed
high
aluminous
Tungsten
96-93
94-90
na
0.5-0.9
0.2-1.0
na
0.15-0.6
0.1-0.4, up to 2
Mayo
Fairbanks
aluminous
calc-alkaline
alkaline>peralk
alkaline>peralk
magnetite
na
105, 93
92-90
92-90
92-90
0.09
0.33 Fort Knox
pluton only
na
na
1.8
na
South Seward
Tanacross
Tombstone
Livengood
74
Mid-Cretaceous Plutonic Province Suites
Chapter 3
Chapter 3
Mid-Cretaceous Plutonic Suites
Ar geochronology of uncertain reliability yields dates as young as 125 Ma (Chugach suite),
but also a U-Pb date as old as 153 (Plafker et al., 1989): most dates are from 145 to 135 Ma.
Tonalite-trondhjemite series plutons in the western Chugach Mountains, ranging from 135
to 125 Ma (Pavlis et al., 1988), may also belong to this belt. Plutons of the belt have high
aeromagnetic responses, which together with their tonalitic calc-alkaline arc affinities (Plafker
et al., 1989), designates them as magnetite series.
The region underlain by this plutonic suite has a high density of small intrusion-related
mineral occurrences that are dominated by Cu-Au-Ag porphyry-style, skarn, and vein-style
mineralisation. Among these occurrences are the Midas, London, and Cape porphyries, and
the calcic iron skarns in the McCarthy area (Moffit and Mertie, 1923; MacKevett, 1976). The
porphyry occurrences, which are associated with the Chitna Valley batholith, are interpreted to
have formed in an offshore oceanic arc at ≥ 146-138 Ma and then were subsequently accreted
to the continental margin (Young et al., 1997). Strontium initial ratios <0.7045 for the arc
rocks (Arth, 1994) are consistent with a primitive oceanic origin.
Nutzotin-Kluane Belt (118-108 Ma)
Bounded to the north by the Denali fault system, the plutons of the Nutzotin-Kluane belt
form a narrow, 800-km-long belt (Nutzotin-Chichagof belt of Hudson, 1979) that occurs
throughout the northern Wrangell Mountains in Alaska (i.e., the Chisana arc of Barker, 1987)
and the Kluane Ranges suite in southeastern Yukon (Fig. 2), and into the Alaskan panhandle,
where they form the Chichigof belt of Richter et al. (1975a). They mainly intrude rocks of the
Wrangellia and Alexander terranes, as well as adjacent Jura-Cretaceous flysch. The plutons
are typically large epizonal plutonic complexes and stocks that are elongate and parallel with
the regional fabric (Woodworth et al., 1991). They are dominated by hornblende-biotite
granodiorite, with lesser quartz diorite and diorite, calc-alkaline, I-type intrusions (Richter
et al., 1975a; Campbell and Dodds, 1983; Dodds and Campbell, 1988). Plutons have a wide
range of K-Ar dates from 147 to 108 Ma and thus some included igneous bodies may be part
of the older Tosina-St. Elias suite. Dodds and Campbell (1988) define the age range of Kluane
Range suite intrusions to be 117-108 Ma. Aeromagnetic maps indicate that these rocks in
Yukon have a highly magnetic character. To the south, in the southeastern Alaska panhandle,
the Klukwan-Duke belt comprises 110-100 Ma mafic-ultramafic Ural-Alaska-type complexes
that were deeply emplaced mainly within the Jura-Cretaceous flysch belt. This 500 km x 40
km belt consists of rocks that are concentrically zoned from dunite cores, outward through
peridotite, to olivine and hornblende pyroxenite (Foley et al., 1997).
Numerous intrusion-related mineral deposits in the northern Wrangell Mountains part to
this belt include the quartz diorite- and granodiorite-associated Baultoff, Carl Creek, Horsfeld,
Bond Creek and Orange Hill porphyry copper and copper-molybdenum deposits (Young et
al., 1997). The adjacent Triassic limestones host gold-bearing skarn deposits (Newberry et al.,
1997). These deposits have ages of 114 to 105 Ma (Richter et al., 1975a, b; Richter, 1976).
The main eight porphyry deposits have a combined resource of about 1,000 Mt averaging
0.20-0.35% Cu, as well as grades of about 0.02% Mo and minor silver and gold enrichments
(Hollister et al., 1975). All are presently unmined, in large part due to their relatively remote
75
Chapter 3
Mid-Cretaceous Plutonic Province Suites
location. Calcic copper, iron, and/or gold skarns occur on both the northern and southern,
more accessible flanks of the Wrangell Mountains (Richter et al., 1975a; MacKevett, 1976),
with the Nabesna deposit being the economically most significant deposit. Almost 2 t Au have
been recovered from pyrite lenses in fractured limestone associated with garnet-pyroxene
skarn (Wayland, 1943). The paleotectonic setting for these intrusion-related deposits in
the Wrangell Mountains is uncertain, but where the belt continues into the northern part of
southeastern Alaska (i.e., Bruce Hills porphyry deposit), magmatism is interpreted to have
taken place near the paleo-continental margin (Young et al., 1997). Potentially large iron
resources (e.g., Klukwan, 3.2 billion tonnes of rock averaging 16.8% Fe) occur as magnetite
in the outer hornblende pyroxenite zone of many of the Klukwan-Duke intrusions (Goldfarb,
1997), as well as possible economic occurrences of Ti and PGE. Potential intrusion-related
mineral resources in the Kluane Range of this belt are unknown because they are sited in a
national park.
St. Lawrence-Seward-Koyukuk Belt (113-99 Ma)
Early Cretaceous plutons form an east-trending belt from St. Lawrence Island in the
Bering Sea, east to the southern/central Seward Peninsula and across the northern and eastern
Koyukuk Basin (Fig. 2). If the Koyukuk-Seward-St. Lawrence belt is interpreted as a single
belt, then it links several diverse lithotectonic terranes, and may also be viewed as the eastern
extension of the Okhotsk-Chukotsk plutonic belt in eastern Russia (Miller, 1989). The St.
Lawrence Island and East Seward plutons intrude the Proterozoic and Paleozoic pericratonic
assemblages that comprise the Seward terrane, whereas the West and East Koyukuk plutons
intrude the accreted Koyukuk volcanic arc terrane and the overlapping Cretaceous terrigenous
sedimentary rocks. The belt often follows near the tectonic contact between the Koyukuk and
Seward terranes. The St. Lawrence Island, East Seward, West and East Koyukuk plutons are
all slightly alkaline in composition, with hornblende- and clinopyroxene-bearing monzonites
and syenites, but some plutons are more typical calc-alkaline granodiorites (Miller, 1972,
1989; Patton and Csejtey, 1980; Patton and Box, 1989; Amoto et al., 2002). Accessory
magnetite and titanite are reported for most plutons (Csejtey and Patton, 1974; Miller, 1989).
The alkaline plutons typically yield dates of 113 to 99 Ma, with most between 109 to 105 Ma
(Miller, 1989). The belt likely developed in response to north-dipping subduction.
In addition, a volumetrically small, but significant suite of highly alkaline, silicaundersaturated ultrapotassic intrusive bodies occur, mainly within the area of the West
Koyukuk suite (Miller, 1989). West Koyukuk plutons apparently have mixed oxidation states,
as indicated from both Fe2O3/FeO ratios (Fig. 3) and aeromagnetic signatures. Most of the
plutons have relatively modest and flat aeromagnetic responses, but the large, ultrapotassic
Selawik Hills pluton is defined by a notable aeromagnetic low and a few plutons (e.g.,
Shiniliaokok Creek pluton) have very high aeromagnetic signatures. The mixed signature may
reflect the overlapping distributions and differing sources of the alkalic and ultrapotassic suites.
Two other, smaller, plutonic suites have also been identified within this belt (Miller, 1994).
Plutons in East Seward, including the 96-91 Ma Darby pluton and other associated plutons
to the north, are evolved silicic granites with moderate to high aeromagnetic responses, high
Fe2O3/FeO ratios (Miller and Bunker, 1976) and moderately radiogenic strontium ratios (0.708-
76
Chapter 3
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0.711, Arth, 1987). The slightly younger East Koyukuk plutonic suite forms two 150-km-long
belts (east- and southwest-trending) that are mostly defined by large, hornblende>biotite,
calc-alkaline, granodioritic to granitic plutons in the eastern Koyukuk terrane (the east half
of the Hogatza plutonic belt of Miller et al., 1966). These bodies yield K-Ar ages of 89-79
Ma, have high Fe2O3/FeO ratios, and low initial strontium ratios of 0.7038-0.7056 (Arth et al.
1989a; Miller 1994). Although intrusions of both the East Seward and East Koyukuk suites
are mainly metaluminous, there are a few weakly peraluminous examples. Associated aplite
dykes, tourmaline-rich zones, miarolitic cavities, coeval volcanic rocks, and broad hornfels
aureoles characterize many intrusions. Abundant accessory magnetite and titanite have been
reported (Miller 1989), and the plutons have high to moderate aeromagnetic signatures, and
are oxidized, as defined by Fe2O3/FeO ratios from 1 to 3 (Fig. 3). Isotopic and trace element
data indicate that there is no continental crustal component to these shallowly emplaced East
Koyukuk bodies (Miller, 1989; Arth et al., 1989a).
In addition, a 200-km-long east-trending belt of younger, silicic plutons in southern Seward
Peninsula, which forms the core of the Bendeleben and Kigluaik Mountains, yields 87-81 Ma
K-Ar dates that represent cooling ages related to uplift of the granites and gneissic country
rocks (Armstrong et al., 1986). The granites and high-grade metamorphic rocks developed in
response to an Albian Barrovian metamorphic event between 105 and 92 Ma and the former
are thus interpreted as anatectic plutons (Armstrong et al., 1986; Miller et al., 1992; Amato
et al., 1994). The large Kachuik and East Bendeleben Mountains plutons have low and flat
aeromagnetic signatures, but ages of 100-90 Ma that are similar to the rest of this suite.
The mainland region is characterized by small and relatively unstudied, intrusion-related UTh, Be, REE-bearing pegmatite, and polymetallic Ag-Sn-Bi-Pb-F vein occurrences. There are
also a few small tin showings in the southern part of St. Lawrence Island (e.g., Kangukhsam
Mountain). Widespread, but weakly disseminated, molybdenite is known from the Sevuokuk
quartz monzonite pluton (e.g., Poovookpuk Mountain and Booshu Camp) on westernmost
St. Lawrence Island (Patton and Csejtey, 1980), but the most significant mineralisation is the
Poovookpuk Mountain and West Cape Cu-Mo porphyry occurrences, which are associated
with plutons in the north.
Gold mineralisation in the southern Seward Peninsula developed in response to a ca. 110
Ma metamorphic event that localized small orogenic lode-gold deposits in the greenschist
facies rocks in the southern part to the peninsula (Goldfarb et al., 1997). Although gold vein
formation is coeval with the onset of magmatism, the deposits are located 40-50 km south and
west of the main magmatic bodies. These historically uneconomic lodes were eroded to form
the main beach and river placers that yielded almost 200 t Au in the early 1900’s. Silver-rich,
base-metal vein deposits in the higher grade metamorphic rocks of the Seward Peninsula (i.e.,
Independence, Omalik) show a closer spatial association, and are isotopically compatible,
with mid-Cretaceous magmatism (Goldfarb, 1997), although some workers suggest that these
deposits represent Paleozoic syngenetic mineralisation (e.g., Schmidt, 1997). The Darby
granite porphyry in the southeastern part of the peninsula is highly enriched in uranium
(e.g. Eagle Creek deposit) and was the obvious source for uraniferous alluvium that was
remobilized into an Eocene roll-front type deposit (Thompson, 1997).
77
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Figure 3. Whole rock ferric:
ferrous ratios for selected midCretaceous plutonic suites in
Alaska and Yukon. Data from:
Richter et al. (1975a); Luthy et
al. (1981); Blum (1983); Miller
(1989); Solie et al. (1990); Burns
et al. (1991, 1993); Newberry et
al. (1990a), Newberry and Solie
(1995); Patton and Moll-Stalcup
(2000); and Hart (unpublished).
The dashed line represents the
boundary between magnetite- and
ilmenite-series granitoids.
The most notable Koyukuk intrusion-related occurrences are the small Wheeler and Clear
Creek U-Th-F prospects, which are associated with an ultrapotassic intrusion (Miller and
Elliott 1977), and the Placer River molybdenite occurrence. Mineralisation associated with
the other intrusions form the Hogatza and Hughes mineral districts, which include many
very small polymetallic vein, scheelite vein, and gold placer showings. Placer thorianite of
the Hogatza deposit is associated with the Zane Hills (101-84 Ma) biotite-hornblende quartz
monzonite pluton (Staatz, 1981), and there is similar mineralisation downstream from the
Indian Mountain (84 Ma) hornblende granodiorite pluton.
78
Chapter 3
Mid-Cretaceous Plutonic Suites
Tanacross-Dawson Range-Whitehorse Belt (112-91 Ma)
Extending from east-central Alaska through western Yukon, south to the British Columbia
border, the Tanacross-Dawson Range-Whitehorse plutonic belt (Fig. 2) consists of plutons
in the Tanacross area, the Gardiner pluton of eastern Alaska (Richter et al., 1975a), and
batholiths of the Dawson Range in eastern Yukon (Klotassin batholith of Tempelman-Kluit and
Wanless, 1975, 1980). It also can likely be extended, by lithological similarity, to include the
Whitehorse plutonic suite in southern Yukon (Morrison et al., 1979; Hart 1995, 1997). Most
intrusions form large northwest-trending batholiths that intrude metamorphic rocks of the
Yukon-Tanana terrane, except for the Whitehorse suite that comprises smaller epizonal plutons
mainly intruding rocks of the Stikine terrane (Fig. 1). Coarse-grained, euhedral hornblendebiotite granodiorite is the dominant lithology, with locally more mafic diorite and gabbro,
and locally fractionated bodies of quartz monzonite and biotite granite (MacIntyre and Coffee
Creek suites in Yukon; Hart, 1997), and even more felsic components such as leucocratic
quartz monzonite (Richter et al., 1975a). These rocks are metaluminous in composition and
follow a calc-alkaline fractionation trend. They decrease in age to the west, with rocks of the
Whitehorse suite dated from 112-109 Ma, Dawson Range from 106-99 Ma, and Tanacross
from 99-91 Ma. Initial strontium ratios average 0.7045 and 0.706 for the Whitehorse and
Dawson Range intrusions, respectively (Hart, 1995; Selby et al., 1999). All rocks have coarse
primary magnetite and high aeromagnetic responses. The Dawson Range and Whitehorse
suites have moderate to high magnetic susceptibilities and high Fe2O3/FeO ratios (Figs. 3 and
4). Thus, all plutons of this belt are classified as magnetite-series.
Intrusion-related mineralisation is mostly limited to small molybdenite and copper
porphyry and skarn occurrences (e.g., Toni-Tiger), and small gold-bearing veins, porphyries
and placers (e.g. Mt. Freegold/Antoniuk in the eastern Dawson Range). Significant
mineralisation, however, is associated with rocks of the Whitehorse plutonic suite, which
generated the numerous Cu-Au-Ag skarns of the Whitehorse Copper Belt that produced 7t Au,
23t Ag and 123,000 t of Cu prior to 1983 (Tenney, 1981).
Ruby-Kaiyuh-Nyac Belt (117-108 Ma)
The Ruby-Kaiyuh-Nyac belt of interior Alaska is dominated by the northeast-trending,
400-km-long batholiths of the Ruby geanticline (Fig. 2). The plutons intrude Proterozoic
and Paleozoic metamorphic rocks of the Ruby terrane and the Paleozoic and Mesozoic
oceanic rocks of the Angayucham-Tozitna terranes. Batholiths are dominated by coarsegrained to porphyritic leucocratic biotite granite, with lesser granodiorite (mainly in the
north), muscovite-biotite granite, syenite, and monzonite. Plutons are moderately to weakly
peraluminous with >0.8% normative corundum (Miller, 1989), particularly in the larger
plutons to the south, although neither cordierite, nor garnet, have been identified. Initial
strontium and neodymium ratios are generally radiogenic, but decreasingly so to the north
(Arth et al., 1989b). Assorted U-Pb, Ar-Ar, and K-Ar ages range from 112 to 108 Ma. The
plutons have generally low ferric:ferrous ratios (~0.5 at 70% SiO2 ; Fig. 3), low magnetic
susceptibilities, and generally low aeromagnetic responses, particularly in more southerly
batholiths. Ilmenite and lesser titanite have been reported (Miller, 1989), and the batholiths
are likely mainly ilmenite-series. Geochemical and isotopic characteristics are consistent with
79
Chapter 3
Mid-Cretaceous Plutonic Province Suites
their formation by melting of, or contamination with, Paleozoic continental crust (Miller, 1989;
Arth et al., 1989b). Fewer data are available for the smaller plutons in the Kaiyuh (112 Ma)
and Nyac (117-108 Ma) regions, but they are interpreted to form the more southerly extensions
of the Ruby suite, as they are similar-aged plutons, with similar lithologies and geochemistry
(Patton et al., 1984; Miller, 1989).
Intrusion-related mineral deposits in the Ruby-Kaiyuh-Nyac belt are sparse, with a few
small tungsten skarns, tin greisens (i.e., Sithylemenkat), and assorted tin, niobium, and gold
placers (i.e., Ruby, Melozitna and Tofty districts). The widespread occurrence of cassiterite
in placers suggests that the small, but numerous, lode occurrences represent the remains of
a larger tin province that was eroded during unroofing of the geanticline (Hudson and Reed,
1997). Minor copper-molybdenum-tungsten showings occur in the north of the belt, where
some of the granites are more oxidized (Patton and Miller, 1970; Clautice, 1983; Clautice et
al., 1993; Baker and Foley, 1986; Blum et al., 1987). The most significant mineralisation is a
series of Ag-As-Au±Sn-Zn-Pb prospects and deposits immediately south of the Kaltag Fault,
including the 113 Ma Illinois Creek deposit and the Honker and Perseverance silver-bearing
vein occurrence (Flanigan, 1998).
Yukon-Tanana Uplands Plutons (109-102, 93 Ma)
Voluminous Cretaceous intrusions in the central Yukon-Tanana Uplands, mainly the
area between the Shaw Creek and Volkmar faults, are characterized by large batholiths that
intrude amphibolite-grade Paleozoic metasedimentary and metaigneous rocks of the YukonTanana terrane (Fig. 2). Slightly to moderately peraluminous plutons vary from fine-grained
to coarse-grained, porphyritic leucocratic granite and granodiorite, to less-common quartz
monzonite (Smith et al., 1999; Dilworth, 2003; Day et al., 2003). Biotite dominates over
sparse hornblende, and muscovite is rare (e.g., Newberry et al., 1990b). Most reported U-Pb
dates for batholiths and plutonic rocks are from 109 to 102 Ma, but many dates from dykes
and smaller and more metaluminous/mafic bodies are about 93 Ma, which is similar to a large
number of Ar-Ar dates on igneous and hydrothermal micas (Smith et al., 1999; Dilworth et
al., 2002; Selby et al., 2002; Day et al., 2003; Werdon et al., 2004). The nature of the plutonic
rocks, large batholiths, and high regional metamorphic grade suggest that the intrusions were
emplaced at mid-crustal levels. The batholiths all have low and flat aeromagnetic signatures
and mostly have Fe2O3/FeO ratios <0.5 (Fig. 3). Limited work on the Goodpaster batholith
indicates that ilmenite and titanite are dominant over magnetite, and it has low magnetic
susceptibilities (0.1-0.2) and Fe2O3/FeO ratios (Dilworth, 2003; Werdon et al., 2004).
Mineralisation in the region is dominated by the high density of occurrences that form the
40-km-long, southeast-trending Pogo-Blue Lead gold belt located south of the Goodpaster
batholith. Mineralisation includes the world-class Pogo deposit (5.6 Moz Au; Smith et
al. 1999), but the genetic relationship between gold mineralisation and the intrusions is
controversial (Goldfarb et al., 2000; Hart et al., 2002; Groves et al., 2003; Rhys et al.,
2003). The 104 Ma age of the Pogo deposit (Selby et al., 2002) is the same as the age of
the dominant plutonic event in the region, but is also only a few million years younger than
the regional metamorphic event that affected the host rocks (Day et al., 2003). Some smaller
80
Chapter 3
Mid-Cretaceous Plutonic Suites
gold occurrences in the area have the same age as the Pogo deposit (i.e., Blue Lead, 105
Ma, Newberry et al., 1998), whereas, other gold prospects have been dated at ca. 94 Ma (i.e.
Shawnee Peak, Prospect 4021; Selby et al. unpublished). The uplands region is geologically
complex and poorly studied, and, except for a few small molybdenum- and tungsten-rich
porphyry and skarn prospects (i.e. Rainy Mountain, Boulder Creek, Section 21, and Lucky 13
occurrences), also with the same bimodal age distribution (e.g., Newberry et al., 1998), the role
of the granitoids in generating economically significant gold mineralisation is equivocal.
Anvil-Hyland-Cassiar Belt (110-96 Ma)
The Anvil (110-98 Ma), Hyland River (106-96 Ma) and Cassiar (110-100 Ma) plutonic
suites (Fig. 2) comprise a >900-km-long belt (reconstructed across Tintina Fault) of large, likeaged plutons and batholiths with similar, peraluminous-leaning geochemical and lithological
characteristics (Pigage and Anderson, 1985; Driver et al. 2000; Heffernan and Mortensen,
2000) (Fig. 2). All suites intrude Paleozoic and Neoproterozoic stratigraphy that comprises
the ancient North American continental margin (Selwyn Basin) or displaced pieces of the
margin (Cassiar Terrane). Lithologies are dominated by variably porphyritic, peraluminous to
slightly peraluminous biotite granite and granodiorite, with local muscovite-bearing phases.
The Anvil suite of plutons consists of three phases: peraluminous muscovite-biotite granites
and two groups of metaluminous to peraluminous hornblende-biotite granodiorites to granites.
The Cassiar batholith includes biotite>hornblende granodiorite and quartz monzodiorite in the
south and muscovite-biotite granite in the north. As is typical in these plutons, there are a wide
range of, dominantly radiogenic, isotopic ratios (i.e., initial strontium ratios of 0.706 to 0.740;
εNd=-2.7 to -17, δ18O of 10 to 12 per mil are reported for the Cassiar batholith; Driver et al.,
2000).
Magnetic susceptibilities for Hyland River rocks are wide-ranging but mostly moderate
(Fig. 4), as are aeromagnetic responses which show that the Anvil suite rocks are low and less
magnetic than Hyland or Cassiar suites. Ferric:ferrous ratios of Quiet Lake batholith rocks,
considered part of the Cassiar suite, are between 0.5 and 1.0 (C. Hart, unpublished). Biotite of
the Seagull suite, a highly fractionated sub-suite of the Cassiar suite (Liverton and Alderton,
1994), has compositions that indicate reduced fO2 between NNO and QFM (Liverton, 1999).
Together, most of these data are consistent with an ilmenite-series classification suite, but
Driver et al. (2000) note that titanite is common in most of the Cassiar batholith, and that
magnetite is dominant over ilmenite except in the northern parts of the batholith. Heffernan
and Mortensen (2000) suggest that magnetite is the common oxide mineral in Hyland
intrusions. We interpret the Anvil suite to be ilmenite-series, whereas the Cassiar and Hyland
suites although leucocratic and generally containing low amounts of Fe-Ti oxides, are weakly
oxidized, and possibly weakly magnetite-series. The Seagull suite intrusions are reduced and
ilmenite-series.
Associated mineralisation is characterized by numerous tungsten±molybdenum skarns
and veins, with distal Ag-Pb-Zn veins. In particular, the Cassiar suite intrusions and adjacent
calcareous strata define a particularly favorable environment for scheelite skarn formation,
with dozens of small occurrences and a few small deposits (Risby, Obvious, Stormy). The
81
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Figure 4. Magnetic susceptibility
values for selected Yukon
plutonic suites. The magnetic
susceptibility boundary between
magnetite and ilmenite series
granitoids is approximately 3.0
(Ishihara, 1977).
most significant mineral occurrences are a series of tin skarns, including the JC tin skarn (1.25
Mt of 0.2 SnO2; Layne and Spooner, 1991) that is associated with the Seagull suite, and the
Logtung porphyry tungsten deposit (162 Mt of 0.14% WO3) intrusion also within the Cassiar
belt.
Southern Alaska Range (100-88 Ma)
A poorly exposed cluster of mid-Cretaceous granodiorite, diorite and pyroxenite intrusions
occurs in the southern Alaska Range of southwestern Alaska, with the full areal extent of the
plutons uncertain. These intrusions, nonetheless, are recently recognized as being associated
with important mineralisation in that they host the very large Pebble porphyry copper-goldmolybdenum deposit (Bouley et al., 1995), as well as other similar occurrences such as the
Neocola deposit. Recent company estimates suggest the Pebble deposit contains 16.5 billion
82
Chapter 3
Mid-Cretaceous Plutonic Suites
pounds of copper and 26.5 million ounces of gold within an inferred resource of 2.74 billion
tones grading 0.27% Cu, 0.015% Mo, and 0.3 g/t Au, and hydrothermally altered rocks occur
over an area of approximately 90 km2 (Northern Dynasty Minerals, Ltd., January 21, 2004).
The intrusions are non-radiogenic and likely have a high mantle contribution (Young et al.,
1997). Dating of the Pebble and Neocola deposits and host intrusions indicate ages are mostly
between 100 and 88 Ma (Young et al., 1997; Schrader et al., 2001).
Tok-Tetlin Belt (101-93 Ma)
This 600-km-long northwest-trending Tok-Tetlin belt consists of numerous elongate
plutons and small batholiths that occur north of the Denali fault system, from the northernmost
Alaska Range to western Yukon (Fig. 2). Igneous rocks include the Tok-Tetlin and Cheslina
intrusions in Alaska (Richter et al., 1975a), and some of the Nisling Range granodiorite plutons
in Yukon (Tempelman-Kluit, 1976), which have intruded rocks of the southern Yukon-Tanana
and Windy-McKinley terranes. These rocks are poorly studied, but consist of hornblendebiotite granodiorite, quartz monzonite, quartz monzodiorite, and pyroxenite. The belt may
extend further north to the Bonnifield area where similarly reduced granitoids occur, or
alternatively, plutons in this region may define a southerly extension of the Fairbanks-Salcha
suite. Ages are mostly 101-93 Ma, but there are a few outliers at 107 and 88 Ma. Despite their
high iron contents and apparent metaluminous compositions, the plutons generally have low
aeromagnetic responses and low Fe2O3/FeO ratios (<0.3; Fig. 3) indicating a reduced character.
As such, these rocks are interpreted to be ilmenite-series. Only a few small stibnite- and goldbearing vein prospects have been recognized in association with this suite of plutons.
Tungsten Plutonic Suite (97-94 Ma)
Forming a 300-km-long northwest-trending belt of small to moderate-sized plutons along
the eastern margin of the Selwyn basin, the Tungsten plutonic suite straddles the border of
western NWT and eastern Yukon (Fig. 2). The 97-94 Ma plutons consist of leucocratic biotite
granite, monzogranite, and quartz monzonite. Biotite is dominant, hornblende is present
locally, and sparse muscovite is reported (Gordey and Anderson, 1993), but garnet is rare,
and cordierite not reported. Intrusions are weakly to moderately peraluminous (Hart et al.,
2005). Initial strontium ratios are high, typically >0.720. All plutons have low aeromagnetic
responses, low whole rock Fe2O3/FeO ratios (0.1-0.3; Fig. 3), and low magnetic susceptibilities
(<1; Fig. 4). Associated skarn assemblages are very reduced (Dick and Hodgson, 1982),
skarn-forming fluids were methane-bearing (Bowman et al., 1985; Gerstner et al., 1989), and
biotite compositions of the granitoids have very low Fe3+/Fe2+ ratios and, therefore, low fO2
(Keith et al., 1990). Intrusions of the Tungsten suite are characterized by accessory ilmenite,
and thus represent ilmenite-series granitoids. World-class scheelite skarn deposits at Mactung
and Cantung (57 Mt of 0.95% WO3 and 9 Mt of 1.4% WO3, respectively), and smaller deposits
at Clea and Lened (Sinclair, 1986) are the most significant intrusion-related deposits. These
deposits have associated sub-economic copper, and local zinc and tin enrichments. Small tin
greisens, and distal lead-zinc and antimony vein occurrences are also associated with this suite
of igneous rocks (Hart et al., 2005).
83
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Mayo Plutonic Suite (95-92 Ma)
Approximately one hundred small (1-20 km2), 95-92 Ma plutons comprise the 400-kmlong Mayo plutonic suite (Hart et al., 2005) that spans across central Yukon (Fig. 2). The
plutons intrude the northern margin of the Neoproterozoic to Paleozoic Selwyn basin. The
plutons are dominated by porphyritic to coarse-grained leucocratic quartz monzonite, but also
include granodiorite and biotite granite (Murphy, 1997). Biotite is dominant, although locally
hornblende and clinopyroxene are present. Most plutons are metaluminous, with high SiO2
plutons verging to weakly peraluminous compositions. Lamprophyre dykes compose a small,
but important component of most of the Mayo plutons. Initial strontium isotopic values of
0.710 to 0.730 indicate that the rocks are highly radiogenic. All plutons contain titanite as the
dominant Fe-Ti phase, and rare magnetite and ilmenite. The plutons have low aeromagnetic
responses (Hart et al., 2000), low Fe2O3/FeO ratios (0.2-0.5; Fig. 3), and low magnetic
susceptibilities (0.09-0.4, Fig. 4).
Intrusions of the Mayo suite are associated with numerous gold occurrences, both intrusionand contact aureole-hosted, including deposits at Dublin Gulch, Scheelite Dome, and Clear
Creek (Murphy, 1997; Hart et al., 2000). Economically significant gold placer deposits have
been worked for more than one hundred years in areas downstream of the intrusion-related
gold lodes. The intrusions are also responsible for numerous tungsten skarn deposits (Ray
Gulch, Kalzas) and a few small tin occurrences (Sinclair, 1986). In addition, the Ag-Pb-Zn
vein deposits of the Keno Hill district, which produced >200 Moz Ag, are proximal to one of
these intrusions, but the genetic relationship is unclear (Lynch et al., 1990).
Fairbanks-Salcha Plutonic Suite (94-90 Ma)
Clusters of small plutons, forming a 150-km-long belt from the Fairbanks to the Richardson
areas (Fig. 2), have several associated intrusion-related mineral occurrences. The FairbanksRichardson plutons all intrude the dominantly greenschist grade, early Paleozoic and older
Fairbanks Schist. Compared to intrusions of the Yukon-Tanana Uplands, the Fairbanks
and Salcha plutonic suites represent sparse and isolated plutons rather than vast batholiths
exposed south of the Shaw Creek fault. The Fairbanks-Salcha granitoids may be the western
continuation of the Mayo plutonic suite that has been offset dextrally along the Tintina Fault.
Fairbanks area plutons are dominated by the Gilmore Dome pluton, the smaller Pedro
Dome pluton, and the gold-rich Fort Knox (Vogt) stock (Newberry et al., 1996). These
intrusions are characterized by weakly peraluminous porphyritic, biotite>hornblende
granite and fine-grained granodiorite (Forbes, 1982; Blum, 1983, 1985; Newberry et al.,
1990b; McCoy et al., 1997). Strongly peraluminous aplite and pegmatite phases are also
present. Biotite is the dominant mafic mineral, with subordinate hornblende in some plutons,
although hornblende is particularly common in the Fort Knox stock. Reliable dating from the
region indicates a narrow range of 94-90 Ma for pluton ages, which is similar to ages of the
Richardson district intrusions (McCoy et al., 1997). Initial strontium ratios are approximately
0.712 (Blum, 1985; McCoy et al., 1997). The plutons have low magnetic susceptibilities, flat
aeromagenetic signatures that are indiscernible from the background schists, low Fe2O3/FeO
ratios of 0.1-0.4 (Fig. 3), low fO2 values of -17 to -14 (McCoy et al., 1997), and lack magnetite
84
Chapter 3
Mid-Cretaceous Plutonic Suites
as a primary accessory mineral, but contain ilmenite (McCoy et al., 1997) and appreciable
titanite (Blum, 1985). Newberry et al. (1990b) define these as subduction-related, variably
fractionated I-type plutons, which have been contaminated with perhaps 5-15 percent crustal
material.
Plutons in the adjacent Richardson district consist of a few sparse ca. 90 Ma high-level
biotite granodiorite and quartz-feldspar porphyry stocks and dykes (McCoy et al., 1997).
Intrusive rocks in the Richardson district consist of high-level quartz-feldspar porphyry
intrusions (McCoy et al., 1997), which are generally more oxidized than most of the Fairbanks
plutons (Fig. 3). Few studies exist on the plutons in the Salcha River area, but the igneous
rocks include granite, granodiorite, and-two mica granites that yield two populations of ages of
ca. 106 and ca. 94 Ma (Werdon et al., 2004). They also have low aeromagnetic signatures, low
magnetic susceptibilities, and contain ilmenite (Werdon et al., 2004), although they are slightly
more oxidized than the Fairbanks intrusions (Fig. 3).
Mineralisation in the Fairbanks area is dominated by the low-grade, high-tonnage,
producing Fort Knox gold deposit (5.5 Moz), which is hosted in a small and variably
porphyritic granite stock (Bakke, 1995). Sheeted, low-sulfide quartz±feldspar veins, dated
at 92.4 Ma by Re-Os (Selby et al., 2002), contain most of the low grade (~0.93 g/t Au) gold
resource, and are reported in places to be transitional with mineralised feldspar-rich veins that
also cut the various phases of the Fort Knox stock (Bakke, 1995; McCoy et al., 1997). Also in
the Fairbanks area, the sheared margins of a small hornblende-biotite tonalite stock host ca. 90
Ma arsenopyrite-rich gold mineralisation at the Ryan Lode deposit. Other ca. 93 Ma plutons
in the Fairbanks area are spatially associated with small, sub-economic tungsten skarns (i.e.,
Stepovich and Spruce Hen; Allegro, 1987; Newberry et al., 1997). Additionally, the region has
yielded >8 Moz of placer gold (Bundtzen et al., 1996) and contains a large number of schisthosted gold veins that have been genetically related to distal 94-90 Ma intrusions (McCoy
et al., 1997), although this relationship has been questioned by some workers (i.e., Goldfarb
et al., 1997; Hart et al. 2002). Similarly, the undated, presently-producing True North gold
deposit, a thrust-hosted stockwork to disseminated-style deposit, is several kilometres from an
intrusion and lacks intrusion-related characteristics as defined by Lang et al. (2000) and Hart et
al. (2002). The few significant Au±Ag, As, Bi, Te, Sb, and Pb occurrences in the Richardson
district are related to an 88 Ma biotite granodiorite and quartz feldspar porphyry stock (McCoy
et al., 1997).
Livengood - Tombstone Belt (92-90 Ma)
A narrow, 300-km-long east- to northeast-trending belt of quartz-alkalic plutons that intrude
rocks of east-central Alaska (Reifenstuhl et al., 1997a; quartz-alkalic plutons of Newberry,
2000, herein called the Livengood suite) is correlated across ~400 km of dextral displacement
along the Tintina fault system to correlate with the Tombstone plutonic suite of west-central
Yukon, as shown in Fig. 2 (Anderson, 1987; Hart et al., 2005). Each of the suites consists of
approximately ten compositionally zoned bodies that are dominated by moderately alkalic
phases, such as biotite-, hornblende-, and/or pyroxene-bearing monzonite and syenite, with
lesser quartz-monzonite, monzodiorite, monzogabbro, and silica-undersaturated high-K phases,
85
Chapter 3
Mid-Cretaceous Plutonic Province Suites
such as tinguaite (Anderson, 1987). All bodies have ages between 92 and 87 Ma (Reifenstuhl
et al., 1997b; Anderson, 1987), and elevated initial strontium ratios of approximately 0.710
(Reifenstuhl et al., 1997a; Lang, 2001). Magnetite-bearing, quartz-poor marginal phases have
been recognized in some plutons (Hart et al., 2005), locally with zonations to magnetite-barren
quartz-rich interiors (Abercrombie, 1990; Reifenstuhl et al., 1997a). Most plutons have a
moderate to low, but internally variable, aeromagnetic response, locally with concentric zoning
and highly magnetic phases (Hart et al., 2000). Tombstone suite rocks have a wide range of
Fe2O3/FeO (Fig. 3) and magnetic susceptibility values (Fig. 4), and include many igneous
bodies that are oxidized, and they are, therefore, assigned to the magnetite-series.
Uranium-Th-REE mineralisation (Roy Creek in Alaska, Tombstone/Ting in Yukon) is most
commonly associated with igneous rocks in both suites of the belt, but the Tombstone suite is
also associated with the Brewery Creek gold deposit and the Marn and Horn Au-Cu-Bi skarns
in Yukon (Brown and Nesbitt, 1987). In addition, the Wolverine and Sawtooth Mountain
occurrences in Alaska and the similar Antimony Mountain occurrence in Yukon are each
dominated by stibnite veins with anomalous precious metal concentrations.
Redox State Characteristics
Aeromagnetic Character
Total field aeromagnetic signatures are responsive to magnetite content, such that
rocks with larger volumes of magnetite generally yield elevated aeromagnetic responses
(Grant, 1985). This is particularly the case for mid-Cretaceous rocks that cooled through
the Currie temperature during the Cretaceous normal superchron, such that they are not
affected by inverted dipole effects from cooling during magnetic reversals. As such, regional
aeromagenetic surveys over the mid-Cretaceous magmatic rocks in Yukon and Alaska allow
for first-order interpretations about the magnetic character and, therefore, the redox series of
plutons. Aeromagnetic data (e.g., Saltus 1997; Saltus et al. 1999; Lowe et al. 2003) indicate
that some suites, such as the Nutzotin, Cassiar, or Tungsten suites, show consistent responses,
be they either high, moderate, or low, respectively. Others responses, such as for the East
Seward suite, are variable (Fig. 5), potentially indicating complexities in either the redox
state of the magmas of the suite, or problems in the assignment of particular plutons to a
specific suite. This is particularly a problem in Alaska, where less detailed mapping and less
precise age-dating make suite assignment of individual plutons less certain. Complex redox
states may also be a characteristic intrinsic to peralkaline rocks, which are typically hotter
and remain above the solidus longer, and thus have a greater capacity to incorporate reducing
sedimentary rocks. Additionally, sub-solidus effects could cause the oxide mineralogy to
change as a function of the fO2 of the magmatic fluids.
Magnetic Susceptibility
Magnetic susceptibility measurements are essentially a direct indicator of the average
magnetite volume in a rock. Determinations are herein presented for many of the plutons in
the Yukon suites (Fig. 4), but few are available for Alaskan granitoids. Our measurements
86
Chapter 3
Mid-Cretaceous Plutonic Suites
Figure 5. Plutons of the East Seward
and eastern South Seward plutonic belts
are shown with underlying total field
aeromagnetics (white is high, black is
low) to highlight their aeromagnetic
character and variability. Plutons are
named.
were made on several freshly broken, unoxidized, surfaces using a KT-9 handheld magnetic
susceptibility meter, with a pin-spacer to account for analytical variations from uneven
surfaces. All values presented here are averages of 6-10 determinations per rock, at x10-3 SI
units. Ishihara et al. (2000) suggested that the value of 3.0 (equivalent to 100x10-6 emu/g in
Ishihara, 1979) be used to separate magnetite- from ilmenite-series granitoids, at 70% SiO2.
Some plutonic suites have restricted ranges of magnetic susceptibility, whereas many have
wide ranges of values. The calc-alkaline Dawson Range is the most magnetic, with a mean
value of 5.1 that supports the presence of primary magnetite. However, among the wide range
of values for the Dawson Range, there are many of <0.1 from highly fractionated, iron-poor
rocks. Similarly, plutons from the alkalic Tombstone suite yield a wide range and a slightly
elevated average value of 1.8, supporting the occurrence of primary magnetite in some samples
(Hart et al., 2005). The Mayo and Tungsten plutonic suites are the least magnetic. All
measurements, exclusive of those for a few pyrrhotite-bearing rocks from the latter suite, are
<1.0, and average values are 0.09 and 0.16, respectively. The Hyland River and Cassiar suites
have wide ranges, with average values of 2.3 and 0.77 respectively. These rocks, although
dominantly peraluminous with highly radiogenic isotopic signatures (i.e., Driver et al., 2000),
locally host titanite and minor magnetite and have slightly elevated aeromagnetic signatures
and are thus classified as weakly magnetite-series.
Measurements of plutons directly associated with mineral deposits are more restricted in
range (Fig. 6). Plutons at Dublin Gulch, Mactung, Cantung and Logtung are all associated
with tungsten skarns and yield low values, mostly <0.5 representing ilmenite-series suites.
The values for the intrusion at Mactung are particularly low, averaging only 0.03. The goldenriched Fort Knox pluton has similar low values, as does the Dublin Gulch pluton, both
87
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Figure 6.
Magnetic
susceptibility values for
selected plutons associated
with significant intrusionrelated mineral deposits.
are gold-dominant systems but have associated tungsten mineralisation. This low magnetic
character and reduced state of plutons associated with gold deposits helped provide the
basis for the reduced intrusion-related gold deposit model (Lang et al., 2000; Thompson and
Newberry, 2000). The Whitehorse pluton has high magnetic susceptibility values (average
13) that support the presence of considerable primary magnetite and indicates that associated
copper-gold skarn mineralisation is related to a highly-oxidized, magnetite-series calc-alkaline
pluton.
Most of the reduced plutons and suites have a high proportion of determinations between
0.1 and 0.5. Although a value of 3.0 has been suggested as the division between magnetite
and ilmenite-series granitoids (Ishihara, 2000), the reduced rock units measured in this study
generally do not exceed 0.5, and this value may best provide an empirical division between
magnetite and ilmenite-series granitoids for the rocks in the northern Cordilleran midCretaceous plutonic province.
88
Chapter 3
Mid-Cretaceous Plutonic Suites
Ferric:Ferrous Ratios
An approximation of the oxidation state of a magma can also be obtained from
determinations of whole-rock ferrous and ferric iron contents. Magnetite and ilmenite-series
granitoid groups have been divided at an Fe2O3:FeO ratio of 0.5, at 70% SiO2 (Ishihara, 2000).
Compiled Fe2O3:FeO whole-rock ratios, determined from major element geochemistry on
mid-Cretaceous granitic rocks in Alaska and Yukon, range from as low as 0.06 to as high as
8.1 (Fig. 3). Many plutonic suites yield narrow ranges of ratios that increase with increasing
silica. Suites with large data ranges and poor positive correlations with silica may partly
result from incorrect suite assignment of plutons due to poor age control or insufficiently
detailed mapping. The most reduced determinations were on plutons of the poorly understood
Tok-Tetlin belt, where two of the ratios are 0.06, with others ≤ 0.3, but together showing
decreasing redox conditions with increasing silica content. Plutons in the Dawson Range and
East Koyukuk are among the most oxidized, with Fe2O3:FeO ratios consistently >1.0, and
positive correlations with silica. The generally lower values for the Tanacross intrusions (0.41.0) suggest that they may not be part of the same suite with the Dawson Range plutons as
suggested above.
Data from the Fairbanks area have a wide data range (0.1-2.0), with both increasing and
decreasing ratios with increasing silica. Ratios from Salcha suite rocks are all higher than
those from the Fairbanks suite for a given silica content. Rocks from the Yukon-Tanana
Uplands also have a wide range of ratios (0.08-1.0), probably indicative of a several plutonic
suites that have yet to be individually distinguished in this region.
Plutons from western Alaska are similarly varied. Rocks from the East Koyukuk suite have
ratios clearly indicative of oxidized magmas (1-3), and ratios that increase with silica. West
Koyukuk data are mostly from the ultrapotassic suite of intrusions and show wide oxidationstate variations (0.3-5), but are generally oxidized (>0.8) without any trends with varying silica
contents. The Ruby intrusions mostly have ratios between 0.4 and 1.0, and show a broader
trend of decreasing redox state with increasing silica. These values are higher than expected
for these mainly crustal melt-derived plutons that lack magnetite (Miller, 1989; Arth, 1994).
The lower Fe2O3:FeO ratios for the Kaiyuh intrusions may result from significant amounts of
contamination in the magmas by carbonaceous sedimentary country rocks.
Central and eastern Yukon plutons show clear trends of increased redox ratios with
fractionation, with the alkalic Tombstone suite (0.2-2) being more oxidized and mostly within
the limit typical of magnetite-series rocks. The plutons from the Mayo (0.15-1) and Tungsten
(0.1-0.3) suites show values clearly below the dividing limit of 0.5. Hart et al. (2005) indicate
that only the Tungsten suite rocks contain ilmenite and that the Mayo suite is dominated by
titanite. However, according to Ishihara (1977, 1981), both may be classified as ilmenite-series
due to their overall low contents of iron-oxide minerals.
89
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Space, Time and Metallogenic Distribution of
Magnetite/Ilmenite Series Plutons
Distribution Patterns
The mid-Cretaceous plutonic epoch in Alaska and Yukon is represented by several, mostly
northwest-trending, plutonic belts that form a magmatic province locally as wide as 700 km,
although this may, in part, result from duplication by displacements along the Denali and
Tintina fault systems, as discussed below. Individual belts are generally 50- to 200-km-wide
and follow the regional structural fabric. Magnetite-series plutonic belts are preferentially
hosted in more seaward tectonic settings, whereas ilmenite-series intrusions are hosted in
more landward settings (Fig. 7), presumably farther from paleo-subduction zones. Magnetiteseries granitoids are mostly emplaced into the primitive arc terranes, whereas ilmenite-series
intrusion are dominantly intruded into the ancestral North American continental margin, or
displaced or metamorphosed pericratonic equivalents. There are, however, notable deviations
from these associations. The magnetite-series plutons of the St. Lawrence Island, East Seward,
and Tanacross-Dawson Range belts are hosted in pericratonic terranes (Seward and YukonTanana terranes) and the magnetite-series Livengood and Tombstone intrusions are in North
American continental margin strata, as are the weakly magnetic but slightly peraluminous
Hyland and Cassiar suites of plutons. Only the ilmenite-series intrusions of the Tok-Tetlin belt
occur in a seaward position and, importantly, ilmenite-series rocks are not hosted in primitive
accreted terranes. These deviations indicate that neither the basement terranes, nor the
immediate host rocks, may be the controlling element in determining magmatic redox state.
Plutonic suite distribution patterns in the northern Cordillera are complicated by the large
dextral strike-slip displacements which, when reconstructed, would place the Insular Belt
rocks 350 km south of their current position with respect to the Intermontane Belt rocks
due to movement along the Denali fault system (Fig. 8). Similarly, Intermontane Belt rocks
would, in turn, be about 430 km south of their current position with respect to ancestral North
American rocks due to movement along the Tintina fault system. These reconstructions would
have the effect of removing the telescoping and duplication of the more outboard magnetiteseries plutonic belts, such that the northern end of the Nutzotin belt may correlate with the
Whitehorse plutons, and the northern Cassiar suite would correlate with the southern Hyland
intrusions. As a result, the plutonic belts may have originally been thinner, perhaps as much as
one-half of their current 700 km width (Fig. 8).
Another complicating feature specific to plutonic belts in westernmost Alaska, is that they
appear to be related to a different subduction zone than that that generated plutonic belts in
southern and central Alaska. However, it is likely that, at least the St. Lawrence, East Seward,
and Koyukuk belts are a single belt that has been bent across an oroclinal bend as shown in
Fig. 7 (North Alaskan orocline of Johnston, 2001). The belt may continue to the southwest,
under-cover, as the region is characterized by along-strike deep magnetic highs (Saltus et al.,
1999). The belt likely bends again, this time to the east across the Kulukbuk Hills orocline.
This east-trending belt potentially joins with the plutons in the southern Alaska Range making
it possible that they may all have originally been part of the same magmatic belt (Fig. 7).
90
Figure 7. Distribution of magnetite and ilmenite-series mid-Cretaceous granitoids suites in Alaska and Yukon.
Chapter 3
Mid-Cretaceous Plutonic Suites
91
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Figure 8. Reconstruction of dextral displacements along the Tintina and Denali fault systems to show
approximate alignment of magnetite and ilmenite-series belts as they would have been in the midCretaceous, prior to early Tertiary offset.
Circum-Pacific magnetite and ilmenite-series plutonic belts have varied distributions
(Ishihara 1998). Much of the eastern Asian continental margin comprises coastal magnetiteseries, and inboard and more regionally extensive ilmenite-series granitoids. There are notable
variations and uncertainties in these patterns that result from tectonic complexities such as
the formation of marginal basins (Sea of Japan) and oceanic arcs (Philippines), and the broad,
but submerged, continental shelf. On the Japanese Islands, paired fore-arc ilmenite-series
and back-arc magnetite-series belts typify the Late Cretaceous-Paleogene granitoids and this
pattern is repeated by the overprinting Miocene-Quaternary magmatic arcs (Ishihara 1981).
The belts in Japan tend to be narrow (80-100 km), but on the Asian mainland, they are as broad
as several hundred kilometers.
The Pacific coast of the Americas is dominated by magnetite-series granitoids belts, locally
with thin coastward ilmenite-series belts such as in Northern Chile (Ishihara et al., 1984) and
the Sierra Nevada batholith (Bateman et al. 1991). However, the opposite pattern is apparent
in the Peninsular Ranges where ilmenite-series are dominant and are inboard from coastal
magnetite-series granitoids (Gastil et al., 1990). In Peru, a thick coastal magnetite-series
92
Chapter 3
Mid-Cretaceous Plutonic Suites
plutonic belt dominates with more restricted, inboard ilmenite-series belt (Ishihara et al.,
2000).
The patterns presented in this paper for the northwestern North American Cordillera
emphasize a broad, coastal magnetite-series belt with a broad, landward ilmenite-series belt,
and a thin, innermost magnetite-series belt. The pattern is generally similar to that in the
Peninsular Ranges and the eastern Asian continental margin but is broader in distribution, more
restricted in time, and reflects a differing tectonic setting.
Temporal Variations
The Early to mid-Cretaceous magmatic epoch spans approximately 55 million years, from
145 to 90 Ma, and was represented by episodic plutonism across the northwestern Cordillera.
This was initially dominated by the widespread emplacement of magnetite-series plutons,
subsequently followed by widespread ilmenite-series plutons, and terminated with a final
minor episode of magnetite-series plutons. The oldest magnetite-series components (145125 Ma) may have been pre-accretionary and coeval with Late Jurassic oceanic arc-building
events prior to their accretion to form part of the Insular terranes on the outermost parts of the
orogen. Subsequent magmatism (118-99 Ma) was also magnetite-series, with metaluminous,
arc-type, calc-alkaline plutonism occurring further inboard, through the eastern Insular terranes
to Stikinia in the Intermontane belt, and to the far northwest through the West Koyukuk belt
and across St. Lawrence Island. Ilmenite-series granitoids were first emplaced at about 113
Ma, likely in response to crustal thickening in response to terrane accretion. These granitoids
include the intrusion of the large Ruby-Kayuh-Nyac system, and then the belts of peraluminous
granitoids that characterize the Yukon-Tanana Uplands. The later-stage suites (109-96 Ma;
Cassiar, Hyland) are slightly more oxidized than those in the other ilmenite-series belts and are
classed here as weakly magnetite-series. These were followed by the emplacement of reduced,
ilmenite-series suites (98 to 92 Ma; Tungsten, Mayo, Fairbanks, Salcha). The last plutonic
event is defined by the 92-90 Ma alkalic, variably magnetite-series Livengood and Tombstone
suites (Figs. 2 and 7).
There is an overall inland younging trend in the ages of the plutonic belts regardless of their
oxidation state. But magnetite-series plutonic belts were active 5-10 million years prior to
the initiation of significant ilmenite series plutonism at ca. 109 Ma. Otherwise, magnetite and
ilmenite series plutonism was contemporaneous during the period from 109 to about 95 Ma.
However, ilmenite-series plutonism begins to dominate late in this episode (97-92 Ma), and
to have been followed by the small, late, alkalic magnetite-series event. In broad agreement,
Ishihara (1981) notes that magnetite-series suites in Japan are older than ilmenite-series
granitoids, in both the Cretaceous and Miocene belts.
Figure 9 (following page). Distribution of significant mineral deposits and occurrences associated with
mid-Cretaceous plutons in Alaska and Yukon. Metallogenic patterns developed emphasize a Cu-Fe-Mo
tenor associated with outboard magnetite-series granitoids. Inboard ilmenite series granitoids have
significant associated tungsten mineralisation, as well as notable gold associations. Alkalic rocks also
have associated U-Th mineralisation. The dashed line is the main contact between magnetite and ilmenite
series granitoids.
93
Chapter 3
94
Mid-Cretaceous Plutonic Province Suites
Chapter 3
Mid-Cretaceous Plutonic Suites
Metallogeny
Magnetite and ilmenite-series granitoid belts are characterized by distinctly different
metallogenic associations (Ishihara 1977, 1981; Blevin and Chappell 1992; Blevin et al.,
1996). These associations characterise the Alaska and Yukon plutonic suites (Fig. 9), but
there are important variations from the more classical examples. Details of the mineral
deposits are summarized in Table 2. Mineral occurrences displaying a Cu-Au-Fe tenor are
strongly associated with the magnetite-series plutons in the Insular terranes and in the St.
Lawrence-Seward-Koyukuk belt. Molybdenum prospects are also present in the latter belt
of igneous rocks. Tungsten mineralisation best characterizes rocks of the ilmenite-series, but
is also associated with the weakly magnetite-series plutons that together form a >1000-kmlong belt through the Cassiar, Hyland, Tungsten, Mayo, and Fairbanks plutonic belts, after
reconstruction of the Tintina fault system displacement (Fig. 8). Magnetite-series alkalic
plutonic belts each contain intrusion-related U±Th occurrences, locally also with gold and
copper enrichments, as is displayed by the Tombstone, Livengood and alkalic parts of the
Koyukuk suites.
Gold mineralisation also has an association with the ilmenite-series plutonic belts,
particularly those of the Mayo and Fairbanks suites (Fig. 9). This association is counter to
most granite-series metallogenic schemes (i.e. Blevin et al., 1996), but forms the basis of the
intrusion-related gold system model (Thompson et al., 1999; Lang et al., 2000; Thompson
and Newberry, 2000) that emphasizes an association between gold and moderately reduced
intrusions. This association had been originally recognized empirically by Leveille et al.
(1988) for intrusions in central Alaska.
Despite the variability of igneous rock compositions and metallogenic associations in the
mid-Cretaceous plutonic epoch, significant intrusion-related tin mineralisation is rare, except
for the JC and similar deposits associated with those very reduced intrusions of the western
Cassiar suite (Seagull suite). Polymetallic mineralisation, characterized by Sn-Ag but also
typically including Cu-Pb-Zn-Bi-Au, is represented by a few greisens and vein deposits, such
as Illinois Creek in Alaska and Logan in Yukon.
Silver -Pb-Zn-bearing, Sb-rich, and other varieties of polymetallic vein occurrences are
difficult to confidently characterize as intrusion-related, but in many regions, including the
world-class Keno Hill silver district, mineralisation is associated with ilmenite-series plutons.
The polymetallic occurrences are likely coeval with intrusion-related gold and tungsten
mineralisation, and appear to be deposited as part of a cooling paragenetic hydrothermal
sequence distal to the plutons. Polymetallic ores are also related to magnetite-series plutonism
in accreted terranes, such as surrounding porphyry copper deposits of the Nutzotin plutonic
belt in the Wrangell Mountains. However, in some districts polymetallic mineralisation is
of uncertain affinity, particularly those occurrences on the ancient continental margin where
syngenetic origins for polymetallic ores are also possible (e.g., East Seward, West Koyukuk).
Silver-rich Pb-Zn mantos are preferentially developed above unroofed plutons that intrude
North American strata (e.g., Midway Ag), and some have associated gold mineralisation (e.g.
Ketza River Au). The redox state of those plutons is uncertain because they are not exposed,
but they are likely to be ilmenite or weakly magnetite-series plutons of the Cassiar suite.
95
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Table 2. Intrusion-related mineral deposits and significant occurrences associated with Early and mid-Cretaceous
felsic intrusions in Alaska and Yukon. Data compiled from Newberry et al. (1997), Nokelberg et al. (1994); Young et
al., (1997), Sinclair (1986), Hart et al. (2000), Deklerk (2003) and specific references cited in table. Bold=deposit,
not-bold=prospect/showing. *=district. Fairbanks district includes Gil, Yellow Pup, Stepovich, Spruce Hen, Pedro,
Cleary; East Interior district includes Mitchell, Deer Creek, Oscar, Champion, Iron Creek; Charley River district
includes Wolverine, Cresent Ck., Twin Mtn. and Creek Mtn.; Lucky 13 district includes Bonanza, Beef Ridge, Valley
Ridge; Upper Chena includes MR45, MR52, N. Fork Salcha.
Miner’n
Age
(Ma)
Deposit
(*=district)
Commodity,
Style
Mt. Hayes/
Rainey Creek
McCarthy
Fe (Cu, Au) skarn
150
Nutzotin
Fe (Cu, Au) skarn
140
Tosina
London & Cape
Midas
Zackly
Rambler
Nabesna
Bond Creek
Horsfeld
Baultoff
Carl Creek
Poovookpuk Mtn
Cu porphyry
Cu-Au porphyry
Cu-Au skarn
Au skarn
Au-Cu-Ag skarn
Cu porphyry
Cu porphyry
Cu porphyry
Cu porphyry
Cu-Mo porphyry
140
140
130
114
114
114
111
111
111
108
Tosina
Tosina
Nutzotin?
Nutzotin
Nutzotin
Nutzotin
Nutzotin
Nutzotin
Nutzotin
St. Lawrence
West Cape
Orange Hill
108
105
St. Lawrence
Nutzotin
Neacola
Mo-Cu porphyry
Cu porphyry (minor
skarn)
Cu-Mo porphyry
94
Pebble
Cu porphyry
90
Southwest Alaska
Range
Southwest Alaska
Range
Whitehorse
Copper Belt
Antoniuk/Mt.
Freegold
Blue Lead
Illinois Creek
Paternie
Forty Mile
Table Mountain
Cu skarn
Intermontane Belt
111
Whitehorse
Tenney (1981)
Au porphyry, veins
108
Dawson Range
Deklerk (2003)
Au
Ag-Sn, Au veins
Cu-Mo-Au porphyry
Au-Cu
W Au skarn
105
105
103
102
90
YT Uplands
Kaiyuh
Livengood/Fairbanks
Liberty Bell
Fairbanks*
East Interior*
Au-Ag-Bi-Cu skarn
W skarn
Cu skarn
92
93
90
Tok-Tetlin
Fairbanks
YT Uplands
Upper Chena*
W skarn
92
Salcha
Charley River*
W skarn
Lucky 13*
W skarn
McLeod
Fort Knox
Ester Dome
Democrat
Roy Creek
Elephant
Mo porphyry
Au sheeted veins
Au veins
Au
Felsic U
Au
McCoy et al. (1997)
Flanigan (1998)
McCoy et al. (1997)
McCoy et al. (1997)
Newberry (1987); McCoy et al.
(1997)
Yesilyurt (1994)
Allegro (1987)
Burleigh et al. (1994); Newberry
et al. (1997)
Albanese (1984); Newberry et al.
(1997)
Foley & Barker (1981); Newberry
et al. (1997)
Burleigh et al. (1994);
Newberry et al. (1997)
West (1954)
Bakke (1995)
McCoy et al. (1997)
McCoy et al. (1997)
Burton (1981)
McCoy et al. (1997)
96
Associated
Plutonic Suite
Insular Belt
YT Uplands
92
94
90
90
90
YT Uplands
Kiayuh
Fairbanks
Fairbanks
Salcha
Livengood
Livengood
References
Rose (1966), Newberry et al.
(1997)
MacKevett (1976), Newberry et
al. (1997)
Nokleberg et al. (1987)
Nokleberg et al. (1987)
Ford (1988)
Weglarz (1991)
Weglarz (1991)
Richter et al. (1975b)
Richter et al. (1975b)
Richter et al. (1975b)
Richter et al. (1975b)
Bundtzen et al. (1994), Patton
and Csejtey (1971, 1980)
Patton and Csejtey (1971, 1980)
Richter et al. (1975b)
Reed and Lanphere (1969, 1972)
Bouley et al. (1995), Young et
al. (1997)
Chapter 3
Mid-Cretaceous Plutonic Suites
Table 2. (cont.)
Miner’n
Age
(Ma)
Deposit
(*=district)
Commodity,
Style
Placer River
Zane Hills
Mo porphyry
Cu-Au porphyry
Clear Creek
Wheeler Creek
Billiken
Eagle Creek
Independence
Quartz Creek
Perseverance/
Beaver Creek
Purcell Mtn.
Windy Creek
Kanuti
Felsic U
Felsic U
Fe skarn
Felsic Plutonic U
Ag-Pb-Zn veins
Pb-Zn-Ag veins
Pb-Zn-Sb veins
Logtung
W porphyry
Ketza River
Silver Hart
Risby
Quartz Lake
Hyland Au
Bailey
Keno Hill
Sa Dena Hes
Logan
JC
Mactung
Au-Ag manto
Ag-Zn-Pb skarn
W-Cu skarn
Ag-Zn-Pb manto
Au disseminated
W-Cu skarn
Ag-Pb-Zn veins
Pb-Zn-Ag-Sn skarn/
manto
Pb-Zn-Ag ?
Sn skarn
W-Cu skarn
Cantung
W skarn
95
Tungsten
Ray Gulch
Dublin Gulch
W skarn
Au sheeted veins
95
94
Mayo
Mayo
Scheelite Dome
94
Mayo
Clear Creek
Clea
Lened
Midway
Kalzas
Brewery Ck.
Au veins, sheeted
disseminated
Au veins, sheeted
W skarn
W skarn
Ag-Pb-Zn manto
W vein/stockwork
Au disseminated
93
~93
~93
91
92
Mayo
Tungsten
Tungsten
Cassiar
Mayo
Tombstone
Marn/Horn
Ting/Noting
Au-Cu skarn
U felsic pluton
92
90
Tombstone
Tombstone
Mo-Cu porphyry
Mo porphyry
W skarn
Associated
Plutonic Suite
Koyukuk Belt
103
Seward/Koyukuk
~100
East Koyukuk
~100
~100
100
97
91?
References
East Koyukuk
East Koyukuk
East Seward
East Seward
East Seward
West Koyukuk
Koyukuk
Miller (1976), Miller and Elliot
(1977)
Miller (1976)
Miller (1976)
Gamble and Till (1993)
Miller and Bunker (1976)
Hudson et al. (1977)
Bundtzen et al. (1994)
Nokleberg et al (1994)
Koyukuk
East Seward
Ruby
Hudson et al. (1977)
Clautice (1983)
North American Margin
112
Cassiar?
108
103
103
-
Cassiar
Cassiar
Cassiar
Hyland?
Hyland
Hyland
Mayo
Hyland
~102
~101
95
Cassiar
Cassiar (Seagull)
Tungsten
Abbott (1986); Noble et al.
(1984)
Fonseca (1998)
Abbott (1983)
Deklerk (2003)
Vaillancourt (1982)
Deklerk (2003)
Lynch et al. 1990
Deklerk (2003)
Deklerk (2003)
Layne and Spooner (1991)
Dick and Hodgson (1982),
Atkinson & Baker (1986)
Mathieson and Clark (1984);
Bowman et al. (1985)
Lennan (1986)
Hitchins and Orssich (1995),
Maloof et al. (2001)
Mair et al. (2000)
Marsh et al. (2003)
Godwin et al. (1980)
Glover and Burson (1986)
Bradford and Godwin (1988)
Lynch (1989)
Diment and Craig (1999);
Lindsey et al. (2000)
Brown and Nesbitt (1987)
Deklerk (2003)
97
Chapter 3
Mid-Cretaceous Plutonic Province Suites
The plutonic belts with little genetically-associated metallic mineralisation are those that
form ilmenite-series batholiths such as in Ruby and Yukon-Tanana Uplands, and the magnetiteseries batholiths of the Tanacross-Dawson Range. All of the plutons in these belts intruded
amphibolite-grade metamorphic rocks, after peak deformation and probably at mid-crustal
levels. The associated voluminous, ilmenite-series magmas, were low temperature, viscous,
and mainly anhydrous, and are therefore, unlikely to have exsolved a metal-bearing magmatic
hydrothermal fluid. The highest densities of intrusion-related mineral deposits are associated
with the reduced and radiogenic, ilmenite-series Fairbanks and Mayo suites, and the oxidized
and primitive, calc-alkaline, magnetite-series Nutzotin-Kluane belt. Magmas related to
mineralisation in these belts intruded to form small plutons at shallow crustal levels; many
remain partly or completely unroofed. Small-batch, higher-temperature hydrous magmas
emplaced at high crustal levels, where pressure and temperature gradients are steep, appear to
best dictate prospectivity. Given these conditions, magmatic processes responsible for volatile
phase separation and metal-enrichment operate efficiently.
Tectonic Setting of Redox-Series Granitoids
The regional and temporal variations in redox-series granitoids reflect their evolving
tectonic settings, source materials, and magmatic processes. It is widely recognized that
oxidized magmas are mostly the calc-alkaline and metaluminous products generated by
partial melts in the mantle wedge above a subducting slab. These melts can be modified by
assimilation-fractional crystallization (AFC) processes during ascent and interactions with
overlying, non-carbonaceous crust (Gill, 1981; Ishihira, 1981). Oxidized alkalic (potassic)
magmas are derived through melting of metasomatized mantle, typically during postcollisional to extensional regimes in rifting environments (Bailey, 1983). Ilmenite-series
magmas are typically generated in compressional settings through the melting of, or interaction
of melts with, reduced, carbonaceous sedimentary rocks that are typical of continental margin
sequences (Ishihira, 1981).
Most of the magnetite-series plutonic belts in Alaska and Yukon are typical calc-alkaline,
metaluminous, I-type magmas that were originally generated in the mantle wedge or mafic
lower crust in response to subduction-related hydration melt. The plutons are mostly
uncontaminated because they intruded primitive arc terranes, except for the plutons of the
Tanacross-Dawson Range belt that intruded Paleozoic and older metasedimentary rocks
of the Yukon-Tanana terrane and have elevated initial strontium ratios (0.706). Despite
the contamination, these radiogenic rocks are still oxidized and mostly strongly magnetic.
The Cassiar and Hyland suites are locally weakly magnetic and categorised as weakly
magnetite-series. They are slightly peraluminous, have highly radiogenic isotopic ratios, have
characteristics akin to S-type granitoids, and were generated far inboard from the continental
margin. These features indicate a substantial contribution from continental crust, which
typically would result in a more reduced melt. However, large parts of the Neoproterozoic
crust underlying this part of Yukon consist of oxidized, hematitic sandstone and siltstone
(Hyland Group; Gordey and Anderson 1993), and there are limited amounts of carbonaceous
materials. In addition, a mafic igneous component has been suggested as a partial source
for these weakly oxidized magmas (Driver et al. 2000). Similarly, the variably magnetite-
98
Chapter 3
Mid-Cretaceous Plutonic Suites
series Livengood and Tombstone suites intruded Selwyn Basin strata on the ancestral North
American margin and were not reduced. The oxidized and alkalic melts were, however, likely
derived through partial melting of ancient metasomatized mantle and had little interaction with
the mid to upper crust (Hart et al., 2005).
Ilmenite-series magmas in Alaska and Yukon developed from melting of continental crust
in response to crustal thickening associated with terrane collision, accretion, and obduction
(Miller, 1989, 1994; Arth, 1994). This is most likely the case for the large batholiths that
comprise the Ruby geanticline and Yukon-Tanana Uplands batholiths, as the country rocks in
both regions are amphibolite-grade, with voluminous magmatism immediately following peak
metamorphism. The once-continuous Fairbanks-Mayo-Tungsten ilmenite-series belts appear
to have had a more complex origin that involved interactions of mantle-derived components
with the crustal melts (Hart et al., 2005). Smaller belts, such as the South Seward and some
in the Chugach region, appear to have been localized in shear zones during thrusting, and were
synchronous with metamorphism or orogenic collapse (e.g. Amato et al., 1994). The most
enigmatic ilmenite-series belt is the Tok-Tetlin belt that is very reduced, yet sited between
nearly contemporaneous magnetite-series belts. These anomalous rocks could result from the
melting of a large flysch component, which is interpreted to exist at depth between the YukonTanana Terrane and Insular terranes (Stanley et al., 1990; Beaudoin et al., 1992) following
early Cretaceous thickening. Isotopic evaluation of granitic rocks in central Alaska by
Aleinikoff et al. (2000) failed to find flysch generated melts, but only one of their samples were
from the Tok-Tetlin belt.
The tectonic setting and processes required to form reduced magmas are much more varied
than those needed for oxidized magmas (Ishihara 1981). The melting of, or contributions
from, a continental lithosphere containing carbonaceous materials is the most general example
for generating reduced melts. However, other scenarios include in-situ reduction of oxidized
magmas due to assimilation of wall-rocks (Ishihara and Sasaki, 1989), a depth-dependent
mechanism whereby volatiles and, therefore, fO2, are lower for magmas generated below
the zone of slab dehydration (Gastil et al., 1990), or that deeply-emplaced non-radiogenic
and primitive magmas may locally source from reduced materials to become ilmenite-series
(Bateman et al., 1991).
The temporal and spatial distributions of magnetite- and ilmenite-series granitoids in Alaska
and Yukon can be explained by a model in which latest Jurassic to Early Cretaceous, arcgenerated, I-type, magnetite-series granitoids (Tosina-St. Elias) are built upon offshore terranes
prior their accretion to form the Insular Belt. Initial closure of the Gravina Ocean and collision
of those terranes with the edge of the continental margin resulted in a short magmatic lull (135120 Ma), which may have been coincident with a change in subduction polarity. Subduction of
Pacific plates (probably the Farallon plate) beneath the new continental margin initiated in the
late Early Cretaceous generated magnetite-series granitoids (Nutzotin-Kluane). Magmatism
then migrated further landward (Whitehorse -Dawson Range-Tanacross) in response to final
closing of the Gravina basin (Fig. 8) between the Insular and Intermontane tectonic elements.
The collision of the Insular terranes resulted in compression and tectonic thickening behind
the arc, which gave rise to metamorphism and partial melting of the lower crust beneath the
99
Chapter 3
Mid-Cretaceous Plutonic Province Suites
Yukon-Tanana and Ruby terranes. This initially generated syn-metamorphic orthogniesses
at ~120-112 Ma. However, during a subsequent or continued period of uplift, extension, and
orogenic collapse, decompression-assisted melting generated vast, locally foliated but mainly
undeformed, ilmenite-series (Ruby, Yukon-Tanana Uplands, Anvil) and weakly magnetiteseries (Hyland, Cassiar) batholiths from ~112 Ma until ~97 Ma. The inboard components of
post-collapse, upper crustal associated extension provided the framework for emplacement of
small upper crustal plutons of the Fairbanks, Mayo and Tungsten suites at 95 Ma. The final
gasp of inboard extension resulted in magnetite-series alkalic plutons of the Livengood and
Tombstone suites at 92-90 Ma, and the lamprophyre dykes associated with the Mayo suite.
Northern Cordilleran magmatism was mostly a response to subduction of the Pacific
oceanic plates beneath the North American margin. Mid-Cretaceous plutonism ended
abruptly at 90 Ma, probably in response to oceanic plate vector readjustments at ca. 95 Ma
(Engebretson et al., 1995). The Kula plate migrated rapidly northward after detaching from the
Farallon plate at 85 Ma (Engebretson et al., 1985). Significant coupling of crustal fragments
along the continental margin with the Kula plate resulted in at least 500 km, and possibly as
much as 2000 km (Irving and Wynne, 1991; Johnston et al., 1996), of net displacement along
the Tintina fault system and allied structures. During the Paleocene, displacement jumped
seaward to the Denali fault system, and resulted in an additional 350 km of displacement of
Insular terrane elements. These rapid displacements resulted in the buckling of the Koyukuk
and Intermontane tectonic elements in western Alaska resulting in the development of the the
North Alaska and Kulukbuk oroclines (Johnston, 2001).
Conclusions
The mid-Cretaceous plutonic episode (145-90 Ma) in the northern North American
Cordillera is the most widespread, voluminous, diverse and metallogenically significant
plutonic event in the northern Cordillera, with a current width of nearly 700 km. The plutonic
province was emplaced across an accretionary margin that includes primitive seaward
terranes added onto an ancient continental margin. Geological, geochemical, isotopic, and
geochronometric attributes of the mid-Cretaceous plutons are compiled to define 23 plutonic
suites or belts that represent this magmatic province. The oxidation states of these plutonic
suites and belts, as indicated by their aeromagnetic signatures, magnetic susceptibilities, and
whole-rock ferric:ferrous ratios, define distinct and continuous magnetite and ilmenite-series
belts that are today bent and offset by post-emplacement structural complexities.
Magnetite-series plutonic belts are preferentially hosted in the more seaward tectonic
settings, whereas ilmenite-series intrusions are hosted in more landward settings, farther
from paleo-subduction zones. Magnetite-series granitoids mainly intruded the primitive arc
terranes, and ilmenite-series intrusions were dominantly emplaced into the ancestral North
American continental margin or into displaced or metamorphosed pericratonic equivalents.
However, there are important exceptions to these generalizations. One exception is the
presence of magnetite-series plutons in continental rocks, but its correlative of ilmenite-series
plutons in primitive terranes or coastal settings is rare.
100
Chapter 3
Mid-Cretaceous Plutonic Suites
Overall temporal trends define magnetite-series belts that young inland, with younger
ilmenite-series belts further cratonward. The oldest and most seaward plutonic episodes
(145-125 Ma) are arc-type magnetite-series and may include pre-accretionary oceanic arc
formation. Successive continental arc magmatism (118-99 Ma) is also magnetite-series, with
metaluminous calc-alkaline plutonism occurring inland of the 145-125 Ma belts. Ilmeniteseries plutonism, initiated at about 112 Ma in response to crustal thickening, was dominated by
the formation of large, slightly peraluminous batholiths. Later plutonic suites (109-96 Ma) are
similarly slightly peraluminous, but are more oxidized and designated as weakly magnetiteseries belts. A final magmatic episode led to the emplacement of widely scattered ilmeniteseries granitoids during a minor extensional event at 98 to 92 Ma. In the final phases of
extension, magnetite-series alkalic plutonic suites and lamprophyres were emplaced at 92-90
Ma in the most inland locations.
Mineral deposits dominated by copper, gold, and iron are mainly associated with
magnetite-series plutons, there are also some local examples of molybdenum occurrences
associated with these intrusions. Magnetite-series alkalic plutonic belts are characterized
by gold and copper enrichments, as well as by uranium and thorium occurrences. Tungsten
mineralisation best characterizes the ilmenite-series plutons. Perhaps uniquely, intrusionrelated gold mineralisation in the northern Cordillera is associated with ilmenite-series plutonic
belts, most particularly in the Mayo and Fairbanks suites. Such an association is counter to
most granite-series metallogenic schemes (i.e. Blevin et al., 1996), but forms the basis of the
reduced intrusion-related gold system model of Thompson et al. (1999) and Lang et al. (2000).
Base-metal (Pb-Zn±Ag) mineralisation directly associated with mid-Cretaceous plutons is not
common, but small polymetallic occurrences are found distal to both magnetite and ilmeniteseries mineralizing plutons. Significant associated Sn mineralisation is rare. The plutonic
belts with the fewest mineral occurrences are those of both ilmenite-series and magnetite-series
that are dominated by large batholith complexes. Plutonic suites with the highest density of
associated mineral deposits are those of the highly-radiogenic ilmenite-series Fairbanks and
Mayo suites and the magnetite-series Nutzotin-Kluane belt, which consist of mainly small
plutons that were emplaced at upper crustal levels.
Acknowledgements
The senior author (CJRH) would like to thank Shunso Ishihara for his encouragement and
patience in awaiting completion of this manuscript. In addition, the attendance of JLM to
the Ishihara symposium and of CJRH to the Hutton symposium are acknowledged as pivotal
events in making this manuscript viable. David Groves and an anonymous reviewer are
thanked for their contributions for improving the manuscript. Furthermore, the support of the
Centre for Global Metallogeny, Yukon Geological Survey and the US Geological Survey are
appreciated.
101
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Mid-Cretaceous Plutonic Province Suites
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116
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Source and Redox Controls
Chapter 4
Source and Redox Controls on Metallogenic
Variations in Intrusion-Related Ore Systems,
Tombstone-Tungsten Belt,
Yukon Territory, Canada
Craig J.R. Hart1,2, John L. Mair1, Richard J. Goldfarb3 & David I. Groves1
Centre for Global Metallogeny
School of Earth and Geographical Sciences
University of Western Australia
Nedlands, WA, 6009Australia
1
Yukon Geological Survey
Box 2703 (K-10),
Whitehorse, Yukon Y1A 2C6 Canada
2
United States Geological Survey
Box 25046 (MS964)
Denver, Colorado, 80225 USA
3
117
Chapter 4
Source and Redox Controls
Preface to Chapter Four
This paper reflects a presentation made at the Fifth Hutton Symposium on the Origin of Granites
and Related Rocks in Toyohashi, Japan in May 2003. “Source and Redox Controls of IntrusionRelated Metallogeny, Tombstone-Tungsten Belt, Yukon, Canada” will be published shortly in the
“Fifth Hutton Symposium Volume on the Origin of Granites and Related Rocks” in Transactions
of the Royal Society of Edinburgh: Earth Sciences, and jointly distributed as a Special Paper by the
Geological Society of America. The spelling and format for citations and references follow those
that are used by the Royal Society of Edinbugh.
Justif cation of authorship: This manuscript results from field work and observations from
across the Tombstone-Tungsten belt, with follow-up rock sampling, analyses and compilation of a
significant amount of whole-rock, trace, rare-earth and redox geochemical data as part of the Ph.D.
study. Mr. John Mair assisted with the petrology of rock samples and in presenting the geochemical
diagrams, and provided insight into the interpretations. Dr. Richard Goldfarb was involved in the
fieldwork and provided editorial assistance. Dr. David Groves suggested the study, facilitated the
interpretations through discussions, and provided editorial assistance.
118
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Source and Redox Controls
Source and Redox Controls on Metallogenic Variations in
Intrusion-related Ore Systems, Tombstone-Tungsten Belt,
Yukon Territory, Canada
Abstract
The Tombstone, Mayo and Tungsten plutonic suites of granitic intrusions, collectively
termed the Tombstone-Tungsten Belt, form three geographically, mineralogically,
geochemically and metallogenically distinct plutonic suites. The granites (sensu lato) intruded
the ancient North American continental margin of the northern Canadian Cordillera as part of
a single magmatic episode in the mid-Cretaceous (96-90 Ma). The Tombstone suite is alkalic,
variably fractionated, slightly oxidized, contains magnetite and titanite, and has primary, but
no xenocrystic, zircon. The Mayo suite is sub-alkalic, metaluminous to weakly peraluminous,
fractionated, but with early felsic and late mafic phases, moderately reduced with titanite
dominant, and has xenocrystic zircon. The Tungsten suite is peraluminous, entirely felsic,
more highly fractionated, reduced with ilmenite dominant, and has abundant xenocrystic zircon.
Each suite has a distinctive petrogenesis. The Tombstone suite was derived from a enriched,
previously-depleted lithospheric mantle, the Tungsten suite intrusions from continental crust
including, but not dominated by, carbonaceous pelitic rocks, and the Mayo suite from similar
sedimentary crustal source, but is mixed with a distinct mafic component from an enriched
mantle source.
Each suite has a distinctive metallogeny that is related to the source and redox characteristics
of the magma. The Tombstone suite has a Au-Cu-Bi association that is characteristic of most
oxidized and alkalic magmas, but also has associated, and enigmatic, U-Th-F mineralisation.
The reduced Tungsten suite intrusions are characterized by world-class tungsten skarn
deposits with less significant Cu, Zn, Sn, and Mo anomalies. The Mayo suite intrusions are
characteristically gold-enriched, with associated As, Bi, Te, and W associations. All suites also
have associated, but distal and lower temperature, Ag-Pb- and Sb-rich mineral occurrences.
Although processes such as fractionation, volatile enrichment, and phase separation are
ultimately required to produce economic concentrations of ore elements from crystallizing
magmas, the nature of the source materials and their redox state play an important role in
determining which elements are effectively concentrated by magmatic processes.
Introduction
Regional-scale metallogenic and ore-element variations of intrusion-related ore systems
have been ascribed to a variety of processes. The tectonic setting, magma composition, degree
of fractional crystallization, and redox state all potentially play a role in determining the
metallogeny of an intrusion-related mineral deposit district (e.g. Ishihara 1981; Titley 1982,
1991; Blevin and Chappel 1992; Blevin et al. 1996; Barton 1996; Lang and Titley 1998).
Controversies have typically focused on the relative importance of either the source materials or
magmatic-hydrothermal processes in enriching a system in a particular metal suite. Economic
geologists commonly emphasize the role that magmatic-hydrothermal processes play in
intrusion-related metal enrichments, whereas igneous petrologists emphasize the importance of
the nature of the source materials.
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Source and Redox Controls
The Tombstone-Tungsten Belt (TTB) in Yukon Territory, Canada, forms a 750-km-long
belt of dominantly felsic intrusions that were emplaced across the ancient western North
American continental margin from 96 to 90 Ma. The TTB is the innermost and youngest of
numerous plutonic belts that intruded the continental margin in the mid-Cretaceous. Although
magmatism occurred over a short time interval and across a single tectonic element, pluton
compositions vary considerably along the belt, as do the associated mineral occurrence types.
Several deposits and numerous occurrences of gold, molybdenum, silver, tin, copper, and
uranium, as well as world-class tungsten deposits, are associated with plutons of the TTB
(Hart et al. 2000). This well-mineralized region represents the eastern part of the Tintina Gold
Province (Hart et al. 2002), which notably hosts numerous intrusion-related gold systems
across Yukon and interior Alaska (Thompson et al. 1999; Goldfarb et al. 2000; Lang et al.
2000). Based on pluton distribution, nature, and composition, the belt can be divided into
three main groups of intrusions. From west to east, they comprise the Tombstone, Mayo and
Tungsten plutonic suites, with each having its own distinctive metallogenic signature.
This paper presents data on the geological, mineralogical, and geochemical features that
characterise each of the plutonic suites. In particular, petrology and major, minor, and trace
element geochemical and isotope data are applied to characterise and compare the three
suites. This is used to constrain the nature of the potential source materials that generated their
magmas, and the processes that contributed to the diversity of lithologies. In addition, the
metallogeny of each suite is evaluated in the context of these magmatic source variations, with
particular emphasis on the role of oxidation state and fractionation.
Tectonic Setting
The northern North American Cordillera is underlain by numerous tectonic elements that
include outboard accreted island arc and oceanic terranes, accreted terranes with pericratonic
affinities, and variably displaced or deformed portions of the ancient continental margin
(Gabrielse et al. 1991). Whereas the mechanisms of terrane assembly and accretion remain
contentious, most of the assembly and growth of the current continental margin occurred
during ongoing convergent tectonism between Early Jurassic and mid-Cretaceous time
(Gabrielse and Yorath 1991; Plafker and Berg 1994). Following the waning of collision and
accretionary tectonics, extensive magmatism occurred across much of the newly assembled
continental margin during mid-Cretaceous time (Armstrong 1988). Hundreds of batholiths
and plutons of the mid-Cretaceous magmatic episode are distributed in broadly orogenparallel belts and geographically restricted clusters. Plutons within each belt have similar
characteristics and ages, and, as such, comprise plutonic suites (Mortensen et al. 2000; Hart et
al. 2004a; Fig, 1). There is a large variation in the characteristics between the plutonic suites:
some are typical arc-related, metaluminous and calc-alkaline I-types (e.g., Whitehorse suite,
Hart 1997), others comprise peraluminous granitoids that were mainly derived from crustal
melts (e.g., Cassiar suite, Driver et al. 2000), and others consist of variably alkalic, locally
silica-undersaturated plutons (e.g., Tombstone suite, Anderson 1982).
Among the most interesting and economically prospective of the mid-Cretaceous plutonic
suites are those that comprise the most inboard plutonic belt, the Tombstone, Mayo and
Tungsten suites (Fig. 1). These plutons, which form the TTB, were emplaced into a thick
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Figure 1 Regional tectonic elements of Yukon and the distribution of mid-Cretaceous plutonic
suites, shown in various shaded tones. The Tombstone-Tungsten Belt (TTB) includes the
distribution of the Tombstone, Mayo and Tungsten plutonic suites. Light shade is distribution of
Selwyn Basin. Dotted lines are distribution limits of plutonic suites. Dark lines are fault, with
teeth are thrust faults. Light lines are tectonic boundaries.
package of variably calcareous, quartzose, and argillaceous Neoproterozoic, and locally
carbonaceous Paleozoic strata, which were deposited within the Selwyn Basin along the ancient
North American margin. Although carbonate platforms comprise much of the ancient North
American margin, the Selwyn Basin developed in response to Neoproterozoic and Paleozoic
rifting events that resulted in an attenuated crust, and the development of euxinic shale basins
and associated Pb-Zn sedimentary exhalative deposits (e.g., Faro; Abbott et al. 1986). Selwyn
Basin strata were structurally thickened and locally metamorphosed to lower greenschist
facies as a result of outboard terrane accretion immediately prior to mid-Cretaceous plutonism
(Murphy 1997). Numerous features associated with the plutons, such as east-trending dyke
swarms and parallel sub-vertical extensional veins, suggest that the region underwent a period
of post-collisional extension immediately following compression. As such, the TTB intrusions
were emplaced during post-collision in an inboard part of the continental margin.
Characteristics of the Tombstone-Tungsten Belt
More than 100 plutons, as well as numerous dykes and sills, of the TTB form the most
inboard plutonic belt of the mid-Cretaceous magmatic episode. The plutons intruded the
northern Selwyn Basin, to the south of the long-lived, and likely basin-controlling Dawson
thrust fault (Fig. 2; Abbott 1995). Within the TTB, the Tombstone, Mayo, and Tungsten
plutonic suites are defined on the basis of their distributions and lithological similarities. The
term “plutonic suites” is used, as it is common in the North American lexicon for a regional
group of coeval and lithologically similar plutons. This approach differs somewhat from the
121
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Source and Redox Controls
use of “suites” in the Australian context, which is more dependent upon the magma source
according to geochemical and isotopic parameters, and typically refers to several lithologies
within a single or numerous plutons (White 1995; Chappell 1996).
Each of these three plutonic suites in the TTB forms a linear belt, with associated dykes
paralleling their trends. The Tombstone suite is mostly located along a 120-km-long, southeast
trend that parallels the Tintina Fault (Fig. 1). The plutons were discordantly emplaced into
folded and faulted Neoproterozoic to Mesozoic sedimentary rocks in the western part of the
Selwyn Basin (Fig. 2). Notably, the plutonic belt cuts the major Cretaceous structural features,
such as the Robert Service and Tombstone thrust faults (Fig. 2), indicating a negligible role
played by the faults in magma focussing. However, the trends of the Tombstone and Mayo
suites are interpreted to reflect structural trends associated with Neoproterozoic rift events.
The Mayo suite of intrusions forms an ESE-trending, 370-km–long belt from the Tintina
Fault to the eastern border of the Yukon Territory (Fig. 3). Country rocks are dominantly
folded and thrust-faulted quartzose and argillaceous strata of the Neoproterozoic Hyland
Group, with carbonaceous shale and chert of the Devono-Mississippian Earn Group in the east.
The Tungsten suite of intrusions forms an almost 200-km-long, SE-trending linear belt
from Macmillan Pass to south of the townsite of Tungsten (Fig. 4). The plutons intrude
carbonaceous and calcareous Paleozoic strata and lesser quartz-rich and pelitic Neoproterozoic
strata of the easternmost Selwyn Basin.
In east-central Alaska, intrusions of equivalent age and lithology to the Mayo and
Tombstone suites form the Fairbanks-Salcha and Livengood suites, respectively, which crop
out in the world-class Fairbanks gold district south of the Tintina Fault (Hart et al. 2004).
These intrusions are interpreted as the western parts of the Mayo and Tombstone suites that
were offset by ~430 km of dextral transcurrent displacement on the Tintina Fault during the
Late Cretaceous and Early Tertiary (Mortensen et al. 2000; Murphy and Mortensen 2003).
Plutons throughout the TTB have well-developed, resistant-weathering, contactmetamorphic aureoles that are about half-the-width of adjacent intrusions. None of the
plutons have associated volcanic rocks. Equilibrium contact metamorphic assemblages and
fluid inclusion data from syn-plutonic quartz-veins constrain pluton emplacement pressures to
mostly 1.0 to 2.5 kbar (Baker and Lang, 2001; Marsh et al., 2003). Extensive geochronology
(in Gordey and Anderson 1993; Murphy 1997; Mortensen et al. 2000; Coulson et al. 2002;
Hart et al. 2004b; Mortensen unpublished) indicates that the TTB plutons were emplaced over
a narrow time interval of 99 to 90 Ma with the Tombstone suite of intrusions being emplaced
at the younger end of the range. Essentially contemporaneous 40Ar-39Ar dates from magmatic
biotite and hornblende with U-Pb zircon dates (Coulson et al. 2002; Hart et al. 2004b) indicate
that the region has not been affected by post-intrusion thermal events.
Plutonic Suites
Intrusions of the TTB commonly occur as isolated plutons, particularly within the
Tombstone and Tungsten suites. However, the Mayo intrusions typically occur in clusters,
commonly with numerous associated dykes, which collectively comprise intrusive complexes.
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Source and Redox Controls
Figure 2 Distribution of Tombstone plutonic suite
and associated intrusion-related mineral occurrences.
Pluton names are italicized. Significant mineral
occurrences are named. Geological information from
Gordey and Makepeace (2003). Mineral deposit
information from Yukon Minfile (2003).
Figure 4 Distribution of the Tungsten plutonic
suite in eastern Yukon and westernmost Northwest
Territories (NWT) and associated intrusion-related
mineral occurrences. Outlined, unshaded plutons
do not belong to the Tungsten plutonic suite,
O’Grady Batholith is alkalic and a distal member
of the Tombstone suite. Pluton names are italicized.
Significant mineral occurrences are named. Mineral
occurrences often have the same name as the pluton.
Geological information from Gordey and Makepeace
(2003). Mineral deposit information from Yukon
Minfile (2003).
Figure 3 Distribution of the Mayo plutonic suite across central Yukon and associated intrusion-related
mineral occurrences. Clear Creek, Scheelite Dome and Dublin Gulch also have significant placer gold.
Pluton names are italicized. Significant mineral occurrences are named. Mineral occurrences often have
the same name as the pluton. Unfilled plutons belong to other suites. Geological information from Gordey
and Makepeace (2003). Mineral deposit information from Yukon Minfile (2003).
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Source and Redox Controls
The Tombstone plutons are well-zoned, comprising several distinct lithological phases that
form nested concentric zones or phases adjacent to each other. The Mayo intrusions most
commonly consist of a main phase stock with gradational lithological variations potentially
indicative of multiple intrusive phases. In addition, these plutons characteristically have
a variety of associated dykes, including intermediate to mafic lamprophyres, porphyritic
granitic dykes, and subordinate aplites and pegmatites. The Tungsten suite intrusions
have comparatively minor, within-pluton, lithological phase variations and lack associated
intermediate to mafic phases. However, plutons of the Tungsten suite commonly have
associated pegmatites, aplites, and greisens. Intrusions of all suites are wallrock inclusionpoor, with only sparse occurrences of microdiorite enclaves. The Tungsten and Mayo
intrusions are weakly porphyritic, with white megacrystic alkali feldspar. Characteristics of
the plutonic suites are summarized in Table 1.
Nomenclature
After first designating those plutons along the border between Yukon Territory and
Northwest Territories (NWT) as the Selwyn plutonic suite (Anderson 1983), Anderson
(1988) and Woodsworth et al. (1991) subsequently included all mid-Cretaceous plutons in the
Selwyn Basin within this group. The plutons were subdivided into two-mica, transitional and
hornblende-bearing phases. Anderson (1988) further encouraged the additional use of more
regional names, such as Anvil plutonic suite (Fig. 1).
The Tombstone plutonic suite has traditionally referred to those geographically restricted
plutons in the westernmost part of the TTB that have alkalic affinities (Tempelman-Kluit 1970;
Woodsworth et al. 1991), with the Tombstone Mountain batholith being the type locality.
Tombstone plutonic suite was subsequently used by Mortensen et al. (1995) and Murphy
(1997) to refer to all plutons that intruded along the northern margin of the Selwyn Basin
between Dawson City and the NWT border (Fig. 1). Furthermore, Mortensen et al. (1995,
2000) renamed the Selwyn plutonic suite intrusions near the NWT border, the Tungsten
plutonic suite, but included only those smaller plutons that were circa 94 Ma, which thus
excluded the batholiths that Anderson (1988) had included within the Selwyn plutonic suite.
The term Tombstone plutonic suite is herein retained as originally applied to the dominantly
alkalic intrusive rocks near the Tintina Fault, but in contrast to Murphy (1997) and Mortensen
et al. (1995), does not include the east-trending belt of intrusions in central Yukon Territory.
These plutons, between the Tombstone and Tungsten suites, are designated the Mayo plutonic
suite. The Tungsten plutonic suite, as applied to the small peraluminous intrusions near
the Yukon Territory-NWT border by Mortensen et al. (2000), is retained. Together, these
three suites form the TTB. The Tombstone suite intrusions are characteristically alkalic and
metaluminous; the Mayo intrusions are sub-alkalic and mostly metaluminous, and distinctive
from the more felsic Tungsten suite intrusions that are dominantly peraluminous.
Tombstone Plutonic Suite
Tombstone suite intrusions are characterized by their alkalic character, limited distribution,
and larger, well-zoned plutons (Anderson 1987, 1988; Gordey and Anderson 1993;
Abercrombie 1990; Smit et al. 1985; Duncan et al. 1998). They include six main intrusions, as
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Chapter 4
Source and Redox Controls
Table 1 Summary of characteristics of the plutonic suites
Tombstone
Mayo
Tungsten
References
92-90 Ma
95-92 Ma
97-94 Ma
Dominant
lithologies
Alkali feldspar syenite to
quartz syenite
Monzonite to granodiorite
Granite to monzogranite
Pluton size
Moderate
Small
Small
Zoned, maf c margins felsic
cores
Simple, later maf c phases
Simple, textural
variations, some
fractionated phases
Coarse, cumulate
Medium to f ne grained,
locally porphyritic
Medium grained, weakly
porphyritic
Pyroxene (aegerine-augite)>
hornblende>biotite
Biotite>hornblende
clinopyroxene common
Biotite>>muscovite
Magnetite>titanite
Titanite
Ilmenite>>titanite
Epidote, allanite, melanite,
apatite, f uorite, zircon
Allanite, apatite, zircon
Garnet, apatite,
monazite, tourmaline,
allanite, zircon
4, 7
50-70 %
55-75 %
66-76 %
3, Table 3
Metaluminous, except where
highly fractionated
(0.65-1.1)
Metaluminous to weakly
peraluminous
(0.6-1.15)
Weakly peraluminous
(1.0-1.2)
Alkaline to peralkaline
subalkaline
subalkaline
0.2-1.1
0.15-0.45
0.1-0.3
Fig. 11
1.79
0.11
0.16
Fig. 10
Most <20, some 20-80
Most 20-100, some 100600
Most <20
3, 10
None
Some
Considerable
0.710-0.720
0.7115-0.7140
0.717-0.737
3, 4, 7, 8
-7 to -9
-8 to -13
-13 to -15
3, 5, 10
9-11
11-14
9-13
6, 7, 10
820°C
780°C
750°C
Table 3
Au-Cu-Bi
U-Th-F
Au-Bi-Te, W, As
Ag-Pb
W-(Cu-Zn-Mo)
Alkalic, slightly oxidized,
metaluminous, radiogenic,
syenite cumulates
Metaluminous, moderately
reduced, radiogenic,
biotite granodiorite
Weakly peraluminous,
reduced, radiogenic,
biotite granite
Ages
Plutons
Grain size
Maf c phases
Dominant Fe-Ti
indicator mineral
Accessory minerals
SiO2 range
ASI
Alkalinity
Fe2O3/FeO
Average magnetic
susceptibility
(x10-3SI)
Cr (ppm)
Inherited zircons
Initial Sr ratio
ENdT
Oxygen isotopes
ZST
Associated
mineralization
Characterization
1, 2,9,10
7
Notes: ASI-Aluminum saturation index; ZST – Zircon saturation temperature. References: 1-Anderson 1993
(in Gordey and Anderson 1993); 2-Murphy 1997; 3 – Lang 2000, 4-Abercrombie 1990, 5-Farmer et al. 2000,
6-Marsh et al. 2003, 7-Anderson 1988, 8-Gareau 1986; 9-Coulson et al. 2002; 10-Hart and Mair unpublished.
Na-not available.
125
Chapter 4
Source and Redox Controls
well as swarms of sills, dykes, and small stocks along their trend (Fig. 2). Two outliers from
the linear distribution are the isolated Emerald Lake pluton (Fig. 3) and O’Grady batholith
(Fig. 4), which occur several hundred kilometres east of the other intrusions in the suite.
Tombstone plutons are typically circular to sub-circular, relatively large (10-80 km2 in
plan), and typically composed of multiple concentric zones of lithologically distinct phases.
The dominant rock types are coarse-grained alkali-feldspar syenite and quartz syenite. Many
plutons have mafic marginal phases, such as pyroxenite and hornblende diorite, as well as
central, more felsic, monzonitic or granitic phases that are nested in the syenite. Tombstone
intrusions also locally contain distinctive alkalic, quartz-undersaturated phases such as
tinguaite (feldspathoid syenite).
The dominant mafic minerals include augite, aegerine, hornblende, local biotite, and lesser
melanitic garnet. Hornblende (arfvedsonite) typically replaces earlier formed augite. Fe-Ti
accessory phases are magnetite and titanite; other accessory minerals include zircon, fluorite,
apatite, and nepheline (Anderson 1988; Abercrombie 1990).
Mayo Plutonic Suite
Mayo suite plutons are generally small in the west (1-5 km2), larger in the east (20-80
km2), but notable is the anomalously large Roop Lakes pluton (125 km2) (Fig. 3). This suite
has a characteristic spatial and temporal associations with intrusion-related gold deposits.
Mayo intrusions typically form isolated plutonic centres, or clusters of small felsic to mafic
plutons. Intrusions are typically dominated by a main porphyritic quartz monzonite that
characteristically includes less-evolved magmatic phases and locally forms weakly composite
plutons (e.g., Dublin Gulch). Alternatively, some main phase plutons are cut by monzonite to
diorite stocks and dykes (e.g., Scheelite Dome). Three small plutons in the Clear Creek area
are dominated by a main-phase quartz monzonite, whereas more mafic lithologies dominate
three others in the same area. A variety of porphyritic, aplitic, and pegmatitic felsic dykes, as
well as calc-alkaline lamprophyre dykes, typically cut all plutons and their hornfelsed aureoles.
The early, mainly quartz monzonite plutons are weakly to moderately porphyritic, with
white potassium feldspar phenocrysts that are locally >1cm in length. The phenocrysts
occur in a fine- to medium-grained matrix of predominantly quartz and feldspar, with lesser
biotite. Biotite is more abundant than hornblende, and clinopyroxene, although typically
rare, is common in some plutons. Accessory minerals include titanite, allanite, apatite, and
zircon. Intermediate intrusions are mineralogically similar to the earlier, more felsic main
phase intrusions, but with a greater proportion of clinopyroxene, biotite, hornblende, and
titanite. These more mafic phases locally contain unusual textures, with two generations of
clinopyroxene. Early clinopyroxene is typically Mg-rich augite (Mg# >80) and is poorly
preserved. Finer euhedral diopside grains (Mg # ~ 60) occur amongst quartz, and are
interpreted to have formed late in the crystallisation sequence. Lamprophyre dykes include
both sub-alkalic amphibole-rich spessartites and alkalic phlogopite/biotite-rich minettes (cf.
Rock et al. 1991). Deuteric alteration is most extensive in intermediate to mafic intrusive
rocks, with pyroxenes altered to amphibole and biotite. Titanite is ubiquitous and magnetite
is locally present only in late granitic dykes, whereas ilmenite is rare, occurring only as fine
inclusions within mafic phenocrysts in intermediate to felsic intrusions.
126
Chapter 4
Source and Redox Controls
Tungsten Plutonic Suite
Intrusions of the Tungsten suite are distinguished by their felsic, more homogeneous, and
consistently peraluminous character, as well as their strong association with tungsten skarn
mineralisation (Anderson 1982, 1983, in Gordey and Anderson 1993). Plutons are typically
small, generally less than 15 km2 in surface exposure.
Tungsten suite plutons are dominated by medium-grained quartz monzonite to monzogranite.
Notably, these plutons lack both amphibole and clinopyroxene, which are common in quartz
monzonite intrusions of the Mayo suite. Plutons typically feature gradational zoning to more
felsic or peraluminous phases. Late-stage aplite or pegmatite dykes are locally common, as
are tourmaline veinlets and rusty-weathering greisen-like alteration on joint planes in pluton
cupolas.
Biotite, the dominant mafic mineral, ranges from brown to reddish, and commonly contains
black pleochroic halos from radiation damage. Although biotite is typically the sole mica,
primary muscovite is locally present in fractionated phases. Orthoclase forms both irregular
coarse ophitic grains that incorporate earlier-formed biotite and plagioclase, as well as euhedral
to subhedral simple-twinned laths. Ilmenite, the dominant oxide phase, typically occurs as
fine grains in biotite. Subhedral plagioclase grains commonly feature oscillatory zoning,
with sericitised grain cores. Accessory phases include allanite, zircon, apatite, and ilmenite,
with trace amounts of garnet and tourmaline in more highly fractionated phases, aplites, and
pegmatites. Magnetite is not present. Secondary ilmenite is strongly anhedral and associated
with areas of chloritised biotite. Weak to moderate deuteric alteration is characterised by
chloritisation of biotite, sericitisation, and carbonate alteration of plagioclase.
Intrusion-Related Metallogeny
Each of the three plutonic suites has its own distinctive metallogenic association (Table 2),
although a wide range of factors, such as P-T of emplacement, nature of country rock, structure,
and water-rock conditions, result in the diverse range of gold and/or tungsten-rich mineralisation
styles as described by Hart et al. (2000). All suites, however, appear to have spatially associated
Ag-Pb-Zn- and As-Sb-bearing veins that are peripheral to the plutons and are products of the
final stages of intrusion-related hydrothermal activity.
The alkalic Tombstone plutons tend to be associated with mineral occurrences that are
dominated by either a Au-Cu-Bi or U-Th-F association (Fig. 2). Enrichments of native gold
(to tens of ppm) are hosted in pyrrhotite-rich skarns that contain percent-level copper and
anomalous bismuth concentrations, such as characteristic of the Marn (Brown & Nesbitt 1987)
and Horn deposits. An arsenic-rich epizonal gold deposit, Brewery Creek (0.3 Moz Au; Lindsay
et al. 2000), is localized within, and adjacent to, monzonitic Tombstone suite sills and stocks
(Fig. 2). Four of the Tombstone suite intrusions host U-Th-F mineralisation, with two of the
occurrences being large-tonnage, low-grade deposits in disseminated and veinlet-hosted styles.
Locally, highly-fractionated parts of the Tombstone intrusions contain Au-, Cu-, and Bi-bearing
quartz veins and vug fillings, such as in the Emerald Lake pluton (Fig. 3), and Sn- and Ag-rich
tourmaline greisens, such as in the Syenite Range pluton (Fig. 2). Mineralisation associated
with the Tombstone suite is characterized by high halogen contents, as indicated by the presence
of fluorite, scapolite, and, locally, sodalite.
127
Chapter 4
Source and Redox Controls
Table 2 Examples of mineralization associated with the plutonic suites
Deposit/
Occurrence
Metals
Mineralization
Style
Ore Minerals
Gangue
minerals
Reference
Brown & Nesbitt
(1987)
Tombstone Plutonic Suite
Marn
Au-Cu-Bi
Skarn
Pyrrhotite,
chalcopyrite,
bismuthinite, electrum
Hedenbergite,
almandine garnet
Horn
Au-Cu-Bi
Skarn
Pyrrhotite>pyrite,
chalcopyrite, gold,
bismuthinite,
Hedenbergite,
garnet, f uorite
Brewery CreekMoosehead Zone
Au-As-Sb
Vein stockwork
Pyrite, arsenopyrite
Kaolinite
Lindsay et al.
(2000)
Ting/Noting
U-Th-F
Disseminated, veins
Uraninite, molybdenite
Fluorite
Yukon Minf le
(2003)
Mayo Plutonic Suite
Dublin Gulch
Au-Bi-W
Sheeted veins
Po, bismuthinite,
scheelite
Quartz, K-spar,
sericite
Maloof et al.
(2001)
Scheelite Dome
Au-As
Replacements,
stockworks
Arsenopyrite, pyrite,
pyrrhotite
Quartz, sericite,
carbonate
Mair et al. (2000)
Clear Creek
Au-Bi-W
Sheeted veins
Pyrite, bismuthinite,
scheelite
Quartz, K-spar
Marsh et al.
(2003)
Ray Gulch
W
Skarn
Scheelite
Diopside
Brown et al.
(2002)
Keno Hill
Ag-Pb
Veins
Galena
Quartz, siderite
Lynch et al.
(1990)
Tungsten Plutonic Suite
Mactung
W-Cu
Skarn
Scheelite,
chalcopyrite,
pyrrhotite
Diopsite, garnet,
actinolite
Atkinson & Baker
(1986)
Cantung
W-Cu
Skarn
Scheelite,
chalcopyrite,
pyrrhotite
Diopside,
epidote, garnet
Bowman et al.
(1985)
Clea
W-Cu
Skarn
Scheelite,
chalcopyrite,
pyrrhotite
Diopside, garnet,
vesuvianite,
biotite, f uorite
Yukon Minf le
(2003)
The Mayo intrusions are dominantly metaluminous, are most consistently associated with
gold mineralisation, and have the highest abundance of associated mineral occurrences (Fig.
3). Approximately 1 Moz of placer gold have been recovered from creeks that drain areas
underlain by the intrusions of the Mayo suite and surrounding hornfels. Gold occurrences
include intrusion-hosted Au-Bi-Te-W quartz-alkali-feldspar sheeted vein arrays such as at
Dublin Gulch (4 Moz Au; Hitchins & Orssich 1995; Maloof et al. 2001), Au±W skarns, and
country rock-hosted Au-As quartz vein arrays such as those at Scheelite Dome (Mair et al.
2000). Solitary, shear- and fissure-hosted Au-As veins are also common features proximal
to, or within, Mayo intrusions. Other types of mineralisation include tungsten skarns, AgPb-Zn±Sb veins, and tin-bearing greisens. The area containing the Mayo suite of intrusions
also hosts significant silver-rich quartz veins, with >200 Moz Ag recovered from veins of the
Keno Hill district (Lynch et al. 1990). However, the genetic relationship of these veins to the
intrusions is unclear. The Fort Knox gold deposit in Alaska (7 Moz Au) is hosted in a Mayo
suite intrusion that has been offset by the Tintina Fault.
128
Chapter 4
Source and Redox Controls
The peraluminous Tungsten suite intrusions are associated with the largest tungsten deposits
in North America: the Cantung deposit (Bowman et al. 1985; Gerstner et al. 1989) hosts
approximately 9 Mt of 1.4% WO3 and the Mactung deposit (Dick & Hodgson 1982; Atkinson
& Baker 1986; Fig. 4) contains approximately 57 Mt of 0.95% WO3. Both deposits are
developed in Lower Cambrian carbonates adjacent or above Tungsten suite intrusions. Some
intrusions have highly fractionated phases that contain garnet, muscovite, tourmaline. Each
deposit consists of several stratabound ore zones. The best ores (>1.5% WO3 and 0.2% Cu)
are characterized by pyrrhotite-rich (>15%) pyroxene skarn with scheelite. The molybdenum
content of the scheelite is low. Garnet-bearing skarns are much less common, contain less
pyrrhotite and are lower grade. The skarns locally have sub-economic grades of zinc and
molybdenite. Elsewhere, Tungsten suite intrusions generate small Sn, Mo and Au enrichments.
Geochemistry
The three plutonic suites are characterized by distinct major, minor, and trace element
geochemical signatures (Table 3). The Tombstone and Mayo intrusions have wide
compositional variations, with SiO2 contents ranging from 50 to 75 wt %, whereas Tungsten
intrusions have more restricted SiO2 contents from 65 to 75 wt % (Fig. 5A). The Tombstone
suite rocks are distinguished from those of the Mayo suite by their higher alkalinity and lower
MgO contents (and Mg#), particularly at ≤65% SiO2 (Fig. 5B). The Tombstone intrusions that
contain <58% SiO2 are commonly nepheline normative, mainly due to their elevated alkali
contents.
Aluminum saturation is also variable within the different suites (Fig. 5C). The Tombstone
and Mayo suites display a wide range of aluminum saturation index (ASI) values, which is
consistent with their broad range in SiO2 (cf. Chappell and White 2001). Generally, rocks with
<65% SiO2 are metaluminous, with an ASI value <1, whereas those Mayo intrusions with SiO2
values >70% have ASI values slightly >1, as do those few highly fractionated Tombstone suite
examples. Tungsten suite intrusions are entirely weakly peraluminous, with all values slightly
>1.
The use of Rb, Sr, and Ba as fractionation indices is complicated by the relatively enriched
nature of the intermediate to mafic intrusions of the Tombstone and Mayo suites (Fig. 6).
Intrusions of the Tungsten suite, which have a restricted SiO2 range, feature a strong increase in
rubidium content with increasing SiO2. Similarly, intrusions of the Tombstone suite show a clear
increase in rubidium content with increased SiO2 content, although this occurs over a greater
SiO2 range. In contrast, Mayo suite intrusions are characterized by only a slight increase in
rubidium with increased SiO2. Strontium shows strong depletion with increasing SiO2 content
in both the Tombstone and Tungsten suites, whereas the Mayo suite have no well-defined trends.
Notably, the strontium contents of Mayo suite intrusions are considerably higher than those of
the Tungsten intrusions at a given SiO2 content.
Interpretation of primitive mantle-normalised trace-element diagrams (Fig. 7) indicates that
all groups are strongly enriched in large ion lithophile elements (LILE) and light rare earth
elements (LREE). In addition, all suites have distinct negative titanium and niobium anomalies
(Fig. 7A). Most intrusions of the TTB also exhibit a negative phosphorous anomaly, but it
is absent in mafic phases of the Mayo intrusions. Chondrite-normalised REE plots indicate
129
Chapter 4
Source and Redox Controls
Table 3 Representative geochemical data for Tombstone (Deadman pluton), Mayo (Dublin Gulch pluton) and Tungsten (Mactung pluton) plutonic suites.
Sample
DEADMAN 1 DEADMAN 2 DEADMAN 3 DEADMAN 4
SiO2
Al2O3
CaO
MgO
Na2O
K2O
FeO (total)
Fe2O3*
FeO*
MnO
TiO2
P2O5
Cr2O3
LOI
H2O+
Total
57.8
19.3
3.76
0.11
3.59
8.6
2.85
1.65
1.2
0.13
0.304
0.03
0.005
0.65
0.5
98.8
64.7
17.6
2.34
0.24
5.84
5.43
1.92
0.92
1.0
0.05
0.198
0.02
0.005
0.6
0.3
99.4
Mg#
0.04
F
S %
Cl
CO2 %
B
1050
0.005
347
0.06
47
V
Cr
Co
Ni
Cu
Zn
Ga
Ge
As
Rb
Sr
Y
Zr
Nb
Mo
Ag
In
Sn
Sb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
W
Tl
Pb
Bi
Th
U
38
-20
1
22
-10
87
22
1.2
7
197
3,380
11.4
175
12.8
-2
-0.5
-0.1
2
0.6
5.0
8,590
34.0
69.7
9.12
36.8
6.16
1.43
3.57
0.42
1.80
0.29
0.85
0.096
0.78
0.111
4.3
0.74
0.6
1.29
55
0.1
10.6
3.93
DUBLIN
DIKE 1
DUBLIN
DUBLIN QTZ- PEGMATITE
SYENITE
DKE
58.6
21.1
0.85
0.23
2.98
12.1
2.53
0.53
2.0
0.06
0.303
0.02
0.005
0.4
0.7
99.4
68.3
16.1
1.53
0.26
5.38
4.9
1.8
1.0
0.8
0.04
0.175
0.01
0.005
0.65
0.2
99.4
59.6
14.2
5.49
4.71
2.14
5.44
5.12
0.92
4.2
0.1
0.551
0.16
0.03
1.05
0.8
98.9
69.5
16.0
2.63
0.66
2.57
4.37
1.9
0.5
1.4
0.02
0.343
0.08
0.006
1.05
1.0
99.5
61.0
16.7
3.94
1.61
2.0
9.81
1.64
0.34
1.3
0.04
0.592
0.16
0.005
1.8
0.4
99.8
0.11
0.08
0.13
0.48
0.26
479
0.005
30
0.3
45
1520
0.005
157
0.01
29
10
0.005
69
0.4
37
271
0.005
165
0.06
20
478
0.005
25
0.005
31
23
-20
1
-20
-10
-30
24
1.2
-5
178
1,110
17.2
414
22.9
-2
-0.5
-0.1
2
0.5
3.5
2,760
43.4
75.5
7.69
25.3
4.57
1.12
3.37
0.57
2.87
0.54
1.53
0.207
1.38
0.217
9.8
1.15
0.6
0.63
13
-0.1
29.7
6.44
15
-20
2
-20
-10
108
32
1.5
-5
705
599
30.4
518
40.3
2
-0.5
-0.1
3
0.7
67.6
857
95.4
164
15.8
49.9
8.63
1.69
5.60
1.02
5.00
0.96
2.64
0.366
2.24
0.312
10.8
2.40
3.2
2.51
73
-0.1
68.5
11.0
21
-20
1
-20
-10
-30
27
1.3
-5
211
637
15.5
287
24.0
-2
-0.5
-0.1
2
0.2
3.4
1,660
41.9
71.9
7.01
22.1
3.88
0.894
2.84
0.48
2.40
0.48
1.30
0.190
1.14
0.173
7.9
1.15
0.8
0.73
13
-0.1
27.4
10.7
91
272
18
60
-10
85
19
1.4
5
238
467
22.9
204
15.5
-2
-0.5
-0.1
3
0.6
6.6
1,620
59.6
99.4
10.4
36.4
6.42
1.34
4.34
0.78
3.99
0.76
2.01
0.276
1.71
0.249
5.6
1.24
0.5
1.27
22
-0.1
31.6
4.65
Zircon Saturation Temperatures
M
2.08
1.88
Temp K
1018
1108
Temp C
745
835
1.71
1144
871
1.53
1102
829
2.41
1008
735
DUBLIN
DIKE 2
DUBLIN
MACTUNG
MACTUNG
MAIN PHASE
DIKE
TOURM PEG
130
MACTUNG
DIKE IN
STOCK
68.1
15.5
3.18
1.13
2.68
4.45
3.53
0.63
2.9
0.07
0.487
0.16
0.005
0.95
1.0
100.4
72.3
14.2
1.96
0.43
2.66
5.17
1.89
0.39
1.5
0.04
0.176
0.05
0.005
0.5
0.5
99.5
59.8
14.7
6.41
4.37
2.41
4.28
5.58
1.08
4.5
0.1
0.673
0.17
0.02
1.1
1.0
99.8
67.2
15.2
3.46
1.42
2.59
4.5
3.07
0.57
2.5
0.04
0.459
0.13
0.005
0.85
0.7
99.2
75.7
13.1
1.07
0.17
3.19
4.31
1.01
0.21
0.8
0.05
0.054
0.0066
0.005
1.4
0.6
100.1
72.6
15.9
0.65
0.14
4.41
5.32
0.34
0.24
0.1
0.05
0.02
0.02
0.005
0.45
0.6
99.9
0.50
0.44
0.32
0.14
0.29
0.24
0.19
10
0.005
194
0.39
27
1790
0.03
124
0.32
22
930
0.005
200
0.23
24
441
0.005
25
0.01
35
117
0.005
25
0.06
381
2350
0.005
68
0.08
27
561
0.005
25
0.005
25
21
-20
2
-20
-10
124
22
1.3
10
213
476
4.6
152
13.9
-2
-0.5
-0.1
4
3.8
11.2
2,930
22.5
39.4
4.09
14.3
2.68
1.01
1.55
0.22
0.88
0.13
0.32
0.040
0.25
0.032
4.5
1.10
1.7
1.56
30
0.2
13.2
7.79
40
37
3
23
-10
-30
19
2.8
-5
302
640
32.6
296
22.6
-2
-0.5
-0.1
8
0.2
8.4
3,780
62.1
111
12.3
44.9
9.18
1.61
6.70
1.13
5.81
1.11
2.94
0.427
2.75
0.446
8.4
1.53
377
0.79
8
-0.1
29.1
5.21
83
178
16
65
12
89
20
1.5
15
221
438
23.0
191
14.0
-2
-0.5
-0.1
4
0.8
8.1
1,370
51.6
94.7
9.88
34.2
6.17
1.25
4.58
0.76
3.89
0.76
2.07
0.284
1.83
0.279
5.3
1.15
2.1
1.12
20
-0.1
25.7
6.58
32
30
7
21
-10
59
20
1.2
-5
198
465
18.1
224
14.2
-2
-0.5
-0.1
1
0.3
7.5
1,380
49.4
88.4
9.22
32.0
5.86
1.28
4.35
0.68
3.31
0.62
1.59
0.203
1.27
0.184
6.2
1.20
2.2
0.90
16
-0.1
25.3
5.66
11
-20
2
-20
-10
33
17
1.3
-5
206
262
18.9
135
12.1
-2
-0.5
-0.1
5
0.5
14.2
603
56.2
100
11.5
40.7
7.62
1.38
5.69
0.74
3.49
0.63
1.79
0.230
1.59
0.231
4.3
1.73
0.6
1.19
26
-0.1
25.8
15.4
-5
-20
-1
-20
-10
-30
28
5.0
-5
468
70
58.0
29
26.4
-2
-0.5
-0.1
7
0.2
21.4
56
12.4
28.7
3.69
15.3
7.17
0.175
7.79
1.86
9.46
1.72
4.64
0.830
5.55
0.843
4.4
18.4
2.8
2.63
51
0.5
13.2
15.0
47
57
5
136
-10
100
23
2.1
-5
275
365
20.9
198
16.4
-2
-0.5
0.1
12
0.5
27.9
993
55.2
103
11.2
38.8
6.99
1.46
4.82
0.75
3.60
0.69
1.88
0.239
1.57
0.226
5.5
1.48
0.8
2.11
38
1.4
27.4
7.30
-5
-20
1
-20
-10
-30
20
2.2
-5
355
89
45.9
78
18.5
-2
-0.5
-0.1
11
-0.2
16.7
119
15.3
31.1
3.74
14.6
5.22
0.433
6.06
1.39
7.48
1.52
4.09
0.668
3.81
0.572
4.1
4.09
0.9
1.97
44
-0.1
21.2
20.9
1.29
1063
790
2.18
1055
781
2.18
1055
781
2.18
1055
781
1.26
1055
781
1.33
933
660
1.50
1070
797
1.39
1369
1096
Notes: Whole-rock data by XRF at XRAL-SGS, Canada. Trace element data by ICP-MS at Actlabs, Canada. FeO determined by titration. Cl and F by specific ion.
*Fe2O3 is calculated from FeO total and FeO.
MACTUNG
MAIN
STOCK
Chapter 4
Source and Redox Controls
Figure 6 Variation diagrams of Ba, Sr and
Rb vs. silica for the Tombstone, Mayo, and
Tungsten plutonic suites emphasizing varying
source compositions and fractionation trends
between the suites.
Figure 5 Geochemical plot for the Tombstone,
Mayo and Tungsten plutonic suites. All
geochemical plots composed using selected
data of C. Hart and J. Mair (unpublished),
Lang (2000) and Abercrombie (1990), and that
in Table 3. A – Alkali-silica plot. ASI=molar
Al/Ca+Na+K. B) MgO-silica plot. C) ASIsilica plot.
131
Chapter 4
Source and Redox Controls
Figure 7 Multi-element variation
diagrams for representative plutons
of the Tombstone, Mayo and
Tungsten plutonic suites. Data are
normalized to primitive mantle
values of Sun and McDonough
(1989). A) LILE-data. B) REE.
Data from Table 3, and associated
unpublished data.
Figure 8
Plot of Cr (ppm) vs
SiO2 for Tombstone, Mayo and
Tungsten plutonic suite granitoids
emphasizing the Cr-enrichment
in Mayo intrusions. High values
are interpreted to come, at least in
part, from xenocrystic Cr-diopside.
Overlapping data points with low
Cr are at or below detection limit of
10 ppm.
132
Chapter 4
Source and Redox Controls
strong enrichments of LREE, with negative slopes for intrusions of all groups (Fig 7B). The
Tombstone and Mayo suites generally have no obvious europium anomalies, whereas more
fractionated intrusions of the Tungsten suite have pronounced negative europium anomalies.
On the trace-element discrimination diagrams of Pearce et al. (1984), each suite overlaps both
the syn-collisional and volcanic-arc fields.
The variation in chromium concentration is pronounced (Fig. 8). Almost all Tungsten suite
intrusions have concentrations below the analytical detection level (e.g., <20 ppm Cr). The
Tombstone suite intrusions similarly contain low chromium levels, although more primitive
intrusions contain as much as 100 ppm Cr. The Mayo suite intrusions have elevated, but
variable chromium levels; felsic intrusions mostly contain <50 ppm, intermediate phases
show 50-200 ppm, and mafic phases contain as much as 700 ppm Cr. Chromium content is
concomitant with the presence of chromium diopside.
Isotopic Data
Isotopic (Sr, Nd, and O) compositions can reflect the nature of magma source regions in the
middle to lower crust or mantle, and/or the extent of contamination from upper crustal sources
(DePaolo et al. 1992). Isotopic data for plutons from the TTB are currently limited (Lang 2000),
with most available data from intrusions in the Syenite Range (Abercrombie 1990), Clear Creek
(Farmer et al. 2000; Marsh et al. 2003), Emerald Lake (Smit et al. 1985) and Scheelite Dome (J.
Mair, submitted). Initial strontium isotope ratios for Tombstone suite rocks are 0.710 to 0.713
and εNd values range from -7.6 to -10.3 (Abercrombie 1990; Lang 2000). Our unpublished
whole rock oxygen isotopic values range from 9 to 11 per mil. The Mayo intrusions have
somewhat similar initial strontium isotope ratios from 0.712 to 0.714 (Kuran et al. 1982; Lang
2000) and εNd values of -8.3 to -12.5 (Farmer et al. 2000; Lang 2000). Whole-rock oxygen
isotope values have been determined only for Clear Creek intrusions and range from 11.4 to
13.9 per mil (Marsh et al. 2003). Tungsten suite intrusions have very high initial strontium
isotope ratios of 0.712 to 0.748 (Godwin et al. 1980; Gareau 1986; Lang 2000), and two rocks
have εNd values of -13 to -15 (Lang 2000). Oxygen isotopes for Tungsten suite rocks include
values of 9-13 per mil for the Cantung pluton (Bowman et al. 1985; Anderson 1988).
Both strontium and neodymium isotopic ratios are highly radiogenic in intrusions of all three
plutonic suites. Notably, the more mafic and basic compositions, such as diorite, tinguaite and
lamprophyres, also yield similar highly radiogenic values. However, Tombstone rocks may be
slightly less radiogenic than Mayo suite rocks, and Tungsten suite rocks are significantly more
radiogenic (Lang 2000). This suggests that the more easterly plutons of the Tungsten suite
have larger contributions from the middle to upper crust. Limited analyses of Neoproterozoic
sedimentary rocks in the Selwyn Basin indicate initial strontium isotope ratios (at ~95 Ma) of
0.720 to 0.760 (Kuran et al. 1982) and εNd values of ~-16 to -25 (Garzione et al. 1997; Creaser
and Erdmer 1997). Consequently, the parent magmas to the plutons appear to be composed of
large proportions of highly radiogenic crustal melts, but the plutons are less radiogenic than the
country rocks (Fig. 9).
133
Chapter 4
Source and Redox Controls
Figure 9 Plot of εNd vs initial Sr ratio for
Tombstone, Mayo and Tungsten plutonic
suites using data compiled from Godwin
et al. (1980), Kuran et al. (1982), Gareau
(1986), Abercrombie (1990), Lang (2000),
Farmer et al. (2000). Hyland Group field
from data of Garzione et al. (1997), and
Creaser & Erdmer (1997). Broad arrow
indicates direction of increasing crustal
component to magma.
Redox State
The oxygen fugacity of a granitic magma can reflect the redox state of the source region,
as well as influence the processes of metal enrichment in the magma (Ishihara 1981; Candela
1989; Carmichael 1991; Blevin & Chappell 1992). Several features are indicative of the
oxidation state of a pluton; these include its magnetic character, Fe-Ti mineralogy, and Fe2O3/
FeO ratio. The Mayo and Tungsten suite intrusions have, for the most part, low and flat
aeromagnetic signatures that are impossible to differentiate from the low background values
of the sedimentary country-rock sequences (Hart et al. 2000). However, where these plutons
intruded carbonaceous (i.e., reduced) sedimentary strata, they have contact metamorphic
aureoles with elevated magnetic signatures, due to pyrrhotite development in hornfels. In
contrast, Tombstone intrusions have an inherently zoned or heterogeneous magnetic character
reflecting their varied lithologies, some of which do contain magnetite.
These aeromagnetic data are supported by magnetic susceptibility studies of the granitic
rocks. Measurements using a KT-9 hand-held magnetic susceptibility meter on freshly
broken rock surfaces indicate that the Tombstone suite rocks have a higher average magnetic
susceptibility (1.79x10-3 SI) and a wider range (Fig. 10) than Mayo and Tungsten suite rocks
(average 0.11 and 0.16x10-3 SI, respectively).
The dominant Fe-Ti-bearing accessory mineral can also reflect the oxidation state of a
magma and be characterized as magnetite or ilmenite-series (Ishihara 1977, 1981). Most of the
intrusive rocks in the region are titanite-bearing, but some Tombstone rocks contain significant
magnetite. In the Mayo intrusions, titanite is dominant and ilmenite is rare. Magnetite is
generally restricted to late granite dykes and occurs as scattered grains in some mafic dykes.
Ilmenite is common in the Tungsten suite intrusions, whereas magnetite and titanite are absent.
Whole rock ferric:ferrous ratios of fresh granitic rocks provide an approximation of the
redox potential for rocks lacking subsolidus oxidation (Burkhard 1991). Generally, the
134
Chapter 4
Source and Redox Controls
Figure 10
Histograms of magnetic
susceptibilities for intrusive rocks of the
Tombstone, Mayo and Tungsten plutonic
suites. Measurements were made determining
averages of 6-10 readings of various, freshly
broken rock surfaces of a single sample
using a handheld KT-9 Kappameter with a
pin spacer to account for surface roughness.
Most plutons are represented by at least 4-5
samples.
Figure 11 Changes in oxidation state (Fe2O3/FeO ratio) of Tombstone, Mayo and Tungsten intrusive
rocks with fractionation, as represented by SiO2. Note the increased oxidation state with fractionation for
all three suites. Scale is in log units to give greater resolution at low levels. The anomalously elevated
values likely reflect partial secondary oxidation from weathering. The approximate ranges for oxidation
states of porphyry copper deposits (PCD) and tin-tungsten deposits (SnW) are indicated by grey shading.
The calculated approximate positions of selected oxygen buffers are shown (NNO=nickel-nickel oxide;
QFM=quartz-fayalite-magnetite; Hem-Mag=hematite-magnetite) and were derived using the MELTS
(Ghiorso and Sack 1994) calculator for multi-component silicate liquids by inputting absolute chemical
compositions of all three suites, and constrained with T=850°C and P=2000 bars.
135
Chapter 4
Source and Redox Controls
intrusions have relatively low Fe2O3/FeO ratios, but a general trend of increased Fe2O3/FeO
with increasing SiO2 content suggests increased fO2 with progressive magmatic differentiation
for rocks of all three suites. The Tombstone intrusions have the highest Fe2O3/FeO ratios,
whereas the Tungsten suite intrusions have the lowest (Fig. 11). In the Tombstone intrusions,
the higher proportion of Fe2O3/FeO is expressed mineralogically as Fe3+-bearing silicates
(andraditic garnet, hornblende), as well as local enrichments of magnetite. The least-felsic
Tungsten suite intrusions plot below the QFM buffer (Fig. 11), and pyrrhotite is locally
present in some highly-fractionated Tungsten suite aplitic phases. The Mayo intrusions are
slightly reduced, with the greatest number of samples near the QFM buffer. Although many
samples plot above the QFM buffer, most do not contain magnetite, which suggests that other
parameters, in addition to oxygen fugacity, control the nature of the Fe-Ti minerals. Also
notable is the dominance of titanite in the Mayo suite rocks that are below the QFM buffer,
which is uncommon in most igneous rocks.
Zircon Character
Several authors have suggested that the presence and nature, or lack of, inherited zircon are
useful guides for interpretations of the nature and thermal state of the source region for granitic
melts (e.g., Watson & Harrison 1983; Chappell et al. 1987, 1998). Zircons in Tombstone suite
intrusions are large (to mm length), typically anhedral, and show only weak element zonation
(Fig. 12), with a notable absence of inherited cores (Fig. 12A). Zircons in rocks of the Mayo
suite are dominantly euhedral and contain variable proportions, as much as 10%, of ancient
inherited cores that are overprinted with thick, oscillatory-zoned, euhedral magmatic zircon
rims (Fig. 12B). Tungsten suite intrusions contain zircons that are similar to those of the Mayo
intrusions, but are smaller with thinner oscillatory-zoned rims, and contain a higher percentage
of old inherited cores, many lacking overgrowths (Fig. 12C). Although the xenocrystic zircons
may reflect the nature of the source material, their presence likely reflects low zircon saturation
temperatures (Miller et al. 2003). Zircon saturation temperatures are modelled estimates
of zircon solubility conditions in a melt that depends, in part, on melt composition and
zirconium concentrations (Watson & Harrison 1983). It provides the best measurement of the
temperature of a magma at its source (Miller et al. 2003). The Tombstone suite intrusions have
zircon saturation temperatures of about 820°C, which is about 40o and 70o higher than average
Mayo and Tungsten suite temperatures, respectively.
Discussion
Source Regions of Magmas
Constraints on the source regions and redox states of each plutonic suite can be related to
the contrasting element characteristics of their associated mineralisation.
The Tombstone intrusions have a wide range of silica content that are indicative of a
protracted fractional crystallization history. Variation diagrams of rubidium and strontium
versus SiO2 also support significant fractionation (Fig. 6). The lack of negative europium
anomalies in the REE diagrams is interpreted to reflect the suppression of early plagioclase
growth due to the high alkalinity or internal water pressure of the parental magma. Cumulate
136
Chapter 4
Source and Redox Controls
Figure 12 Backscatter SEM images of zircons. A)
Tombstone suite zircons are large, mainly anhedral to
subhedral and have faint internal variations, B) Mayo suite
zircons have large, finely oscillatory rims on early cores,
some of which are xenocrystic, C) Tungsten suite zircons
have thinner, oscillatory rims on a greater proportion of
xenocrystic cores, but also include xenocrystic zircons
lacking overgrowths.
textures suggest that density separation of mineral phases has strongly influenced the
compositional variations. All rocks are alkalic, with ultrapotassic compositions for those
that host accumulations of potassium feldspar. Silica undersaturated phases are feldspathoidbearing. Non-cumulate mafic phases that contain approximately 50-55% SiO2 are shoshonitic in
composition. The strong enrichment of LILE, LREE, and volatiles, and the high alkalinity and
high neodymium and strontium isotopic values, suggest that a parental mafic melt was derived
from a lithospheric mantle source that had been long-ago enriched by metasomatism prior to
fusion and melt generation. However, the relatively low Mg#, and low chromium and nickel
contents of these rock are best explained by early fractionation of olivine, pyroxene and spinels
(Nelson et al., 1986).
The nature of the enriched components in intrusions of the Tombstone suite suggests that
metasomatism was subduction related, because the mafic rocks, as well as the more evolved
intrusions, feature distinct negative Nb-Ti anomalies - a characteristic commonly attributed
to melt generation in a subduction-related setting (Pearce 1982; Tatsumi & Eggins 1995).
More-evolved phases likely resulted from fractionation, potentially with a degree of crustal
assimilation. This is supported by U, Th and Zr contents that all increase significantly as SiO2
increases from 50% to 60%. Rocks with >70% SiO2 represent highly fractionated, quartz-rich
intrusions and, at >75% SiO2, are weakly peraluminous and locally contain tourmaline. Zircon
saturation temperatures for the Deadman pluton range from 750-850°C (Table 3), and the lack
of inherited zircon suggests that any assimilation of crustal material occurred at temperatures
137
Chapter 4
Source and Redox Controls
>850°C. In addition, many of the zircons for the Deadman pluton are anhedral, and are
interpreted to have formed late in the crystallisation sequence, where they concentrated
available uranium and thorium.
The Tungsten suite intrusions have numerous characteristics to indicate that they are
mainly derived from an ancient crustal source. The abundance of inherited zircon, the variable,
although highly radiogenic, strontium and neodymium isotope values, the positive δ18O values,
the moderately to strongly reduced state of the rocks, a lack of associated intermediate and
mafic phases, and the elevated ASI values are all consistent with a dominantly sedimentaryrock crustal source. However, the intrusions are only weakly peraluminous, are generally
less peraluminous than typical S-type granites (cf. Chappell and White 2001), and are
relatively rich in calcium and strontium for granitoids derived from metasedimentary rocks.
These factors could be due to either a mafic crustal component, or a significant volume of
meta-carbonate (calc-silicate) rocks amongst the quartzo-feldspathic and pelitic strata in the
source region. Deep seismic profiles of the region (Snyder et al. 2002) show that Proterozoic
platform sedimentary rocks are imaged at the Moho boundary. It is likely that these strata were
involved in melting to re-establish the Moho after depression through deep crustal isotherms
in response to upper crustal thickening in the Late Jurassic to Early Cretaceous. Zircon
saturation temperatures calculated for the main phases of the Mactung pluton range from 770
– 800°C (Table 3). Inherited ancient zircon cores are abundant in the Mactung intrusion. The
temperature likely corresponds to the point at which the inherited zircons were entrained, and
does not necessarily rule out a more protracted petrogenesis.
Early, main-stage quartz monzonites to monzogranites of the Mayo suite are geochemically
similar to Tungsten suite plutons (e.g., Fig. 5). They have positive δ18O values and evolved
neodymium and strontium isotope characteristics. However, many early Mayo plutons
contain at least a small proportion of hornblende, and some contain abundant clinopyroxene.
This mineralogical difference, in combination with a lower proportion of inherited xenocrystic
zircon, and an association with intermediate to mafic intrusions, indicates that either a mafic
crustal component was included in the magma, or that early intrusions contained a component
of both mantle- and crustally-derived material. In addition, Mayo suite intrusions contain
elevated chromium contents, and are generally enriched in strontium relative to the Tungsten
suite intrusions for a given silica content. Zircon saturation temperatures for felsic rocks of
the Mayo suite range from 770 –790°C (Table 3), and the presence of inherited zircon suggests
that they were incorporated during crustal melting close to, or slightly above, the temperatures
estimated for zircon saturation. These zircon characteristics, along with the less pronounced
fractionation trends in rubidium and strontium data, suggest that Mayo suite intrusions had
considerably more interaction with crustal rocks than did the bodies of the Tombstone suite.
Mafic rocks of the Mayo suite have similarities to some Tombstone suite rocks in that they
are alkalic in nature, volatile-rich, and formed relatively late in the plutonic history of the
region. However, the mafic rocks of the Mayo suite typically have relatively higher Mg#’s
and contain higher chromium concentrations. These are related to the common occurrence of
xenocrystic chromium-diopside, which indicates that these mafic Mayo phases are unlikely
to have undergone considerable fractionation, (Mair et al. in press). Intermediate lithological
138
Chapter 4
Source and Redox Controls
phases typically contain elevated chromium and, in some cases, have textures indicative of
mixing mafic and felsic magmas. These phases likely result from the interaction between
evolving felsic magmas in the crust and mantle-derived mafic magmas.
Isotope data for rocks of the Mayo plutonic suite are intermediate between those for the
Tombstone and Tungsten suite plutons. The variations between the suites are interpreted to
reflect the proportion of mantle-sourced material, with the Tombstone intrusions having the
greatest component of material derived from a mantle source, followed by the Mayo suite.
The radiogenic nature of mantle-derived phases is attributed to ancient metasomatism of the
lithospheric mantle, which also enriched LILE, LREE, volatiles, and other incompatible phases
(cf. Carlson & Irving 1994; Nelson et al. 1986). In addition, neodymium and strontium ratios
may well have been shifted further to more-radiogenic values by increased crustal contributions.
However, the values for mantle-derived rocks are considerably lower than ratios for both upper
crustal strata of the Selwyn Basin and intrusions of the Tungsten suite.
Redox Characteristics of Source Materials
Intrusions of the TTB range from magnetite-dominant relatively oxidized, to ilmenitedominant reduced bodies. These variations are interpreted to primarily reflect the varying
proportion of magma contributed from different sources. The Selwyn Basin crustal rock
package is dominated by marine siliciclastic rocks, some of which are basinal or off-shelf facies
that contain graphite, and thus can be considered as strong reductants (Ishihara 1977, 1981). In
contrast, mantle-derived phases are typically relatively oxidised, due to oxidising metasomatic
agents that are introduced to the mantle wedge above subduction zones (cf. Carmichael et al.
1996). In mafic intrusions of the Mayo suite, the presence of sparse magnetite, rare barite
inclusions in mafic phenocrysts, and high Mg# pyroxenes suggest that these rocks may have
originally been oxidised. The Tombstone suite rocks are the most alkalic, show the least
evidence of crustal interactions, and are the most oxidised. Conversely, the isotope data suggest
a greater component of middle to upper crustal material in the Tungsten suite intrusions - these
rocks will typically contain a higher proportion of reduced carbon and are therefore the most
reduced.
Relationships Between Redox State of Magmas and Metallogeny
The associations between the oxidation state and the metallogenic expression of intrusionrelated hydrothermal systems have been well documented (e.g. Ishihara 1977, 1981; Blevin
and Chappell 1992). Ishihara (1977, 1981) indicated that the compatibility and partitioning
behaviour of metals is strongly influenced by the oxidation state of a magma, which can be
estimated based on the dominance of either magnetite or ilmenite. The chalcophile Cu±Au
association is generally associated with oxidized mantle-derived magmas generated in a
subduction zone setting (Candela 1989). A high oxidation state is optimal for preventing
early formation of sulphide globules in a magma, which can efficiently sequester chalcophile
elements such as copper and gold (Burnham and Ohmoto 1980). Molybdenum enrichment is
also favoured by oxidised conditions, but at a redox state lower than is typical of that associated
with porphyry copper deposits (Ishihara 1981).
Numerous studies have indicated that the lithophile metal association of Mo, W and Sn is
also strongly influenced by redox conditions (Ishihara 1981, Blevin and Chappell 1992; Candela
139
Chapter 4
Source and Redox Controls
1995). Redox conditions influence the valency of some elements, which, in turn, determines
the compatibility of such elements in accessory minerals. For example, tin enrichment in
fractionating melts is favoured under reduced conditions, where it occurs mostly as Sn2+,
rather than as Sn4+. In more oxidised magmas, Sn4+ is favoured, but readily substitutes for Fe3+
and Ti4+ in mafic silicate minerals and in Fe-Ti phases (such as titanite), thus preventing tin
enrichment (Ishihara 1978). Tungsten enrichment is similarly favoured by reduced conditions,
although associated intrusions span a broader range of redox environments (Ishihara 1981).
At higher fO2, tungsten has affinities with molybdenum and copper, being concentrated in
minerals such as molydoscheelite (Sato 1980); under more reducing conditions, tungsten has
affinities with tin (Ishihara 1981).
Other ore-associated elements that are variably enriched in the TTB systems are tellurium
and bismuth. Bismuth is commonly strongly enriched in reduced gold and tungsten skarns,
and by inference, the concentration of bismuth through magmatic processes appears to be
favoured by an intermediate to reduced oxidation state (Meinert 1992). Conversely, tellurium
is commonly enriched in epithermal mineralisation associated with oxidised alkalic intrusive
complexes (Jensen and Barton 2000; Cooke and McPhail 2001), suggesting that its enrichment
may be controlled by factors other than just redox effects. However, controls of bismuth and
tellurium concentrations are equivocal, as they are anomalous in a variety of different deposit
styles that developed under contrasting redox conditions (e.g. Ciobanu et al. 2003).
The precise role that redox conditions play in generating gold-enriched magmas remains
enigmatic. Although gold is a siderophile element, it has strong chalcophile tendencies, and
the formation of early sulphide globules in the melt, particularly copper- and iron-bearing
sulphides, could effectively reduce the gold concentration in the residual melt (Cygan and
Candela 1995). As a consequence, magmas too oxidized to precipitate sulphide minerals
may be more effective at concentrating gold during fractionation, and indeed, gold-enriched
magmatic-hydrothermal systems are recognized as being associated with highly oxidized
magmas (Candela 1989). However, other workers have suggested that gold in oxidized
melts can be sequestered by magnetite, as Tilling et al. (1973) report values >100 ppm Au
in magnetite. Supporting this concept are intrusions in central Alaska with Fe2O3/FeO ratios
below 0.6, which typically lack magnetite and plot below the NNO buffer, and appear to
be more prospective for gold mineralisation (Leveille et al. 1988; Newberry & Solie 1995;
McCoy et al. 1997). Recently however, sequestering of gold by magnetite has been shown
experimentally to be only moderately efficient in felsic melts, even those that are goldsaturated (Cygan & Candela 1995; Simon et al. 2002, 2003). Despite these contradictory
factors, gold is known to be highly incompatible in most silicate phases. Taken together, these
factors indicate that redox state likely has a large influence on the partitioning of gold in the
magmatic environment during fractionation
The intrusions and associated mineral occurrences of the TTB show metal correlations
that are mostly consistent with the redox associations outlined above. The relatively moreoxidised Tombstone suite intrusions are associated with mineralisation dominated by Cu-Au
(± Bi) and U-Th signatures. Although the Tombstone intrusions are considerably less-oxidised
than those related to typical porphyry copper deposits, they are sufficiently oxidised to prevent
140
Chapter 4
Source and Redox Controls
early sulphide formation in the melt, into which copper and gold could partition (Cygan and
Candela 1995). Uranium-Th mineralisation is interpreted to result from fluids generated during
extended fractionation of undersaturated melts, which, at high temperatures, suppresses zircon
crystallisation and uranium sequestering.
The Tungsten suite intrusions are reduced and associated with large tungsten deposits that
also have minor enrichments of Cu, Mo, Zn, and Sn. Tungsten enrichment in the melts is
favoured by the precipitation of ilmenite over titanite, as W4+ may substitute for Ti4+ in titanite
due to their common valency (Ishihara 1978). The lack of extensive tin mineralisation may be
due to insufficient fractionation of Tungsten suite intrusions, stripping of Sn2+ from the melt by
biotite crystallization, or a lack of tin in the melt source area. North America, as a whole, is
anomalously lacking in significant tin deposits (Ishihara 1981; Taylor 1979).
The Mayo intrusions are generally characterised by an intermediate oxidation state. They
are only slightly more oxidized than intrusions of the Tungsten suite and slightly more reduced
than those of the Tombstone suite. Consequently, they have a metal association that is broadly
intermediate between that of the other suites, such that many Mayo intrusions are predominantly
associated with gold deposits, but also includes subordinate tungsten mineralisation. The Mayo
suite related gold deposits are characteristically associated with enrichments of Bi, Te, and
As, and these ore systems are also generally copper-poor. In addition, tungsten mineralisation
is widespread in the Mayo intrusions, but, unlike deposits associated with Tungsten suite
intrusions, high tonnage resources are not recognized.
Conclusions
Among the numerous potential controls on the regional metallogeny of intrusion-related ore
systems, the nature of the source rocks and the corresponding redox state of the magmas are
among the most important. The TTB forms a single magmatic belt that was emplaced within
the ancient North American continental margin in the mid-Cretaceous. Magmatism post-dates a
period of terrane collision and crustal thickening. The nature of the plutons and their associated
mineral deposits vary considerably across the TTB, becoming more reduced and radiogenic,
and tungsten-rich/gold-poor to the southeast. On the basis of these variations, the belt is
divisible into three plutonic suites, which, from west to east, are the Tombstone, Mayo, and
Tungsten suites. The plutons are compositionally variable, but comprise dominantly granites
to monzonites. There is a general trend from more metaluminous intrusions in the northwest
(Tombstone suite) to peraluminous- intrusions in the southeast (Tungsten suite). All plutons
have high initial strontium isotope ratios (0.710-0.730) and high whole-rock oxygen isotope
values (+11 to +13 δ18O), and most plutons have low magnetic susceptibilities and low primary
oxidation state (Fe2O3/FeO ratio ~ 0.2-0.3).
The Tombstone suite intrusions are alkalic and metaluminous, and include silica-saturated
and undersaturated phases that result from fractionation and the effects of density separation of
minerals to form cumulates. These relatively oxidized plutons have accessory magnetite and
have associated Au-Cu-Bi and U-Th metallogeny.
Plutons of the Tungsten suite are peraluminous and have many features of S-type granitoids,
such as locally abundant muscovite, garnet, and tourmaline. These plutons are reduced, as they
141
Chapter 4
Source and Redox Controls
lack magnetite and titanite, but contain minor ilmenite and locally pyrrhotite. The plutons
have significant xenocrystic zircon. Plutons of the Tungsten suite are associated with worldclass tungsten deposits, but lack association with significant gold occurrences.
Intrusions of the Mayo suite have some characteristics that are difficult to reconcile within
the generalizations of “granite-series” models (e.g. Ishihara 1981; Blevin et al. 1996). They
are characterized by titanite as the dominant iron- and titanium-bearing accessory mineral,
whereas magnetite and ilmenite are typically too scarce to be diagnostic. As such, they are
too reduced to be magnetite-series, but the titanite suggests the magmas are too oxidized
to be considered ilmenite-series. They have characteristics that support neither an I-type,
nor an S-type designation, in the sense of Chappel and White (1992). They are prodigious
mineralizers despite being weakly-reduced, highly radiogenic, and locally peraluminous, lowtemperature granites. The plutons, however, commonly contain amphibole and some contain
clinopyroxene. Although felsic intrusions are volumetrically dominant, they are intimately
associated with intermediate and mafic intrusions considered to represent hybrid phases
developed through mixing with mafic magmas. The Mayo suite intrusions host significant
gold mineralisation, with elevated Bi, As, W, and Te.
The distinctive lithological, geochemical, and isotopic parameters that characterise each
plutonic suite are interpreted to reflect the varying proportions of different source materials.
The main sources are sialic crust, and a mantle source that is strongly enriched in LILE, LREE,
volatiles, and other incompatible phases, and locally contains depleted element enrichments.
The influence of the crustal source gradually increases from northwest to southeast across
central Yukon Territory, such that most granites in the southeast crystallized from melts
that had a predominantly crustal source. The different source regions had a strong primary
influence on the oxidation states of the parent magmas. Redox effects likely controlled
the behaviour of metals during fractionation of the magmas, either promoting or inhibiting
their residual concentration. Although the magma compositions, redox state, and nature of
associated mineralization of TTB intrusions are relatively compatible with existing models of
intrusion-related ore systems (e.g. Ishihara 1981; Blevin et al. 1996), it is remarkable for such
variation in intrusion compositions and metallogeny to occur along a single contemporaneous
magmatic belt.
Acknowledgements: Lara Lewis compiled the magnetic susceptibility data and assisted in
the figure preparation. Continued support from the Yukon Geological Survey is appreciated
by Craig Hart, and Rich Goldfarb recognizes the support of the U.S. Geological Survey’s
Minerals Program. Previous Hutton Symposium volumes provided much of the basis for the
senior author’s granite acumen, and works by Shunso Ishihara and Phil Blevin established
foundations for integrating magmas and metallogenic processes. We thank K. Sato and an
anonymous reviewer for their time and effort in reviewing this manuscript. The editorial
assistance and patience of Vikki Ingpen is appreciated. This manuscript represents a portion of
the senior author’s PhD dissertation at the Centre for Global Metallogeny at the University of
Western Australia.
142
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Source and Redox Controls
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Integrated Geochronology and Duration
Chapter Five
Integrated U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar
Geochronology of Mineralizing Plutons
in Yukon and Alaska:
Duration of Magmatic-Hydrothermal Systems
C.J.R. Hart1, 2, M.E. Villeneuve3, J.L. Mair1, D. Selby4, and C. Wijns1
Centre for Global Metallogeny
School of Earth and Geographical Sciences
University of Western Australia
Crawley, WA, 6009, Australia
1
Yukon Geological Survey
Box 2703 (K-10)
Whitehorse, Yukon, Y1A 2C6, Canada
2
Geological Survey of Canada
601 Booth Street
Ottawa, Ontario, K1A 0E8, Canada
3
Earth and Atmospheric Sciences
University of Alberta
Edmonton, Alberta, T6G 2E3, Canada
4
151
Chapter 5
Integrated Geochronology and Duration
Preface to Chapter Five
This paper is a publication based on a presentation made at the SEG2004 Meeting in Perth,
Australia, under the theme “Cutting Edge Developments in Mineral Exploration”. The presented
paper, “Integrated U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar Geochronology of Mineralising
Plutons in Yukon and Alaska; Duration of Magmatic-Hydrothermal Systems” has subsequently
been submitted to a special issue of Mineralium Deposita as an invited paper, and is currently
in the review process. The spelling and format for citations and references follow those that are
used in Mineralium Deposita.
Justif cation of authorship: This manuscript represents the culmination of several years of
geochronological efforts to understand the evolution of the belt as based on new data generated
as a result of this Ph.D. study. Dr. Mike Villeneuve provided Ar-Ar analyses of various minerals.
Mr. John Mair has been involved with on-going collaborations as part of his Ph.D., and has
supplied some additional Ar-Ar dates and SHRIMP analyses. Dr. David Selby is a long-time
collaborator who has undertaken the Re-Os analyses in Dr. Robert Creaser’s isotope laboratory
in response to a project established by the candidate during the Ph.D. study. Mr. Chris Wijns
undertook the thermal modeling in response to variables suggested by the author. All sampling
for SHRIMP and Ar analyses were undertaken by the candidate, except those at Scheelite
Dome where were made in partnership with Mr. Mair. All compilations, tables, figures and
interpretations are those of the candidate, or of the candidate in collaboration with one of the
co-authors.
152
Chapter 5
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Integrated U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar
Geochronology of Mineralizing Plutons
in Yukon and Alaska:
Duration of Magmatic-Hydrothermal Systems
Abstract
The mid-Cretaceous plutons that underlie the Tombstone-Tungsten Belt of Yukona dn
Alaska have a high proportion of associated mineral deposits and occurrences. Establishing
the age relationships of these complex ore-bearing magmatic-hydrothermal systems requires
the integration of several geochronological methods such that robust mineral species
representing differing stages of magma crystalisation and hydrothermal activity can be
effectively and precisely dated.
SHRIMP zircon U-Pb, mica Ar-Ar, molybdenite Re-Os, and TIMS U-Pb data are compiled
to establish the timing of magmatism and mineralisation (97-92 Ma), and the duration of
magmatic-hydrothermal systems within intrusion-related gold and tungsten systems of the
Tintina Gold Province, in Yukon and Alaska (1.1 to 4.2 m.y). Thermal modeling of these
systems indicates that the plutons and their associated hydrothermal systems should cool to
below the Ar-mica closure temperature (~250°C) very quickly (~30 k.y.) but episodic magma
pulses can keep the systems viable for much longer periods (0.3 to 1.0 m.y). Establishing
genetic relationships between mineralization and magmatism requires that the timing of these
events be proven to within this narrow time-frame.
Existing TIMS U-Pb dating indicated that most plutons hosting mineralisation were
emplaced circa 92±1 Ma. However, much of the zircon U-Pb data (and zircon SEM imagery)
show evidence of ancient inheritance and Pb-loss, which cast doubt on the accuracy of the
individual dates. SHRIMP dating of zircon was employed to avoid Pb-loss and inheritance.
Resulting dates are up to 4 m.y. older than the TIMS dates which suggests that the zircons have
notable, though inconspicuous Pb-loss, even from highly abraded fractions.
Ar-Ar dates on magmatic minerals indicate that most of these plutons cooled quickly as the
dates are within one million years of the best U-Pb dates. Ar-Ar dates on hydrothermal biotite
and muscovite from mineralisation within or adjacent to the plutons are generally within
agreement, within the ±1 m.y. uncertainty of the SHRIMP dates, but are sometimes 2-3 million
years younger than the magmatic mica dates. This suggests that Ar retention in hydrothermal
micas may differ than that in magmatic micas. Re-Os molybdenite dates, both model dates
and isochrons, are also in agreement, and within the uncertainty of the SHRIMP dates for the
host plutons.
An integrated approach to documenting the duration and events within a magmatichydrothermal history seems valid, but many complicating factors conspire to make resolution
of individual events within a one million year time-frame difficult, if not impossible. In
addition to the within-system problems, uncertainties in the absolute values of the decay
constants, and the compounding of decay constant errors across isotopic systems further
increase the uncertainties. Until these problems are reduced, and calibrations between methods
153
Chapter 5
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are improved, the slotting-in of events within a one million year window is best done as a
process of relativity from within a single isotopic system. In systems lacking complex cooling
histories, Ar-Ar is likely most effective at discerning events within timeframes of less than one
million years,
Introduction
The age of formation of the intrusion-related gold and tungsten systems across the
Tombstone-Tungsten belt (TTB) and within the Fairbanks District is an important component
for understanding both regional and local chronologies. Establishing the timing of plutonism
aids both our understanding of emplacement with respect to local structural and metamorphic
events and the plutonic responses to far-field tectonic processes. As well, it may highlight
systematic trends or variations along the belt that relate to causative factors affecting magma
generation and emplacement. In addition, if these are truly intrusion-related deposits, the
timing of mineralization should be essentially co-eval with magmatism, and any younger
dates should relate to the duration of their associated hydrothermal systems. Discrepancies in
such timing relations could relate to other factors, be they intrinsic to the systems or extrinsic
responses to more regional events.
However, magmatic-hydrothermal systems are notoriously complex, typically with several
magmatic phases and vein phases that may be mutually cross-cutting. These different phases
may each have different minerals that are useful for dating. As a result, integrating numerous
chronometers and different mineral species from different parts of the geological environment
appears to be an effective method to understand the evolution of magmatic-hydrothermal
systems.
Herein, new SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon and ArAr mica ages are integrated with existing conventional TIMS (thermal ionization mass
spectrometry) U-Pb zircon and recently published Re-Os molybdenite age data. These data
have been variably obtained from magmatic minerals from host plutons, gangue and ore
minerals in mineralized rocks and alteration minerals in host rocks. This array of settings,
materials and methods will allow us to establish temporal relationships between igneous
intrusions and their associated hydrothermal mineralization. The data will better define the
duration of magmatic-hydrothermal activity responsible for formation of intrusion-related gold
and tungsten systems in the Tintina Gold Province, Yukon and Alaska. As well, the different
isotopic methods will be evaluated and compared as chronometers in magmatic-hydrothermal
systems.
Regional Geology - Tintina Gold Province
The Tintina Gold Province is a vast region of Alaska and Yukon that comprises numerous
(>15) individual gold belts and districts with a total gold resource/production of more than
70 Moz (Hart et al. 2002). Much of this mineralization is spatially, and locally genetically
associated with suites of mid-Cretaceous granitic plutons. Prolific among the plutonic suites
are the Tombstone-Mayo and Tungsten suites of mid-Cretaceous plutons that form the 600km-long Tombstone-Tungsten belt (Figure 1). This belt comprises numerous plutons with
associated gold deposits in the Fairbanks area of east-central Alaska, USA, and in the Tintina
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Chapter 5
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Figure 1. Distribution of mid-Cretaceous granitoids (red) in Yukon and eastern
Alaska which comprise part of the Tintina Gold Province. The youngest plutons
form the Tombstone-Tungsten Belt (TTB-yellow shaded) and are associated with
significant gold and tungsten mineralization and are located along margins of the
Selwyn Basin (blue). The Fort Knox pluton and deposit are part of this belt, but have
been displaced 430 km by the Tintina Fault.
Gold Belt in central Yukon, Canada. Together, these deposits form the basis of the intrusionrelated gold model (Lang et al. 2000; Thompson and Newberry 2000). In addition, this
belt includes world-class tungsten-rich skarn deposits that are also associated with the midCretaceous plutons.
The plutons were emplaced within, but near, the northern and eastern margins of
the Selwyn Basin, a Neoproterozoic to Mesozoic clastic basin that formed within the
surrounding carbonate platforms (Fig. 1). Clear Creek, Scheelite Dome and Dublin Gulch
plutons are hosted in pelites, psammites, quartzites and calc-silicate that comprise the of the
Neoproterozoic Hyland Group (Murphy 1997; Stephens et al. 2000). The strata have locally
undergone Cretaceous ductile deformation and greenschist grade metamorphism to form the
Tombstone Strain Zone. These rocks cooled through ~250°C approximately 105 million years
ago (Mair et al. in review a). The Fort Knox pluton formed in a similar setting but has been
dextrally offset 430 km to the northwest along the Tintina Fault (Fig. 1), and is considered
to be within the Yukon-Tanana terrane. Mactung and Cantung plutons intruded deformed,
but essentially unmetamorphosed Neoproterozoic to early Paleozoic strata. A few Late
Cretaceous McQuesten Plutonic Suite intrusions crop out 5-10 km from the Clear Creek and
Scheelite Dome areas (Murphy 1997), but have had no obvious any thermal effects.
The gold-dominant plutons at Fort Knox, Clear Creek, Dublin Gulch and Scheelite
Dome which are associated with gold deposits, are metaluminous to slightly peraluminous,
moderately reduced ilmenite-series granitoids that form part of the Mayo Plutonic Suite (Hart
et al. 2004, 2005). Similar plutons in the Fairbanks area, such as Fort Knox, are the faultoffset equivalents of the Mayo Suite. The Cantung and Mactung plutons are further east, more
peraluminous, more reduced, ilmenite-series granitoids and are representatives of the Tungsten
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Integrated Geochronology and Duration
Plutonic Suite (Hart et al. 2004). All plutons consist of a main phase lithology, normally
granite or quartz monzonite, but include other magmatic phases with subtle lithological and
textural variations. All plutons are radiogenic, with initial Sr values >0.71. Most plutons have
well-developed and resistant, rusty-weathering contact metamorphic aureoles up to 1-2 km
wide of biotite hornfels with andalusite and biotite porphyroblasts and disseminated pyrite and
pyrrhotite.
The plutons at Fort Knox, Clear Creek, Dublin Gulch and Scheelite Dome are dominated
by several styles of gold mineralization, have placer gold in the creeks that drain them, and
also have associated tungsten mineralization, mostly as small scheelite skarns. The Mactung
and Cantung plutons are responsible for generating the largest, richest and most significant
tungsten deposits in North America (Dick and Hodgson 1982; Matheson and Clark 1984). The
plutons and their associated intrusion-related deposits were mostly formed between 8 and 4 km
depth (McCoy et al. 1997; Baker and Lang 2001). All plutons, except for that at Fort Knox,
lack evidence of significant post-emplacement thermal perturbations, although late-stage
lamprophyres have been recorded at Clear Creek, Scheelite Dome and Cantung.
Geology of the Plutons
Fort Knox
The Fort Knox pluton (or Vogt stock) hosts the Fort Knox gold deposit which is ~40
km northeast of Fairbanks in east-central Alaska (Bakke 1995; Bakke et al. 2000). The
easterly elongate 1.1x0.6 km pluton intrudes previously-deformed, quartzose and pelitic
metasedimentary rocks of the Fairbanks schist in the Yukon-Tanana terrane (Fig. 2a). The
pluton consists of three textural phases that are mostly of sparsely K-feldspar porphyritic
biotite>hornblende granites with late aplite and pegmatite dykes. Mineralization consists of
low-sulphide sheeted and stockwork veins, and quartz shear veins with pyrite, bismuthinite,
scheelite and lesser molybdenite as the main ore minerals. The Fort Knox deposit is hosted
entirely within the intrusion and has approximately 5.5 Moz of contained gold at ~0.9 g/t
(Bakke et al 2000). The pluton also has associated, but small, tungsten skarns in its periphery.
The Fort Knox pluton is clearly post-deformational. It lacks deformational fabric, cuts the
regional fabric and overprints it with dykes and a hornfels assemblage. The metasedimentary
host rocks yield white-mica Ar cooling ages that are variable but mostly between 120 and
105 Ma; biotite ages show more scatter (Douglas et al. 2002). The pluton has, however, been
subjected to a younger thermal event as indicated by ca. 50 Ma zircon fission-track data (Murphy
and Bakke 1993).
Clear Creek
The Clear Creek area is characterized by six small plutons with diameters that range
from 0.2 to 3.5 km and, together with their associated hornfels, produce a resistant highland
that forms the headwaters to Clear Creek (Fig. 2b). All plutons cut the fabric in the host
metasedimentary rocks. Pluton compositions range from quartz monzonite to quartz
diorite. The Pukelman stock is mainly porphyritic biotite>hornblende quartz monzonite, but
pegmatite, aplite and lamprophyre dykes have also been recognized. The Clear Creek area
hosts numerous occurrences and styles of gold mineralization as well as small tungsten skarns
156
Chapter 5
A
Integrated Geochronology and Duration
D
B
E
F
C
Figure 2. General geological maps of the: a) Fort Knox; b) Clear Creek; c) Sheelite
Dome; d) Dublin Gulch; e) Mactung; and f) Cantung areas.
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Chapter 5
Integrated Geochronology and Duration
(Stephens et al. 2000; Marsh et al. 2003). Intrusion-hosted mineralization is dominated by
arrays of thin (0.1-2 cm) sheeted, variably auriferous quartz ± K-feldspar veins with 1-2%
pyrite, arsenopyrite, and rare scheelite, molybdenite, chalcopyrite and bismuthinite, in its
cupola. Alteration is weak, typically comprising thin selvages of vein proximal silicification.
More than 129,000 oz of placer gold has been recovered from creeks draining the Clear Creek
plutons (Allen et al. 1999).
Scheelite Dome
The Scheelite Dome area is dominated by the east-trending 4 x 2 km quartz-monzonite
intrusion which intrudes the ENE-trending, moderately SSE-dipping fabric that is developed
in the Neoproterozoic Hyland Group (Fig. 2c). The Scheelite Dome stock, and adjacent
metasedimentary rocks, are intruded by widespread east-trending dikes of monzonite, quartz
monzodiorite, granodorite and lamprophyre, with subordinate aplite and pegmatite (Mair
et al. in review b). The main phase quartz-monzonite contains clinopyroxene and biotite as
the dominant mafic phases, with minor hornblende and accessory apatite, titanite, allanite
and zircon. Lamprophyres include both spessartite and minette varieties. A well-developed
contact metamorphic aureole, characterized by biotite, andalusite and potassium-feldspar in
pelitic horizons, and wollastonite, diopside and plagioclase in calcareous horizons, extend for
up to 500 m from the pluton margins. Associated mineralization is variable and includes a
intrusion-hosted sheeted quartz veins, fissure and shear-type country-rock-hosted auriferous
veins, tungsten and gold-tungsten skarns, silver-lead-zinc veins, and considerable placer gold
(Mair et al. 2000).
Dublin Gulch
The Dublin Gulch pluton is an elongate, northeast-trending 5.5x2 km body that intrudes
and cuts greenschist-grade quartzite and phyllite of the Neoproterozoic Hyland Group (Fig.
2d). The pluton is dominated by quartz monzonite with granite, granodiorite and lesser aplite.
Mineralization is dominated by intrusion-hosted sheeted auriferous quartz vein arrays such
as the Eagle and Olive zones, and the Ray Gulch (also known as Mar) scheelite skarn deposit
(Lennan 1983; Hitchins and Orssich 1995; Maloof et al. 2001; Brown et al. 2002). The Olive
zone consists of numerous, 0.5 to 10 cm thick quartz-dominant veins with up to a few percent
ore minerals that are dominated by arsenopyrite, pyrrhotite and pyrite, with lesser bismuthinite,
scheelite and molybdenite. The veins have 1-5 cm wide alteration selvages of quartz and
muscovite, with lesser Fe-carbonate±chlorite. Molybdenite occurs rarely on vein margins
and in alteration selvages. The Olive zone is similar in most respects to the Eagle zone but
generally has thicker veins, coarser-grained sulphide minerals and more arsenopyrite.
Mactung
The Mactung pluton (or Cirque Lake stock) is a 2.4x1.2 km pluton that has intruded
deformed, but unmetamorphosed clastic rocks of the Neoproterozoic Hyland Group, and
carbonate strata of the Cambrian Rabbitkettle Formation (Fig. 2e). The pluton comprises
three phases, megacrystic, equigranular and aplitic granitoids, with the equigranular phase
commonly occurring as apophyses into the pelitic units (Anderson 1982). The main pluton
is dominated by biotite quartz-monzonite, but includes granite, monzogranite and aplite
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Integrated Geochronology and Duration
(Anderson 1982). Hornblende is absent, but the stock locally contains garnet, muscovite and
tourmaline. Tourmaline occurs as coatings on fractures and in vein selvages (Anderson 1982;
Atkinson and Baker 1986).
Skarn mineralization is dominated by pyroxene, actinolite ± garnet, biotite skarn
overprinted by pyrrhotite, scheelite, chalcopyrite ± molybdenite (57 Mt at 0.95 % WO3;
Atkinson and Baker, 1986). The skarn and hornfels are crosscut by quartz veins that contain
scheelite and molybdenite with a greisen-type alteration (quartz-muscovite-tourmaline ±
biotite).
Cantung
The Cantung pluton occurs on the easternmost margin of Selwyn Basin in the westernmost
Northwest Territories. The 5x1 km, northwest –trending pluton is the exposed apex of the
flat top of a pluton where it is downcut and exposed in the Flat River valley (Fig. 2f). The
intrusion consists of weakly peraluminous, medium-grained biotite granite, with later aplitic
dykes. Locally muscovite is evident, particularly proximal to the skarn mineralization where
the pluton displays phyllic alteration. The cylinder-like Circular stock is lithologically
similar, although coarser grained, and crops out 1.5 km to the north (Fig. 2f). The main ore
zones, the Open Pit and E-Zone, host > 9 Mt of 1.4% WO3 hosted mainly as pyrrhotite-rich,
pyroxene±biotite skarns (Mathieson and Clark 1984). Scheelite and lesser chalcopyrite are the
main ore minerals. A late stage lamprophyre dyke cuts the skarn zones in the open pit.
Geochronology
Initial geochronology, using the K-Ar and Rb-Sr methods, began in the 1970s when
the region was widely explored for tungsten (as compiled in Sinclair 1986, and Gordey
and Anderson 1993). These data indicated a general mid-Cretaceous age, but gave widely
scattered results between 106 and 76 Ma. More recent exploration has further highlighted
the importance of understanding these plutonic rocks. A significant TIMS U-Pb zircon
campaign was undertaken in the region in the mid-1990s, in response to mapping efforts and
the increased recognition of the gold potential of these intrusions (Murphy 1997; J. Mortensen
unpub., in Lang 2001; Coulsen et al. 2002;). This work resulted in a large number of precise
determinations that indicated that many of the plutons were emplaced over the short time
interval at 92 ± 1.0 Ma (Mortensen et al. 2000). However, most of the U-Pb analyses were
discordant due to the high proportion of xenocrystic zircon and potentially some degree of Pbloss, thus complicating the zircon systematics and making interpretations equivocal. Argon-Ar
methods were used to date various magmatic and mineralized rocks at Fort Knox and other
mineral occurrences in the Fairbanks area (McCoy et al. 1997), and at the Emerald Lake pluton
in eastern Yukon (Coulsen et al. 2002).
More recently, a campaign of Re-Os molybdenite dating was initiated to determine the age
of mineralization at some Tintina Gold Province deposits (Selby et al. 2002, 2003). These
authors provided additional, and still more precise, TIMS U-Pb data for the Dublin Gulch,
Clear Creek and Mactung plutons, but discrepancies between U-Pb, Ar-Ar and Re-Os dates
became apparent.
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Chapter 5
Integrated Geochronology and Duration
In an effort to improve on the accuracy of determinations on the timing of crystallization,
SHRIMP U-Pb methods were applied and are presented below. This method, which utilizes a
focused ion beam, allows targeting of portions of a zircon which lack both ancient inheritance
and Pb-loss, to obtain a more accurate date of zircon crystallization. SHRIMP U-Pb analyses
were undertaken on the Fort Knox, Clear Creek, Scheelite Dome, Dublin Gulch, Mactung
and Cantung plutons (Appendix 2). The timing of magmatism and mineralization was also
evaluated using Ar-Ar analyses on micas, which are well-developed in both magmatic and
hydrothermal environments. Argon-Ar analyses from Scheelite Dome, Dublin Gulch, Mactung
and Cantung are presented in Appendix 3.
Fort Knox
Several geochronological techniques have been applied at Fort Knox (Table 1, Fig. 3a).
Various plutons and various types of intrusion-related mineralization in east-central Alaska
and the Fairbanks area have returned Ar-Ar dates of 91 to 83 Ma (McCoy et al. 1997).
Several Ar-Ar white-mica dates from mineralized Fort Knox stockwork veins have returned
similar ages from 88.1 to 86.8 Ma (as reported in McCoy et al. 1997). Argon-Ar analyses
of magmatic phases yield dates of 88 Ma on biotite from the pluton and 87 Ma on white
mica from a pegmatite (McCoy et al. 1997; D. McCoy and P.Layer unpub.). These dates are
essentially concordant with the mineralization ages, and together are interpreted to represent an
approximate 1 m.y. duration of magmatism and mineralization (McCoy et al. 1997). However,
a U-Pb TIMS date of 92.5 ± 0.2 Ma was reported for the Fort Knox pluton (J. Mortensen
unpub., reported in Bakke 1995, and Selby et al. 2002) and suggested a four to six million year
difference between magmatism and mineralization.
In order to provide a more robust age for mineralization, Selby et al. (2002) applied the
Re-Os method to three samples of molybdenite associated with gold mineralization. Six
determinations yield a restricted range of dates from 93.0 to 91.8 Ma, and a mean of 92.6 ± 0.9
Ma which is the same as the reported U-Pb TIMS date for the plutons, defining approximately
contemporaneous magmatism and mineralization. SHRIMP U-Pb analyses of zircons from
the Fort Knox pluton yield a slightly older date of 93.5 ± 1.0 Ma. This older SHRIMP date
likely results from avoidance of zones of Pb-loss which may cause a conventional U-Pb to
yield a slightly younger result. However, the TIMS, SHRIMP and Re-Os dates all overlap at
the two-sigma error level.
The Ar-Ar dates are up to six million years younger than the U-Pb dates. However, these
younger dates are not indicative of the timing of mineralization as there are several indications
that they are the result of partially resetting. Some Ar-Ar spectra from the Fairbanks area
have early steps that indicate a period of ca. 53 Ma Ar-loss and others have stepwise increasing
spectra that indicate resetting (D. McCoy pers. comm.; McCoy et al. 1997). Younger thermal
activity at the Fort Knox pluton and environs is indicated by fission track ages of about 50 ±
5 Ma on zircon, indicating that temperatures exceeded -190°C (240 ± 50°C), and 48-36 Ma from
apatite, suggesting rapid cooling to <120°C (Murphy and Bakke 1993). Eocene subaerial
volcanic rocks (56-53 Ma) are recognized in the Fairbanks area, and currently crop out about
12 km downstream from Fort Knox (Roe and Stone 1993), and were likely responsible for
partially resetting the Ar-Ar systematics of the magmatic and ore-related micas.
160
Chapter 5
Integrated Geochronology and Duration
A
B
C
161
Chapter 5
Integrated Geochronology and Duration
D
E
F
Figure 3. Graphical representations of compiled dates from the: a) Fort Knox; b) Clear
Creek; c) Sheelite Dome; d) Dublin Gulch; e) Mactung; and f) Cantung areas. Ages shown
as U-Pb in zircon and titanite are TIMS ages. Abbreviations: musc=muscovite, bio=biotite,
moly=molybdenite, peg=pegmatite.
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Chapter 5
Integrated Geochronology and Duration
Table 1. Compilation of geochronological data from Fort Knox. Abbreviations; alt’n=alteration,
musc=muscovite, qtz-quartz.
Sample
Geology
Method/Material
Date (Ma)
Reference
Several
samples
Country rock schist
Ar-Ar muscovite
120-105
Douglas et al.
2002
Mort FK
Pluton
U-Pb TIMS zircon
92.5±0.2
Hart FK1
Pluton
U-Pb SHRIMP
zircon
93.5±1.0
Mortensen
unpub., in Bakke
1995; Selby et al.
2003
This paper
FK1-1
Intrusion-hosted qtz
vein+musc alt’n
Intrusion-hosted qtz
vein+musc alt’n
Fracture coating
Fracture coating
Fracture coating
Intrusion-hosted qtz
vein+musc alt’n
Re-Os molybdenite
93.0 ± 0.4
Selby et al. 2002
Re-Os molybdenite
92.2 ± 0.5
Selby et al. 2002
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
92.9 ± 0.4
92.6 ± 0.4
91.8 ± 0.3
92.9 ± 0.4
Selby et al. 2002
Selby et al. 2002
Selby et al. 2002
Selby et al. 2002
Re-Os molybdenite
Re-Os molybdenite
92.6 ± 0.9
92.4 ± 1.2
Selby et al. 2002
Selby et al. 2002
Stockwork vein
Stockwork vein
Stockwork vein next
to chalcedony
Pluton
Ar-Ar white mica
Ar-Ar white mica
Ar-Ar white mica
88.1 ± 0.4
87.9 ± 0.4
86.8 ± 0.4
McCoy et al. 1997
McCoy et al. 1997
McCoy et al. 1997
Ar-Ar biotite
88.9 ± 0.5
Pegmatite
Hornfels
Ar-Ar white mica
Ar-Ar biotite
87.4 ± 0.3
88-93 steps
Pegmatite
Ar-Ar amphibole
95 Ma saddleshaped prof le
McCoy et al.
unpublished
McCoy et al. 1997
McCoy et al.
unpublished
McCoy et al.
unpublished
FK1-2
FK2-1
FK2-2
FK2-3
FK3
Mean of 6
Isochron of 6
93RNFN3
93RNFN4
93RNFN6B
93RNFN5
Clear Creek
Geochronological results from Clear Creek are compiled in Table 2 and displayed in Figure
3b. Of the six plutonic stocks at Clear Creek, two are dated by U-Pb TIMS methods. The
Pukelman stock has a U-Pb zircon date of 91.4 ± 0.8 Ma and a U-Pb titanite age of 91.5 ± 0.6
Ma, and the larger Rhosgobel stock yields a U-Pb zircon date of 91.4 ± 0.3 Ma (Murphy 1997).
Subsequent U-Pb TIMS zircon dates of the Pukelman pluton by Selby et al. (2003) yield dates
of 91.4 ± 1.9 and 92.3 ± 0.9 Ma. Most of the U-Pb determinations are discordant, such that the
absolute zircon dates were determined by lower intercept regressions with concordia, but these
type of dates typically have large inherent uncertainties (Ludwig 2001).
Initial Re-Os isotopic determinations on molybdenite within the granitoids indicate
mineralization ages of ca. 98 Ma which are clearly in error as the dates are older than the host
granitoids (Selby et al. 2003). Subsequently, two molybdenite samples associated with gold
mineralization in the Pukelman pluton yielded mean Re–Os dates of 93.6 ± 0.3 and 92.5 ± 0.3
Ma, and a similar mean of 93.4 ± 0.4 Ma was determined for molybdenite from the nearby
Saddle zone (Selby et al. 2003).
These imprecise U-Pb dates, and conflicting Re-Os dates encouraged SHRIMP U-Pb
zircon determinations to be undertaken. The result was a date of 93.6 ± 1.8 Ma which is
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Chapter 5
Integrated Geochronology and Duration
approximately 1 m.y. older than the U-Pb TIMS zircon dates, and remarkably similar to the
Pukelman Re-Os dates. An Ar-Ar date determined on biotite from hornfels adjacent to the
Pukelman stock gave a date of 91.7±0.4, but two middle steps in the plateau are 92.6 ± 0.6
Ma (Marsh et al. 2003), similar to the Re-Os dates. An Ar-Ar date of muscovite from a quartz
vein in the Rhosgobel zone gives a date of 90.0±0.3 Ma. This date may be partly reset 65 Ma
Vancouver Creek stock which is 6-7 km to the south.
Table 2. Compilation of geochronological data from Clear Creek, with emphasis on the
Pukelman pluton. Abbreviations: qtz=quartz; moly=molydenite; py=pyrite; musc=muscovite
Sample
Geology
Method/Material
Date (Ma)
Reference
92DH-151
92DH-151
CC-1
DS7-01
PUK1
Pluton
Pluton
Pluton
Pluton
Pluton
U-Pb zircon
U-Pb titanite
U-Pb zircon
U-Pb zircon
SHRIMP U-Pb zircon
91.4 ± 0.8
91.5 ± 0.6
91.4 ± 1.9
92.3 ± 0.9
93.6 ± 1.8
Murphy 1997
Murphy 1997
Selby et al. 2003
Selby et al. 2003
This paper
DS7-01-1
DS7-01-2
DS7-01-3
DS7-01 mean
CC-3-1
CC-3-2
CC3 mean
CC-2-1
CC-2-2
CC2 mean
Qtz-moly vein
Qtz-moly vein
Qtz-moly vein
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
Selby et al. 2003
Selby et al. 2003
Selby et al. 2003
Qtz-moly vein
Qtz-moly vein
Re-Os molybdenite
Re-Os molybdenite
Saddle zone
Saddle zone
Re-Os molybdenite
Re-Os molybdenite
93.6±0.3
93.6±0.3
93.7±0.3
93.6 ± 0.3
92.6±0.3
92.4±0.3
92.5 ± 0.3
93.4±0.4
93.5±0.3
93.4 ± 0.4
98CCPU2
98CCPU2
89CCRG13
Hornfels
Ar-Ar biotite
Ar-Ar biotite, 2 steps
Ar-Ar muscovite
91.7±0.4
92.6 ±0.6
90.0±0.3
Marsh et al. 2003
Marsh et al. 2003
Marsh et al. 2003
Qtz-py-musc vein
Selby et al. 2003
Selby et al. 2003
Selby et al. 2003
Selby et al. 2003
Dublin Gulch
Dublin Gulch results are compiled in Table 3 and displayed in Figure 3c. The Dublin Gulch
pluton has TIMS U-Pb zircon and titanite ages of 93.5+5.8/-0.5 Ma and 92.8±0.5, respectively
(Murphy 1997). The U-Pb zircon data indicate significant inheritance in most zircon fractions,
such that the date is determined from a single, nearly concordant fraction whose upper error is
determined from its 207/206 age which results in an imprecise upper error limit. The absolute
date however, is only slightly older that its titanite age. A more precise TIMS U-Pb zircon age
of 94.0±0.3 Ma was largely based on two overlapping concordant fractions (Selby et al. 2003).
Further support for this date is provided by a SHRIMP U-Pb zircon date of 94.2±1.0 Ma from
this study.
Mineralization was dated using Re-Os in molybdenite analyses which yields a range of
ages from 95.2 and 93.1 Ma. The age of 93.2±0.3 Ma is considered the best analysis due to
its reproducibility and larger sample size (Selby et al. 2003). Coarse-grained biotite from
auriferous quartz veins associated with the same molybdenite that was dated by Re-Os, gives
an Ar-Ar date of 96.5±1.3 Ma. The analysis, however, is too old and an inverse isochron plot
indicates that it hosts a component of excess Ar.
164
Chapter 5
Integrated Geochronology and Duration
Table 3. Compilation of geochronological data from Dublin Gulch
Sample
Geology
Method/Material
Date (Ma)
Reference
94SD-240
94SD-240
DS16-01
Pluton, Eagle zone
Pluton, Eagle zone
Pluton, Olive zone
U-Pb zircon
U-Pb titanite
U-Pb zircon
93.5+5.8/-0.5
92.8±0.5
94.0±0.3
Murphy 1997
Murphy 1997
Selby et al. 2003
00CH DG1C
00CH DG1C
Pluton, Olive zone
Pluton, Olive zone
U-Pb SHRIMP zircon
Ar-Ar biotite
94.2±1.0
96.5±1.3
This paper
This paper
DG1-1
DG1-2
mean
DS2-01-1
DS2-01-2
mean
Olive zone
Olive zone
Re-Os molybdenite
Re-Os molybdenite
Selby et al. 2003
Selby et al. 2003
Olive zone
Olive zone
Re-Os molybdenite
Re-Os molybdenite
94.3±0.3
95.2±0.4
94.8
93.1±0.3
93.2±0.3
93.2
Selby et al. 2003
Selby et al. 2003
Scheelite Dome
Geochronological data for Scheelite Dome are compiled in Table 4 and displayed in Figure
3d. The Scheelite Dome pluton yields a TIMS U-Pb zircon date of 92.1±0.5 Ma and a titanite
age of 91.2±0.9 Ma (Murphy 1997) which are considerably more precise than previous K-Ar
ages of between 90 and 99 Ma (Kuran et al. 1982). SHRIMP U-Pb zircon dates of 94.4 ± 1.6
and 94.6 ± 1.0 Ma are slightly older. Mair et al. (in review b) present several Ar-Ar dates for
the Scheelite Dome area, including 92.7 ± 0.2 Ma on magmatic biotite, and 92.9 ±0.2 Ma on
hydrothermal biotite from an auriferous quartz vein. A date of 91.2 ± 0.6 Ma is determined for
biotite from an auriferous, granite-hosted quartz vein (Appendix 3).
Table 4. Compilation of geochronological data from Scheelite Dome
Sample
Geology
Method/Material
Date (Ma)
Reference
94DM-240
94DM-240
SD Main
SDMonz
Pluton
Pluton
Pluton
Pluton
U-Pb zircon
U-Pb titanite
U-Pb SHRIMP zircon
U-Pb SHRIMP zircon
92.1±0.5
91.2±0.9
94.4 ± 1.6
94.6 ± 1.0
Murphy 1997
Murphy 1997
Mair 2004
Mair 2004
00CH SD-1
Magmatic
Skarn
Hydrothermal
Qtz-molyscheelite vein
Pluton
Pluton
Ar-Ar biotite
Ar-Ar biotite
Ar-Ar biotite
Ar-Ar biotite
92.7±0.2
93.8±1.3
92.9±0.2
91.2±0.6
Mair 2004
Mair 2004
Mair 2004
This paper
K-Ar biotite
K-Ar biotite
90.4±5.8
99.3±3.8
Kuran et al. 1982
Stevens et al. 1982
GS-101
SYA79-84
Mactung
Geochronological data for Mactung are compiled in Table 5 and presented in Figure 3e.
Five K-Ar isotopic dates for the Mactung pluton on biotite and muscovite are in the range
90-89 Ma (Wanless et al. 1974; Hunt and Roddick 1987). A TIMS U-Pb zircon date of 92.1
± 0.4 Ma was recently obtained by Selby et al. (2003) and indicates only slight discordance
from, but much higher precision than, the K-Ar dates. However, six Re-Os determinations on
three molybdenite samples give a narrow range of older, but precise dates of 96.9 to 97.6 Ma,
suggesting a five million year discordance with the U-Pb date. Selby et al. (2003) interpreted
165
Chapter 5
Integrated Geochronology and Duration
this as evidence of earlier mineralation event that was coincidently intruded by the younger
Mactung pluton, supporting a similar suggestion by Atkinson and Baker (1986) on the basis of
alteration zones that were discordant to the position of the Mactung pluton.
SHRIMP U-Pb analyses of Mactung zircons return a date of 96.4 ± 1.0 Ma which is much
more in line with the Re-Os in molybdenite dates and do not require an earlier mineralizing
event. Three Ar-Ar dates were obtained on differing types of mineralization. Fresh, coarsegrained biotite from biotite-rich pyrrhotite skarn gives a date of 96.7 ± 0.6 Ma. In addition,
coarse-grained muscovite from quartz-tourmaline-muscovite-molybdenite veins yield a flat
plateau of 95.2 ± 0.9 Ma for veins cutting the pluton and 95.5 ± 0.6 Ma for veins cutting
the hornfelsed country rocks. The Ar-Ar dates are also older than the TIMS zircon date and
support a single magmatic and metallogenic event. Since the SHRIMP, Re-Os, and oldest ArAr mica dates overlap at the two sigma uncertainty level, the SHRIMP determination may be
slightly too young.
Table 5. Compilation of geochronological data from Mactung.
Sample
Geology
Method/Material
Date (Ma)
Reference
DS10-1
CH MT-1
Pluton
Pluton
U-Pb zircon
U-Pb SHRIMP
92.1±0.4
96.4±1.0
Selby et al. 2003
This paper
MT-02-1
MT-02-2
DS8-01-1
DS8-01-2
DS9-01
DS12-01
Mean of 6
Isochron of 6
Quartz greisen vein
Quartz greisen vein
Quartz greisen vein
Quartz greisen vein
Quartz greisen vein
Quartz greisen vein
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
Re-Os molybdenite
97.2±0.5
96.9±0.5
97.1±0.4
97.1±0.4
97.2±0.4
97.6±0.4
97.2±0.2
97.5±0.5
Selby et al 2003
Selby et al 2003
Selby et al 2003
Selby et al 2003
Selby et al 2003
Selby et al 2003
Selby et al 2003
Selby et al 2003
00CH MT03
00CH 15
Skarn
Greisen vein in
pluton
Greisen vein in
hornfels
Ar-Ar biotite
Ar-Ar muscovite
96.7±0.6
95.2 ± 0.9
This paper
This paper
Ar-Ar muscovite
95.5 ± 0.6
This paper
Pluton
Pluton
Pluton
Aplite dyke
K-Ar biotite
K-Ar biotite
K-Ar biotite
K-Ar muscovite
89±4
89±2
90±2
90±1
Wanless et al. 1974
Hunt and Roddick 1987
Hunt and Roddick 1987
Hunt and Roddick 1987
00CHMT14A
FJ68-320-2
ANMT-81-8-1
ANMT-81-8-1
ANMT82-331-1
Cantung
Geochronological data for the Cantung area are compiled in Table 6 and presented in
Figure 3f. Early K-Ar biotite dates for the pluton at Cantung give an age date of 93.6±2.6 Ma
(Archibald et al. 1978). A SHRIMP date on the same quartz monzogranite yields an older date
of 96.6±1.2 Ma. Three Ar-Ar mica dates on skarn mineralization and associated veins range
from 95.8 to 93.9 Ma, defining a narrower range than the mica K-Ar dates of 97.3 to 92.2 Ma.
There are no available TIMS U-Pb data nor Re-Os analyses.
166
Chapter 5
Integrated Geochronology and Duration
Table 6. Compilation of geochronological data from Cantung.
Sample
Geology
Method/Material
Date (Ma)
Reference
02CH CT-1
Cantung pluton
96.6±1.2
This paper
AHC-11
Circular stock, pluton
SHRIMP U-Pb
zircon
K-Ar biotite
93.6±2.6
Archibald et al. 1978
02CH CT1-6
02CH CT1-5
02CH CT2-21
Biotite skarn
Qtz-bio-moly vein
Muscovite in quartzpyrrhotite vein
Ar-Ar biotite
Ar-Ar biotite
Ar-Ar muscovite
95.8±0.6
94.4±0.6
93.9±0.6
This paper
This paper
This paper
U43-19E
ZKA-1
U43-20A
Biotite skarn
Greisen pluton
Tremolite skarn
K-Ar biotite
K-Ar muscovite
K-Ar biotite
94.6±2.5
92.2±2.4
97.3±2.7
Archibald et al. 1978
Archibald et al. 1978
Archibald et al. 1978
Summary
Newly acquired SHRIMP U-Pb zircon and Ar-Ar mica data from Tintina Gold Province
plutons and mineral deposits are integrated with previously published TIMS U-Pb, Ar-Ar and
Re-Os geochronological determinations (Murphy 1997; McCoy et al. 1997; Selby et al. 2002,
2003) and presented in Table 7. All presented dates are given with their two-sigma errors, but
some published data do not have indicated precision.
Table 7. Compilation of ages for six plutons and associated magmatic-hydrothermal
mineralization in Yukon and Alaska. Dates are compiled from data sources referenced in the
Tables 1-6.
Location
(west to
east)
U-Pb
zircon
TIMS
U-Pb
zircon
SHRIMP
Re-Os
molybdenite
Ar-Ar
magmatic
biotite
Ar-Ar
hydrothermal
muscovite
Ar-Ar
hydrothermal
biotite
Fort Knox
92.5 ± 0.2
93.5 ± 1.0
92.4±1.2
87.4 ± 0.4
muscovite
88.1-86.8 ± 0.4
n/a
Clear Creek
92.3 ± 0.2
91.4 ± 0.8
93.6 ± 1.8
92.5-93.4 ±
0.5
91.7 ± 0.4
90.0 ± 0.3
n/a
Scheelite
Dome
91.2 ± 0.9
94.6 ± 1.4
94.4 ± 1.0
n/a
92.7± 0.2
n/a
92.9± .2
Dublin
Gulch
94.0 ± 0.3
92.8 ± 0.5
94.2 ± 1.0
95.2-93.1 ±
0.4
n/a
n/a
96.5 ± 1.3
Mactung
92.1 ± 0.4
96.4 ± 1.0
97.5±0.5
96.7 ± 0.6
skarn
95.2 ± 0.9
95.5 ± 0.6
Cantung
n/a
96.6 ± 1.2
n/a
95.8 ± 0.6
skarn
93.9 ± 0.6
94.4 ± 0.6
The accuracy of any individual geochronological method is difficult to judge in isolation. It
is only from integration of age dates from other methods or minerals, where the systematics are
different or better understood, that accuracy can be interpreted. For example, nearly co-eval
SHRIMP and Re-Os dates and slightly younger Ar dates provide a pattern that is expected and
predictable, and thus increases the reliability of the absolute dates. Contemporaneous Re-Os
and Ar-Ar dates on hydrothermal minerals that are older than zircon U-Pb dates indicate a lack
of accuracy of some U-Pb dates. This is particularly apparent at Mactung.
The near concordance of Re-Os molybdenite and the best U-Pb zircon dates suggest either
a very short interval between magma crystallization and hydrothermal mineralization, or that
167
Chapter 5
Integrated Geochronology and Duration
Figure 4. Plot of sample size vs. age for molybdenite samples from Clear
Creek showing the increase in reproducibility for sample sizes greater
than 20 mg, using data of Selby et al. (2003). Similar results have been
recorded by Creaser and Selby (2002) and mechanisms accounting for this
variability are presented by Selby and Creaser (2004). Data points shown
are listed in Table 2.
intercalibration of the isotope systematics may be skewed. Both Re-Os in molybdenite and
Ar-Ar in vein mica dates record the hydrothermal event, but the Re-Os dates are consistently
older. The younger Ar dates could result from the cooling of the hydrothermal system
as opposed to the timing of crystallization, or may indicate that there are intercalibration
problems between the two systems.
Comparison of TIMS zircon dates on similar rocks from different laboratories indicates
variations up to 1 m.y. although these dates are commonly expressed with uncertainties less
than this range. Rhenium-Os dates on molybdenite are reproducible within various aliquots
from the same sample, but variations of up to 1 m.y. are apparent between different samples
from the same location (e.g. Clear Creek). A detailed evaluation of erratic Re-Os data has
revealed the importance of sample homogenization which is best achieved with larger sample
sizes (Fig. 4). These points and others emphasize that the precision expressed in the age dates
is simple analytical precision, but is far outweighed by other, mainly geological, uncertainties.
168
Chapter 5
Integrated Geochronology and Duration
Discussion
Comparison of data
Before comparing the geochronological data, it is necessary to evaluate the TIMS UPb analyses. The TIMS U-Pb zircon dates on plutons associated with mineralization were
determined using conventional methods, whereby zircon populations consisting of tens to
hundreds of grains with similar size and magnetic characteristics were analysed. These dates
indicate that most of the plutons were emplaced at ca. 92±1.0 Ma (Murphy 1997; Lang et al.
2000; Mortensen et al. 2000; Coulson et al. 2002). However, most of the zircon U-Pb fractions
give discordant and complex results. Part of this complexity is due to the inclusion of some
ancient zircons which mostly occur as cores with magmatic overgrowths within the analysed
population (Fig. 5a). This interpretation is supported by scanning electron microscope
backscatter-emission (BSE) imagery of similar zircons in which there are obvious xenocrystic
zircon cores represent a significant part of the zircon population, from 1-20% depending on
the sample. The resulting analysis of these samples is discordant yielding an older 207Pb/206Pb
age (Fig. 6). This requires the application of regression techniques to determine an age on
the basis of a lower intercept with concordia. This method results in large inherent errors,
particularly for young (i.e. Mesozoic) zircons (Ludwig 2001).
In addition, many of the analysed zircon populations had high uranium contents, and BSE
examination of similar zircons from the same plutons indicates that many zircons have internal
damage to their structure (Fig. 5b). These metamict regions result from radiation damage
and are sites that are susceptible to loss of radiogenic product lead. Analysis of a fraction
containing Pb-loss will result in dates that are too young. However, since the concordia curve
is nearly parallel to the Pb-loss curve, the zircons may still appear to be concordant (Fig. 6). It
is likely that most of the analysed fractions had a mixed component of both inherited zircon
and Pb-loss. Most of the zircon populations that were analysed were subjected to abrasion to
reduce Pb-loss from outer zones, but removal of such zones will only concentrate the effect
of internal Pb-loss. It will also increase the effect of xenocrystic cores on the determined age.
Improvements to the TIMS dates can be made by more judicious selection of fewer zircons, for
example, demonstrated at Dublin Gulch (Selby et al. 2003).
Given that many of the TIMS zircon dates are predicated on models sensitive to
assumptions regarding Pb-loss and xenocrystic components, such as lower regressions, there
are reasonable grounds to question the accuracy of some of the dates. In order to avoid regions
of a crystal that may have a contribution of each of these components, it is necessary to use the
high spatial resolution of SHRIMP analysis, with supporting BSE imaging. The six SHRIMP
U-Pb zircon dates presented here yield ages between 93.6±1.8 and 96.6±1.2 Ma. The absolute
dates are all equivalent to, or up to 4.3 m.y. older than, their corresponding TIMS date,
confirming that the TIMS ages are modified by from some degree of Pb-loss.
The Re-Os molybdenite results are available from four locations (Selby et al. 2002, 2003)
and they are typically within the uncertainty of the SHRIMP U-Pb zircon dates, with few
analyses deviating more than one million years. Argon-Ar dates on magmatic biotite indicate
that most of these plutons (except Fort Knox) cooled relatively quickly, as the dates are just
slightly younger than the SHRIMP U-Pb dates. This suggests that argon closure temperatures
169
Chapter 5
Integrated Geochronology and Duration
Figure 5. SEM backscatter-emission images of zircons showing: a) magmatic overgrowths
on ancient inherited zircon cores; and b) regions of internal damage which are likely loci of
Pb-loss. These examples are all from Scheelite Dome. The scale bars are all 100 microns.
for the biotite (about 250°C) were reached within 1-2 m.y. of zircon crystallization. Biotite
Ar-Ar dates from skarns show similarly good concordance with magmatic biotite and
the best U-Pb zircon ages. Hydrothermal biotite and muscovite Ar-Ar dates from vein
mineralization, within or adjacent to the plutons, are 1.0-3.4 m.y. younger than the SHRIMP
U-Pb zircon dates, and are consistently younger than the magmatic mica dates. These patterns
are integrated and displayed in Figure 7. Argon-Ar dates from micas at Fort Knox deviate
considerably from these trends, with dates consistently 4-6 m.y. younger than the U-Pb zircon
and Re-Os molybdenite dates, in response to partial resetting in response to a ca. 50 Ma
thermal event specific to the Fairbanks area.
170
Chapter 5
Integrated Geochronology and Duration
Figure 6. Concordia plot showing hypothetical data points from TTB plutons
whereby analysed zircon fractions have inheritance and Pb-loss. The result
is an array of possible lower-intercept regression ages and an array of mixed
nearly-concordant fractions, which are difficult to interpret accuratly.
By virtue of its ability to avoid regions of inheritance and Pb-loss, the SHRIMP U-Pb in
zircon dates are considered more accurate than the conventional TIMS U-Pb dates, although
at the expense of analytical precision. This increased accuracy is supported by the closer
coincidence of the SHRIMP dates with the Re-Os dates, and Ar-Ar dates on magmatic
zircons, most notably as displayed at Mactung and summarized in Figure 7. The younger
Ar-Ar dates on hydrothermal micas suggest: a) that the region experienced slow cooling; b)
that mineralization was associated with a younger magmatic pulse; or c) that Ar retention
in hydrothermal micas may be less robust than in magmatic micas. Slow cooling may be
ruled out as the muscovite ages, which have a higher closure temperature to Ar diffusion,
would yield older dates than the biotite. However, at Mactung, Cantung and Clear Creek,
the muscovite dates are younger than the biotite dates, potentially indicating deviations in the
long-held dogma of the higher Ar closure-temperature of muscovite.
Duration of the Magmatic-Hydrothermal Systems
The direct comparison of dates representing magmatic and hydrothermal events from
the same deposits provides an opportunity to determine the evolution and duration of the
magmatic-hydrothermal systems. This is possible since these systems, except Fort Knox, have
not been affected by post-emplacement thermal effects, and thus the data can be interpreted to
reflect the cooling history that represents the magmatic to hydrothermal transition. For Fort
Knox is interpreted without the Ar data. For each system, direct comparisons are made of the
best absolute ages representing magmatic and hydrothermal events to determine the absolute
171
Chapter 5
Integrated Geochronology and Duration
Figure 7. Generalized summary of relationships between age determinations
from a typical TTB pluton using different methods and materials.
durations. Additionally, the data are compared at the extremes of the two-sigma uncertainty
level to determine the potential durations. Anomalous data points have been excluded, which
means that some of the TIMS data and the young Ar-Ar analyses that do not overlap other
analyses have been omitted.
The TIMS U-Pb zircon and Re-Os molybdenite dates from Fort Knox indicate a
contemporaneous magmatic-hydrothermal event at ~92.5 Ma. If weight is given to the
SHRIMP U-Pb zircon date, an apparent absolute duration of 1.1 m.y., or 3.3 m.y including
the two sigma errors of all three isotopic systems, is indicated. At Clear Creek, the absolute
apparent duration is 1.9 m.y., or 4.0 m.y. at the two sigma error limit. At Scheelite Dome,
by excluding the TIMS zircon analyses, the gap between 94.6 and 92.7 indicates an absolute
duration of 1.1 m.y. and a potential duration of 2.2 m.y at the two-sigma error limits. At
Dublin Gulch, excluding the Ar-Ar mica analysis which has excess Ar and the oldest Re-Os
molybdenite determination which is from too-small a sample, the absolute range of dates is
between 94.2 and 93.1 Ma for an absolute duration of 1.1 m.y, but a potential duration of 2.5
m.y. At Mactung, excluding the TIMS zircon analysis, the apparent duration as indicated
by the absolute dates between the oldest U-Pb zircon determination at 96.4 and the youngest
hydrothermal phase at 95.2 Ma, is 1.2 m.y. However, this is complicated by older Re-Os
molydenite determinations at ~97.3 Ma, suggesting that the system could have a duration of
as long as 3.7 m.y. including the broadest extremes of the two sigma errors. The Cantung
magmatic-hydrothermal system evolved from 96.6 to 93.9 Ma for an apparent duration of 3.3
m.y, or 4.2 m.y. including the two sigma error limits.
The range of absolute ages from zircon crystallization to hydrothermal micas of the six sites
suggests that duration of the magmatic-hydrothermal systems were 1.1 to 3.3 m.y., increasing
to 2.2 to 4.2 m.y if the extremes of the uncertainties are included. Argon-argon in mica and
U-Pb zircon data for magmatic phases from another TTB pluton, the Emerald Lake pluton,
suggest that magmatism lasted less than one million years (Coulson et al. 2002).
172
Chapter 5
Integrated Geochronology and Duration
Table 8. Compilation of TTB data to assess the durations of magmatic-hydrothermal systems.
Location
Oldest
magmatic
date
Youngest
hydrothermal
date
Absolute
duration
Duration
within
two-sigma
Fort Knox
93.5±1.0
92.4±1.2
1.1
3.3
Clear Creek
93.6±1.8
91.7±0.4
1.9
4.0
Scheelite Dome
94.6±0.9
92.7±0.2
1.1
2.2
Dublin Gulch
94.2±1.0
93.1±0.4
1.1
2.5
Mactung
Cantung
96.4±1.0*
96.6±1.2
95.2±0.9
93.9±0.6
1.2
3.3
3.7
4.2
Ranges
93.5-96.6
1.1-3.3
2.2-4.2
Note
Ar data reset and
excluded
Anomalously young
vein excluded
Anomalously young
vein excluded
Excess Ar and small
sample size Re-Os
samples excluded
* Oldest magmatic date could be up to 1 my older which would increase the absolute duration but not the
duration based on two-sigma errors.
Modeled Durations of Cooling Plutons
The reliability of the estimates of duration of magmatic-hydrothermal events indicated
above can be evaluated by several methods. One method is by comparison with thermal
models for similar geological settings. Several types of data indicate that small (up to 10
km3) magmatic bodies emplaced in the upper crust tend to cool to background temperatures
in periods far less than 106 years (Cathles 1981). Much of the published thermal modeling is
based on porphyry copper deposits that were emplaced as small high-level plutons. These data
indicate rapid cooling to only slightly above background temperatures in as few as 10 k.y. to
30 k.y. (Elder 1977; Cathles 1977). The systems can be maintained for longer by increasing
the size of the plutonic pluton (i.e. to 60 k.y.; Norton 1982) such that a body twice the size
will take four times as long to cool (Cathles 1981). Most models simulate emplacement of
magmas, as 1-2 km diameter cylinder-shaped plutons, into the cool, upper few kilometres of
crust, with high permeability and fluid fluxes resulting in significant effects from convective
cooling. Conductive cooling is, in some cases, up to two orders of magnitude slower. Cathles
(1981) suggested that any magmatic body emplaced at 800°C regardless of size, cannot
maintain a thermal anomaly greater than 200°C for more than 4 million years.
The typical TTB systems are unlike porphyry systems as they were emplaced at depths
of 8-5 km and lack evidence of external fluid interactions. As a result, a conductive cooling
model is erected of a typical TGP pluton as a cylindrical body, 2 km in diameter, emplaced
immediately at 800°C into isotropic host rocks with an ambient temperature of 200°C
(equivalent to a depth of 6 km with a 35°C/km thermal gradient). Results are shown in Figure
8a. Using only conductive cooling, the margin of the pluton cools to about 250°C in about
65 k.y. This short interval persists despite biasing every variable towards system longevity.
Modeling of a much larger TTB pluton, using slightly different variables, gave similar results
(Coulson et al. 2002). These time intervals are not significantly different from the convective
cooling models, since conductive cooling is dominant until the system is below 400°C when
convective cooling can operate. Therefore, even conductive cooling models cannot adequately
account for magmatic-hydrothermal system durations in the range of 106 years.
173
Chapter 5
Integrated Geochronology and Duration
Figure 8. a) Thermal models
of a pluton represented by an
infinite length, 2 km diameter
cylinder of magma emplaced
at 800°C into an environment
of 200°C. These conditions
best represent those of the
Tombstone-Tungsten Belt. The
pluton Note different scales and
colour indicators of distance
on the two figures. b) Same
variables previous model, with
three additional magma pulses
of equal volume to the original
at 15 ka intervals.
In the models, although the most effective method of maintaining the system in the hot
hydrothermal realm (>300°C) longer is to have higher background temperatures, regional
mineral assemblages do not indicate elevated temperatures. Another method to sustain
magmatic-hydrothermal systems is the episodic injection of fresh pulses of hot magma. The
model in Figure 8b demonstrates that a magmatic system can be maintained at >350°C for
125 k.y. and >250°C for almost 250 k.y. by three additional pulses of magma of equal volume
to the initial mass, at intervals of 15 k.y. The timing between pulses is key, as short intervals
will allow cooling of the system to be too rapid whereas too-long intervals require energy to
be used to reheat an already cooled system and behave as many solitary systems. Sustaining
hydrothermal systems is best-maintained by cumulative thermal episodes separated by
intervals short enough to prevent rapid cooling and may be capable of keeping systems alive
for a million years or so. Some locations, such as Clear Creek which consists of six individual
stocks may have such a history. Other TTB systems that are represented by a single pluton,
174
Chapter 5
Integrated Geochronology and Duration
particularly those of the Mayo Plutonic Suite such as Scheelite Dome and Dublin Gulch, are
actually mixtures of at least two batches of magma, one from the mantle (Mair et al. 2005;
Hart et al. 2005). Such systems may also be episodically recharged and maintain magmatic
heat.
Durations of Cooling Plutons from Empirical Data
Several empirical indicators of the duration of magmatic-hydrothermal systems have been
documented. Contemporary geothermal systems are recognized as having short lifespans
of 10 k.y. to 200 k.y. (White and Heropolos 1986). However, some system, such as The
Geyers in California, have been active for at least 1.1 m.y., and despite numerical modeling
that suggests it should have cooled 0.5 m.y. ago, it is still >300°C (Norton and Hulen 2001).
Exposed plutonic rocks in active arc environments are locally less than one million years old,
particularly in porphyry copper deposit environments.
Detailed geochronological research on ancient magmatic-hydrothermal systems indicates
variable lifespans. For example, quartz-sericite alteration at Chuquicamata may have been 2-3
m.y. later than potassic alteration (Reynolds et al. 1998), a 1.8±0.4 m.y duration is indicated
at Rosario in the Collahuasi District (Masterman et al. 2004), and a 3.2 m.y lifespan is likely
at La Escondita (Richards 1999). The Bingham porphyry was also constructed over at least
1.5 m.y. of magmatic and hydrothermal activity (Parry et al. 2001). Contrasting with these
relatively long-lived events is the short magmatic to hydrothermal episode at Porgera which
was possibly as short as 0.1 m.y, and 0.4 m.y. at most (Ronacher et al. 2002). Volcanism
and mineralization at Round Mountain occurred in a period possibly as short as 0.05 m.y.,
but certainly within 0.5 m.y. (Henry et al. 1997), and hydrothermal activity associated with
porphyry and epithermal mineralization at Luzon lasted only 0.3 m.y. but is bracketed within
1.3 m.y. of volcanic activity (Arribas et al. 1995). Similarly, at Batu Hijau, 2.5 m.y. of episodic
magmatic activity was followed by a short, 80 k.y. hydrothermal event (Garwin 2002). So
although hydrothermal activity associated with hydrothermal mineralization may have a
limited duration, it commonly occurs within a longer period of protracted magmatism.
Limitations of Data Integration
An integrated approach to documenting magmatic-hydrothermal evolution and duration is
valid, but many complicating factors conspire to make resolution of individual events within
a one or two million year time-frame difficult, if not impossible. Most obvious are variations
in the nature of the minerals that are dated, such as cryptic inheritance or Pb-loss in zircons.
Different micas may have variable capacities for argon retention, which may be dependent on
simple factors such as volatile content (e.g. water) and grain size. Similarly, factors involved
with the decoupling of rhenium and osmium in molybdenite are not fully understood and
sample selection may significantly affect age determinations (Creaser and Selby 2002; Selby
et al. 2003; Selby and Creaser 2004). Additionally, features specific to the Yukon and Alaska
intrusion-related systems may not be fully understood, such as geothermal gradients and the
number and timing of successive magmatic pulses.
There are, in addition, within-system problems that include uncertainties in the absolute
values of the decay constants and the compounding effects of comparing decay constant errors
175
Chapter 5
Integrated Geochronology and Duration
across isotopic systems, which further conspire to increase the uncertainties. Until these
problems are reduced, and calibrations between methods are improved, the elucidation of
events within a 1 m.y. window is probably best done from within a single isotopic system. The
U-Pb TIMS dates are precise enough, but zircons with composite histories decrease the ability
to establish accuracy with confidence, and SHRIMP is incapable of achieving <1 m.y. levels of
precision.
The Road Ahead
The U-Pb method has only been applied to hydrothermal dating in rare cases, mainly due to
the lack of uranium-bearing minerals suitable for analysis. However, recent developments in
U-Pb phosphate dating are making inroads in this direction (Vielreicher et al. 2003). Improved
understanding of Re-Os systematics (e.g. Selby and Creaser 2004) and applications (Morelli
et al. 2003) are important research avenues. Recent improvements in double-spiking of Re-Os
analyses have increased the precision of this method, yet such techniques are only applicable
in special situations (e.g., very young or in molybdenite with very low rhenium contents).
Further, most magmatic-hydrothermal systems lack multiple generations of magmatic and
hydrothermal molybdenite providing fewer opportunities for “event” determinations. In
upper crustal systems lacking complex cooling histories, Ar-Ar may be one of the most
applicable methods because datable minerals are typically available in both the magmatic and
hydrothermal regime. Precision of absolute ages are limited by the “standard-based” nature
of the method, but carefully chosen analytical protocols can result in relative ages determined
with precisions less than ±0.1% (i.e. <0.1 m.y. at 100 Ma), potentially making the method
capable of differentiating events that are 0.3 m.y. apart.
Acknowledgements
Thanks to all those who have contributed to the geochronology of these systems, particularly
to Jim Mortensen for his contributions in this region. The Yukon Geological Survey is
thanked for continuing support of this research. Thanks also to Dan McCoy and Paul Layer
for allowing inclusion of some unpublished data. We appreciated the contribution of Rob
Creaser for access and developments in his Re-Os lab. Ar-Ar dates were performed at the
Geological Survey of Canada in Ottawa. SHRIMP data were acquired on the SHRIMP II facility,
which is operated by a consortium consisting of Curtin University of Technology, the Geological Survey
of Western Australia and the University of Western Australia with the support of the Australian Research
Council. Zircon imaging was done at the Centre for Microscopy and Microanalysis at The
University of Western Australia. This manuscript benefits from a thorough review by David
Groves.
176
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Chapter 6
Conclusions
Chapter Six
Conclusions
Evaluation of intrusion-related mineralisation and the associated plutonic framework of
the Tintina Gold Province, with emphasis on the Tombstone-Tungsten Belt, has resulted in a
number of conclusions. These are briefly presented and interpreted with respect to the goals
and aims as indicated in Chapter One.
Tintina Gold Province
The Tintina Gold Province (TGP) is a composite of numerous placer gold and lode gold
belts and districts that occur throughout central Alaska and Yukon. All factors combine to
indicate a gold endowment of about 70 Moz, although this estimate is variable and dependent
on changing resource estimates for the large Donlin Creek deposit. Approximately half of the
endowment and most of the production is from placer gold, emphasizing the immaturity of the
region. However, exploration activity for lode resources increased dramatically in the early
1990s in response to discoveries at Fort Knox, Brewery Creek, Pogo and Donlin Creek.
The belts and mineral districts that make up the TGP contain gold deposits that formed as a
result of several different mechanisms and are not necessarily dominated by intrusion-related
mineralisation. Much of the gold formed in response to geological events that ranged from
Late Triassic-Early Jurassic to as young as early Tertiary. Plutons responsible for generating
intrusion-related gold systems are all Cretaceous, probably mostly mid-Cretaceous, and are
mostly in the Tombstone-Tungsten belts (TTB) and the Fairbanks district, regions previously
considered more prospective for tungsten mineralisation.
Previous authors sought to include as much of the TGP gold mineralisation within a single,
complex intrusion-related model, but the deposits are best understood by separating them
according to their dominant geological setting. As such, gold and gold-related deposits and
occurrences form three groups representing differing styles of mineralisation – namely shear
hosted, epizonal and intrusion-centred deposits.
Recognizing the complexities within these regionally diverse mineral occurrences, a
more refined, intrusion-centred ore-deposit model can be defined and developed according to
observations mostly made in the TTB and Fairbanks district. This model emphasises spatial
and temporal proximity to a central mineralising pluton and highlights outwardly-zoned
metallogenic associations. Within this model, there is a large variation in the styles of gold
mineralisation, largely in response to differing host rock types, structural variables, and the
position and distance with respect to the source pluton. However, this intrusion-centred model
results in a predictable geological and geochemical model that can be effectively used in
exploration targeting.
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Conclusions
Exploration methodologies targeting the apical regions of these moderately reduced,
mid-Cretaceous intrusions are considered to be most successful. Regional aeromagnetic
and geochemical surveys are efficient mechanisms for recognizing these intrusions and
metalliferous haloes, but direct identification of deposits within these regimes remain more
elusive.
Different mineralisation styles correlate to differing grades of the deposits. Intrusionhosted deposits, such as Fort Knox, approximate 1 gpt Au but can be very large (200 t
Au). Epizonal deposits may have refractory ores, such as at Donlin Creek, but may also be
oxidized and more economic, such as at Brewery Creek. Shear-hosted vein deposits have
grades equivalent to orogenic deposits (10-20 gpt) but their genetic association with a central
mineralising intrusion is less certain. The largest deposit of this type in the TGP, Pogo, is
particularly controversial in terms of its genesis.
Northern Cordillera Mid-Cretaceous Plutonic Province
Twenty-five Early and mid-Cretaceous (145-90 Ma) plutonic suites are defined within the
complex geological collage that underlies Alaska and Yukon, on the basis of their lithological,
geochemical, isotopic and geochronometic characteristics. The oldest suites are in the most
outboard of the accretionary terranes and there is a general younging trend cratonward, such
that the youngest suites intrude the ancient continental margin. Plutons associated with
intrusion-related gold mineralisation in the TTB and Fairbanks area have a narrow midCretaceous age range of 96-92 Ma.
Redox characteristics are such that the older and more outboard suites are clearly
magnetite-series, whereas those intruding the ancient continental margin, or its displaced or
metamorphosed equivalents, are much more reduced. Of these, most are ilmenite-series, but
some suites are slightly too oxidised, and/or magnetic to be ilmenite-series and are classified as
transitional to weak magnetite-series.
The time-space-redox distribution of the plutonic suites allows reconstruction of the
mid-Cretaceous plutonic episode. It was originally subduction dominant, with oxidised,
metaluminous and isotopically primitive plutons. Vast regions of reduced, slightly
peraluminous and radiogenic batholiths were generated during subsequent protracted collision
and crustal thickening. Waning stages of plutonism gave rise to sparse and smaller, slightly
reduced plutons in more cratonward positions. The final magmatic episode involved the
emplacement of the most inboard, variably oxidised and alkaline plutonic suites during weak
extension.
Associated mineralisation largely follows classical redox associations, with a Cu-Au-Fe
metallogenic tenor with the magnetite-series plutons, and W-(Sn) mineralisation with the
ilmenite-series plutons. However, there are notable differences, as intrusion-related Ag-PbZn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold
deposits and occurrences are associated with ilmenite-series plutons in interior settings and
form the basis for the reduced intrusion-related gold deposit class.
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Conclusions
Source and Redox of Tombstone -Tungsten Belt Intrusions
Within the mid-Cretaceous TGP plutonic suites, the youngest and most cratonward are
amongst the most metallogenically prolific, hosting and/or generating numerous gold, tungsten
and silver deposits. This 750-km-long belt of plutons is divisible into the Tombstone, Mayo
and Tungsten plutonic suites.
The Tombstone suite is alkalic, variably fractionated, slightly oxidized, contains magnetite
and titanite, and has primary, but no xenocrystic, zircon. The Mayo suite is sub-alkalic,
metaluminous to weakly peraluminous, fractionated, but with early felsic and late mafic
phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten
suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite
dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The
Tombstone suite was derived from an enriched, previously-depleted lithospheric mantle, the
Tungsten suite intrusions from continental crust including, but not dominated by, carbonaceous
pelitic rocks, and the Mayo suite from a similar sedimentary crustal source, but mixed with a
distinct mafic component from an enriched mantle source.
Each suite has a distinctive metallogeny that is related to the source and redox
characteristics of its magmas. The Tombstone suite has a Au-Cu-Bi association that is
characteristic of the most oxidized and alkalic magmas, but also has associated and enigmatic
U-Th-F mineralisation. The reduced Tungsten suite intrusions are characterised by world-class
tungsten skarn deposits with less significant Cu, Zn, Sn, and Mo occurrences. The Mayo suite
intrusions are characteristically gold-enriched, with associated As, Bi, Te, and W associations.
All suites have associated, but distal and lower temperature, Ag-Pb- and Sb-rich mineral
occurrences.
The distinctive lithological, geochemical and isotopic parameters that characterise
each suite result from varying proportions of magma from different sources, with lesser
mantle influence in more easterly plutons. These variations in source components influence
the oxidation state of the magma which, in turn, influences the nature of their associated
metallogeny. Magma composition, redox state and metallogeny are relatively consistent with
existing intrusion-related ore-deposit models. However, intrusion-related gold deposits are
only one component of the wide variation in granitoid composition and associated metallogeny
across this single, broadly coeval magmatic belt.
Integrated Geochronology and Duration of TTB Systems
New SHRIMP U-Pb and Ar-Ar data are combined with existing TIMS U-Pb dates and
recently published Re-Os dates for five TTB magmatic-hydrothermal systems including
Fort Knox (93.5-92.4 Ma), Clear Creek (93.6-91.7 Ma), Scheelite Dome (94.6-92.7 Ma),
Dublin Gulch (94.2-93.1 Ma), Mactung (96.4-95.2 Ma) and Cantung (96.6-93.9 Ma). The
data indicate ages that are up to four million years older than those first determined by TIMS
U-Pb of zircon analyses, suggesting that the latter may be inaccurate, likely in response to a
combination of a xenocrystic zircon component and Pb-loss from radiation-damaged zircons.
185
Chapter 6
Conclusions
Comparison of the dates from the different geochronological methods on different minerals
within a single system provide consistent and predictable results. The best zircon U-Pb
dates of the granitoids reflect magma crystallization and are slightly older than the Re-Os
molybdenite dates that reflect hydrothermal mineralisation. These Re-Os dates are however,
older than the Ar-Ar dates on hydrothermal mica phases, suggesting that the magmatichydrothermal system remained above the closure temperature of the micas (~250°C) for some
time (0.2-2 m.y.) after their crystallization.
Evaluation of most-robust geochronological data indicates that these magmatichydrothermal systems were active for as little as 1.1 m.y. or as long as 4 m.y. These durations
are similar to those defined geochronologically for other intrusion-related systems, but are
considerably longer than those indicated by thermal modeling. Episodic magmatism may be
the best mechanism to maintain thermal energy within these systems for longer periods and
explain this apparent discrepancy.
Final Remarks
Complexities in the intrusion-related gold system (IRGS) model, as presented by some
authors, result from too broad a perspective. Previous authors attempted to place a wide array
of gold deposits sited throughout the Tintina Gold Province into a single model, whereas
only those uncontested intrusion-related deposits of the Tombstone-Tungsten belt and the
Fairbanks district should be used to define a robust IRGS model (see Chapter 2). Only when
the critical features of the model are defined, should other deposits be considered for inclusion
in the model. A key to recognizing intrusion-related mineralization is the presence of a central
mineralising pluton, within and around which there are predictable spatial patterns of different
styles of mineral occurrences. The roles that such mineralizing plutons play in relation to
epizonal and shear-hosted gold mineralisation remain contentious, and require further study for
resolution.
There is considerable variation in the nature of magmatism and the nature of economic
significance of mineralisation associated with the mid-Cretaceous plutonic episode. The
most prospective mineralising plutons are those that result from emplacement of small
batch magmas and are partly unroofed, as opposed to large magma-volume batholiths that
were emplaced at deeper levels. Gold is associated with both primitive highly-oxidised
plutons and radiogenic relatively-reduced plutons, but the oxidised plutons have subordinate
gold associated with copper mineralisation, whereas reduced intrusions have a gold-only
paragenesis. This emphasizes the importance of the nature of the melt source and its redox
state to the metal associations of the magmatic-hydrothermal systems.
The most prospective plutonic suites for intrusion-related gold systems are those that were
emplaced furthest inboard of the subduction zone and proximal to the ancient cratonic margin.
The Tombstone-Tungsten Belt, is in this position, was the final product of a protracted 30
m.y.-long magmatic event, and was emplaced during weak extension. Significant along-strike
variations in the belt are the result of differing proportions of differing source components.
Extension in the central and western part of the belt promoted the involvement of deeper,
186
Chapter 6
Conclusions
more-enriched mantle melts to mix with, and hybridise crustal melts which dominate in the
eastern part of the belt. This mantle contribution essentially converted reduced Tungsten Suite
tungsten-associated granites into moderately-reduced Mayo Suite gold-associated granites in
the central part of the belt. That the mantle-derived component was gold-enriched is indicated
by the gold-enrichments in the most westerly, mostly mantle-derived Tombstone Suite. The
magmatic and metallogenic associations in the Tintina Gold Province should be tested in
similar tectonic settings and plutonic belts considered to be prospective for similar mineral
deposits, worldwide.
187
Chapter 6
188
Conclusions
Appendix
Appendix i
Geochonology Sample Locations
00CH MT-03
Mactung mine NTS 1050/8
Sample taken from mine dump
Zone 9 441815E 7017680N
Biotite-rich region in W-skarn
indicative of potassic alteration. Biotite
is coarse-grained and intergrown with
pyrrhotite.
00CH MT-14A
Mactung mine area, ~1.1 km north
of adit NTS 105O/8
Zone 9 447316E 7293548N
Muscovite in quartz-tourmalinemuscovite-molybdenite veins cutting
hornfels. Re-Os sample as well.
00CH MT-15
Mactung mine area, ~ 300m east
of adit NTS 105O/8
Zone 9 428103E 7017812N
Muscovite in greisen on quartztourmaline coated joints in Mactung
pluton.
00CH DG-1C
Dublin Gulch deposit NTS 106D/4
East side of upper Olive Gulch
Zone 8 462000E 7100850N
Coarse-grained biotite in quartzmolydenite vein in granite, same as ReOs sample.
00CH SD-1
Scheelite Dome NTS 115P/16
Sheeted zone on north side of gulch
Zone 8 437350E 7073650N
Coarse-grained muscovite in quartzmolybdenite vein in granite, same as
Re-Os sample.
02CH CT1-5
Cantung tungsten mine NTS
105H/16
Underground
Zone 9 539333E 6870553N
Coarse-grained biotite from quartz
biotite ±molybdenite veins cutting granite
and aplite. Age of f uid exsolution.
02CH CT 1-6
Cantung tungsten mine NTS
105H/16
Underground
Zone 9 5393333E 6870553N
Biotite from a coarse-grained
biotite skarn cross cut by pyrrhotite
and chalcopyrite veinlets. Age of skarn
mineralization.
02CH CT2-20
Cantung area, NTS 105H/16
Circular stock above Cantung open pit
Zone 9 538388E 6871405N
Coarse-grained muscovite from
muscovite-pyrrhotite greisens on fracture
surfaces on the Circular stock. Age of
mineralization, minimum age for granite.
02CH CT 2-21
Cantung open pit, NTS 105H/16
Zone 9 539231E 6870208N
Coarse-grained greenish sericite
developed in 1 cm diameter circular
fragment in quartz-pyrrhotite vein cutting
aplitic dyke at Cantung open pit. Age of
hydrous mineralization.
189
Appendix
Appendix 2
SHRIMP Data
&ORT+NOX0LUTON
Pmlq
CH+.*.
CH+.0*.
CH+./*.
CH+..*.
CH+.5*.
CH+.4*.
CH+/*.
CH+.3*.
CH+0*.
5PPM
4HPPM
/-1
1-.
034
.214
.212
..26
5/6
.425
46-
66
.5
2/
./2
.-1
52
./6
165
/5
/0/Qe
,/05R
-+2-+-2
-+.2
-+-5
-+-4
-+-5
-+.3
-+/6
-+-1
/-4`lo o
/-3M_,/05R
4c/-3 "
.+.36/
-+1.--+4101
-+-5/4
-+.2/-+/.4/
-+1631
-+/-16
-+004.
-+-.11/5
-+-.1124
-+-.12/.
-+-.121/
-+-.1333
-+-.1343
-+-.1423
-+-.15-0
-+-.154/
.σ
boolo
+---.21
+---.03
+---.1+---./2
+---./3
+---./4
+---.00
+---./4
+---.0/
/-4`lo o
/-3M_,/05R
6/+0 ı
6/+2 ı
6/+6 ı
60+. ı
60+6 ı
60+6 ı
61+1 ı
61+4 ı
62+/ ı
.+-+6
-+6
-+5
-+5
-+5
-+5
-+5
-+5
-EANÕ;=CONF
7TDBYDATAPTERRSONLYOFREJ
-37$PROBABILITY
0UKELMAN0LUTON#LEAR#REEKAREA
Pmlq
MH+2*.
MH+.-*.
MH+.-*/
MH+6*/
MH+..*.
MH+.0*.
MH+.1*.
MH+.2*.
MH.4*.
MH.4*0
MH.4*1
MH+.5*.
MH+.6*.
5PPM
4HPPM
..66
.-6.053
.615
.251
135
..44
./55
.-16
65.0/6
.46/
//24
/6/
/.05.
/1/
..3
66
/1.
.24
42
.04
/-5
20.02
/0/Qe
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-+/2
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-+.0
-+-5
-+//
-+/.
-+.0
-+-4
-+.1
-+.3
-+0.
-+-3
/-4`loo
/-3M_,/05R
4c/-3 "
-+-5/1
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-+.464
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-+./13
-+.30.
-+//34
-+.1./
.+22./
-+-320
-+./5-
-+-.2-0-+-.2-0-+-.1652
-+-.142.
-+-.12.-+-.120-+-.11.3
-+-.1363
-+-.1001
-+-.11.5
-+-.2//-+-.133/
-+-.1423
.σ
boolo
+---016
+---023
+---014
+---01+---010
+---010
+---002
+---01+---000
+---003
+---026
+---005
+---01-
/-4`loo
/-3M_,/05R
!GEÕσ
63+/ ı
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/+/
/+/
/+.
/+/
/+.
/+.
/+0
/+.
/+/
-EANÕ;=CONF
7TDBYDATAPTERRSONLYOFREJ
-37$PROBABILITY
$UBLIN'ULCH0LUTON
Pmlq
AD+.*.
AD+/*.
AD+0*.
AD+1*.
AD+2*.
AD+3*.
AD+4*.
AD+5*.
AD+6*.
AD+.-*.
AD+..*.
AD+./*.
190
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06.
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640
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52
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1+6-/6
.+4160
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.+22--
/-4`loo
/-3M_,/05R
-+-.146-+-.15--+-.131/
-+-.11/3
-+-.1340
-+-.1521
-+-.13.0
-+-.1243
-+-.1542
-+-.14/4
-+-.2-43
-+-.1540
.σ
boolo
+---0-5
+---021
+---04/
+---0.2
+---034
+---0/2
+---0/3
+---0.6
+---0..
+---/40
+---/64
+---/43
/-4`loo
/-3M_,/05R
!GEÕσ
61+3 ı
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60+4 ı
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62+/ ı
61+/ ı
63+2 ı
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Appendix
AD+.0*.
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-+-.16-4
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+---/36
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+---0-.
+---/4/
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-EANÕ;=CONF
7TDBYDATAPTERRSONLYOFREJ
-37$PROBABILITY
-ACTUNG0LUTON
Pmlq
JQ+6+1*.
JQ+6+.*.
JQ+6+/*.
JQ+2+.*.
JQ+3+0*.
JQ+5+/*.
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5PPM
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.413
.506
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6.
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/-4`loo
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-+-.131-+-.145/
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-+-.2-16
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-+---/25
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-+---/30
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-+---/36
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/-4`loo
/-3M_,/05R
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60+4 ı
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65+6 ı
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.+3
.+4
.+4
.+4
.+4
.+4
.+4
.+5
.+4
.+4
.+3
.+.
-EANÕ;=CONF
7TDBYDATAPTERRSONLYOFREJ
-37$PROBABILITY
#ANTUNG0LUTON
Pmlq
@Q+.*.
@Q+/*.
@Q+0*.
@Q+1*.
@Q+3*.
@Q+4*.
@Q+.-*.
@Q+./*.
@Q+.1*.
@Q+.2*.
@Q+.4*.
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-+.446
.+6-6.
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-+-.1324
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+---0/0
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-EANÕ;=CONF
7TDBYDATAPTERRSONLYOFREJ
-37$PROBABILITY
191
Appendix
Appendix 3
Argon-Argon data
Selected samples were processed for 40Ar/39Ar analysis of bioitite, muscovite or
sericite by standard mineral separation techniques, including hand-picking of clearest,
least altered grains in the size range 0.5 to 1 mm. Individual mineral separates were
loaded into aluminum foil packets along with a single grain of Fish Canyon Tuff Sanidine
(FCT-SAN) to act as flux monitor (apparent age = 28.03 ± 0.28 Ma; Renne et al., 1998).
The sample packets were arranged radially inside an aluminum can. The samples were
then irradiated for 12 hours at the research reactor of McMaster University in a fast
neutron flux of approximately 3x1016 neutrons/cm2.
Laser 40Ar/39Ar step-heating analysis was carried out at the Geological Survey of
Canada laboratories in Ottawa, Ontario. Upon return from the reactor, samples were split
into one or two aliquots of one to four grains each, and loaded into individual 1.5 mmdiameter holes in a copper planchet. The planchet was then placed in the extraction line
and the system evacuated. Heating of individual sample aliquots in steps of increasing
temperature was achieved using a Merchantek MIR10 10W CO2 laser equipped with
a 2 mm x 2 mm flat-field lens. The released Ar gas was cleaned over getters for ten
minutes, and then analyzed isotopically using the secondary electron multiplier system
of a VG3600 gas source mass spectrometer; details of data collection protocols can be
found in Villeneuve and MacIntyre (1997) and Villeneuve et al. (2000). Error analysis
on individual steps follows numerical error analysis routines outlined in Scaillet (2000);
error analysis on grouped data follows algebraic methods of Roddick (1988).
Corrected argon isotopic data are listed below, and presented as spectra of gas release
or on inverse-isochron plots (Roddick et al. 1980). Each gas-release spectrum plotted
contains stepheating data from one or two aliquots. Such plots provide a visual image of
reproducibility of heating profiles, evidence for Ar-loss in the low temperature steps, and
the error and apparent age of each step.
Neutron flux gradients throughout the sample canister were evaluated by analyzing
the sanidine flux monitors included with each sample packet and interpolating a linear fit
against calculated J-factor and sample position. The error on individual J-factor values
is conservatively estimated at ±0.6% (2σ). Because the error associated with the J-factor
is systematic and not related to individual analyses, correction for this uncertainty is not
applied until calculation of dates from isotopic correlation diagrams (Roddick, 1988). If
there is no evidence for excess 40Ar the regressions are assumed to pass through the 40Ar/
36
Ar value for atmospheric air (295.5) and are plotted on gas release spectra. Blank were
measured prior and after each aliquot and levels vary between 40Ar=2.5-3.6x10-7 nm,
39
Ar=4.2-13.3x10-9 nm, 38Ar=0.4-1.7x10-9 nm, 37Ar=0.4-1.7x10-9 nm, 36Ar=0.7-1.3x10-9 nm,
all at ±20% uncertainty. Nucleogenic interference corrections are (40Ar/39Ar)K=0.025±.005,
(38Ar/39Ar)K=0.011±0.010, (40Ar/37Ar)Ca=0.002±0.002, (39Ar/37Ar)Ca=0.00068±0.00004,
(38Ar/37Ar)Ca=0.00003±0.00003, (36Ar/37Ar)Ca=0.00028±0.00016. All errors are quoted at
the 2σ level of uncertainty.
192
Appendix
References
Renne P. R., Swisher C. C., Deino A. L., Karner D. B., Owens T. L., and DePaolo D. J.,
1998a. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar
dating. Chem.Geol. 145, 117-152.
Roddick, J.C., 1988. The assessment of errors in 40Ar/39Ar dating. In: Radiogenic Age and
Isotopic Studies, Report 2. Geological Survey of Canada, Paper 88-2, pp. 7–16.
Roddick, J.C., Cliff, R.A. and Rex, D.C., 1980. The evolution of excess argon in alpine
biotites — a 40Ar/39Ar analysis. Earth and Planetary Science Letters, 48: 185–208.
Scaillet, S., 2000. Numerical error analysis in 40Ar/39Ar dating, Chemical Geology v.162,
269–298.
Villeneuve, M.E. and MacIntyre, D.G, 1997. Laser 40Ar/39Ar ages of the Babine
porphyries and Newman Volcanics, Fulton Lake map area, west-central British
Columbia. In: Radiogenic Age and Isotopic Studies, Report 10. Geological Survey
of Canada, Current Research 1997-F, pp.131–139.
Villeneuve, M.E., Sandeman, H.A. and Davis, W.J., 2000. A method for the
intercalibration of U-Th-Pb and 40Ar/39Ar ages in the Phanerozoic. Geochimica et
Cosmochimica Acta, v. 64, p. 4017-4030.
193
Appendix
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./+6 63+55ı-+14+563+45ı-+33
3+563+50ı-+35
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./+1 63+20ı-+21
#(-4!-USCOVITE*:—.—%&
!LIQUOT!
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./
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!LIQUOT!
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!LIQUOT"
/0+150ı -+/0.4+340ı -+-22
.4+42-ı -+/-5
.4+312ı -+-64
.4+33-ı -+-54
194
0AGEOF
c06_ !PPARENT!GE
J^ `
%"&
.+1 64+-2ı /+1/
0/+6 62+33ı-+//+5 62+56ı-+54
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.+1
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1 62+15ı.+0/
//+4 62+0-ı -+0.+2 62+2-ı .+20+662+/3ı -+34
2+662+/5ı -+3.
Appendix
4ABLEOF!RGON$ATA
Mltbo
^
Slirjb 06>o
u.- *.. CC
03
>o,06>o
04
>o,06>o
05
>o,06>o
1-
>o,06>o
" 1- >R
>QJ
' 1- >o,06>o
#($'#"IOTITE*:—.—%
!LIQUOT!
/+1
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0
0+2
0+6
1+/
1+3
2
./
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#(3$"IOTITE*:—.—%
!LIQUOT!
/+1
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!LIQUOT"
-+-.0
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#(#4"IOTITE*:—.—%
!LIQUOT!
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c06_ !PPARENT!GE
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61+04ı-+561+12ı-+32
61+02ı-+4.
61+05ı-+02
61+06ı-+00
195
0AGEOF
Appendix
4ABLEOF!RGON$ATA
Mltbo
^
Slirjb 06>o
u.- *.. CC
03
>o,06>o
04
>o,06>o
05
>o,06>o
1-
>o,06>o
" 1- >R
>QJ
' 1- >o,06>o
#(#4"IOTITE*:—.—%
!LIQUOT!
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A!SMEASUREDBYLASERINOFFULLNOMINALPOWER7
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C%RRORSAREANALYTICALONLYANDDONOTREFLECTERRORINIRRADIATIONPARAMETER*
D.OMINAL*REFERENCEDTO&#43!.-A2ENNEETAL1&
!LLUNCERTAINTIESQUOTEDATσLEVEL
196
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Appendix
197
Appendix
198