University of Orléans, SEG - Student Chapter field trip report on mineral
deposits and geothermal activity within the basin and range province (Utah
- Nevada - Wyoming, USA)
During the academic year 2015-2016, the SEG-Student Chapter of Orléans University, has
organized a two weeks field trip in the “Basin and Range” Province (Utah-Nevada-Wyoming, USA)
with the support of several academic and industry sponsors. The aim was to broaden our outlooks and
to provide an international dimension to our young Student Chapter.
The main scientific objective of such a project was to study and better characterized the links
between (i) ore forming processes (mostly Au and Cu), (ii) post orogenic extension (e.g. emplacement
of a metamorphic core complex) and (iii) fluid circulation (e.g. active and inactive hydrothermal
systems). Thanks to the invaluable help of our American correspondents, we have chosen several places
in the Basin and Range Province. In addition, this field trip gave to us the opportunity to make
relationships between our Student Chapter and those of Utah and Nevada and to present our respective
works/researches in order to develop future scientific collaborations between universities and the mining
companies. Finally, it was also a great opportunity for the Master degree students to apply theoretical
concepts to real world operations.
The “Basin and Range” province
The “Basin and Range” Province is bounded eastward by the Wasatch fault in central Utah and
westward by the Sierra Nevada Mountain in western Nevada. This Province is characterized by northsouth trending faulted mountains separated by large flat valleys. This very particular landscape results
from a specific geodynamic evolution which can be summarized as follow:
(i)
From Late Jurassic to the end of Cretaceous, the convergence between the oceanic Farallon
plate and the North American plate increase and led to low angle subduction (Dickinson,
1976; Malavieille,1993) inducing a limited subduction-related magmatism. The resultant
shortening have generated crustal thickening which caused the Laramide orogen elevation
(Coney and Harms, 1984).
(ii)
At the early Eocene (~52 Ma), a drastic modification of the conditions (slow convergence
rate and/or the retreat of the oceanic subducted slab) lead to the oceanic plate break up,
causes a large reduction in east-west horizontal compressive stress. Thus, from ~50 to 20
Ma, kinematic of the subduction dynamic shifted toward a “normal subduction” and backarc extension (Zoback et al., 1981; Malavieille,1993) associated with an extensional collapse
in the “Basin and Range” Province. The gravitational collapse was then followed by a rapid
exhumation (dated between 50-20 Ma), which is characterized by the formation of many
Metamorphic Core Complexes (Dickinson, 2002; e.g. the Ruby Mountain, Fig.1). This
exhumation was accommodated by low angle normal faults and implied a crustal thinning
across the all Basin and Range Province. Locally, kilometric high-angle and listric faults
(e.g. the Wasatch Fault, Fig.1) controlled the structures and Neogene deposits of the basins.
Contemporaneous at this time, magmatism and volcanism appear in this Province.
Several mechanisms are responsible for the wide range of mineral deposits within the Basin and
Range Province (e.g. Alta skarn, Tintic mining district, Carlin type deposits, Fig.1). At the first
glance, geodynamic processes and related tectonic, seem to control deposits. In addition, magmatism
and related hydrothermal fluids played a key role in metals transportation (e.g. active hydrothermal
system of Yellowstone Park).
Thus, the famous Basin and Range Province allowed us to have an overview of this unique
geodynamic/tectonic setting and to better understand both the mechanisms and types of deposits related.
UTAH - Salt Lake City
Gold Quarry
Mine
Long Canyon
Mine
Ruby Mountain
MCC
Alta skarn
The Wasatch
fault view
Tintic Mining
district
Figure 1 : Location of the visited spots over Utah and Nevada.
Regional/local scale geological context and related ore deposits have been illustrated both on
the field and in the University of Utah (Salt Lake City) by Professor Erich Petersen, Professor J. R.
Bowman, D. A. Hedderly-Smith and the members of the Utah Student Chapter. On May 3, we had the
great opportunity to visit the Frederick A. Sutton building located in the Utah University, Salt Lake City
and had interesting shearing with Professor E. Petersen and the Student Chapter members of Utah. In
the morning, Professor J. R. Bowman introduced us the Alta copper skarn contact aureole and the related
problematics trough a talk illustrated by numerous samples (We could not go on the field because of the
snow).
The Alta stock contact aureole:
Figure 3 : Example of samples studied during the Lab work on the Alta stock contact aureole.
Figure 2 : Geological map of the Alta stock contact aureole showing isograds as it were originally been defined by
Crittenden, 1965 and Baker et. al., 1966.
The Alta Stock granodiorite is one of several mid-Tertiary plutons in Utah’s central Wasatch
Range located between Alta and Brighton. The stock has intruded and contact metamorphosed
Precambrian and Paleozoic sedimentary sequences made of quartzites, pelitic units and carbonate rocks
(Fig. 2). All along the morning, Professor J. R. Bowman takes the time to explain the geological context
of the stock emplacement and the metamorphic reactions occurring in the surrounding rocks. The
following reactions have been classified in terms of distinct metamorphic zones depending on the
distance to the contact (from the outer zone to the contact). In details, these reactions are bed controlled,
and several reactions may coexist in the same area thus questioning about fluids circulation in the
surrounding rocks and lithological control on the reactions. Hence, this lab works were very instructive
as we document the changes in mineral assemblage within carbonates as a consequence of progressive
metamorphism in the presence of fluids (Fig. 3).
Figure 3: example of samples studied during the Lab work on the Alta stock contact aureole
On the afternoon, a brief visit of the University of Utah Seismograph stations (Fig. 4) was
followed by a little field work on the Wasatch fault zone (Fig. 5).
Figure 4 : Pictures of Fault Lane Park and the University of Utah seismograph stations.
The Wasatch fault zone:
The Wasatch fault zone (Fig. 5) represent the eastern boundary
of the Basin and Range province and it is the longest active normal
fault in the United States (about 343km long). This fault zone
represents the largest earthquake risk for 80 percent of the Utah’s
population. Ten discrete segments have been identified (Machette and
al., 1991) including five active medial segments with Holocene slip
rates of 1-2 mm/year and with recurrence intervals of 2000-4000
years.
During the afternoon, we visited Fault Line Park in Salt Lake
City which is a playground for children, built just on the fault scarp
(Fig. 4). Then, we drove to south to observe the fault zone in the
landscape from G. K. Gilbert Geologic View Park (fig. 5). This visit
was very interesting both because it represents a major structure of the
Basin and Range system and provides a very good example of postorogenic extensional structure.
On May 4, we drove southward to work on the Tintic mining
district to see the well-developed alteration mechanisms. We focus on
Figure 5 : Location of the Wasatch
fault zone (after Duross et. al., 2016)
and a landscape view of the fault.
this area for two main reasons. First, it represents a good analogous of
the deeper Yellowstone geothermal activity and secondly, it was one
of the largest productive district in Utah.
The Tintic mining district:
The Tintic mining district, located to the south of Salt Lake City, was discovered in 1869 and expanded
quickly since the railroad arrival in 1878. This district became the second largest historic mining district
in Utah. The last mine operating, the Trixie mine in the East Tintinc district, was closed in 2002.
However some exploration activities persist.
The Tintic mining district is characterized by a thick Palaeozoic sequences which have been
deformed during the Cretaceous Sevier orogeny into large north-south trending anticlines and synclines,
and then, intruded calc-alkaline Oligocene and Miocene monzonitic stocks, dykes and sills and covered
by cogenetic volcanic rocks related to the southern caldera (Krakuhec and Briggs, 2006). Most rocks in
the district are hydrothermally altered to some degree. Based both on the geology and the distribution
of alterations and ore occurrence this district has been divided into four subdistricts (Krakuhec and
Briggs, 2006): (i) the Main subdistrict (Fig. 6) represents the biggest production area with the
exploitation of sub-vertical copper-gold-silver chimneys and sub-horizontal, carbonate-hosted, leadzinc-silver ore in the form of replacement deposits; (ii) the East subdistrict (Fig. 6) corresponds to thin
sheets of volcanics above Paleozoic host strata. Ore occurrences consist in carbonate-hosted lead-zincsilver replacements and high-sulfidation copper-gold veins; (iii) the Southern subdistrict is characterized
by a subeconomic porphyry copper system which was mainly exploited for copper, silver and lead in
high sulfidation veins; (iv) and finally, the North part hosts zinc-rich replacement ores.
During the first part of the day, we discovered
the breccia type deposit type from the Main district,
crossing the layered Oligocene volcanic rocks
(latite). Breccia features mostly include chimneys /
dikes composed of rounded clasts of different rock
types set in a fine-grained clastic matrix (Fig. 7).
They are characterized by angular and rounded quartz
derived from the deeper quartzitic deposit and by a
fine grained of quartz, volcanic rocks, pyrites and
chalcopyrites. Due to the rounded nature of such
clasts, they have been called pebble dikes. These
types of deposit are probably associated with a
strongly
explosive
mechanism
allowing
deep
material to go up at around 70km per hour. The rest
Figure 6 : Location of Main, East and Southwest Tintic
subdistricts and of ore bodies, after Krahulec and Briggs,
2006.
of the day, was devoted to the typical study of several
alteration processes that encountered in porphyry
copper deposit setting. At least six alteration types
could be recognized on the field: pervasive latite kaolinisation, Cambrian dolomite silification forming
jasperoid, plagioclase replacement by calcite, alunite alteration, propylitic alteration (QSP: Quartz
Sericite-Pyrite) and pyrophyllitic alteration. At the end of the day, we had the occasion to have a look
on drill core in order to see especially the pyrite-chalcopyrite-molybdenite veins related to the system.
Figure 7 : Illustration
of the chimneys and
pictures of the group.
May 5, dedicated to oral presentations in the University of Utah (Fig. 8), was the occasion to
introduce our SEG Student Chapter to the Utah’s members and to the researchers, and also to discuss
about scientific problematics addressed in each university and PhD.
Figure 8 : Group picture in the University of Salt Lake City
And so, Professor E. Petersen organized a morning of conferences involving several researchers
and students. Professor J. N. Moore made an interesting presentation on geothermal systems; Lihai Hu
(graduate assistant) presented an original application of LA-ICP-MS on the distribution of trace
elements in hair; M. Jorgensen introduced magnetotellurics as a useful tool for ore prospection; Wei Lin
(graduate assistant) presented the induced polarization effect; C. Jones, geysers geothermal field, S. J.
Hill talked about humic acid and bacterial controls on uranium and J. Porter made a presentation on the
famous Bingham Cu-Mo-Au porphyry. On our side, two PhD students presented a part of their works:
Sylvain Delchini introduced the use of Raman Spectroscopy on Organic Matter as a reliable method to
investigate thermal history of ore deposit and Vincent Roche presented his work that dealing with the
connexion between subduction dynamics and distribution of geothermal resources with the example of
the Menderes Massif in Turkey. Likewise, the academic advisor of the Orléans SEG Student Chapter,
Johann Tuduri, made a talk on the geodynamic setting of Neoproterozoic magmatic-hydrothermal ore
deposits in Morocco. We also had the occasion to discuss about some posters from both the University
of Utah and the University of Orléans (Fig. 8).
After a long driving day toward Nevada on May 6, we finally arrived at the campsite located at
the foot of the Ruby Mountain. Everything was generously organised by Franck Valli and one of his
colleague from Newmont Mining Corporation. On May 7, the day began at 5am, direction the Carlintype deposits until May 9.
NEVADA – Carlin-type Deposit
The Carlin Trend deposit and the Long Canyon Deposit are characterized by “hidden gold”.
Gold deposit is located in carbonates layers and may contained into an alteration level. These deposits
show an enrichment in Arsenio-pyrite, antimony, mercury, barium and pyrite. However, there is a
difference between the Carlin Trend and the Long Canyon deposits (Fig. 9). While the gold
mineralization are located in the carbonate slope units, in turbidites levels, slides or slumps (Carlin Trend
type), the Long Canyon deposit is characterized by carbonate platform formations (Cook, 2015). For the
first type (deep-water), the associated faults and fractured stratigraphic conduits for gold-bearing
hydrothermal fluids, provide an excellent recipe for Carlin-type gold deposits. Concerning the second
type, the transformation of the
carbonate platform in carbonate
karsts in shallow-water is the major
element to develop gold-host rocks.
Figure 9 : schematic cross section of passive
margin sedimentation showing Long Canyon
and Carlin units, after Cook and Corboy,
2004.
1) Long Canyon
Long Canyon (Fig. 10) area was discovered in 1991,
but its exploitation started in 2005. One more time, we
attached to understand the structural control of the location
of such deposits. In this site, we focus on the main
deformation which contains or localizes the gold deposit
(personal communication of Franck Valli). We have seen
different ductile structures such as metric to decametric
anticlines and synclines folds and boudinage in schists and
marbles levels. Here, brittle deformation is also welldeveloped. Metric damaged zone of faults, sometimes
filled of calcite and strongly altered, are observed during
the field (Fig. 11). Locally, we saw lamprophyres and few
hydraulic breccia.
Figure 10 : Long Canyon
geological map and cross
section after Smith et al.
2013.
Figure 11 : Pictures illustrating the different geological features seen on the field in the Long Canyon mining area.
2) Carlin Trend
The Carlin Trend deposit is the most prolific
gold production area in the North America since the
beginning of the exploitation in 1955. According to
Muntean et al. (2011), the Nevada’s Carlin Trend
gold deposit could be a magmatic-hydrothermal
origin, however there is still no consensus in the
scientific community. In this area, gold is generally
associated to Arsenio-pyrite. In addition, during the
geologic and structural presentation of the open pits,
we saw numerous incoherencies explaining the
current revision of the geologic and structural map
(Fig. 12 and 13). So, during the guided study we
focused our attention on the structural control of this
area to better characterize the gold precipitation (Fig.
12).
Figure 12 : Cross sections through the Carlin type deposits, after
Rhys et al., 2015.
Figure 13: Pictures from the Carlin Trend deposit open pit
Our last day in Nevada, May 9, was dedicated to the study of a great example of Metamorphic Core
Complex (MCC): The Ruby Mountain.
NEVADA – The Ruby Mountain (3500m): a Cenozoic MCC
This cross-section aims to study an “MCC” (Metamorphic Core Complex) and started with the
Eocene-Miocene sediments and finished with the metamorphic rocks dated Paleozoic (Fig. 14). In
addition, some of units are the equivalent of Carlin Trend formations and allows us to do the comparison
and study how geological units propagated them in space and why gold deposition is not found in Ruby
Mountain.
Figure 14: Genesis gold Mine and group picture with Franck Valli
from Newmont, members of the SEG Student Chapter of Nevada
and of Utah.
Figure 14 : Geologic map and schematic cross section of the Ruby Mountain core complex domain, after Howard, 2003.
The Ruby Mountain is qualified as MCC principally because of three major elements: (i) a major
ductile shear zone with development of mylonitic rocks, (ii) exhumation of middle-crust rocks with a
stretching lineation through an elongate dome, and (iii) an igneous and melting activity associated
(Wernicke, 1981). This type of structure took place in a back-arc extensional context.
Two major points was studied during this exercise: (1) the chronology of the events and (2) the
main structures accommodating the deformation.
(1) We could see two main generations of igneous rocks, a first group foliated and another none
deformed. There also is several composition and different geometries. Thus, with dating, two
main events are distinguish, ca 85 Ma and ca 29 Ma. The first shows rocks foliated and
metamorphism, with gneiss and migmatites. The second one is characterized by granites and
pegmatites. We could deduce that the subduction episode occur between 85Ma and 29Ma, and
with the type of igneous rocks, that the second event, at 29Ma correspond to an important crustal
thinning associated to crustal melting. In ending to this extensional event, some dykes took place
during the Miocene.
(2) Regarding the structural aspect, we observed shearing criteria (Fig. 15), all top to the west, in
agreement with the regional extension. An important point draws our attention, the Ogilvie
thrust. This structure is described as a thrust folded during the compression and segmented
during the extension. But on the field, it is not visible and the evidences of its existing are quite
weak. It is used to explain an unusual lithological series: Limestone (Ordovician-Devonian) /
Quartzite (Cambrian) / Limestone (Ordovician-Devonian).
Figure 15: Group picture looking toward the Ruby Mountain; top to the west shearing criteria (simple shear) close to the low
angle normal fault; pure shear inside the MCC.
Yellowstone National Park
The visit of the Yellowstone National Park was a great occasion to make the link between the
active geothermal system in the Yellowstone caldera representing what is happening at the surface level
and the Tintic copper porphyry system analogous of the deeper system.
The Yellowstone region was the birthplace of three huge eruptions since the last 2.1 million
years, each of them resulting in the formation of a large caldera. The most recent caldera forming
eruption occurred 640 000 years ago and created the Yellowstone caldera of 55km wide and 80km long
(Fig. 16). In such systems, the calderas are considered formed by upward movement of highly viscous,
dissolved-gas-enriched magmas coming from a shallow magma chamber. When rising, the magma
stresses the surrounding rocks and generate earthquake.
All along the two days of
work, we had the opportunity to
have a look on the Caldera walls,
which are exclusively composed
of rhyolitic rocks, and to discover
the many types of geothermal
manifestations within the most
recent caldera.
Hot springs are the most
common thermal features in the
park (Fig. 17 - Grand Prismatic hot
spring).
Those
thermal
manifestations come from water
Figure 16: USGS geological map of the Yellowstone National Park.
convection. Indeed, cooler water at
the surface seeps into underlying bedrock, become superheated at depth and rise back to the surface
through an open plumbing system. The geysers features (Fig. 17 - Old Faithful geyser) are also hot
springs but when the superheated water rise back to the surface; it encountered constrictions in the
plumbing system preventing a free circulation and creating overpressure in the vent.
The mudpots (Fig. 17 - Fountain Paint pot), mud volcanoes or sulphur caldron are also visible
in several places. In those cases, surface water is collected into shallow clay-rich depression heated by
deep forming steam rising through the ground. Sulfuric acid is produced by organisms at depth, when
raising it breaks down the surrounding rocks into clay.
Finally, the travertine terraces, as the Mammoth ones (Fig. 17), are formed when thermal water
rises into limestones. When it reaches the surface, the carbon dioxide is released and the calcium
carbonate transported into hot water is deposited forming travertine terraces.
Figure 17: Rhyolitic caldera edge, Grand Prismatic hot spring, Old Faithful Geyser, Fountain Paint Pot, Mammoth hot
spring travertine terraces, group picture close to Lower Falls.
The discussions, meetings, exchanges…
This field trip of two weeks allowed us to discover one of the most famous geological province
in the world: “the Basin and Range”, to understand the different geological processes related to this
province and the links with the mineralizations and to discussed with many geologists in various fields
(economic geology, geochemistry, structural geology…) both belonging to the industrial world and to
the research world (Fig. 18). We also had the opportunity to meet the members of the Geological Society
of Nevada with whom we could discuss of our different experiences (Fig. 18).
Figure 18: Illustration of the different exchanges and group fieldwork having occurred during the field trip.
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