JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 5
PAGES 977^1009
2014
doi:10.1093/petrology/egu013
Regional-scale Metasomatism in the Fortescue
Group Volcanics, Hamersley Basin, Western
Australia: Implications for Hydrothermal Ore
Systems
ALISTAIR J. R. WHITE*, RAYMOND E. SMITH, PATRICK NADOLL
AND MONICA LEGRAS
CSIRO EARTH SCIENCE AND RESOURCE ENGINEERING, AUSTRALIAN RESOURCES RESEARCH CENTRE, 26 DICK
PERRY AVENUE, KENSINGTON, WA 6151, AUSTRALIA
RECEIVED JULY 13, 2013; ACCEPTED MARCH 7, 2014
Mafic to intermediate volcanic rocks of the Fortescue Group form the
lowermost stratigraphic unit of the 100 000 km2 Hamersley Basin
on the southern margin of the Archaean Pilbara Craton, Western
Australia.These represent one of the oldest (2·8^2·7 Ga) known examples of a continental flood basalt sequence. A regional burial
metamorphic gradient extends across the basin from prehnite^pumpellyite facies in the north to epidote^actinolite greenschist facies in
the south and west. Superimposed on this metamorphic gradient, regional-scale metasomatism has affected extensive areas of the
Fortescue Group. Metasomatized mafic lavas are characterized by
well-developed assemblages dominated by pumpellyite^quartz or epidote^quartz associations. The mineral associations of metasomatic
domain types broadly match the distribution of metamorphic isograd
indicator minerals with a southward and westward increase in the
proportion of epidote. A continuum exists between least altered rocks
that preserve the regional metamorphic signature and the most intensely altered metasomatized rocks. Metasomatism is essentially
continuous over a stratigraphic strike length of 100 km and across a
strike width of 20 km. Regionally, metasomatically altered volcanic
rocks occur widely across the Hamersley Basin and its outliers, over
an area of some 450 km by 200 km. Metasomatic alteration is most
conspicuous in the lower-grade metamorphic zones because pumpellyite- and epidote-rich rocks are green to yellow^green in outcrop.
Whole-rock geochemical data indicate that metasomatism is associated with strong depletions in alkalis (Na, K, Li, Rb), alkali
earths (Mg, Sr, Be, Ba) and heavy first transition series metals
*Corresponding author. Telephone: þ61 8 6436 8735. Fax: þ61 8 6436
8555. E-mail: [email protected]
(Mn, Fe, Co, Ni, Cu, Zn), with a significant enrichment in Si.
Calcium shows more variable mobility. Such geochemical trends, particularly depletions in Fe, Mn and base metals (Co, Ni, Cu, Zn), together with the metasomatic mineral assemblages, are comparable
with those associated with the sub-sea-floor circulation of seawater,
particularly in relation to the metal-depleted root zones of base
metal deposits. Petrographic features indicate that the development
of metasomatic mineral associations post-dates the formation of regional metamorphic assemblages. Consequently, it is interpreted that
the hydrothermal fluid flowed through the buried pile after sufficient
time to allow for the metamorphic mineral assemblages to approach
equilibrium with ambient P^T conditions. The hydrothermal
depletion in Fe observed across the Fortescue Group mafic lavas is
intriguing given the abundance of iron ore in the overlying Hamersley
Group, and a possible connection cannot be ignored. Furthermore, the
scale of fluid flow observed in the Fortescue Group, which occurred
through zones of inherent permeability, such as vesicular and brecciated
lava flow tops, without the aid of major cross-cutting structures, has
significant implications for the size of metasomatic systems in other
mafic volcanic terranes, with potential consequences for the exploration
for hydrothermal mineral deposits.
KEY WORDS: Hamersley Basin; hydrothermal alteration; hydrothermal
ore deposits; low-grade metamorphism; metasomatism
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JOURNAL OF PETROLOGY
VOLUME 55
I N T RO D U C T I O N
Metasomatism, the chemical alteration of rocks through
interaction with fluids (Harlov & Austrheim, 2013), is an
important open-system process in the crust (Etheridge
et al., 1983). Metasomatism plays a fundamental role in regional metamorphic processes from the sea floor to deep
crustal settings (Bickle & McKenzie, 1987; Banerjee et al.,
2000; Engvik et al., 2011) and is commonly intimately
linked to the formation of hydrothermal ore deposits
(Einaudi et al., 1981; Carten, 1986; Powell et al., 1991).
Indeed, the haloes of metasomatic alteration around ore
deposits are valuable exploration indicators exploited in a
range of geological environments: zonation of potassic,
phyllic, argillic and propylitic alteration around porphyry
copper systems (Lowell & Guilbert, 1970); addition of
K2O, CO2, As and S around orogenic gold deposits
(Bierlein et al., 2000); addition of Sb and trends in oxygen
and sulphur isotopes around volcanic-hosted massive sulphide deposits (Large et al., 2001). Alteration haloes are
particularly useful for deposits hosted in mafic greenstone
terranes such as the Eastern Goldfields Superterrane of
the Yilgarn Craton, Western Australia (Eilu & Groves,
2001).
At a regional scale, however, the application of this technique relies on the ability to distinguish geochemical variations that are, or are not, associated with economic
mineralization, which in turn requires an understanding
of the background geochemical heterogeneity in the surrounding rocks. This is a particular challenge in large
mafic igneous provinces that can be subjected to hydrothermal alteration in a range of settings, many of which do
not result in economic mineralization. Different processes
in each of these settings will result in a distinct mineralogical and geochemical signature occurring at a range of
spatial scales.
The Fortescue Group, in the Hamersley Basin of the
Pilbara Craton, Western Australia, represents, along with
the Ventersdorp Supergroup in South Africa, one of the
oldest known continental flood basalt provinces on Earth.
The exceptional exposure, low degree of weathering and
general lack of cover across the Hamersley Basin make
the Fortescue Group one of the best locations to study the
petrology, metamorphism and metasomatism of a continental flood basalt province on the scale of hundreds of kilometres. In this study, we present field, petrological, and
geochemical data used to investigate the nature and origin
of regional-scale metasomatism in the mafic volcanic
rocks of the Fortescue Group. The scale and lithological
homogeneity of the mafic rocks, over a range of burial
depths, in the Fortescue Group affords an ideal opportunity to investigate the effects of a post-eruption, regionalscale hydrothermal system within a continental large igneous province.
Furthermore, the size, age and composition of the
Fortescue Group are comparable with those of the more
NUMBER 5
MAY 2014
intensely deformed and metamorphosed, and highly
mineralized greenstones of the Eastern Goldfields
Superterrane, Yilgarn Craton, Western Australia. The
Fortescue Group may, therefore, be considered broadly
analogous to the Eastern Goldfields Superterrane prior to
orogeny and mineralization. As such, the nature and scale
of metasomatism discussed here have important implications for the size of mineralized systems in other large igneous provinces.
GEOLOGIC A L S ET T I NG
The Fortescue Group is a sequence of ultramafic to felsic
volcanic rocks and associated sedimentary rocks up to
6·5 km thick, located on the southern margin of the
Pilbara Craton, Western Australia (Thorne & Trendall,
2001). It is the oldest component of the Mount Bruce
Supergroup, which makes up the depositional Hamersley
Basin, an essentially continuous basin that crops out over
an area of some 100 000 km2 (Fig. 1; Trendall, 1990). The
Mount Bruce Supergroup was deposited over an extended
period between c. 2·8 and 2·4 Ga (Arndt et al., 1991;
Trendall et al., 1998; Blake et al., 2004; Hall, 2005; Hassler
et al., 2011). The Fortescue Group is conformably overlain
by the widespread, 2·5 km thick, Hamersley Group, which
is characterized by banded iron formations and associated
major iron ore deposits, with subordinate shale and dolomite (Taylor et al., 2001).
Thorne & Trendall (2001) offered the most comprehensive study to date of the Fortescue Group. They provided
an integrated stratigraphic framework, summarized the
published geochronology and described the sedimentary
and volcanic rocks in terms of four tectono-stratigraphic
units, based on facies analysis, positions of unconformities,
and geochemical data: (1) Mount Roe Basalt and earlier
sedimentary formations (2·78 Ga); (2) Hardey Formation,
predominantly comprising sedimentary rocks (2·76 Ga);
(3) Kylena, Boongal, Tumbiana, Pyradie, Maddina and
Bunjinah Formations, which are primarily mafic^intermediate volcanic rocks (2·72 Ga); (4) Jeerinah Formation,
consisting mostly of argillite and fine-grained volcaniclastic rocks (2·70 Ga). Approximate age data are from Blake
et al. (2004). The Fortescue Group is typically interpreted
as having been deposited in an extensional tectonic setting
(Blake & Groves, 1987; Thorne & Trendall, 2001; Blake
et al., 2004), showing subaerial or non-marine affinities in
the northern Hamersley Basin, with marine conditions
inferred in the south (Arndt et al., 2001; Bolhar & Van
Kranendonk, 2007).
Mafic to intermediate flood lavas of tectono-stratigraphic unit 3 are widely distributed across the north side
of the Hamersley Basin in the Maddina Formation, with
lateral equivalents in the Bunjinah Formation in the central and southern parts of the basin (Fig. 1; Thorne &
Trendall, 2001). These formations, along with additional
978
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
Fig. 1. Simplified geological map of the Fortescue Group in the Hamersley Basin, showing the regional metamorphic isograds of Smith et al.
(1982) and the locations of the samples considered in this study. Metamorphic isograds are defined by the appearance of epidote (ZI^ZII) and
actinolite (ZII^ZIII), and the disappearance of pumpellyite (ZIII^ZIV) (After White et al., 2014).
mafic material from the Pyradie Formation, are the primary subject of this study. Throughout, the term ‘mafic’ is
applied to all volcanic rocks ranging from basalts through
to basaltic andesites.
The dominant style of lava flow emplacement is as extensive sheet flows, traceable along strike for several tens
of kilometres, and showing common development of columnar jointing. Flow tops in the northern part of the
basin are typically highly vesicular, implying subaerial or
shallow sub-aqueous eruption. In the south, there is extensive development of pillows and compound sheet flows
with a lower degree of vesicularity, implying eruption
under deeper water.
Although not considered specifically here, the Pyradie
Formation also contains significant proportions of ultramafic material including extensive sequences of differentiated flows with olivine and pyroxene cumulates with
both pyroxene and olivine spinifex upper zones, along
with homogeneous undifferentiated pyroxene spinifex textured flows. These are particularly well-developed in the
southern part of the basin between Paraburdoo and the
southeastern edge of the Rocklea Dome (Fig. 1).
Authigenic mineral growth in mafic volcanic rocks defines a regional metamorphic gradient that extends across
the Hamersley Basin from prehnite^pumpellyite facies in
the north to epidote^actinolite greenschist facies in the
south and west (Fig. 1; Smith et al., 1982). Four metamorphic zones are defined north to south: zone 1 (ZI),
prehnite^pumpellyite zone; ZII, prehnite^pumpellyite^
epidote zone; ZIII, prehnite^pumpellyite^epidote^actinolite zone; ZIV, (prehnite)^epidote^actinolite zone (Fig. 2).
Smith et al. (1982) estimated that metamorphic grade is
consistent with burial depths of 2^3 km in the north to
8^10 km in the south, as determined from reconstructed
cross-sections and published petrogenetic grids for lowgrade mafic rocks (Frey et al., 1991). White et al. (2014)
applied thermodynamic modelling of metamorphic and
metasomatic mineral assemblages to estimate pressure^
temperature (P^T) conditions. They estimated greater
burial depths of metamorphism in the north, at around
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JOURNAL OF PETROLOGY
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KARRATHA
ROEBOURNE
21°0'0"S
ROEBOURNE
21°0'0"S
KARRATHA
VOLUME 55
ZI
21°30'0"S
ZII
ZIII
XRD (wt%)
Epidote
<10
10-20
>20
PANNAWONICA
0 10 20 30 Kilometres
116°30'0"E
Qualitative petrography
Epidote-poor
Epidote-rich
Metamorphic isograd
Fortescue Group
Town
(a)
117°0'0"E
ZII
ZIII
0 10 20 30 Kilometres
117°30'0"E
116°30'0"E
ZII
22°30'0"S
XRD (wt%)
Pumpellyite
<10
10-20
>20
PANNAWONICA
ZIII
117°0'0"E
ZIII
PARABURDOO
117°30'0"E
TOM PRICE
23°0'0"S
23°0'0"S
ZIII
0 5 10 15 Kilometres
117°30'0"E
ZII
TOM PRICE
ZIV
Qualitative petrography
Pumpellyite-poor
Pumpellyite-rich
Metamorphic isograd
Fortescue Group
Town
(b)
22°30'0"S
21°30'0"S
ZI
XRD (wt%)
Epidote
<10
10-20
>20
Metamorphic isograd
Fortescue Group
Town
(c)
ZIV
ZIII
0 5 10 15 Kilometres
118°0'0"E
PARABURDOO
117°30'0"E
118°0'0"E
ZII
22°30'0"S
22°30'0"S
ZII
XRD (wt%)
Pumpellyite
<10
10-20
>20
Metamorphic isograd
Fortescue Group
Town
(d)
ZIII
ZIII
ZIV
ZIII
0 5 10 15 Kilometres
PARABURDOO
117°30'0"E
TOM PRICE
23°0'0"S
23°0'0"S
TOM PRICE
XRD (wt%)
Actinolite
<10
10-20
>20
Metamorphic isograd
Fortescue Group
Town
(e)
ZIV
ZIII
0 5 10 15 Kilometres
118°0'0"E
PARABURDOO
117°30'0"E
XRD (wt%)
Chlorite
<10
10-20
>20
Metamorphic isograd
Fortescue Group
Town
(f)
118°0'0"E
Fig. 2. Simplified geological maps of the Fortescue Group showing the distribution of epidote (a, c), pumpellyite (b, d), actinolite (e) and chlorite (f) in different metamorphic zones. High epidote and/or pumpellyite contents pick out regions of metasomatic alteration. Mineral proportions are semi-quantitative estimates of X-ray diffraction (XRD) spectra interpreted with the Bruker DIFFRAC.SUITE EVA software package.
5^6 km depth, as well as estimating consistent conditions
of metasomatism at depths of around 8^9 km across the
entire basin. This also involves a higher T metasomatic
overprint in the low-grade zones, and a lower T overprint
in the higher-grade zones. White et al. (2014) used this
to suggest that metasomatism occurred after, or synchronous with, regional deformation. Shibuya et al. (2010)
interpreted the regional metamorphic sequence as
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WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
stratigraphy-related, although this is incompatible with the
lowest metamorphic grades being observed in basal stratigraphic formations of the Fortescue Group in the northern
Hamersley Basin. In the north of the basin, the lower
metamorphic grades are related to higher structural levels
(i.e. shallower burial) than in the south, and are not due
to being at higher stratigraphic levels.
Structural deformation in the Hamersley Basin essentially comprises broad, open, east^west-trending folds that
intensify southwards with lesser faulting. Folding is interpreted to post-date development of regional metamorphic
mineral assemblages, as evidenced by the folding of metamorphic isograds in the southern portion of the basin
(Fig. 1). Faulting is restricted to sets of syn-Fortescue rift
faults that trend approximately ESE^WNW to east^west,
parallel to the strike of the basin stratigraphy (Thorne &
Trendall, 2001). These faults are laterally extensive, stretching right across the Hamersley Basin, although they do
not significantly cross-cut stratigraphy and there is a notable absence of the subvertical shear zones commonly
observed in higher-grade mafic terranes.
Superimposed on the ambient burial metamorphic paragenesis is widespread, pervasive metasomatic alteration
characterized by marked bulk compositional change and
primarily identified by the development of epidote^
quartz^titanite or pumpellyite^quartz^titanite rocks
(Smith et al., 1982). What were once individually homogeneous lavas are now markedly heterogeneous in outcrop
appearance, metamorphic mineral association and chemical composition. Here we discuss the nature, origin and
implications of this regional-scale metasomatism.
E P I D O T E , P U M P E L LY I T E A N D
M E TA S O M AT I S M I N M A F I C
RO C K S
Epidote and pumpellyite occur in a wide variety of mafic
rocks. Both minerals are commonly formed during regional burial metamorphism. Progressive zonings of authigenic prehnite, pumpellyite, epidote, albite and actinolite
have been widely described since Coombs (1960) and
Packham & Crook (1960) established what is referred to
as the prehnite^pumpellyite facies of low-grade metamorphism. Well-documented examples of these minerals
in low-grade metamorphic terranes include the Taveyanne
Formation of western Switzerland (Coombs et al., 1976;
Schmidt et al., 1997), the Keweenawan Supergroup of
Minnesota, USA (Pumpelly, 1873; Van Hise & Leith, 1911;
Stoiber & Davidson, 1959; Jolly & Smith, 1972; Schmidt,
1993), the Karmutsen volcanics of British Columbia
(Starkey & Frost, 1990), as well as the Fortescue Group
itself (Smith et al., 1982). In many sequences the minerals
mentioned above form associations that are linked to general depth of burial, some sequences showing grades
ranging from zeolite facies through prehnite^pumpellyite
facies to greenschist facies. Although overlapping the
range shown by pumpellyite, epidote typically occurs at
higher grades around the transition to greenschist facies
and above.
Epidote, and to a lesser extent, pumpellyite, is very
common in mafic rocks that have undergone some degree
of hydrothermal alteration through their interaction with
particular fluid types. Mafic lavas can be subjected to
fluid interaction and metasomatism in a broad range of
geological settings, including, but not limited to, sea-floor
alteration (Donnelly, 1966; Humphris & Thompson, 1978a,
1978b), sub-sea-floor circulation (Evarts & Schiffman,
1983; Alt et al., 1986; Banerjee et al., 2000; Banerjee &
Gillis, 2001), cooling-related deuteric alteration (Raam
et al., 1969) or metasomatism during low-grade (Smith,
1968) or high-grade (Engvik et al., 2011) metamorphism.
Alteration of mafic rocks through interaction with seawater results in various mineralogical and geochemical
changes dependent upon the precise temperature and
fluid regime at the time of alteration. Element mobility, as
both enrichment and depletion, has been documented for
most major, minor, and commonly considered trace elements (Hart et al., 1974, 1999; Humphris & Thompson,
1978a, 1978b; Teagle & Alt, 2004; Nakamura et al., 2007).
Greenschist-facies rocks dominated by chlorite, with lesser
epidote, formed at elevated temperatures below the sea
floor, commonly show depletions in Si and Ca, with variable behaviour of alkalis and Mg (Humphris &
Thompson, 1978a; Teagle & Alt, 2004). In contrast, more
epidote-rich subtypes commonly display increased Ca and
a higher Fe-oxidation ratio, with decreased Mg
(Humphris & Thompson, 1978a).
Mafic rocks that have undergone intense epidotization,
pumpellyitization and silicification have been described
from a range of locations and geological settings, such as
in flood lavas of the Keweenawan Supergroup of
Michigan, USA (Stoiber & Davidson, 1959; Jolly &
Smith, 1972), massive Ordovician mafic lavas of New
South Wales, Australia (Smith, 1968), the Noranda District
of the Abitibi region in Quebec, Canada (Jolly, 1980;
Gibson et al., 1983; Lesher et al., 1986; Hannington et al.,
2003), ophiolite sequences such as the Troodos ophiolite of
Cyprus (Evarts & Schiffman, 1983; Richardson et al., 1987;
Jowitt et al., 2012), the modern Tonga forearc (Banerjee
et al., 2000; Banerjee & Gillis, 2001) and many others (e.g.
Galley, 1993). Such rocks, occasionally termed ‘epidosites’,
are characterized by epidote/pumpellyite þ quartz þ titanite assemblages, where the primary igneous texture has
typically been partly or largely destroyed. Such epidoterich and pumpellyite-rich rocks occur in a range of
morphologies, from patchy replacements of massive lavas
(Smith, 1968), pillows (Vallance, 1969) and amygdaloidal
or brecciated flow tops, through localized sheets and pipes
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JOURNAL OF PETROLOGY
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(Richardson et al., 1987) to semi-conformable, regionalscale alteration zones (Stoiber & Davidson, 1959; Smith
et al., 1982; Galley, 1993; Hannington et al., 2003).
Since the 1980s, those rocks that have undergone intense
epidotization and silicification have commonly been interpreted as representing hydrothermal upflow zones associated with the circulation of seawater above volcanic
centres, as determined by both geological context and
stable isotope data (Gibson et al., 1983; Lesher et al., 1986;
Richardson et al., 1987; Banerjee et al., 2000). Alteration is
commonly localized along once-permeable flow zones
such as vesicular or brecciated flow tops (Dickinson, 1962;
Lesher et al., 1986) and can be traced well beyond single
upflow centres forming district-scale convective hydrothermal circulation (Hannington et al., 2003). In some cases,
epidote-rich rocks like those discussed here are closely
linked to base metal deposits and are thought to represent
the now metal-depleted source zone (Lesher et al., 1986;
Richardson et al., 1987; Jowitt et al., 2012).
Epidote-rich rocks in these settings would typically require the addition of Ca, or substantial redistribution of
Ca, through a Ca-rich fluid. Such a Ca-rich, highly saline
fluid has been documented in fluid inclusions associated
with seawater alteration of Palaeoproterozoic basaltic andesites (Gutzmer et al., 2003) and in the Reykjanes hydrothermal system, Iceland (Hardie, 1983). In this case, the
fluid was derived from seawater through intense modification following interaction with the surrounding host-rock.
Alternatively, Smith (1968) interpreted decimetre-scale
epidote-rich and pumpellyite-rich domains in mafic lavas
of New South Wales, Australia, as forming through redistribution of major elements during prograde, regional,
low-grade metamorphism. Jolly (1980), in a similar
manner, demonstrated likely formation of epidote-rich
and associated albite-bearing domains from an Abitibi location on the Ontario^Quebec border, Canada, largely by
redistribution of major elements, augmented in part by
addition of Ca together with subordinate loss of Si.
F I E L D R E L AT I O N S H I P S O F
M A F I C L AVA S A N D
M E TA S O M AT I C A LT E R AT I O N I N
T H E F O RT E S C U E G RO U P
Detailed field relationships of metasomatic alteration are
best seen in the Maddina Formation of the Fortescue
Group in the northern Hamersley Basin. Here, lava flow
units are commonly around 10 m thick (ranging from 1 to
100 m) and dip gently to the south at 58. Single flows are
traceable along strike for several tens of kilometres
(Fig. 3). In contrast, in the southern Hamersley Basin, steeper dips and more intense folding result in less extensive
along-strike outcrop of lava flows in the Bunjinah
Formation.
NUMBER 5
MAY 2014
Metasomatic alteration largely follows the volcanic stratigraphy, particularly along inferred once-permeable flow
zones such as originally vesicular flow tops and flow-top
breccias, and is essentially continuous over a strike length
of 100 km (Fig. 3). In total, sporadic metasomatic alteration
occurs widely across the Hamersley Basin and its outliers,
over an area of some 450 km by 200 km (Fig. 1). A generalized profile of a hydrothermally altered lava flow is shown
in Fig. 4 and is similar to those proposed for altered portions of the Keweenawan Supergroup (Jolly & Smith,
1972; Smith, 1974). The model profile essentially comprises
a flow base with a ‘least altered’ homogeneous central
layer with progressive upward alteration, through a ‘transition’ layer to a highly altered flow top.‘Least altered’ here
refers to those rocks that have not undergone extensive
metasomatism, but does include all regional metamorphic
mineralogical variation, which will be discussed in the following section.
The least altered rocks are dark grey in colour, massive
and typically fine-grained. Coarser-grained varieties do
occur, particularly in metamorphic zones III and IV
where a progressive overprinting of amphibole needles
and blades can be clearly seen. The transition layer is characterized either by distinct millimetre-scale white spots of
prehnite or a more general pale mottling. The intensity of
spotting is heterogeneous at the outcrop scale and there is
a close association with jointing in the lavas, whereby spotting is concentrated along joints and fractures, particularly
columnar cooling joints. Metasomatic alteration of the
flow tops is particularly conspicuous in metamorphic
zones I and II, and parts of zone III owing to the development of green to yellow^green pumpellyite- and epidoterich rocks. Single epidote and pumpellyite crystals are
only rarely seen infilling former vesicles. In contrast to the
lower-grade zones, in zone IValteration is virtually unrecognizable in the field (see discussion below). Altered rocks
show variable relief, with weathering commonly taking advantage of fracturing, brecciation and original vesicularity.
In contrast, areas that have undergone high degrees of silicification are highly resistant to weathering, as are many
of the least altered massive layers.
In addition to the dominant semi-conformable alteration, metasomatism also occurs, to a lesser extent, in discordant features characterized by (1) localized areas of
multiple thin (2^10 m), completely altered lava flows, (2)
equant areas of thoroughly altered amygdaloidal rock,
and (3) kilometre-scale regions hosting numerous metasomatized lava plugs between 1 and 100 m in diameter.
Although field relations are not obvious, these plugs are
tentatively interpreted as volatile escape channels. In each
of these cases, metasomatism is focused on areas of abundant original vesicularity, flow top brecciation, or, for the
plugs, vesicularity and fracturing that form an annulus
around each plug. Figure 3 illustrates how metasomatically
982
1 0 2 4 6 8 10 Kilometres
2°
5°
21°30’S
983
117°00’E
Dip of lava flow units
Railway
Roads/Tracks
River courses
Flow unit used in Fig. 9c/9d
117°00’E
One
sheet
flow
A
B
117°30’E
117°30’E
Basaltic-andesite with abundant ‘plug-like’ features.
Metasomatically altered flow tops of flood lavas.
Massive ‘least altered’ basalt, basaltic-andesite and
andesite lava flows. A more certain than B.
Basaltic-andesitic lava flows with pronounced jointing.
Numerous thin basaltic-andesitic and basaltic lava flows.
Most have metasomatically altered flow tops.
Andesitic and dacitic lava flows.
Dacite, rhyodacite, rhyolite flows and volcanic breccias.
Many are possibly acid plugs and domes.
2°
21°30’S
21°45’S
21°45’S
Fig. 3.. A geological map of mafic lava flows of the Maddina Formation in the northern Hamersley Basin (for location see Fig. 1) illustrating the relationships between metasomatism and flow
morphology. Single flows can be traced along strike for tens to hundreds of kilometres, and metasomatism is generally conformable to the volcanic stratigraphy, occurring predominantly along
flow tops. Developed after Smith et al. (1978).
116°30’E
116°30’E
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
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Flow top breccia
Prominent relief where silicified
Pumpellyite-quartz/
Epidote-quartz
Prehnite spots
One flow unit
10 – 50 m
Soft, easily eroded topographic low
Altered
flow top
Transitional
layer
‘Least altered’
metamorphic assemblages
Massive
layer
Prominent relief
Flow base
Fig. 4. Schematic profile of a metasomatically altered lava flow,
showing a massive, homogeneous ‘least altered’ base and a progressively metasomatized flow top.
altered rock accounts for as much 50% of the total rock
volume in the mapped area (2500 km2 in size).
P E T RO G R A P H I C
R E L AT I O N S H I P S
The petrology and mineralogy of the Fortescue Group volcanic rocks, particularly regarding regional metamorphic
parageneses and characterization of the four metamorphic
zones, have been briefly described by Smith et al. (1982),
and we expand upon these here. First, the regional metamorphic mineral assemblages in the least altered rocks
will be discussed; second, the mineral associations in metasomatically altered rocks will be considered.
Metamorphic mineral assemblages
Smith et al. (1982) described four regional metamorphic
zones across the Hamersley Basin, defined by distinct isograds: epidote-in (Z1^ZII), actinolite-in (ZII^ZIII), and
pumpellyite-out (ZIII^ZIV). These parageneses acts as a
framework for discussing the regional metamorphic mineral
assemblages. In all least altered, mafic lava samples, a primary igneous, holocrystalline to microlitic texture is
visible, despite any metamorphic overprint (Fig. 5).
NUMBER 5
MAY 2014
Randomly oriented subhedral laths of albite (typically up
to 0·5 mm, rarely up to 2 mm) form a dominant framework,
whereas anhedral crystals of augitic pyroxene (in zones I
and II; Fig. 5a and b) or a felt of subhedral actinolite needles
and blades (in zones III and IV; Fig. 5c and d) are the
major ferromagnesian phases along with patches of very
fine (micron-scale) chlorite flakes. Substantial interstitial
K-feldspar and quartz, often intergrown, along with minor
titanite, trace prehnite and rare calcite, are now present.
Quartz and titanite are most probably metamorphic
phases. It is inferred that the original igneous mineralogy
comprised dominant plagioclase and clinopyroxene, along
with primary K-feldspar and Fe^Ti oxides (Fig. 6a).
Sulphides are often present, usually as small (less than
1mm, rarely up to 6^8 mm), scarce crystals of pyrite, pyrrhotite or chalcopyrite, with lesser sphalerite, galena or
pentlandite, all randomly distributed amongst the groundmass phases. Barite occasionally occurs in veinlets or as a
fracture fill. Traces of albite also occur in fracture veins, as
replacement of volcanic glass, and as epitaxial overgrowths
on other albite (formerly plagioclase) crystals in the
matrix. Other accessory phases include ilmenite, rutile,
zircon, apatite and monazite.
Feldspar phenocrysts are albitized and contain abundant
inclusions of chlorite, and pumpellyite (in zones I^III) or
epidote (zones II^IV), occupying 50% of the feldspar
lath volume (Fig. 5). Occurrences of small oligoclase or
andesine patches within albitized phenocrysts are occasionally seen in higher-grade zones where metamorphic
albitization has not gone to completion. Rarely, traces of
white mica (sericite) are also present. Patches of chlorite,
pumpellyite or epidote also occur with quartz in the interstices, whereas large patches of fine chlorite often
form around pyroxene and amphibole crystals. An appropriate reaction for the formation of this inclusion assemblage is
anorthite in plagioclase þ clinopyroxene þ ilmenite
þ H2 O ¼ chlorite þ pumpellyite
þ titanite þ quartz:
The albites with inclusions are interpreted to represent
albitization and re-equilibration of a former, more calcic
plagioclase during metamorphism. The interpreted equilibrium assemblage is represented on an ACF diagram by
a two-phase pumpellyite^chlorite tie-line, with the bulkrock composition balanced by relic clinopyroxene (Fig.
6b). The involvement of an Fe^Ti oxide phase is supported
by a general decrease in Mg-number for the chlorite and
pumpellyite products compared with the reactant clinopyroxene. The origin of fluid (water) in the above reaction
is equivocal although low-grade metamorphism is almost
exclusively a hydrous process (Smith et al., 1982) with fluid
supplied from within the rock package or from some meteoric source.
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REGIONAL METASOMATISM, FORTESCUE GROUP
Pmp
Chl
Ab
Chl
Cal
Qtz
Ms
Ms
Qtz
Di
Ksp
Ab
Di
Ksp
Pmp
30 μm
(a)
80 μm
(b)
Ksp
Chl
Act
Ms
Qtz
Ab
Ab
Ep
Ep
Qtz
Act
Chl
Ksp
(c)
400 μm
(d)
200 μm
Fig. 5. EDS X-ray maps of least altered samples from zones I to IV. All show a relic igneous texture with a metamorphic overprint. Feldspar
phenocrysts are albitized and contain abundant inclusions of chlorite and pumpellyite^epidote. Rocks from zone I [(a) sample HM-20G, and
(b) sample HM-22A] contain relic clinopyroxene, whereas rocks from zones III and IV [(c) sample HM-29H, and (d) sample HM-12C, respectively] contain metamorphic actinolite. Mineral abbreviations are those of Kretz (1983).
Epidote may form directly in more oxidized lithologies
through a similar reaction to the one above, but that includes reactant magnetite or another oxidized phase. The
presence of pumpellyite versus epidote in the least altered
rocks of zones II and III is interpreted to be a function
both of metamorphic grade, with a preference for epidote
at higher temperatures, and of bulk-rock oxidation ratio
(Fig. 6c).
In metamorphic zones III and IV, actinolite becomes
the dominant ferromagnesian phase, along with chlorite.
Actinolite typically forms needles 0·1^1mm in length, although in zone III it is commonly anhedral and apparently pseudomorphs clinopyroxene (Fig. 5c). With
increasing grade, into zone IV, the actinolite overprint becomes more intense, with more subhedral actinolite needles developing (Fig. 5d). Actinolite is commonly
associated with large patches of fine-grained chlorite that
in rare cases appear to reflect alteration and breakdown
of the actinolite. This may represent either the very early
stages of the metasomatic-type alteration discussed below,
or a low-T retrograde metamorphic alteration.
Prehnite is typically absent in the least altered rocks, although it does rarely appear in very prehnite-rich domains
(centimetre-scale) at the expense of other calcic phases
(Fig. 6f). Prehnite predominantly forms as a replacement
of plagioclase, and to a lesser extent clinopyroxene (Fig. 7).
It preferentially replaces coarse-grained (42 mm) plagioclase phenocrysts (Fig. 7a). In these cases, smaller feldspar
laths in the groundmass show significantly less prehnite replacement. Patches of relic calcic plagioclase are occasionally found amongst the prehnite, although they are
generally too small for reliable analyses. It is interpreted,
therefore, that the formation of prehnite is a response to
local increases in bulk Ca content. The large feldspar
phenocrysts may represent liquidus phases that are more
calcic than those plagioclase crystals in the groundmass.
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Al2O3 + Fe2O3 - Na2O - K2O
MAY 2014
A
Ms
Ms
+ Qtz
+ Ab
+ Ksp
+ H 2O
An
Ep
Prh
An
Ep
Mag
Mag
Pump
Prh
Pump
LA
Chl
LA
+ Qtz
+ Ab
+ Ksp
+ Ttn
+ H2O
(a)
Chl
(b)
CaO Cal
Di
Fe-ox FeO
Act
A
Ms
Di
Fe-ox F
Act
A
Ms
+ Qtz
+ Ab
+ Ksp
+ Ttn
+ H2O
+ Qtz
+ Ab
+ Ksp
+ Ttn
+ H2O
Mag
An
Ep
Mag
An
Ep
Prh
C Cal
+ MgO
+ MnO
Prh
Pump
Pump
LA
LA
Chl
Chl
(d)
(c)
C Cal
Di
Fe-ox F
Act
C Cal
Di
A
A
Ms
Ms
+ Qtz
+ Ab
+ Ksp
+Ttn
+ H2O
An
Ep
Prh
+ Qtz
+ Ab
+ Ksp
+ Ttn
+ H2O
An
Ep
Mag
Prh
Pump
LA
Mag
Pump
LA
Chl
(e)
C Cal
Fe-ox F
Act
Chl
(f )
Di
Act
Fe-ox F
C Cal
Di
Act
Fe-ox F
Fig. 6. ACF compatibility diagrams showing typical assemblages in different metamorphic zones. (a) Inferred original igneous assemblage.‘Feox’ probably includes Ti (ilmenite). (b) Zone I and II assemblages are represented by a Pump^Chl tie-line, with the bulk-rock composition
balanced by relic clinopyroxene. (c) Zone II assemblage containing epidote in place of pumpellyite. (d) Pumpellyite- or epidote-bearing zone
III assemblages where relic clinopyroxene is replaced by actinolite. (e) Zone I assemblage for prehnite-rich domains. (f) Metasomatically altered
assemblages containing both epidote and pumpellyite with chlorite (zones I and II) or actinolite (zones III and IV) are not stable until high degrees of metasomatic alteration. The arrow represents the alteration vector. In all diagrams, the square marked ‘LA’ represents an approximate
bulk composition for a least altered rock. Mineral abbreviations are those of Kretz (1983).
986
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
Ab
Qtz
Pl
Prh
Ksp
Pl
Cpx
Cal
Cpx
Chl
Ms
Ksp
Prh
Cal
(a)
Ab
30 μm
(b)
Qtz
80 μm
Fig. 7. EDS X-ray maps of prehnite-rich domains within least altered samples. (a) A large feldspar lath extensively replaced by prehnite and
albite with inclusions of chlorite and sericite (sample HM-23C). It should be noted how smaller feldspar laths in the groundmass contain significantly less prehnite. (b) Partially prehnite-replaced clinopyroxene and plagioclase feldspar, surrounded by prehnitized glass in a segregation
vesicle (sample HM-20G). In both images, small relic patches of calcic plagioclase are present. Mineral abbreviations are those of Kretz (1983).
Metasomatically altered mineral
assemblages
The end-members of metasomatic alteration in the
Fortescue Group volcanic rocks are essentially pumpellyite^quartz or epidote^quartz rocks. However, there
exists a complete continuum between these and the least
altered samples, reflecting the progressive removal of
metamorphic phases and accompanying bulk compositional change (Fig. 8). Geochemical change during alteration is significant and is discussed in detail in the
following section.
As shown in Fig. 4, there exists a transition zone between
the least altered flow bases and the more highly altered
flow tops. This transition zone is characterized by visible
‘spots’ of prehnite. These are clots of fine-grained (10^
20 mm) prehnite that occur sporadically throughout the
rock. In many cases the prehnite is clearly nucleated on
coarse albitized plagioclase feldspar phenocrysts, as in
Fig. 7a, although prehnite growth extends well beyond the
boundaries of the original phenocryst, forming patches up
to a few millimetres across (Fig. 8a). Prehnite also develops
extensively along narrow fractures and quartz^albite veinlets, indicating a direct link with fluid activity. The persistence of prehnite with continued metasomatism is limited
and prehnite spotting gives way to more altered lithologies.
In zones I and II, early stages of alteration in flow tops
are marked by the complete removal of relic clinopyroxene, which is replaced by pumpellyite (Fig. 8b).
Clinopyroxene is metastable with respect to the metamorphic assemblage and it is likely that the addition of
fluid simply allows the clinopyroxene-consuming reaction
to go to completion, irrespective of any metasomatic mass
transfer. All plagioclase feldspar is completely albitized by
this point and no traces of relic calcic plagioclase are
observed. These rocks remain a dark grey colour in the
field and are rather inconspicuous and difficult to identify
without petrographic or geochemical analysis.
Further metasomatism and alteration leads to continued
growth of pumpellyite and/or quartz with progressive loss
of albite and K-feldspar (Fig. 8c). Variable amounts of
chlorite may be present along with traces of titanite, calcite
and sulphides. The presence of pumpellyite versus epidote
broadly follows the regional metamorphic zones, with
pumpellyite generally restricted to zones I^III and epidote
in zones II^IV (Fig. 2). These rocks have undergone extensive recrystallization and very few, if any, traces of the original igneous texture remain. Pumpellyite typically
occurs as masses of fine-grained (10^20 mm) anhedral crystals in the groundmass, intergrown with quartz, whereas
epidote is commonly coarser-grained (up to 0·5 mm).
However, euhedral crystals of fine pumpellyite, or coarse
epidote, commonly grow into quartz-filled amygdales.
Progressive metasomatism beyond this continues to increase the proportion of epidote and/or pumpellyite and
remove other phases, including chlorite, until essentially
pure pumpellyite^quartz or epidote^quartz assemblages
remain, possibly with trace amounts of titanite, rutile, calcite or apatite (Fig. 8d).
The same general alteration paragenesis is observed in
zones III and IV, although with a preponderance of epidote over pumpellyite (Fig. 2). Additionally, actinolite commonly persists through to rather intense levels of
metasomatism (Fig. 8e), often after chlorite has been
reacted out (Fig. 8f). This corresponds to a crossing of the
epidote^actinolite tie-line in Fig. 6 and requires a marked
change in bulk-rock composition (Fig. 6f). However,
actinolite is also ultimately reacted away, as shown by its
partial replacement by both epidote and pumpellyite
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Chl
Di
Ab
Pmp
Prh
Ab
Chl
Qtz
30 μm
(a)
Qtz
Ksp
30 μm
(b)
Qtz
Chl
Qtz
Ep
(c)
200 μm
Pmp
200 μm
(d)
Qtz
Ap
Qtz
Pmp
Ap
Act
Chl
Ep
Pmp
Act
(e)
200 μm
(f)
200 μm
Fig. 8. A photomicrograph and EDS X-ray maps of progressively metasomatically altered rocks. (a) Photomicrograph of a prehnite-spotted
rock with patches of prehnite growing outwards from large, prehnite-replaced plagioclase phenocrysts (sample HM-20C). (b) Partially altered
rock in which pumpellyite has replaced all relic clinopyroxene, but albite and lesser amounts of K-feldspar are still present (zone I, sample
HM-23B). (c) Strongly altered rock dominated by pumpellyite with lesser amounts of chlorite (zone I, sample HM-21D). (d) Intensely altered
rock containing only epidote and quartz (zone II, sample 10044B). (e) Strongly altered rock that still contains abundant actinolite (zone III,
sample HM-29D). (f) Intensely altered rock that still contains minor amounts of actinolite, which is being replaced by epidote and pumpellyite
(zone IV, sample HM-12A). Mineral abbreviations are those of Kretz (1983).
988
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
in Fig. 8f. The abundance of actinolite and generally colourless epidote in many of these rocks means that they
lack the distinctive yellow^green colour seen in zones I
and II. This is particularly true in zone IV, where pumpellyite is essentially absent in all but rare cases (Fig. 2).
Consequently, metasomatic alteration becomes increasingly difficult to identify in the higher-grade zones.
Ultimately, progressive alteration does not require a change
in mineral tie-lines on the ACFdiagrams shown in Fig. 6 until
advanced levels when chlorite is removed.This is because it is
the metastable and indifferent phases that are removed first,
whereas the calc-silicate and ferromagnesian phases persist
and grow in abundance.
G E O C H E M I C A L R E L AT I O N S H I P S
The mineral assemblages and mineralogical changes
described above result from the interplay between regional
metamorphism and progressive bulk compositional
change through metasomatism. Bulk-rock compositional
change is assessed through whole-rock geochemistry,
whereas mineral chemical data, in this case, provide information primarily on processes related to regional
metamorphism.
Analytical methods
A 60-element suite has been obtained for 62 bulk-rock
samples. Data were provided by Genalysis Laboratory
Services, Perth, Australia, using their Lithogeochemistry
LITH/204X package, utilizing a combination of lithium
borate fusion X-ray fluorescence (XRF; major elements,
given as oxides), inductively coupled plasma optical emission spectrometry (ICP-OES; Sc, V) and inductively
coupled plasma mass spectrometry [ICP-MS; Ba, Cs, Ga,
Rb, Sn, Sr, U, Zr, high field strength elements (HFSE),
rare earth elements (REE)], and four-acid digestion ICPOES (Cu, Ni, Zn) and ICP-MS (Ag, As, Be, Bi, Cd, Co,
Ge, In, Li, Mo, Pb, Sb, Se, Tl) techniques. Carbon was
determined by CS analyser. Total volatiles, as loss on ignition (LOI), were determined by single-stage heating at
10008C. Data were standardized against Ore Research
and Exploration standards OREAS 25a, 45d, 44p, 45d,
and 45e, Natural Resources Canada standard SY-4, and
SARM1 from the South African Bureau of Standards.
Analysed samples were restricted to mafic lavas of the
Maddina, Bunjinah and Pyradie Formations to reduce the
effect of primary lithological variation between units of different petrogeneses (e.g. ultramafic lithologies).
Representative whole-rock data for a subset of samples
are presented in Table 1. All whole-rock data and
associated detection limits are available in online
Supplementary Data Table A1 (supplementary data are
available at http://www.petrology.oxfordjournals.org)
and from the CSIRO Data Access Portal (White, 2013).
Additional whole-rock, major- and minor-element data
are incorporated from an older, unpublished dataset of R.
E. Smith (collected in 1978). There is no systematic bias between these data and those obtained during this study and
so the two datasets are treated together. These additional
data were obtained by the General Superintendence
Company Pty Ltd., Perth, Australia. SiO2, Al2O3, TiO2,
total iron as FeO, MnO, CaO, K2O and MgO were determined by borate fusion XRF. P2O5 was analysed by XRF
on pressed powder pellets. Na2O was determined by atomic
absorption spectroscopy after HF digestion. FeO was measured by ceric sulphate titration, thereby allowing for determination of Fe2O3. Volatiles were determined using a Leco
induction furnace at the analytical laboratories of CSIRO
Division of Mineralogy, Perth, Australia. LOI was corrected for oxidation of Fe3þ during ignition by determining
Fe in the residue. ‘Other volatiles’ (from White, 2013) refers
to the difference between the individually determined volatiles and total LOI.
Mineral chemical data were obtained using aJEOL 8530F
Hyperprobe field emission gun electron probe microanalyser
(EPMA), fitted with five wavelength-dispersive spectrometers, at the Centre for Microscopy, Characterisation, and
Analysis at the University of Western Australia. Operating
conditions were an accelerating voltage of 15 kVand a beam
currentof 10 nA.The fine grain size of the samples necessitated
the use of a 2 mm spot size. Data were reduced using the Probe
for EPMA software package. Representative mineral data for
a subset of samples are presented inTable 2. All mineral chemical data are available in online Supplementary DataTable A2
and from the CSIROData Access Portal (White,2013).
The X-ray maps shown in Figs 5, 7 and 8 were obtained
with a Zeiss Ultra Plus field emission gun scanning electron microscope, fitted with a Bruker XFlash 6 energy-dispersive spectrometer, at the Australian Resources
Research Centre, Perth, Australia, using the Hypermap
function in the Bruker Esprit Quantax software package.
Standard analytical conditions were an accelerating voltage of 20 kV and a beam current of 690 pA. The X-ray
maps show raw X-ray counts where each selected element
is given a different colour, in accordance with the labelled
boxes in the lower left corners of the images, such that different phases are easily distinguished based on combinations of those colours.
Semi-quantitative mineral abundances for 178 samples
were determined on whole-rock powders by X-ray diffraction (XRD) using a Bruker D4 Endeavor, fitted with a Co
tube, Fe filter, and a Lynxeye position-sensitive detector.
The measured 2-theta range was 5^908, with a step size of
0·028 and a divergence slit of 18. Spectrum interpretation
was completed using the Bruker DIFFRAC.SUITE EVA
software package by comparing measured spectra against
the Crystallography Open Database. Mineral proportion
determinations are generally considered reliable to a
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JOURNAL OF PETROLOGY
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Table 1: Representative whole-rock analyses for samples from the Bunjinah and Maddina Formations
Representative whole-rock analyses for samples from the Bunjinah Formation (this study)
Sample:
Zone:
HM-15
3
HM-19C
3
Major elements (wt % oxide)
54·80
58·41
SiO2
0·96
0·76
TiO2
14·66
14·27
Al2O3
0·009
0·013
Cr2O3
9·67
9·61
Fe2O3
MnO
0·15
0·11
MgO
5·21
5·26
CaO
8·45
6·88
1·67
0·68
Na2O
1·83
1·04
K2O
0·158
0·071
P2O5
0·022
0·024
SO3
LOI
2·30
3·25
Total
99·89
100·38
Trace elements (ppm)
Nb
10·8
Y
33·3
Zr
170
Co
43·1
Ni
83·3
Cu
78·7
Zn
87
Li
12·9
Be
1·40
Rb
64·5
Sr
212·6
Cs
0·75
Ba
608·0
As
3·3
Sample:
Zone:
HM-49B
3
8·4
25·4
130
42·4
86·7
59·3
95
29·0
1·02
34·3
43·7
0·54
237·4
50·5
HM-54B
3
Major elements (wt % oxide)
60·31
57·56
SiO2
TiO2
1·07
1·23
Al2O3
12·78
13·06
Cr2O3
50·005
50·005
10·73
12·68
Fe2O3
MnO
0·16
0·16
MgO
1·87
3·91
CaO
9·00
5·13
0·29
2·59
Na2O
K2O
1·76
1·53
0·153
0·186
P2O5
SO3
0·009
0·019
LOI
1·55
2·05
Total
99·68
100·11
HM-26B
3
HM-26C
3
HM-27I
3
HM-29B
3
HM-29D
3
HM-29G
3
53·64
0·87
15·29
0·017
9·10
0·15
5·60
8·27
2·71
1·12
0·131
0·022
2·95
99·87
53·13
0·85
15·28
0·018
9·60
0·15
6·00
8·05
2·87
0·72
0·129
0·027
3·08
99·90
60·71
1·16
13·14
50·005
8·23
0·15
2·70
6·14
5·98
0·18
0·180
0·518
0·76
99·85
60·16
0·84
13·57
0·007
6·80
0·05
1·69
12·59
0·25
0·21
0·142
0·018
3·34
99·67
56·09
0·93
13·85
0·006
9·77
0·09
4·03
10·38
0·27
0·13
0·155
0·009
4·08
99·79
51·71
1·05
13·12
0·006
9·50
0·17
3·96
8·22
3·41
0·68
0·172
0·008
7·69
99·70
9·0
24·7
138
50·3
104·4
44·9
79
20·0
1·73
36·1
126·3
0·47
469·8
8·5
8·7
26·9
135
47·2
105·4
58·5
83
20·9
1·62
23·7
144·3
0·41
353·9
1·1
12·7
38·7
193
42·8
49·8
80·9
73
6·7
2·62
5·7
76·2
0·26
81·8
5·5
HM-55
3
HM-11
4
55·31
0·91
14·05
0·012
9·94
0·15
5·18
7·45
2·00
1·65
0·148
0·046
2·42
99·27
54·43
0·91
14·87
0·016
9·72
0·15
5·36
9·04
1·39
1·22
0·138
0·016
2·36
99·63
11·5
35·8
180
39·3
58·3
46·4
87
16·4
1·34
22·6
67·7
0·34
179·5
0·7
HM-29H
3
HM-49A
3
53·78
0·95
15·08
0·013
10·12
0·15
5·63
5·93
1·77
3·31
0·146
0·032
2·80
99·71
60·77
1·15
12·37
50·005
12·26
0·19
2·33
5·33
3·28
0·33
0·165
0·021
1·56
99·76
9·4
26·5
148
12·0
18·1
13·1
21
1·5
1·09
7·3
63·0
0·31
63·9
2·4
10·3
31·6
167
40·2
53·4
20·3
78
14·3
2·38
4·8
56·8
0·27
54·7
1·8
10·4
32·7
163
45·7
97·2
70·5
85
22·4
1·35
118·6
248·0
1·38
1386·8
1·0
12·3
38·1
190
37·3
17·3
21·1
127
10·5
2·61
11·6
94·2
0·94
231·2
2·1
HM-12A
4
HM-12B
4
HM-12C
4
HM-33A
4
HM-34C
4
HM-41
4
68·78
0·50
12·03
0·013
3·41
0·03
1·17
11·23
0·08
0·05
0·065
0·032
2·17
99·56
67·50
0·32
12·10
50·005
4·75
0·06
1·86
9·06
0·82
0·27
0·033
0·061
2·76
99·60
55·74
1·19
13·09
0·007
12·21
0·18
4·36
7·31
2·94
1·23
0·149
0·024
1·66
100·09
56·13
0·93
15·18
0·015
9·06
0·12
5·14
6·80
3·89
0·45
0·144
0·038
1·93
99·83
57·20
0·91
14·27
0·017
8·80
0·13
5·03
7·13
4·06
0·63
0·140
0·014
1·63
99·96
56·03
0·89
14·84
0·016
9·68
0·14
5·16
7·30
2·73
1·03
0·132
0·024
2·09
100·06
(continued)
990
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
Table 1: Continued
Representative whole-rock analyses for samples from the Bunjinah Formation (this study)
Sample:
Zone:
HM-49B
3
Trace elements (ppm)
Nb
12·1
Y
42·2
Zr
186
Co
30·5
Ni
15·5
Cu
10·5
Zn
93
Li
8·6
Be
1·95
Rb
63·3
Sr
165·5
Cs
0·63
Ba
502·8
As
1·6
HM-54B
3
HM-55
3
HM-11
4
HM-12A
4
HM-12B
4
HM-12C
4
HM-33A
4
HM-34C
4
HM-41
4
13·2
40·6
212
44·3
41·8
60·1
111
16·2
1·70
50·5
87·3
0·67
751·2
1·0
10·1
30·9
160
45·5
88·7
72·9
96
17·4
1·57
55·5
159·5
0·77
380·5
2·9
9·4
30·2
155
43·3
100·2
62·2
82
14·6
1·19
48·0
245·0
0·59
444·4
1·6
18·1
58·4
198
11·6
36·3
47·9
96
1·0
2·34
1·7
23·0
0·16
72·3
9·1
47·0
71·0
436
11·0
20·7
8·7
268
6·5
1·94
8·5
89·7
0·23
60·4
1·0
11·7
26·1
183
47·7
84·9
163·9
106
7·4
1·37
35·6
247·8
0·40
695·4
1·4
9·5
31·7
149
43·0
106·7
47·1
77
11·3
1·49
16·0
103·6
0·23
183·5
1·7
9·2
27·2
142
38·7
98·0
87·5
73
7·7
1·62
23·9
59·9
0·38
336·8
5·8
9·0
28·3
142
46·7
107·9
57·1
86
14·3
1·34
37·8
228·6
0·55
442·3
3·0
HM-22A
1
HM-23A
1
HM-23C
1
53·87
0·84
14·78
0·014
10·24
0·14
5·15
6·43
3·38
1·53
0·123
0·016
3·38
99·89
70·90
0·68
12·00
0·010
3·12
0·03
1·53
5·36
1·97
1·66
0·099
0·015
2·11
99·48
53·82
0·79
15·44
0·015
9·74
0·14
5·19
7·49
3·12
0·94
0·101
0·034
3·02
99·84
Representative whole-rock analyses for samples from the Maddina Formation (this study)
Sample:
Zone:
HM-20B
1
HM-20C
1
HM-20D
1
HM-20E
1
HM-20G
1
HM-20I
1
HM-21C
1
Major elements (wt % oxide)
68·66
53·39
SiO2
0·49
0·78
TiO2
9·19
15·55
Al2O3
Cr2O3
0·009
0·015
7·82
9·62
Fe2O3
MnO
0·06
0·11
MgO
4·04
5·26
CaO
5·61
8·80
0·06
2·75
Na2O
0·05
0·83
K2O
P2O5
0·049
0·094
0·013
0·034
SO3
LOI
3·90
3·07
Total
99·95
100·30
53·70
0·86
15·56
0·014
10·27
0·08
6·20
4·75
3·79
1·28
0·116
0·020
3·37
100·01
52·77
0·75
17·10
0·014
7·13
0·04
4·25
12·88
0·04
50·01
0·107
0·007
5·19
100·28
59·27
0·92
14·44
0·012
7·52
0·10
4·01
7·23
2·88
1·57
0·133
0·045
2·36
100·49
54·57
0·80
15·08
0·015
9·64
0·12
5·20
6·51
3·34
1·56
0·105
0·032
2·89
99·86
54·30
0·80
15·68
0·015
9·63
0·12
5·66
4·49
3·58
2·78
0·104
0·015
2·61
99·78
Trace elements (ppm)
Nb
5·0
Y
15·6
Zr
82
Co
33·3
Ni
80·6
Cu
467·6
Zn
226
Li
15·8
Be
0·49
8·6
27·5
132
48·4
95·6
86·5
84
24·8
1·23
7·5
27·7
121
41·9
76·3
88·6
51
11·0
0·99
9·0
30·1
141
48·6
85·6
79·5
85
18·1
1·17
7·8
25·1
121
48·3
94·1
77·1
84
21·3
1·19
7·9
26·0
122
48·6
93·6
75·8
80
21·1
1·00
7·8
25·9
123
50·3
100·4
79·7
88
31·6
1·00
8·5
28·0
132
49·4
91·0
84·2
97
21·9
1·14
6·9
20·2
108
20·5
32·3
38·0
37
4·9
1·30
7·8
25·3
122
49·9
95·8
72·7
90
21·3
1·27
(continued)
991
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 5
MAY 2014
Table 1: Continued
Representative whole-rock analyses for samples from the Maddina Formation (this study)
Sample:
Zone:
HM-20G
1
HM-20I
1
HM-21C
1
HM-22A
1
HM-23A
1
HM-23C
1
0·8
8·7
0·24
48·5
30·1
45·1
192·5
0·87
439·2
13·0
43·0
194·7
0·74
539·3
3·8
79·7
242·7
1·32
1844·3
1·2
43·0
249·7
0·49
750·7
2·2
40·3
37·3
0·34
315·7
10·0
29·6
183·8
0·42
283·6
1·4
HM-71D
2
HM-75A
2
HM-77C
2
HM-78B
2
HM-91B
2
HM-101A
2
HM-101E
2
HM-105
2
Major elements (wt % oxide)
51·6
55·97
SiO2
TiO2
0·94
0·96
14·88
14·89
Al2O3
0·009
0·006
Cr2O3
7·51
7·47
Fe2O3
MnO
0·08
0·07
MgO
2·10
1·80
CaO
16·09
14·27
0·11
0·11
Na2O
K2O
0·07
0·06
0·154
0·151
P2O5
0·027
0·074
SO3
LOI
6·38
3·77
Total
99·3
99·6
53·89
1·02
15·34
0·008
10·38
0·12
5·73
4·93
2·94
1·49
0·168
0·005
4·04
100·18
53·76
1·55
14·47
0·014
10·77
0·14
4·53
7·92
2·06
1·16
0·350
0·096
3·21
100·1
53·08
1·03
15·15
0·021
8·26
0·06
1·86
15·43
0·12
0·08
0·207
0·013
4·54
99·93
52·14
0·74
13·03
0·111
10·45
0·14
8·40
9·59
1·83
0·07
0·088
0·019
3·73
100·41
59·72
0·95
14·04
0·009
10·38
0·11
5·72
1·47
3·48
0·77
0·162
0·004
3·48
100·4
53·88
0·88
14·65
0·018
10·06
0·14
5·54
5·82
3·36
1·93
0·136
0·022
3·47
99·77
57·83
0·76
14·07
0·019
6·12
0·05
2·17
13·88
0·12
0·03
0·118
0·021
4·63
99·66
55·13
0·97
14·39
0·009
10·12
0·12
5·34
6·03
2·81
1·83
0·159
0·021
3·14
100·16
Trace elements (ppm)
Nb
11·4
Y
38·8
Zr
153
Co
48·6
Ni
32·5
Cu
30·7
Zn
63
Li
12·6
Be
3·22
Rb
2·2
Sr
111·1
Cs
0·10
Ba
19·4
As
12·4
12·5
34·4
176
32·1
31·4
140·6
111
11·5
3·13
2·0
260·7
0·12
19
4·3
11·1
35·6
172
47·1
84·1
66·0
90
25·0
1·33
51·1
105·4
0·57
548·8
1·4
10·7
33·8
176
31·3
51·1
69·3
54
5·4
2·0
3·6
17·0
0·15
51·6
4·2
5·5
21·4
96
55·2
202·1
54
60
29·6
0·92
2·3
48
0·60
32·2
4·6
Rb
Sr
Cs
Ba
As
Sample:
Zone:
HM-20B
1
HM-20C
1
HM-20D
1
1·8
12·8
0·22
50·2
3·9
23·5
156·7
0·39
270·1
2·7
35·7
192·9
1·48
688·6
0·8
HM-63B
2
HM-63E
2
11·4
38·8
153
48·6
32·5
30·7
63
12·6
3·22
2·2
111·1
0·10
19·4
12·4
HM-20E
1
16·6
50·9
285
42·6
55·7
65·0
78
23·1
1·62
41·9
155·4
0·53
713·5
3·6
precision of 5 vol. %. All XRD data are available in
online Supplementary Data Table A3 and from the CSIRO
Data Access Portal (White, 2013).
Whole-rock geochemistry
The whole-rock compositional data represent samples that
cover a wide geographical area across the Hamersley
Basin. As such, geochemical variation can be assessed as a
10
31·5
158
43·2
77·6
74·5
85
23·4
1·19
22·8
54·7
0·31
277·4
2·0
9·5
30·7
160
44·3
99·8
64·9
66
28·3
1·05
66·7
163·8
1·15
684·9
3·6
8·1
25·1
134
23·9
42·0
65·2
190
7·9
1·71
1·2
13·7
0·14
11·9
4·5
function of both regional metamorphic grade and metasomatic alteration. Geochemical variation must also be assessed in terms of primary lithological variability. First,
only samples from the Bunjinah and Maddina Formations
of the Fortescue Group are considered, thereby removing
any potential effect of other volcanic units with different
petrogeneses. Second, typically immobile major, minor
and trace elements (Al, Ti, Nb, Zr, Y) in least altered
992
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
Table 2: Representative averaged mineral analyses for pumpellyite, epidote and chlorite
Representative averaged pumpellyite analyses, based on 16 cations per 24.5 oxygens. Fe3þ was estimated via charge balance.
Sample:
Zone:
HM-20A
1
HM-20C
1
HM-20E
1
HM-23C
1
HM-63C
2
HM-67C
2
HM-71D
2
HM-60E
2
HM-75A
2
Wt % oxides
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
38·35
0·04
25·02
3·91
0·03
2·53
23·30
0·01
0·01
93·20
38·46
0·09
25·99
3·36
0·08
3·02
23·39
0·02
0·02
94·43
38·85
0·04
25·25
4·20
0·02
2·54
23·33
0·01
0·01
94·25
38·44
0·06
27·10
3·44
0·13
0·96
23·00
0·15
0·06
93·34
37·02
0·03
23·74
7·39
0·06
1·84
22·88
0·01
0·00
92·97
37·5
0·04
23·22
6·86
0·05
2·36
22·00
0·01
0·24
92·28
38·86
0·06
23·05
7·45
0·09
2·45
21·37
0·15
0·02
93·5
43·59
0·03
23·51
3·23
0·04
0·24
24·34
0·03
0·34
95·35
39·50
0·05
22·34
7·18
0·09
1·50
22·45
0·06
0·06
93·23
Cations
Si
Ti
Al
Fe3þ
Fe2þ
Mn
Mg
Ca
Na
K
6·14
0·01
4·72
0·02
0·50
0·00
0·60
4·00
0·00
0·00
6·05
0·01
4·82
0·06
0·39
0·01
0·71
3·94
0·01
0·00
6·15
0·00
4·71
0·01
0·55
0·00
0·60
3·96
0·00
0·00
6·16
0·01
5·12
0·00
0·46
0·02
0·23
3·95
0·05
0·01
6·01
0·00
4·54
0·43
0·57
0·01
0·45
3·98
0·00
0·00
6·12
0·01
4·47
0·34
0·59
0·01
0·57
3·85
0·00
0·05
6·26
0·01
4·38
0·13
0·88
0·01
0·59
3·69
0·05
0·00
6·91
0·00
4·39
0·02
0·41
0·00
0·06
4·13
0·01
0·07
6·41
0·01
4·28
0·26
0·73
0·01
0·37
3·91
0·02
0·01
Mg/(Mg þ Fe2þ)
Fe3þ/(Fe3þ þ Fe2þ)
0·55
0·04
0·65
0·13
0·52
0·02
0·33
0·00
0·44
0·43
0·49
0·37
0·40
0·13
0·13
0·05
0·34
0·26
Sample:
Zone:
HM-66F
2
HM-101F
2
HM-66E
2
HM-76A
2
HM-29D
3
HM-29E
3
HM-50D
3
HM-12A
4
Wt % oxides
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
39·06
0·03
24·53
6·10
0·07
2·26
22·9
0·00
0·00
94·95
38·92
0·06
23·88
5·77
0·05
2·38
22·45
0·08
0·01
93·60
44·60
0·07
22·02
4·16
0·04
0·45
24·12
0·13
0·01
95·60
38·05
0·04
24·74
5·18
0·08
1·65
22·31
0·04
0·01
92·10
38·42
0·05
23·87
6·54
0·05
2·52
22·96
0·02
0·01
94·44
37·45
0·06
22·75
7·38
0·04
2·22
22·73
0·01
0·01
92·65
39·08
0·04
25·22
5·09
0·04
2·50
23·63
0·01
0·01
95·62
40·75
0·03
25·12
4·65
0·05
2·13
22·68
0·01
0·06
95·48
Cations
Si
Ti
Al
Fe3þ
Fe2þ
Mn
Mg
Ca
Na
K
6·18
0·00
4·58
0·06
0·75
0·01
0·53
3·88
0·00
0·00
6·24
0·01
4·52
0·11
0·66
0·01
0·57
3·86
0·02
0·00
7·07
0·01
4·12
0·01
0·54
0·01
0·11
4·10
0·04
0·00
6·21
0·00
4·76
0·04
0·67
0·01
0·40
3·90
0·01
0·00
6·11
0·01
4·47
0·25
0·62
0·01
0·60
3·91
0·01
0·00
6·09
0·01
4·36
0·37
0·63
0·01
0·54
3·96
0·00
0·00
6·11
0·00
4·65
0·11
0·56
0·01
0·58
3·96
0·00
0·00
6·39
0·00
4·65
0·00
0·61
0·01
0·50
3·82
0·00
0·01
Mg/(Mg þ Fe2þ)
Fe3þ/(Fe3þ þ Fe2þ)
0·41
0·07
0·46
0·14
0·17
0·02
0·37
0·06
0·49
0·29
0·46
0·37
0·51
0·16
0·45
0·00
(continued)
993
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 5
MAY 2014
Table 2: Continued
Representative averaged epidote analyses, based on 12.5 oxygens. All Fe assumed to be Fe3þ
Sample:
Zone:
10044B
2
HM-89
2
HM-101F
2
HM-68A
2
HM-15
3
HM-29D
3
HM-29E
3
HM-29H
3
HM-50B
3
Wt % oxides
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
37·04
0·09
20·55
15·57
0·05
0·02
23·81
0·00
0·00
97·13
39·28
0·10
23·63
12·55
0·10
0·20
22·99
0·00
0·00
98·85
38·19
0·20
23·90
12·06
0·18
0·36
22·99
0·00
0·00
97·88
39·22
0·09
25·21
10·42
0·10
0·01
23·85
0·12
0·02
99·04
37·96
0·04
28·03
6·88
0·05
0·01
24·50
0·00
0·04
97·51
38·17
0·06
27·44
7·35
0·07
0·08
24·13
0·01
0·00
97·31
38·99
0·05
26·40
8·52
0·05
0·02
23·85
0·01
0·01
97·90
38·51
0·05
28·60
6·54
0·06
0·01
24·65
0·01
0·01
98·44
38·33
0·05
28·44
6·50
0·04
0·01
24·69
0·00
0·00
98·06
Cations
Si
Ti
Al
Fe3þ
Mn
Mg
Ca
Na
K
3·01
0·01
1·97
0·95
0·00
0·00
2·07
0·00
0·00
3·06
0·01
2·17
0·77
0·01
0·03
1·92
0·00
0·00
3·02
0·01
2·23
0·72
0·01
0·04
1·95
0·00
0·00
3·05
0·01
2·31
0·61
0·01
0·00
1·99
0·02
0·00
2·97
0·00
2·59
0·41
0·00
0·00
2·06
0·00
0·00
2·99
0·00
2·53
0·43
0·00
0·01
2·02
0·00
0·00
3·04
0·00
2·43
0·50
0·00
0·00
1·99
0·00
0·00
2·97
0·00
2·61
0·38
0·00
0·00
2·04
0·00
0·00
2·98
0·00
2·60
0·38
0·00
0·00
2·06
0·00
0·00
Fe3þ/(Fe3þ þ Al)
0·33
0·26
0·24
0·21
0·14
0·15
0·17
0·13
0·13
Sample:
Zone:
HM-50D
3
HM-3C
4
HM-6A
4
HM-9C
4
HM-11
4
HM-12A
4
HM-12C
4
HM-13B
4
Wt % oxides
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
38·79
0·06
28·74
6·11
0·04
0·02
24·85
0·00
0·00
98·61
38·14
0·05
28·26
6·69
0·05
0·01
24·56
0·01
0·01
97·78
38·27
0·11
29·14
5·62
0·10
0·02
24·61
0·00
0·00
97·87
38·11
0·07
28·77
6·14
0·06
0·02
24·72
0·00
0·01
97·90
38·30
0·05
28·60
6·26
0·06
0·01
24·66
0·01
0·01
97·96
38·25
0·05
28·14
6·00
0·06
0·13
24·03
0·01
0·01
96·68
37·88
0·06
26·68
8·60
0·05
0·00
24·42
0·00
0·00
97·69
38·78
0·05
28·69
6·05
0·03
0·01
24·79
0·03
0·01
98·44
Cations
Si
Ti
Al
Fe3þ
Mn
Mg
Ca
Na
K
2·99
0·00
2·61
0·35
0·00
0·00
2·05
0·00
0·00
2·97
0·00
2·60
0·39
0·00
0·00
2·05
0·00
0·00
2·97
0·01
2·66
0·33
0·01
0·00
2·05
0·00
0·00
2·96
0·00
2·64
0·36
0·00
0·00
2·06
0·00
0·00
2·98
0·00
2·62
0·37
0·00
0·00
2·05
0·00
0·00
3·00
0·00
2·60
0·36
0·00
0·02
2·02
0·00
0·00
2·98
0·00
2·47
0·51
0·00
0·00
2·06
0·00
0·00
2·99
0·00
2·61
0·35
0·00
0·00
2·05
0·00
0·00
Fe3þ/(Fe3þ þ Al)
0·12
0·13
0·11
0·12
0·12
0·12
0·17
0·12
(continued)
994
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
Table 2: Continued
Representative averaged chlorite analyses, based on 14 oxygens. All Fe assumed to be Fe2þ
Sample:
Zone:
Wt % oxides
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
Cations
Si
Ti
Al
Fe2þ
Mn
Mg
Ca
Na
K
Mg/(Mg þ Fe)
Sample:
Zone:
Wt % oxides
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
Cations
Si
Ti
Al
Fe2þ
Mn
Mg
Ca
Na
K
Mg/(Mg þ Fe)
HM-20A
1
HM-20E
1
HM-20G
1
HM-23A
1
HM-23C
1
HM-60E
2
HM-75A
2
HM-101F
2
HM-66E
2
31·35
0·02
17·75
21·63
0·12
15·89
0·48
0·06
0·07
87·37
28·86
0·02
17·55
23·75
0·13
17·44
0·32
0·02
0·02
88·11
28·52
0·01
18·04
26·07
0·20
15·11
0·41
0·03
0·02
88·41
28·32
0·05
18·16
23·24
0·17
16·07
0·23
0·04
0·04
86·32
27·17
0·02
18·70
26·36
0·26
15·26
0·24
0·02
0·00
88·03
27·34
0·02
20·05
20·23
0·20
20·22
0·15
0·01
0·01
88·23
27·46
0·03
18·90
28·63
0·29
13·87
0·22
0·01
0·01
89·42
28·18
0·00
18·82
25·83
0·24
15·11
0·19
0·02
0·03
88·42
26·37
0·10
18·27
32·42
0·33
10·62
0·30
0·01
0·03
88·45
3·21
0·00
2·14
1·86
0·01
2·43
0·05
0·01
0·01
2·99
0·00
2·14
2·06
0·01
2·69
0·04
0·00
0·00
2·98
0·00
2·22
2·28
0·02
2·35
0·05
0·01
0·00
2·99
0·00
2·26
2·05
0·02
2·52
0·03
0·01
0·01
2·87
0·00
2·32
2·33
0·02
2·40
0·03
0·00
0·00
2·78
0·00
2·40
1·72
0·02
3·07
0·02
0·00
0·00
2·89
0·00
2·35
2·49
0·03
2·13
0·03
0·04
0·00
2·94
0·00
2·31
2·25
0·02
2·35
0·02
0·00
0·00
2·87
0·01
2·34
2·95
0·03
1·72
0·04
0·00
0·00
0·57
0·57
0·51
0·55
0·51
0·64
0·46
0·51
0·37
HM-68A
2
HM-76A
2
HM-15
3
HM-29D
3
HM-29H
3
HM-11
4
HM-12C
4
HM-13B
4
26·25
0·02
19·61
31·59
0·36
10·84
0·14
0·01
0·02
88·84
28·09
0·01
18·22
26·72
0·24
15·12
0·27
0·01
0·01
88·69
27·48
0·03
20·04
24·54
0·42
15·43
0·34
0·02
0·02
88·32
26·60
0·01
19·02
26·66
0·30
13·64
0·64
0·01
0·01
86·89
26·81
0·01
20·77
25·18
0·30
15·81
0·09
0·01
0·01
88·99
26·61
0·02
20·28
24·46
0·44
15·60
0·13
0·05
0·01
87·60
25·71
0·02
19·90
31·04
0·52
10·76
0·15
0·01
0·01
88·12
28·81
0·04
19·11
21·95
0·22
18·54
0·14
0·02
0·05
88·88
2·82
0·00
2·49
2·84
0·03
1·74
0·02
0·00
0·00
2·94
0·00
2·25
2·34
0·02
2·36
0·03
0·00
0·00
2·85
0·00
2·46
2·13
0·04
2·39
0·04
0·00
0·00
2·85
0·00
2·41
2·39
0·03
2·18
0·07
0·00
0·00
2·77
0·00
2·53
2·18
0·03
2·44
0·01
0·00
0·00
2·79
0·00
2·51
2·15
0·04
2·44
0·01
0·01
0·00
2·78
0·00
2·54
2·81
0·05
1·74
0·02
0·00
0·00
2·92
0·00
2·28
1·86
0·02
2·80
0·02
0·00
0·01
0·38
0·50
0·53
0·48
0·53
0·53
0·38
0·60
995
JOURNAL OF PETROLOGY
VOLUME 55
samples were used to assess primary lithological variation
as a function of petrogenesis and igneous fractionation^
crystallization processes. No statistically significant variation amongst the least altered samples was identified.
Similarly, preliminary investigations indicated that no notable variation related purely to metamorphism is evident.
16
HM-12A (Metasomatized)
As
0.65 Ga
0.05 V
8
0.2 Rb
Nb
10 Ta
Th
6
Pb
0.4 Sc
0.4 Y
0.1 Cr
15 Sb
10 Mo
CaO
4
100 In
2U
250 SO3
12 Cs 0.1 Cu
Hf
1.5 Na2O
0.1 Ni
0.0075 Ba
LOI
Sn
MgO
25 Cd
30 MnO
10 P2O5
Tl
15 Bi
2 Ge
K2O
0
4 TiO2
0
30
2
(a)
14
6 K2O
4 MgO
2 Na2O
LOI
2 TiO2
2.3 FeO
5
10
15
20
25
‘Least Altered’ (Average of 2 analyses)
0.1 Zn
Hf
2U
4
2 LOI
15 Bi
Sn
0.1 Ni
10 Sb
Fe2O3
Ge 2 Ta
2 Mo
5 TiO2
0.2 Co
0.04 Cu
15 Cd 65 SO3 10 Cs
0.015 Ba
MgO
0.05 Sr
10 P2O5
Li
10 In
15 MnO
2 Na2O 12 Tl
0.3 Rb
3 K2O
2
4
6
8
10
12
HM-12C ('Least altered')
11158A
11158B
11159
11160
11161
11162
11163
11164
11165A
11165B
C. Vol.
All data wt%
25
20
15
3 Na2O
14
16
4 MgO
120 MnO
8 Fe2O3
0.4 SiO2
2.5 FeO
Al2O 3
1.3 CaO
LOI
5
2 TiO2
6 K2O
(d)
0
30
(b)
10
110 MnO
0
0
0.25 Ga
0
(c)
0.05 Zr
0.5 Nb
0.1 Cr
6
0.4 SiO2
10
Sc
0.1 V
30
15
Al2O 3
CaO
8
8 Fe2O3
Al2O 3
Pb
Th
As
16
1.5 CaO
0.2 SiO2
0.2 Y
10
0
20
5
12
2
0.05 Sr
4
6
8
10
12
HM-22A ('Least altered')
Qtz-rich
Pmp-rich
Pmp-bearing
Prh-rich
C. Vol.
All data wt%
25
0.2 Co
0.1 Zn
Fe2O3
0.5 Li
‘Least Altered’ - individual analyses
HM-23A (Metasomatized)
0.1 Zr
10
Weight % oxide
ppm element
Isocon
Constant Volume
14
Al2O 3
2
Metasomatized Rocks
0.2 SiO2
12
MAY 2014
Conversely, there is significant bulk compositional change
owing to hydrothermal alteration.
Isocon plots are a convenient way of assessing elemental
variations between least altered and metasomatized rocks
(Grant, 1986, 2005). Four isocon diagrams are shown in
Fig. 9. Two are for separate sample pairs whereas the third
16
Weight % oxide
ppm element
Isocon
Constant Volume
14
NUMBER 5
0
5
10
15
20
25
‘Least Altered’ (Average of 10 analyses)
30
Fig. 9. Isocon plots constructed for two pairs of least altered and metasomatized samples from zone I (a) and zone IV (b). (a) represents relatively low levels of alteration compared with (b), which is a more highly altered end-member with more pronounced element mobility. The
two samples used in each of these diagrams were located within the same flow unit and are interpreted to have originally had the same composition. Al is assumed immobile and defines the isocons. (c) is a composite isocon for samples from a single lava flow unit (location shown in
Fig. 3), showing a range of different styles of metasomatized rock against an average of two least altered samples (samples 10097 and 10102)
from the same flow. Qtz-rich indicates quartz-rich alteration (sample 10093); Pmp-rich, pumpellyite-rich alteration (sample 10095); Pmp-bearing, pumpellyite-bearing alteration (sample 10092); Prh-rich, prehnite-rich sample (sample 10094). (d) is a composite isocon for samples from
a single lava flow unit (dashed-outline box in Fig. 3), showing single analyses from a single least altered lava flow against their average. The constant volume (C. Vol.) line is a 1:1 line, assuming no change in rock density. Some data are scaled (scaling factor shown) for plotting convenience.
996
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
and fourth are composite diagrams for multiple samples
from single lava flow units. Each of the pairs consists of a
least altered sample and a metasomatized rock located in
close spatial proximity to each other from the same lava
flow unit. Specific, closely spaced pairs are generally preferred to averaged compositions as there is typically a
higher certainty that their original compositions were
identical and they therefore provide a better estimate of
element mobility.
Figure 9a represents relatively low levels of alteration
compared with Fig. 9b, which represents more intense
levels of alteration. In Fig. 9a, the metasomatized rock
(HM-23A) is equivalent to that shown in Fig. 8b; it contains high proportions of quartz and pumpellyite while
albite and K-feldspar are still present. The least altered
rock (HM-22A) is shown in Fig. 5b. Elements fall into
three dominant groups. The first lies along, or close to, the
isocon, which is well defined by the typically immobile
elements Al, Ti, Nb and Zr. Interestingly, Ca lies within
this group and does not show any increase, despite the
abundance of calc-silicate minerals. The second group incorporates those elements that show significant depletions.
These include the majority of the alkali, alkali earth, and
heavier first transition series elements (Mn to Zn). The
final group are those elements showing major enrichment,
which is limited to Si and As.
Figure 9b represents a more intense stage of alteration.
The least altered rock (HM-12C) is shown in Fig. 5d. The
metasomatized rock (HM-12A) contains both epidote and
pumpellyite, with small amounts of partially replaced actinolite, as shown in Fig. 8f. Comparison with Fig. 6f indicates that such an assemblage would require a major
change in bulk-rock composition. Figure 9b shares a
number of similarities with Fig. 9a. Depletions in the majority of alkali, alkali earth and heavy first transition series
elements are more pronounced. However, the group of
elements that plotted close to the isocon in Fig. 9a is much
less well-defined and many elements show upwards scatter,
implying enrichment. Furthermore, the typically immobile
elements no longer define a single isocon, suggesting variable mobility of these elements. Unlike in Fig. 9a, Ca shows
significant enrichment, accommodated by the dominant
calc-silicate phases in sample HM-12A (Fig. 8f).
The isocons in both Fig. 9a and 9b fall below the dashed
line representing constant volume. This line has a gradient
of unity, assuming constant rock density. Therefore, alteration is either accompanied by a volume increase of as
much as 20% (in Fig. 9a) or implies that the altered rocks
are significantly more dense than the least altered rocks,
which does not fit with qualitative observations and is inconsistent with specific gravity determinations for the R.
E. Smith sample set. A volume increase, however, could
be accommodated through recrystallization removing
pore space and through the infilling of vesicles.
Bulk-rock chemical changes are of course intimately
linked to observed mineralogical changes in the metasomatized rocks. This is shown in Fig. 9c, where a variety of
metasomatized rocks with differing proportions of alteration minerals are plotted against the average of two least
altered samples. All samples used in this diagram come
from a single mapped lava flow unit, outlined by the bold
box in Fig. 3. Four metasomatized rocks are considered:
Prh-rich is a prehnite-rich sample (sample 10094) from the
transition zone. This sample generally shows the least
element mobility, with most elements plotting close to the
1:1 constant volume line, which in this case runs through
Al. Pmp-bearing (sample 10092) is a pumpellyite-bearing
rock, somewhat similar to that shown in Fig. 8b. Pmp-rich
and Qtz-rich are both highly altered rocks, but Pmp-rich
contains significantly more pumpellyite than quartz
(equivalent to Fig. 8d), whereas Qtz-rich contains much
more quartz. As seen in Fig. 9c, whereas most elements
show uniform depletions (Na, K, Mg, Mn, Fe), consistent
with the other isocon diagrams, Si and Ca show more variable behaviour and negatively correlate with each other.
As such, the pumpellyite-rich sample shows extreme Ca
addition, with relatively less Si addition, whereas the opposite is true for the quartz-rich sample. This effect is
most pronounced for Ca, which may in part explain its apparent variable mobility, as seen in Fig. 9a and b. For comparison, Fig. 9d shows 10 single samples from the least
altered portion of a single flow unit (outlined by the
dashed box in Fig. 3) plotted against their average. This
shows how uniform least altered compositions are within
a single flow unit.
Although the decrease in total Fe is clear, Fig. 9c shows
the change in iron oxidation ratio during metasomatism.
Whereas all samples show a uniform depletion in FeO,
Fe2O3 does not decrease by a similar factor, resulting in
an overall increase in oxidation ratio. However, in the
case of the pumpellyite-rich sample, ferric iron is seen to
increase, perhaps representing an active oxidation of iron
within the system. It is these varying chemical changes
that result in the different mineral assemblages observed
in metasomatized rocks.
The link between mineralogy and bulk-rock chemistry is
also evident on ACF diagrams (Fig. 10), where a clear
array is visible trending from a ‘main group’of least altered
samples away from the ferromagnesian apex towards calcsilicate mineral compositions. This main group of data
points highlights the consistent geochemistry of the least
altered samples from across the basin. The degree of chemical alteration correlates strongly with the modal proportion of quartz (Fig. 10a), which is consistent with
petrographic observations and the general enrichment in
Si seen in Fig. 9. Those samples containing abundant prehnite form a tight cluster close to the main group, corresponding to the early stages of metasomatism in the
997
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 5
Al2O3 - K2O - Na2O
A
XRD (wt%)
Quartz
<10
10-20
20-30
>30
XRD (wt%)
Prehnite
<5
5-10
>10
Pumpellyite
Prehnite
MAY 2014
Pumpellyite
Prehnite
Epidote
Epidote
Chlorite
(a)
CaO
Chlorite
(b)
Actinolite
FeO + MgO C
- Fe2O3 - TiO2
Actinolite
A
A
XRD (wt%)
Epidote
<5
5-15
15-30
>30
Prehnite
XRD (wt%)
Pumpellyite
<5
5-15
15-30
>30
Pumpellyite
Prehnite
Epidote
Pumpellyite
Epidote
Chlorite
Chlorite
(c)
C
F
(d)
Actinolite
F
C
Actinolite
F
Fig. 10. ACF diagrams showing a trend in bulk-rock compositions away from the ferromagnesian apex towards calc-silicate mineral compositions. It should be noted that the F apex incorporates total Fe, as opposed to Fig. 6, in which FeO is applied to the A apex. Data points are
shaded according to XRD mineralogy: (a) quartz, with a marked increase during progressive alteration; (b) prehnite, which clusters close to
the main group and corresponds to early stages of alteration in the transition zone; (c) epidote group; (d) pumpellyite. It should be noted how
the pumpellyite trend lies above the epidote trend, consistent with Ca/Al ratios in the respective minerals.
transition zone (Fig. 10b). Metasomatized rocks generally
separate into those that are pumpellyite-bearing (Fig. 10c)
and those containing epidote (Fig. 10d). The epidote-rich
samples lie on the Ca-rich side of the array, whereas the
pumpellyite-rich samples fall on the Al side, compatible
with Ca/Al ratios in those minerals.
The same geochemical trend is observed in all metamorphic zones across the sampled portion of the
Hamersley Basin. Figure 11 shows total Fe (as Fe2O3)
plotted against MnO. In Fig. 11a, data points are shaded
according to regional metamorphic zone and it is clear
that the observed geochemical trend has the same orientation and magnitude in all metamorphic zones. In Fig.
11b, data points are shaded according to XRD quartz
content (as in Fig. 10a) and the depletion of Fe and Mn
clearly correlates with increasing quartz content, and
therefore intensity of metasomatic alteration. Figure 11b
shows only new data from this study and excludes the
older dataset of R. E. Smith, for which no XRD data
exist.
Also shown in Fig. 9 is a notable depletion in base metals
(Co, Ni, Cu, Zn) during metasomatism. This relationship
is shown in Fig. 12. Cobalt, Ni, Cu and Zn are plotted
against bulk-rock MnO content and shaded according to
XRD-determined quartz content. Both MnO and quartz
contents serve as effective proxies for the degree of metasomatism. Although absolute concentrations of all of these
elements are low, statistically significant depletion trends
998
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
18
Metamorphic Zone
Zone I
16
Zone II
Zone III
14
Total Fe as Fe2O3 (wt%)
Zone IV
12
10
8
18
6
4
XRD (wt %) Quartz
<10
20-30
>30
10-20
2
0
(a)
0
0
0.025
0.05
0.075
0.1
0.125
MnO (wt %)
0.15
(b)
0
0.25
0.175
0.2
0.225
0.25
Fig. 11. Plots of total Fe (as Fe2O3) vs MnO (wt %) showing a depletion in both elements owing to metasomatic alteration. (a) Data points are
shaded according to metamorphic zone and the same trend, in both orientation and magnitude, is observed for all zones. (b) New data from
this study are shaded according to XRD quartz content, indicating that the element depletion correlates with an increasing degree of metasomatism (see Figs 9 and 10). MnO data are reported to a precision of 0·01wt %.
60
225
50
200
Ni (ppm)
Co (ppm)
175
40
30
20
25
0.05
0.15
0.1
MnO (wt%)
0.2
0
0.25
0
(b)
0.05
0.15
0.1
MnO (wt%)
0.2
0.05
0.15
0.1
MnO (wt%)
0.2
0.25
350
300
250
Zn (ppm)
Cu (ppm)
200
180
160
140
120
100
80
60
40
20
0
0
100
50
(a)
0
125
75
10
0
150
XRD (wt %)
Quartz
<10
10-20
20-30
>30
200
150
100
50
(c)
0.05
0.15
0.1
MnO (wt%)
0.2
0.25
0
(d)
0
0.25
Fig. 12. Variation of Co, Ni, Cu and Zn (ppm) vs MnO (wt %), shaded according to XRD quartz content. All elements show a broad positive
correlation, corresponding to removal during metasomatism.
999
JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 5
MAY 2014
Al
are still visible. Zinc, however, does show slightly more
variable behaviour and is not always as strongly depleted
as the other elements (see Fig. 9b). No explanation for this
behaviour is presently available.
Metamorphic Zone
Zone I
Zone II
Zone III
Zone IV
Mineral compositions
In contrast to the whole-rock geochemical data, mineral
compositions generally show trends that correlate with regional metamorphic conditions, as shown by Smith et al.
(1982). In this section, an overview is given of the chemistry
of the major compositionally variable mineral phases. In
general, the same trends are observed in minerals from
both metamorphic and metasomatic assemblages such
that the two cannot typically be distinguished on the
basis of mineral chemistry alone (except for pumpellyite;
see below).
(a)
0.5
0.5
Fe (total)
Mg
Al
Pumpellyite
(b)
0.5
0.5
Fe3+
Fe2+
1.8
1.6
1.4
Fe (total)
Pumpellyite analyses were recalculated on the basis of 16
cations per 24·5 oxygens, according to the generalized formula of Coombs et al. (1976): W4X2Y4Z6O(20þx)(OH)(8x),
where W ¼Ca, Mn; X ¼ (Mg, Fe2þ, Mn)2x(Fe3þ, Al)x;
Y ¼ Fe3þ, Al; Z ¼ Si. Pumpellyite is commonly idealized
as containing divalent cations on half of the X sites (i.e.
x ¼1), which is the assumption used here. The proportion
of Fe3þ can be estimated through simple charge balance.
However, the calculation will vary depending on the
chosen value of x (i.e. the extent of oxygen protonation).
Pumpellyite analyses fall into two groups (Fig. 13). The
first contains the majority of analyses and comprises pumpellyites containing both Fe and Mg. The second, smaller,
group comprises pumpellyites from zones I and II that
are very Al-rich and are strongly depleted in Fe and Mg.
This second group is a function of bulk-rock composition
as these samples are the most highly metasomatized with
low bulk-rock Fe and Mg.
Smith et al. (1982) showed that with increasing metamorphic grade pumpellyites contain progressively less
Mg. This is only a very weak trend and although new
data from this study (the first group described in the preceding paragraph) are broadly compatible with this, there
is a large degree of overlap between data from different
metamorphic zones and no statistically significant difference between them (Fig. 13a). There is, however, a more
marked increase in estimated Fe3þ content with increasing
metamorphic grade. Figure 13b shows that all pumpellyite
crystals from zone I and a significant proportion from
zone II contain little or no estimated Fe3þ, according to
the formula given above. In contrast, those from zone III
and the remaining zone II samples show a distinct trend
towards increased Fe3þ contents, which is supported by
Fig. 13c, displaying a negative 1:1 correlation between total
Fe and Al cations per formula unit, implying a direct exchange of Al for Fe3þ. However, total oxidation ratio
[Fe3þ/(Fe3þ þ Fe2þ)] never exceeds 50%. This is contrary
1.2
1.0
0.8
0.6
0.4
0.2
(c)
0.0
3.8
4.0
4.2
4.4
4.6
Al
4.8
5.0
5.2
5.4
Fig. 13. Pumpellyite compositional data. (a) Al^total Fe^Mg (cations per formula unit) diagram showing a weak trend towards the
Al^Fe side with increasing metamorphism, compatible with Smith
et al. (1982). (b) Al^Fe3þ^Fe2þ (cations per formula unit) diagram
showing a general lack of estimated Fe3þ in zone I pumpellyites.
Ternary diagram baseline in both (a) and (b) is at 0·5 Al. (c) Al^
total Fe (cations per formula unit) diagram showing a negative correlation indicating substitution of Fe3þ for Al, particularly in zone II
and III pumpellyites.
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WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
to Smith et al. (1982), who described some strongly ferric
pumpellyites, as estimated through an unusual recalculation scheme involving normalizing CaO contents to an
idealized 23 wt %. The apparent disagreement between
these new data and those of Smith et al. (1982) suggests
that x, in the generalized formula above, may be greater
than unity. Those pumpellyite analyses from zone IV are
from the highly metasomatized sample HM-12A, which
also contains epidote (Figs 8f and 9b). As such, the low
estimated Fe3þ contents shown by this sample may be
related to the sequestering of Fe3þ by epidote, which does
not occur in other samples that contain only pumpellyite.
mechanism, where R indicates divalent cations (Fe, Mg,
Mn) (Fig. 15a). In contrast, data from zones I and II scatter
towards elevated Si contents, corresponding to a Si1œ1 R2
substitution mechanism, where œ indicates a site vacancy.
Magnesium numbers of analysed chlorites vary between approximately 0·3 and 0·7, although there is no statistically
significant correlation with either metamorphic grade or
metasomatic alteration. Instead, Mg number is inferred to
be a function of bulk-rock composition.
4
Metamorphic Zone
3.8
3þ
Epidote analyses show a marked spread in Al/Fe , from approximately Ps10 (30 mol % Al2Fe3þ) up to and above 1 Fe3þ
cation per formula unit (Ps33) (Fig. 14). This trend correlates
strongly with regional metamorphism, as higher-grade zones
contain epidotes of more clinozoisitic compositions. This is
consistent with the data of Smith et al. (1982) and studies
from other low-grade regional metamorphic terranes
(Coombs et al., 1976; Raith, 1976; Schmidt et al., 1997;
Hannington et al., 2003). Intra-sample variation can be as
much as 10 Ps units (30 mol % Al2Fe3þ). A well-defined asymmetric miscibility gap is documented in epidote compositions,
between approximately 53 and 72 mol % Al2Fe3þ
(Ps18^Ps24) at greenschist facies, which narrows with increasing metamorphic grade (Raith, 1976). Figure 14 shows a
number of data points plotting in this range, consistent with
the data of Smith et al. (1982).
Zone I
Zone II
Zone III
Zone IV
3.6
5
3.4
ct =
Al
=
∑o
3
Sudoite
2.8
Al
=
2
3
Clinochlore/
Chamosite
5.5
3.2
Si
Al
=
4
∑o
ct =
Epidote
2.6
∑o
ct =
2.2
2
Si1□1R2+-2
6
2.4
(a)
2
Si1R2+1Al-2
Al2□1R2+-3
Amesite
2.5
3
3.5
4
R2+
4.5
5
5.5
6
Chlorite
0.5
Fe3+
Chlorite analyses are plotted in Fig. 15. Data from metamorphic zones III and IV plot in a tight cluster 25% of
the way along the join between tri-octahedral end-members
(clinochlore, chamosite, etc.) and a notional ‘amesite’ endmember, corresponding to an Al2Si1R1 substitution
Ca + Na + K
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
Metamorphic Zone
0.3
Zone II
0.2
Zone III
Zone IV
0.1
0.0
1.5
1.7
1.9
2.1
0.4
K-feldspar/Albite
Saponite
Muscovite Calcite
0.3
Quartz
0.2
0.1
0
2.3
Al
2.5
2.7
2.9
Fig. 14. Epidote compositional variations in terms of Fe3þ vs Al (cations per formula unit). Analyses show a marked trend towards more
clinozoisite-rich compositions in higher metamorphic zones.
(b)
0.3
0.5
0.7
0.9 1.1
Al (iv)
1.3
1.5
1.7
Fig. 15. Chlorite compositional variations. (a) Sum of divalent cations vs Si cations after Wiewio¤ra & Weiss (1990). Analyses from zones
III and IV show a tight clustering along the tri-octahedral^‘amesite’
join. Zone I analyses show broad scatter compatible with a Si1œ1 R-2
substitution mechanism. (b) Interlayer cations (Ca þ Na þ K) vs
tetrahedral Al after Schmidt et al. (1997). A dominant linear array
exists of increasing interlayer cations with decreasing tetrahedral Al,
consistent with the interlayering of a saponite-type mineral.
Additional scatter to high interlayer cations contents is consistent
with significant amounts of Ca originating from calcite.
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More information on the origin of the scatter shown by
zone I and II samples is provided by the abundance of interlayer cations (Ca, Na, K). Figure 15b shows a plot of tetrahedral Al versus the sum of interlayer cations (after Schmidt
et al., 1997). These analyses do not resemble typical chlorites,
containing high Si, low Al and high Na, Ca and K (Table
2; White, 2013). However, all of the analysed grains are optically identifiable as chlorite. The majority of analyses form a
linear array showing a negative correlation between tetrahedral Al and interlayer cations. The gradient of this array is
consistent with the trend shown by Schmidt et al. (1997),
which is associated with the interlayering (or inclusion) of
saponite, smectite or a similar clay mineral. The interlayering of clay minerals is consistent with the lower regional
metamorphic grades in ZI and ZII compared with ZIII
and ZIV. In addition, a few data points scatter upwards to
higher Ca contents, suggesting the inclusion of calcite.
Other minerals
A range of other minerals were analysed, including actinolite, clinopyroxene, titanite, and sulphide phases. In all
cases, compositions were uniform, showing no significant
or systematic variation as a function of either metamorphic
grade or metasomatic alteration, and are therefore not discussed further here.
OR IG I N OF R EG IONA L - S C A L E
M E TA S O M AT I S M I N T H E
F O RT E S C U E G RO U P
The regional metamorphic gradient described by Smith
et al. (1982) is supported by this study. It is well defined by
progressive mineral associations in least altered rocks, and
is supported by compositional trends in those minerals,
particularly chlorite and epidote. These are consistent
with mineral compositions and compositional trends documented in other low-grade metamorphic terranes
(Coombs et al., 1976; Schmidt et al., 1997).
Superimposed on this metamorphic gradient, the mineralogical and geochemical changes described in the preceding sections define a well-characterized association
with regional-scale metasomatic alteration. Petrographic
features show that metasomatism progressively overprints
the metamorphic assemblages. The documented trends in
both mineralogy and whole-rock and mineral compositions are comparable with other documented occurrences
of epidote- and pumpellyite-rich rocks in low-grade mafic
rocks, as described above.
Detailed petrography indicates a development of metasomatic alteration assemblages that progressively replace
pre-existing regional metamorphic assemblages. Calciumrich metamorphic minerals, such as actinolite or relic
clinopyroxene, are replaced first by prehnite (Fig. 8a) and
then by pumpellyite and/or epidote (Fig. 8b). Continued
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alteration results in the removal of feldspars (Fig. 8c^f) as
Na and K are depleted (Fig. 9). The alumina and silica
held in the feldspars are subsequently released, providing
material for further growth of chlorite, pumpellyite^epidote and quartz (Fig. 8c). Finally, extensive removal of Fe
and Mg results in the replacement of chlorite by pumpellyite^epidote (Fig. 8d). More specific replacement textures
can be seen in Fig. 8f, where epidote and pumpellyite are
clearly overgrowing actinolite.
Metasomatic alteration has a negligible effect on mineral
chemistry and the majority of phases maintain their compositions from previous metamorphic assemblages.
Thermodynamic modelling of metamorphic and metasomatic assemblages shows that the observed whole-rock
alteration can be accommodated primarily by changing
modal proportions of phases without the need to extensively re-equilibrate mineral compositions (White et al.,
2014).
The petrographic evidence of metasomatic mineral associations progressively replacing regional metamorphic
assemblages places a number of constraints on the timing
and origin of the metasomatism. Specifically, the metasomatic event must post-date the development of the regional metamorphic mineral assemblages. In the presence
of fluids and high fluid/rock ratios, this time separation
may be short. However, estimates of P^T conditions for
metamorphism and metasomatism suggest that hydrothermal fluid flow occurred synchronous with or after regional
deformation (Fig. 16), which itself must post-date development of the regional metamorphic burial profile. This
therefore precludes an origin of the metasomatism early
in the history of the Fortescue Group, such as autometasomatism during eruption and/or subsequent cooling of lava
flows. Similarly, the metasomatism cannot be related to
early sea-floor alteration or even development coeval with
progressive regional metamorphism, as proposed by
Smith (1968, 1977) for other occurrences of epidote- and
pumpellyite-rich rocks.
The generally semi-conformable nature to the zones of
metasomatic alteration along vesicular and brecciated
lava flow tops indicates that fluid flow was focused along
these horizons. This implies that these horizons possessed
an enhanced primary permeability that allowed for extensive fluid flow, as described by Stoiber & Davidson (1959)
and White (1968) for comparable alteration in the
Keweenawan lavas. The lateral extent of metasomatism in
higher grade zones appears less than in lower grade zones.
Although this is in part due to structural deformation limiting exposed strike length, it may also be related to infilling of porosity, vesicularity and breccias fractures during
metamorphism, thereby restricting fluid flow. However,
brecciated and vesicular rocks, even in ZIII and ZIV, contain amygdales that we interpret to have been open
during metasomatism. There is a distinct lack of major
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WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
Fig. 16. Schematic illustration of some likely characteristics of the metasomatic system operating in the Hamersley Basin. The hydrothermal
fluid is derived from seawater, although specific downflow zones are not preserved. Permeable lava flow tops introduce a strong lateral control
on fluid flow. Fluids are driven northwards by topographic and tectonic gradients generated by deformation to the south. Local upflow of
fluid may contribute a source of Fe to the overlying Hamersley Group that upgrades its iron ore deposits. Modified after White et al. (2014).
cross-cutting structures in the Hamersley Basin; large extensional faults in the southern part of the basin typically
trend parallel to the strike of the stratigraphy (Thorne &
Trendall, 2001). This is particularly notable when compared with higher-grade greenstone terranes, such as the
Eastern Goldfields Superterrane, which contains abundant
10^100 km long shear zones. Consequently, fluid is constrained to flow primarily through permeable zones
within the rocks themselves. This explains why metasomatic alteration is so strongly focused along lava flow tops,
where primary vesicularity and brecciation significantly
enhanced porosity and permeability. This stratigraphic
control on permeability is inferred to have promoted lateral fluid flow, enhancing the geographical distribution of
hydrothermal alteration (Fig. 16).
The field relationships described above contrast with the
more patchy relationships described by Smith (1968),
which were interpreted as being formed via element redistribution during regional metamorphism. The regionalscale metasomatism observed in the Fortescue Group is
incompatible with being the result of a metamorphic fluid.
This is primarily because the spatial extent of metasomatism and volume of rock affected, along with the extent of
element mobility, implies extremely high fluid/rock ratios
and it is unlikely that such volumes of fluid could be
derived through metamorphic devolatilization of rocks
that are still at present hydrated. Specifically, the largest
pulse of fluid production through metamorphic dehydration (certainly in mafic rocks) is typically around the
greenschist^amphibolite facies transition with the dehydration of chlorite (Guiraud et al., 2001; Elmer et al., 2006).
However, the rocks of the Fortescue Group have not
crossed this transition and still contain abundant chlorite
and, consequently, H2O.
The mineralogy of the epidote^quartz and pumpellyite^
quartz rocks, described above, is directly comparable with
the epidote-rich rocks discussed by a range of researchers
(Gibson et al., 1983; Lesher et al., 1986; Richardson et al.,
1987; Galley, 1993; Paradis et al., 1993; Banerjee et al., 2000;
Hannington et al., 2003; Jowitt et al., 2012). Similarly, the
observed geochemical trends are identical to those associated with the epidote-rich rocks described by those same
researchers. Given the comparable petrography and geochemistry between all of these different locations and the
Fortescue Group, it is reasonable to assume that a similar
process, in terms of fluid sources and compositions, produced the metasomatism in the Fortescue Group. The epidote-rich rocks described by the workers listed above are
interpreted as being the result of intense hydrothermal circulation of seawater through mafic rocks, and therefore
the same is probably true in the Fortescue Group.
As previously mentioned, studies of the formation of epidote-rich rocks commonly invoke the involvement of a
Ca-rich fluid. Hardie (1983) and Gutzmer et al. (2003)
described how a highly saline, Ca-rich brine can be
evolved from seawater through its interaction with surrounding rocks. This process requires intense Na metasomatism, whereby Na is precipitated in albite and Ca is
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dissolved into the fluid. Therefore, large areas of albitization (Na metasomatism) would be expected somewhere
within the larger hydrothermal system. Such albitization
was noted by Smith (1968) around decimetre-scale patches
of epidote- and pumpellyite-rich rocks in New South
Wales, Australia. However, no such zones of Na metasomatism are identifiable in the Fortescue Group. Nor have any
such zones been described from elsewhere in the
Hamersley Basin. It is pertinent to remember, first, that
there may have been several kilometres of stratigraphy
above the present-day Hamersley Basin that have since
been removed by erosion (Fig. 16; Smith et al., 1982; White
et al., 2014), and, second, that the Fortescue Group probably
extends southwards underneath the Wyloo Trough (Fig. 1)
and the Ashburton fold belt (Johnson et al., 2013).
Therefore, any original zones of extensive Na metasomatism may not have been preserved to the present day or
are currently buried to the south.
The action of a saline, Ca-rich hydrothermal fluid is
compatible with the metasomatism observed in the
Fortescue Group (and indeed elsewhere), as the high salinity would be an effective way of mobilizing the depleted
elements (alkalis, alkali earths and base metals) as chloride complexes (Seward & Barnes, 1997; Wood & Samson,
1998; Liu et al., 2012). The observed mobility of Ni (Fig.
12b), which typically has a relatively low solubility compared with other base metals (Liu et al., 2012), is a particular indication of extremely high fluid/rock ratios and a
highly saline fluid. Similarly, the apparent mobility of
HFSE (Y, Nb, Th) and other typically immobile elements
(Cr, Sc), as seen in Fig. 9b, may be facilitated by highly
saline fluids at elevated temperatures during sub-sea-floor
hydrothermal alteration (Finlow-Bates & Stumpfl, 1981;
Philippot & Selverstone, 1991; Valsami-Jones &
Ragnarsdo¤ttir, 1997; Jiang et al., 2005). It is unlikely that
such a saline fluid would be derived purely from metamorphic dehydration, and this is further evidence against
such a fluid source.
At present, no direct evidence (e.g. fluid inclusions or
stable isotopes) exists for the composition or source of the
fluid responsible for metasomatism in the Fortescue
Group. This is a focus of continuing research. However,
given that the petrographic and geochemical trends
observed here are directly comparable with those of other
terranes, such as the Noranda District, for which fluid
data do exist (Cathles, 1993; Galley, 1993; Paradis et al.,
1993), it seems reasonable to infer that a similar process
was responsible for the metasomatism in the Fortescue
Group.
An important point to note is that the same style of
metasomatic alteration, in terms of geochemical variation,
occurs across the entire Hamersley Basin, despite mineralogical variation being distinctly less obvious in highergrade zones (particularly zone IV); this is due to the
NUMBER 5
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persistence of actinolite (Fig. 8) and abundant Fe-poor
(colourless) epidote (Fig. 14). The same alteration style is
interpreted to result from the action of a single fluid, or at
least a common fluid source, acting across the entire
basin. This is compatible with the metasomatic fluid being
derived from seawater, as there was an effectively infinite
supply above the rocks of the Hamersley Basin.
Ultimately, the involvement of an external hydrothermal
fluid, probably derived from seawater, is considered to explain the regional-scale metasomatism observed in the
Fortescue Group. Comparisons of modern and ancient
geothermal systems, including ophiolite sequences, indicate that seawater interaction is prevalent at depths of
around 2 km (Evarts & Schiffman, 1983; Alt et al., 1986;
Hannington et al., 2003; Jowitt et al., 2012), which is comparable with the widespread alteration observed in zones
I and II of the Fortescue Group. The reduced extent of
metasomatism observed in higher-grade zones is interpreted, therefore, to relate to the inability of large volumes
of fluid to penetrate to those depths, other than in discrete
locations where faulting or other structural deformation
allow for extensive fluid circulation. Although folding in
the southern Hamersley Basin is related to the
Ophthalmian orogeny, much of the faulting is interpreted
as syn-Fortescue (Thorne & Trendall, 2001) and these
faults would consequently have been available as subvertical fluid pathways at any stage during the metamorphic
and metasomatic evolution of the Fortescue Group. The
link between metasomatic alteration and faulting in the
southern Fortescue Group is equivocal from field relationships and no definitive upflow or downflow zones are identifiable. As discussed above, such zones may have been
eroded from the overlying stratigraphy or be buried to the
south under the Ashburton fold belt. Alteration is, however, clearly still focused along vesicular, pillowed or brecciated flow tops, implying lateral fluid flow. Faulting is
inferred to aid in vertical fluid circulation as loci for downflow and upflow zones, feeding the conformable permeable
horizons. In contrast, the gentle dip of rocks and the relative lack of cross-cutting alteration zones in the northern
Hamersley Basin favour few broad, flat circulation cells,
rather than many steeper cells focused around structures.
Regarding tectonic setting, the geographically extensive
mafic volcanic rocks of the Fortescue Group are broadly
comparable with the widespread lavas of the Noranda
District in Quebec, Canada (Gibson et al., 1983; Lesher
et al., 1986; Hannington et al., 2003), and the other locations
discussed by Galley (1993). The Noranda District contains
a number of felsic to intermediate intrusions that are interpreted as the centres of shield volcanoes (Hannington
et al., 2003) and acted as a heat source driving hydrothermal circulation. The volcanoes themselves were the result
of an elevated geothermal gradient related to crustal rifting. Similarly, one possible heat source affecting the
1004
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
Fortescue Group and driving fluid circulation was an elevated geothermal gradient related to rifting and
volcanism.
A number of continental large igneous provinces have
been interpreted to be the result of mantle plumes
(Campbell, 2007). Although such an origin has not been
proposed for the Fortescue Group, a mantle plume must
be considered as an alternative cause of an elevated
geotherm. In this hypothetical setting, the observed rifting
could be ascribed to tension generated by uplift and
doming above the mantle plume. Furthermore, uplift
above a mantle plume may generate surface topography
providing an additional driving force behind regionalscale fluid flow. The concentration of ultramafic lava flows
in the southern Hamersley Basin corresponds to the highest grade, and therefore deepest [assuming the burial
model of Smith et al. (1982)], portion of the currently
exposed basin. The distribution of ultramafic material
may be taken as a proxy for heat flow, as with the distribution of high-temperature picritic magmas in the centre of
younger continental large igneous provinces that are associated with the axis of a mantle plume (Campbell, 2007).
However, a mantle plume model incorporating uplift and
doming is incompatible with inferred water depths during
eruption of the Fortescue Group. In the southern part of
the Hamersley Basin, where hypothetical uplift would
have occurred, the presence of pillow lavas indicates subaqueous eruption. In contrast, in the north of the basin,
lava flows are primarily subaerial, suggesting a higher
topographic level than in the south.
In both the rifting and volcanism, and the mantle plume
settings described above, the southern Hamersley Basin is
interpreted as being close to the zone of highest heat flow;
either a rift centre or the centre of a mantle plume. In this
case, thermal gradients are the major drivers of hydrothermal fluid flow. It is expected that upwards fluid flow
would occur above the region of highest heat flow, thereby
resulting in inwards flow from the basin margin in the
north. This fluid flow direction is incompatible with
the higher T overprint of metasomatic assemblages in the
north of the basin documented by White et al. (2014).
Alternative drivers of fluid flow other than thermal
gradients must now be considered. Based on consistent
P^T estimates of metasomatism across the Fortescue
Group, alteration occurred synchronous with or after
regional deformation, such that the present-day ground
surface, from which all samples were collected, was at a
constant structural level (depth) (White et al., 2014).
Hydrothermal fluid flow coincident with regional deformation might be subjected to both topographic and tectonic
driving forces, acting from the south, where folding is the
most intense (Figs 1 and 16), towards the north. Such a setting is comparable with the driving force behind fluid circulation responsible for the formation of many Mississippi
Valley-type base metal deposits (Oliver, 1986; Duane &
De Wit, 1988). Other studies that have focused on the
Hamersley Group iron formations (see below) have also
suggested that such a mechanism was active in driving
fluid flow related to the Hamersley iron ore deposits
(Hagemann et al., 1999; Oliver & Dickens, 1999; Powell
et al., 1999). In this model setting, regional-scale hydrothermal circulation is generally northwards, away from the
southern part of basin, and focused along permeable lava
flow tops. It is this permeability structure that promotes regional-scale fluid flow on the scale of tens to hundreds of
kilometres, as is the case in the Noranda District
(Hannington et al., 2003).
Ultimately, the timing, geometry, dynamics and driving
force behind the regional-scale hydrothermal circulation
system are equivocal from field, petrographic and geochemical evidence and a number of hypotheses (as above)
may be proposed. What is clear, however, is that the
Fortescue Group represents one of the oldest continental
flood basalt provinces that has undergone widespread, regional-scale metasomatism with extensive element mobilization. As such, it represents a post-eruptive history not
commonly documented in other continental large igneous
provinces.
I M P L I C AT I O N S F O R
H Y D RO T H E R M A L O R E S Y S T E M S
The regional-scale metasomatism in the Fortescue Group
has some interesting implications for hydrothermal ore
systems, both locally in the Hamersley Basin and more generally. Many of the documented occurrences of epidoterich rocks described above are associated with volcanogenic massive sulphide (VMS) deposits. This is particularly true in the Noranda District and the Troodos
ophiolite, where base metal depletion (as observed in this
study) is taken to indicate that these rocks are the now
metal-depleted root zones of VMS deposits (Richardson
et al., 1987; Hannington et al., 2003; Jowitt et al., 2012). To
date, despite extensive exploration efforts in the
Hamersley Basin, no VMS deposits have been found in
the Fortescue Group or overlying rocks. This may be because they were once hosted in the overlying material that
has since been eroded, because there was no suitable hostrock to promote mineralization, or because fluid flow was
too diffuse and was not concentrated enough to produce
an economic deposit.
Of greater interest is a potential link between regionalscale metasomatism in the Fortescue Group and the iron
ore of the overlying Hamersley Group. A hydrothermal
origin for the Hamersley iron ore has been proposed, although much debated, for many decades (Holland, 1973;
Barley et al., 1999; Powell et al., 1999; Taylor et al., 2001;
Dalstra & Guedes, 2004; Rasmussen et al., 2007; Thorne
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JOURNAL OF PETROLOGY
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et al., 2009; Morris & Kneeshaw, 2011; Evans et al., 2013). The
extent of Fe depletion associated with Fortescue Group
metasomatism cannot be ignored in the light of the worldclass iron ore deposits overlying it. Indeed, some proponents
of a hydrothermal origin for the Hamersley iron ore claim
the involvement of Ca-rich basinal brines, evolved from seawater and with elevated base metal contents, as determined
through fluid inclusion and isotopic studies of the iron formations (Taylor et al., 2001; Rasmussen et al., 2007; Thorne
et al., 2007, 2008, 2009; Evans et al., 2013). This is a similar
fluid type to that proposed here for regional-scale metasomatism in the Fortescue Group. Thorne et al. (2010)
described halogen ratios from iron ore deposits that indicate
that they formed through interaction with highly saline
brines that were generated at the surface through evaporation of seawater. Evans et al. (2013) further discussed this
idea and described ‘brine factories’ in enclosed basins. If
the fluid responsible for metasomatism in the Fortescue
Group formed through a similar process (Fig. 16), that may
explain the apparent lack of extensive Na metasomatism, as
discussed above.
As stated previously, no direct evidence for the fluid responsible for metasomatism in the Fortescue Group exists.
However, indirect evidence of a link between fluid activity
in the Fortescue and Hamersley Groups is present in
terms of likely fluid characteristics: high salinity and rich
in Ca, Fe and base metals. Although speculative, this discussion serves to highlight a potential link between regional-scale Fe depletion in the Fortescue Group and iron
ore formation in the overlying Hamersley Group; a coincidence that should not be overlooked.
Furthermore, the structural control on high-grade iron
ore (associated with faulting) in the southern Hamersley
Basin, as described by Powell et al. (1999) and Taylor et al.
(2001), links directly to our proposal that such faults aided
hydrothermal circulation. Specifically, faults in the southern Hamersley Basin acted as pathways that allowed the
hydrothermal fluid responsible for metasomatism in the
Fortescue Group to rise into the Hamersley Group
(Fig. 16). In addition, as regional deformation increased
from the south over time during the Ophthalmian orogeny,
folding could have led to episodic trapping followed by expulsion of hydrothermal fluids.
The ultimate implication is that the Fortescue Group
may be a source of Fe, as well as base metals, which are collected by the circulating hydrothermal fluid before rising
into the overlying Hamersley Group. Although we do not
advocate the Fortescue Group as a sole source of Fe, it
may contribute to the Hamersley system as an upgrading
mechanism, locally enriching the pre-existing sedimentary
banded iron formation into economic iron ore deposits.
The regional extent of metasomatism in the Fortescue
Group also has much broader implications for other hydrothermal systems in crystalline rocks, including, but not
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restricted to, those associated with hydrothermal ore deposits. The ability of hydrothermal fluids to permeate for
such large distances (4100 km), purely through inherent
permeability, has potential bearing on hydrothermal ore
systems in other greenstone terranes, including the eastern
Yilgarn Craton. In these settings, fluid flow is generally
considered to be focused along major fault structures
(Groves, 1993; Weinberg et al., 2004; Willman et al., 2010), although lateral flow away from conduits should not be
ignored and may be greater than previously realized.
Such lateral fluid migration is of course dependent upon
porosity and permeability, either primary or secondary,
which is itself related to metamorphic or alteration mineral
reactions. Ultimately, this has implications for the exploration of hydrothermal ore deposits, as detectable haloes
may be wider than commonly considered. More importantly, for the Australian setting in particular, the same
principle can be applied to deposits buried under cover,
which may be detected through subvertical fluid flow,
other than along subvertical structures.
CONC LUSIONS
Regional-scale metasomatism in the Fortescue Group has
resulted in the formation of widespread pumpellyite^
quartz and epidote^quartz rocks. Geochemical data show
marked depletions in alkalis, alkali earths, and heavy first
transition series metals (Mn to Zn). Petrographic features
indicate that metasomatic mineral associations overprint
and progressively replace regional metamorphic assemblages. Metasomatism therefore occurred once the regional burial profile had been established. Geological
context and comparison with other studies of epidote- and
pumpellyite-rich rocks suggest that metasomatism is
related to the hydrothermal circulation of seawater,
evolved to form a saline, Ca-rich fluid, capable of leaching
and transporting those depleted elements.
The observed metasomatic system in the Fortescue
Group operated at horizontal length scales of hundreds of
kilometres, and depth scales of as much as 8^10 km. Fluid
flow was predominantly through zones of enhanced primary porosity and permeability, such as vesicular and
brecciated lava flow tops. The spatial extent of metasomatism, and inferred fluid flow, has possible implications for
the origin of ore deposits in the Hamersley Basin itself
(Hamersley Group iron ore) but also for the scale of
hydrothermal systems in other mafic terranes. Specifically,
the haloes around hydrothermal ore deposits may be
larger than typically thought if fluid can migrate
such large distances without the need for major cross-cutting structures. This ultimately has implications for the
exploration for hydrothermal ore deposits in mafic
terranes.
1006
WHITE et al.
REGIONAL METASOMATISM, FORTESCUE GROUP
AC K N O W L E D G E M E N T S
We are grateful for the highly constructive reviews of J.
Glodny and two anonymous reviewers. D. Winchester and
M. Verrall are thanked for their assistance with sample
preparation, XRD and energy-dispersive spectrometry
analysis. The authors acknowledge the facilities, and the
scientific and technical assistance of D. Adams and the
Australian Microscopy & Microanalysis Research Facility
at the Centre for Microscopy, Characterisation &
Analysis, The University of Western Australia, a facility
funded by the University, State and Commonwealth governments. S. Barnes, M. Pearce and B. Godel are thanked
for their useful discussions during the course of this study.
FUNDING
This study was funded through CSIRO capability development fund fellowships to A. White and P. Nadoll.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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