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Journal of Petrology Volume 43 Number 2 Pages 375-402 2002
© Oxford University Press 2002
Decoupling of the Sm–Nd and Re–Os Isotopic Systems in Sulphide-Saturated Magmas in the Halls Creek Orogen, Western Australia
1DEPARTMENT OF EARTH SCIENCES, MONASH UNIVERSITY, CLAYTON, VIC. 3168, AUSTRALIA
2AUSTRALIAN GEOLOGICAL SURVEY ORGANISATION, GPO BOX 378, CANBERRA, A.C.T. 2601, AUSTRALIA
Received May 30, 2000; Revised typescript accepted August 30, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGPALAEOPROTEROZOIC LAYERED...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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We have observed apparent decoupling of the Re–Os and Sm–Nd isotopic systems in sulphide-saturated magmas that suggests that bulk two-component or assimilation–fractional crystallization (AFC) mixing modelling, based on Re–Os isotopic data, is inappropriate for chalcophile isotopic systems in turbulent sulphide-saturated magmas. This behaviour is observed in three Palaeoproterozic layered mafic–ultramafic intrusions in the Halls Creek Orogen of Western Australia. All intrusions clearly have a basaltic parental magma based on primitive olivine and spinel compositions. The intrusions are light rare earth element enriched and define a narrow range of initial Nd-isotopic signatures (
Nd = -0·9 to +0·70), being derived from an enriched mantle source or involving small degrees of crustal contamination. However, a wide variation in Os-isotopic compositions is observed, ranging from (1) slightly enriched for the Panton intrusion (
Os = +11), requiring an enriched-mantle source or a crustally contaminated mantle-derived magma, through (2) enriched for the McIntosh intrusion (Os= +38 to +87), to (3) strongly enriched for the Sally Malay intrusion (Os= +69 to +1300), requiring crustal contamination of a mantle-derived magma. AFC and two-component mixing models can explain the Os- and Nd-isotopic compositions, and in these models an upper crust or radiogenic Lewisian-type lower crust provides a good analogue for the contaminant. Furthermore, the Os isotopic data suggest a picritic parental magma for the Panton and McIntosh intrusions, whereas a basaltic parental magma is indicated for the Sally Malay intrusion. This is in contradiction with the clearly basaltic parental magma implied for all intrusions by olivine and spinel compositions. The inability of two-component mixing and AFC modelling to precisely simulate this system is the result of (1) the magma(s) attaining sulphide saturation, (2) the preferential partitioning of Os into immiscible sulphide and (3) the lowering of Os-isotopic ratios during equilibration between the host magma and immiscible sulphide at high R-factor (high mass ratio of silicate to sulphide melt). Such processes would not affect the lithophile Sm–Nd isotopic system, hence decoupling of the Sm–Nd and chalcophile Re–Os isotopic systems is implied. KEY WORDS: layered mafic–ultramafic intrusions; Palaeoproterozoic; Re–Os isotopes; sulphide saturation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPALAEOPROTEROZOIC LAYERED...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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When the sulphur-carrying capacity of a magma is exceeded, a magma will reach S saturation. In reduced [at oxygen fugacity (fO2) values less than those equivalent to the synthetic fayalite–magnetite–quartz (FMQ) buffer] magmatic systems, this will produce an immiscible sulphide liquid, essentially an Fe–S–O melt (e.g. Fincham & Richardson, 1954). In contrast, in oxidized (fO2 > FMQ) magmatic systems, sulphate (SO42-) is favoured (e.g. Luhr, 1990
). Mafic–ultramafic magmas are reduced magmatic systems, and thus excess sulphur is mostly present in an immiscible sulphide liquid (e.g. Naldrett, 1989). Immiscible sulphide liquids are important because chalcophile elements, such as Ni, Cu, platinum group elements (PGE; including Os) and Re, are concentrated into this sulphide liquid in preference to a silicate liquid (Peach et al., 1990; Fleet et al., 1994). This mechanism is extremely important because (1) it allows for concentration of these valuable chalcophile elements, which may produce an ore deposit if sufficient metals are concentrated into the sulphide liquid, and (2) the resulting metal-rich sulphide liquid is itself concentrated by processes such as gravitational settling (Irvine, 1975; Naldrett, 1989; Naldrett, 1997).
Magmas may attain sulphide saturation by (1) fractionation, (2) changing temperature and pressure and (3) addition of crustal material in the magma (Haughton et al., 1974; Buchanan & Nolan, 1979; Lesher, 1989; Naldrett et al., 1997). Fractionation and changes in temperature and pressure can be discriminated from crustal contamination by identification of crustal geochemical signatures, including negative Nb anomalies, or depleted Nd (Nd < -2) or enriched Os (e.g.Os >
50) isotopic signatures, or by physical features such as sedimentary xenoliths. In particular, the Sm–Nd and Re–Os isotopic systems are both powerful approaches for the identification of crustal contamination. However, both isotopic systems can display extremely disparate behaviour. Sm and Nd are both relatively incompatible lithophile elements, whereas Re and Os are chalcophilic elements, and will be partitioned into any resulting immiscible sulphide in a magma that has attained sulphide saturation.
If a magma attains sulphide saturation and an immiscible sulphide liquid is produced, then the resulting immiscible sulphide must interact with a sufficient quantity of silicate magma from which to scavenge chalcophile elements to increase their metal content to potentially economic concentrations. This can be mathematically described by the R-factor, defined as the mass ratio of silicate to sulphide liquid (Campbell & Naldrett, 1979). In a stagnant magma system, once all nearby chalcophile elements are partitioned into the silicate liquid, then no more chalcophile elements will be further partitioned. In contrast, in a turbulent magmatic system, all nearby chalcophile elements will be partitioned into the immiscible sulphide phase, then the immiscible sulphide droplet will move as the magmatic system is agitated, resulting in additional scavenging of chalcophile elements by the sulphide liquid. Stagnant magma systems are low R-factor magma systems (Campbell & Naldrett, 1979) whereas turbulent magmatic systems are termed high R-factor magma systems. R-factor is thought by Lambert et al. (1998b) to potentially have a strong effect on the Re–Os isotopic system in turbulent, high R-factor magmatic systems. In these systems, the turbulent behaviour of the magmatic system can potentially allow for extensive equilibration between the host magma and crustally contaminated sulphides. This can reduce the apparent initial Os isotopic composition of the sulphides from enriched crustal-like values (e.g. Os >20) to near-chondritic values (e.g. Os 0). This effect has been invoked to explain the apparent low Os observed in PGE ores from the Stillwater Complex, USA (e.g. Lambert et al., 1998b).
We present new trace element and Nd and Os isotopic data from three sulphide-saturated mafic–ultramafic intrusions in the Halls Creek Orogen, a Palaeoproterozoic linear orogenic belt (Hoatson & Blake, 2000). The three intrusions have experienced different magma dynamics, ranging from a stagnant low R-factor system to a relatively turbulent high R-factor system. Thus, these intrusions represent a unique opportunity to document and explore the effects of combined sulphide saturation and variable R-factor on these magmatic systems.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPALAEOPROTEROZOIC LAYERED...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Halls Creek Orogen
The Halls Creek and the King Leopold Orogens form two linear belts that bound the Kimberley Basin (Hancock & Rutland, 1984
). The Halls Creek Orogen is a NNE-trending orogenic belt composed of variably deformed and metamorphosed sedimentary, volcanic and intrusive Palaeoproterozoic rocks, and overlapping Proterozoic and Palaeozoic basinal sequences (Blake et al., 2000). It is bounded to the east by a composite craton of Archaean rocks overlain by Proterozoic rocks (Page & Hancock, 1988), including the Pine Creek Inlier.
The Halls Creek Orogen experienced multiple tectonomagmatic events between 1920 and 1780 Ma (Sheppard et al., 1995). Major tectonothermal events between 1900 and 1600 Ma are also observed in other Proterozoic terrains elsewhere; for example, the Hudsonian Orogeny in Canada and the Eburuian Orogeny in Africa (Etheridge et al., 1987). In particular, significant Proterozoic events are seen in Northern Australian orogenic belts between 1880 and 1850 Ma; for example, the Pine Creek and Mount Isa Inliers (Etheridge et al., 1987).
The igneous and volcanic rocks in the Halls Creek Orogen include igneous and low- to high-grade metamorphic rocks unconformably overlain by sedimentary rocks of the Kimberley Basin (Dow & Gemuts, 1969). The Halls Creek Orogen is divided by three NNE-trending regional faults: the Greenvale Fault to the west, the Springvale Fault in the centre and the Halls Creek–Angelo Faults to the east (Fig. 1). Three zones bounded by these faults are labelled the Western Zone, the Central Zone and the Eastern Zone, respectively (Griffin & Tyler, 1992).
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The Proterozoic history of the Central Zone is complex. The Central Zone is dominated by the Tickalara Metamorphics, comprising some of the oldest rocks that crop out in the Halls Creek Orogen. The Tickalara Metamorphics are a deformed sequence of interbedded mafic volcanic, siliciclastic and calcareous sedimentary rocks, which have undergone low- to high-grade metamorphism (Dow & Gemuts, 1969). Metamorphic grade increases from SW to NE along the zone (Tyler et al., 1994).
The Tickalara Metamorphics have been intruded by felsic, mafic–ultramafic magmas in a number of pulses from 1865 to 1805 Ma. Of most interest to this study are the ultramafic to mafic intrusions (at least 46 are recognized) that intrude the Tickalara Metamorphics and granites of the Bow River Batholith.
The age of the lithosphere underlying the Halls Creek Orogen is controversial. Hancock & Rutland (1984) suggested that underthrust structures of the Halls Creek Orogen dip west underneath the Kimberley Block. This implies that the crust and lithosphere of the Sturt Block to the east may also underlie the Halls Creek Orogen at depth. The oldest recognized rocks from the Sturt Block are Archaean, suggesting Archaean basement also underlies the Halls Creek Orogen. Furthermore, U–Pb SHRIMP zircon analyses from the Eastern Zone have identified detrital zircons with ages grouped at 2·5, 3·3 and 3·6 Ga (Page & Sun, 1991) and the data may indicate the presence of Archaean basement. However, the source of the detrital zircons is unclear, as they may be derived from within the Halls Creek Orogen itself or from adjacent Archaean terrains, such as the Pine Creek Inlier. Furthermore, at present, U–Pb SHRIMP dating of detrital zircons has been undertaken only on sediments from the Eastern Zone. Sm–Nd depleted mantle model ages from the Halls Creek Orogen range from Archaean to Palaeoproterozoic (3·2–2·18 Ga; Page & Sun, 1991) and so are inconclusive. Recent Re–Os isotopic data from xenoliths and chromite separates from the Argyle lamproite and the Seppelt and Maude Creek kimberlite, and chromite separates from the Kimberley Block, provide an imprecise chondritic Re–Os isochron of 3·4 Ga (Graham et al., 1999). Hence, the lithosphere underlying the Halls Creek Orogen may be up to 3·6 Ga in age.
Jacques et al. (1989, 1990) indicated that the trace element data from the Argyle lamproites require a mantle source strongly enriched in incompatible elements. They suggested that the mantle source was enriched as the result of mantle metasomatism (Jaques et al., 1989), which can occur by a variety of processes including subduction of crustal material into the upper mantle, or by migration of small volumes of incompatible element-rich melts (e.g. Frey & Green, 1974; Carlson, 1991). Some Sr, Nd and Pb isotopes indicate that this enrichment event is ancient, or involved addition of an ancient component (McCulloch et al., 1983; Fraser et al., 1985; Nelson et al., 1986; Sun et al., 1986). Nd and Os data for Miocene West Kimberley lamproites and the Argyle lamproites indicate that the enrichment event occurred at 1·7–2·0 Ga (Jaques et al., 1989; Graham et al., 1999), contemporaneous with the mafic–ultramafic intrusive rocks.
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PALAEOPROTEROZOIC LAYERED INTRUSIONS IN THE HALLS CREEK OROGEN |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPALAEOPROTEROZOIC LAYERED...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Hoatson (2000)
divided the layered mafic–ultramafic intrusions of the Halls Creek Orogen into seven major groups on the basis of their age, metamorphic–structural histories, mineralization and geochemistry (Table 1). The Panton and McIntosh intrusions were originally described as sills, but as they are observed to crosscut the enclosing country rocks they are referred to here simply as intrusions.
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The mineralized intrusions of the Halls Creek Orogen occupy two near-parallel NE-trending geographical corridors (Fig. 1; Hoatson, 2000). The first is a PGE–Au corridor (Group I intrusions numbered in Fig. 1 are 26, 30, 32, 33, 39 and 43) and the second is a slightly younger Ni–Cu–Co ± PGE corridor (Group II and V intrusions numbered in Fig. 1 are 3, 4, 7–10, 12–16, 18–20, 24 and 37) 8 km further to the NW. Chilled margins provide estimates of the chemical compositions of the parental magmas of these intrusions. These estimates imply a tholeiitic parental magma, similar to modern intraplate continental flood basalts (Sun et al., 1991), which may represent the products of high degrees of partial melting of the lithospheric mantle in an intraplate environment (Sun et al., 1991).
Hancock & Rutland (1984) suggested that the mafic–ultramafic intrusions represent uplifted fault-bounded segments of larger bodies that had crystallized at depth. However, the presence of chilled margins and contact aureoles, nearby satellite intrusions (Hoatson & Tyler, 1993), feeder conduits, and hybrid mixed zones between mafic magmas and melted country rocks (Blake & Hoatson, 1993; Hoaston & Tyler, 1993) indicate that the intrusions largely crystallized in situ.
Panton intrusion
The 1856 ± 2 Ma Panton intrusion (Page et al., 1995) is a relatively small, elongate mafic–ultramafic body 11 km in strike length with a maximum outcrop width of 2·5 km (Fig. 2; Hoatson; 2000). The intrusion broadly consists of a lower, rhythmically layered, ultramafic series (UMS) of 650 m thickness, and an overlying mafic series (MS) of 900 m thickness. Olivine and subordinate chromite are the only major cumulus phases in the UMS. Chemical and petrographic evidence indicates that cumulus orthopyroxene was absent in the UMS (Hamlyn, 1980). This implies a fundamental difference in the crystallization sequence of the Panton intrusion compared with that observed in the Stillwater Complex (Lambert et al., 1994), Bushveld Complex (Campbell et al., 1983) and Great Dyke of Zimbabwe (Podmore & Wilson, 1987), where bronzitites form thick lithological units (Hamlyn, 1980). The MS is marked by the first major appearance of cumulus plagioclase and consists of gabbronorite and gabbro packages (cumulus plagioclase, augite and rare cumulus orthopyroxene), gradually grading to anorthosite and Fe-rich gabbro packages (cumulus plagioclase, augite and magnetite). The Panton intrusion has a 0·7:1 ratio for the stratigraphic thickness of ultramafic rocks to mafic rocks, similar to the Great Dyke of Zimbabwe (Hamlyn, 1980) but different from the more mafic-dominated Bushveld and Stillwater Complexes (1:4 to 1:6).
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PGE-enriched mineralization in the Panton intrusion is concentrated in chromitite seams that are believed to have formed when new pulses of primitive basaltic magma entered the chamber and mixed with more fractionated resident magma (Perring & Vogt, 1991; Sproule et al., 2000). Mixing between compositionally disparate magmas can form a hybrid magma oversaturated with chromite and sulphide (Irvine, 1975, 1977). This model is supported by reversals in fractionation trends in trace elements across major chromitite seams in the Panton intrusion (Hamlyn et al., 1988; Sproule et al., 2000). The five thickest chromitite seams (A–E) occur in the UMS (Perring & Vogt, 1991). Minor thin chromitites are hosted within dunite lenses in the MS. The A seam, 120 m below the UMS–MS contact, is the thickest (0·8–2·5 m) and contains the highest PGE grades (3·1 ppm Pt, 3·7 ppm Pd, 105 ppb Os). The Panton intrusion appears to have been sulphide saturated stratigraphically above and below the A seam, based on the appearance of small amounts of cumulus sulphide throughout the sequence. Hamlyn et al. (1988) noted that the background S content of the UMS (742 ppm) is more than four times higher than the mean S content of the sulphide-undersaturated ultramafic sections of the Stillwater Complex.
Sally Malay intrusion
The 1844 ± 2 Ma Sally Malay mafic–ultramafic intrusion (Page & Hoatson, 2000) consists of four small layered bodies (Fig. 3) with a total surface area of 2 km2 (Hoatson, 2000). It was intruded after a major metamorphic event at 1850 Ma and was emplaced into variably metamorphosed country rocks of the Tickalara Metamorphics. The Tickalara Metamorphics are lithologically diverse, including migmatites, granulites and minor graphite–pyrrhotite-bearing gneiss horizons (Thornett, 1981; Hoatson, 2000). The most important section of the intrusion is the southern body, which hosts the Ni–Cu–Co sulphide deposit within a thin gabbroic unit separating the overlying peridotite from the underlying footwall migmatites (Thornett, 1981). The gabbroic unit, which varies from 5 to 40 m in thickness, has been divided into a subordinate upper unit of norite containing euhedral pyroxene and a lower unit of ophitic norite and olivine norite (Thornett, 1981).
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Sulphide mineralization exhibits two major textural types: matrix sulphide (5–40% volume) and massive sulphide (90–100% volume). The massive sulphide consists of primary hexagonal pyrrhotite and minor granular pentlandite, chalcopyrite and rounded magnetite (1 mm in diameter). Massive sulphides show little indication of recrystallization and are coarse grained (pyrrhotite 1–3 mm diameter; pentlandite 1 mm diameter). Matrix sulphides extend upward into the basal peridotite and also form the matrix to breccias in the footwall migmatites.
The Sally Malay intrusion shows many similarities to the recently discovered Voisey’s Bay Ni–Cu–Co deposit in Labrador, Canada (Ryan et al., 1995; Naldrett et al., 1996; Hoatson et al., 1997; Lambert et al., 1999; Sproule et al., 1999). Both mineralized intrusions occupy similar tectonic settings, namely Proterozoic 1·85 Ga collisional suture zones (Torngat and Halls Creek Orogens), and both contain troctolites (Ryan et al., 1995; Naldrett et al., 1996). The most mineralized section of Voisey’s Bay is interpreted to represent a feeder conduit to other exposed sections of the intrusion (Ryan et al., 1995; Naldrett et al., 1996). This analogy can also be applied to the sulphide-hosting southern body of the Sally Malay intrusion (Hoatson et al., 1997; Sproule et al., 1999).
McIntosh intrusion
The 1830 ± 3 Ma McIntosh mafic intrusion (Page & Hoatson, 2000) has a total north–south outcrop length of 14 km and an east–west outcrop width of 6 km (Fig. 4). The intrusion is interpreted to have a stratigraphic thickness of 7·8 km and possesses an overall funnel-shape (Hoatson, 2000). The McIntosh intrusion consists of at least six megacycles formed in an open magma system (Mathison & Hamlyn, 1987). A restricted range of lithologies is observed, including troctolite and olivine gabbro and lesser gabbronorite and peridotite. The rocks all possess cumulus textures ranging from adcumulate to mesocumulate with <10% postcumulus minerals (Mathison & Hamlyn, 1987).
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The troctolites consist of cumulus plagioclase and olivine with postcumulus orthopyroxene and augite. Olivine gabbros contain cumulus olivine, plagioclase and augite with rare postcumulus orthopyroxene. The gabbronorites range from olivine gabbronorite to olivine-free gabbronorite and to magnetite gabbronorite. The rare peridotites consist of >80% cumulus olivine and <20% postcumulus plagioclase.
Interstitial sulphides are present in all rocks in varying proportions, attaining 5 wt % in the uppermost olivine–magnetite gabbronorite. Typically, sulphides possess pristine magmatic bleb-like textures, although some recrystallization of sulphides to irregular forms is rarely present in the lower olivine-bearing zone. The dominant sulphide is pyrrhotite with lesser chalcopyrite and pentlandite. On the basis of the presence of interpreted immiscible sulphide droplets throughout the intrusion and high whole-rock S contents (0·05–0·1 wt %), Mathison & Hamlyn (1987) concluded that the McIntosh intrusion was sulphide saturated (or close to saturation) throughout most of the crystallization history.
It should be noted that, overall, the early appearance of plagioclase (e.g. before clinopyroxene) in the crystallization sequence of these intrusions is similar to that of H2O-poor, tholeiitic magmas.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPALAEOPROTEROZOIC LAYERED...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The major element data generated for this study were obtained by X-ray fluorescence spectrometry (XRF) of fused glass disks at the University of Melbourne using the method of Thomas & Haukka (1978)
. Ni, Cu, Cr, V and Zn data were obtained in the same laboratory by XRF of pressed powders using the method of Thomas & Haukka (1978). Selected trace elements, including the rare earth elements (REE), were obtained by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) on a Finnigan MAT ‘Element’ system following high-pressure acid digestion.
Sm–Nd isotopic data were obtained by isotope dilution using a Finnigan MAT 262 multi-collector thermal ionization mass spectrometer at La Trobe University according to the methods described by Elburg & Nicholls (1995), which are similar to those of Maas & McCulloch (1991). Re–Os isotopic data were obtained by isotope dilution negative thermal ionization mass spectrometry (N-TIMS), using the methods of Lambert et al. (1998a).
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPALAEOPROTEROZOIC LAYERED...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Major and trace element data
Major element data are shown in Table 2. On the basis of major element data the magmatic affinities of the intrusions are unclear, as discrimination diagrams are generally based on volcanic rocks and are not suitable for cumulate rocks. Figure 5 shows an AFM diagram indicating that the intrusions have tholeiitic affinities. More convincing, however, is the upward increasing trend of TiO2 and V in the McIntosh and Panton intrusions (Mathison & Hamlyn, 1987
; Hamlyn et al., 1988, which is typical of evolving tholeiitic magmas. The tholeiitic character of the parental magmas is also supported by the Cr, Al and Ti concentrations of spinels in chromitite layers, which are similar to those of tholeiitic magmas (Mernagh et al., 2000; Sun & Hoatson, 2000). Thus, the Panton, Sally Malay and McIntosh intrusions are believed to have tholeiitic to subalkaline affinities.
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, McIntosh intrusion; •, Sally Malay intrusion; 
, Panton intrusion [this study and Hamlyn et al. (1988)
Trace element data are shown in Table 2. Figure 6 shows primitive mantle-normalized trace element variation diagrams for the intrusions with comparisons with other magmas, including mid-ocean ridge basalts (MORB), ocean-island basalts (OIB), continental flood basalts (CFB) and the proposed parental magmas to the Stillwater Complex. We do recognize that the rocks from this study are dominantly of cumulus origin, thus a direct comparison with basalt analyses from other magmas is not appropriate. However, these diagrams are useful in allowing comparison of incompatible elements relative to each other, and to these other magmas. The overall abundance of incompatible elements may vary as a result of dilution; however, the relative slope will not, provided the elements present remain incompatible in the crystallizing assemblage. For this reason, we have selected trace elements that are incompatible in the crystallizing assemblages (olivine, orthopyroxene, plagioclase, clinopyroxene and chromite). Furthermore, the selected trace elements are relatively immobile during alteration and/or metamorphism. To further remove the effects of a cumulus origin, we have also utilized trace element ratios to examine trace element affinities (Fig. 7).
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Trace element abundances for most rocks in all intrusions are generally low, reflecting a cumulus origin. In the Panton intrusion, incompatible trace element abundances generally increase stratigraphically upward. Large ion lithophile elements (LILE), such as Rb, Sr and Ba, show considerable variation that can be related to several factors. First, these elements are controlled by the presence of cumulus plagioclase, with higher abundances in the MS, where cumulus plagioclase is present. Second, the LILE are highly mobile and thus their abundances may have been affected by lower amphibolite metamorphism. The high field strength elements (HFSE) and middle REE (MREE) to heavy REE (HREE), from Nd to Yb (Fig. 6), generally show flat primitive mantle-normalized trace element patterns. Negative Nb and Zr anomalies are observed, which are more pronounced in the UMS than in the MS. Overall higher abundances of LILE/HFSE are recorded (Fig. 6 and Table 3). These characteristics are comparable with those of some CFB (Table 3). [Nb/Th]MN and [Th/Sm]MN in the Panton intrusion (and in fact also for the Sally Malay and McIntosh intrusions; see below) lie on a mixing curve between a typical basalt or picrite from a depleted upper mantle and upper continental crust (Fig. 7).
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Like the Panton intrusion, the Sally Malay intrusion displays an enrichment in the LILE compared with the HFSE; however, this enrichment is more pronounced (Fig. 6). In particular, concentrations of LILE, including Rb, Ba and the light REE (LREE), are intermediate to high considering the magnesian character of the rocks. Furthermore, LILE abundances are more constant in the Sally Malay intrusion than are those in the Panton intrusion. This indicates that the Sally Malay intrusion has not experienced lower-amphibolite-grade metamorphism like the Panton intrusion, and also that the sampled rock types are of a more restricted composition. The enrichment in LILE/HFSE is similar to that observed in OIB, some CFB, siliceous high-magnesian basalts, boninites and the proposed parental magma of the Stillwater Complex. Minor negative Nb anomalies are present in some parts of the intrusion.
The McIntosh intrusion samples show similar trace element features to the other intrusions (i.e. enrichment in LILE compared with HFSE; Fig. 6). The enrichment in LILE/HFSE is not as pronounced as that observed in the Panton and Sally Malay intrusions. Some parts of the intrusion possess negative Nb anomalies.
Figure 8 shows chondrite-normalized REE diagrams for the Panton, Sally Malay and McIntosh intrusions. All samples in the Panton intrusion are LREE enriched ([La/Sm]CN = 1·04–8·3) except the altered chilled margin sample (La/SmCN = 0·76) with flat HREE chondrite-normalized patterns ([Gd/Yb]CN = 0·86–1·8). Samples containing abundant plagioclase (in the MS) possess positive Eu anomalies (Eu/Eu* = 1·22–1·42), whereas samples with minor or no plagioclase (in the UMS) show minor negative Eu anomalies (Eu/Eu* = 0·84–1·06). In the Sally Malay intrusion, moderate LREE enrichments ([La/Sm]CN = 1·8–2·6) and flat HREE patterns ([Gd/Yb]CN = 1·02–1·69) are present. Only minor Eu anomalies are observed (Eu/Eu* = 0·88–1·06). Similarly, for the McIntosh intrusion, a moderate LREE enrichment ([La/Sm]CN = 1·4–3·0) is present, excluding MAC7, which possesses a distinct LREE depletion ([La/Sm]CN = 0·5). All samples have flat HREE patterns ([Gd/Yb]CN = 1·5–1·6 and [Gd/Lu]CN = 1·4–1·7). Distinct positive Eu anomalies are observed for all samples (Eu/Eu* = 1·3–3·55).
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Nd isotopic data
All Sm–Nd isotopic data are shown in Table 4. Sm–Nd isotopic data for the Panton intrusion define a Model 1 errorchron (McIntyre et al., 1966) with an age of 1863 ± 40 Ma and an initial Nd value of -0·9 with a mean square of weighted deviates (MSWD) of 9·14 (Fig. 9). The high MSWD suggests that there is excess scatter about the isochron, more than can be accounted for by analytical uncertainty (McIntyre et al., 1966). A Model 3 isochron regression yields an identical age of 1865 ± 30 Ma and an initial Nd of -0·9, within uncertainty of the U–Pb SHRIMP zircon age of 1856 ± 2 Ma (Page et al., 1995). This suggests that the Sm–Nd isotopic system has remained closed and that there may have originally been minor subtle heterogeneity in initial Nd isotopic composition within the magma chamber. Similarly, for the McIntosh intrusion, the data define a Model 1 errorchron (McIntyre et al., 1966) with an age of 1851 ± 110 Ma and an initial Nd value of 0·1 with an MSWD of 110 (Fig. 8), when the sample MAC7 is excluded. A Model 3 isochron regression yields an age of 1848 ± 61 Ma and an initial Nd of 0·2, when the sample MAC7 is excluded, within uncertainty of the U–Pb SHRIMP zircon age of 1830 ± 3 Ma (Page et al., 1995). This suggests that the Sm–Nd isotopic system has remained closed. MAC7 is from the poorly outcropping margin region of the intrusion, and it was not clear whether this region was part of the intrusion. The range of Sm/Nd values from the Sally Malay intrusion is too small to define an isochron. A limited range of initial Nd values are observed (-0·2 to +0·7), falling within the range of the McIntosh and Panton intrusions.
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Os isotopic data
All Re–O isotopic data are displayed in Table 5. Re–Os isotopic data for samples from the Panton intrusion, excluding one dunite whole rock, the associated chromite separate and the chilled margin sample, define a Model 1 errorchron (McIntyre et al., 1966) with an age of 1780 ± 17 Ma and an MSWD of 1700. This suggests that there is again excess scatter about the isochron, more than can be accounted for by analytical uncertainty (McIntyre et al., 1966). A Model 3 isochron regression yields an age of 1850 ± 40 Ma and an initial Os value of +11 (percent deviation of initial Os isotopic composition from chondritic asthenospheric mantle of the same age; Fig. 10). This isochron age is within error of the Sm–Nd isochron age and the SHRIMP U–Pb zircon age, implying that the Re–Os isotopic system has remained closed. Two samples are observed to deviate significantly from the isochron, with much higher initial Os values. A dunite whole-rock sample and the associated chromite separate have initial Os of 107 and 135, respectively. The dunite sample shows evidence of postcrystallization fluid alteration including extensively altered postcumulus phases and recrystallization of sulphides along grain boundaries and within fractures of olivine and chromite grains. Thus, this sample has probably experienced Re loss. This would lower the Re/Os ratio of the sample and increase the apparent initial Os isotopic composition, as observed for the sample. The chilled margin sample also demonstrates a high positive Os value (+1480). This represents either addition of radiogenic Os or Re loss via metamorphic or hydrothermal fluids. The chilled margin contains secondary quartz from the infiltration of hydrothermal, metamorphic or meteoric SiO2-bearing fluids. These fluids may be responsible for lowering the apparent initial Os isotopic signature by either Os gain or Re loss. Re loss appears unlikely, however, as the Re/Os ratio is very high (16). In fact, Re gain is more likely as the Re/Os ratio is so high (Fig. 9). Addition of Os via fluids is difficult to constrain, as the mobility of Os in fluids is relatively poorly understood. Conversely, wall-rock melting and assimilation may be responsible for the highly radiogenic signature observed, as old crustal material has highly radiogenic Os isotopic signatures.
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, chromitite seams and separates; 
, Mafic Series cumulates. Sally Malay intrusion: 
, massive sulphide; 
, magnetite from massive sulphide. grey square, McIntosh intrusion. Stillwater Complex: 
, J-M Reef chromitite. All Stillwater Complex data are from Lambert et al. (1994)
In contrast, Os isotopic data from the Sally Malay intrusion are significantly more radiogenic. Massive sulphides (including separates of pentlandite, pyrrhotite and magnetite) yield exceptionally radiogenic initial Os isotopic compositions (Os = +950 to +1230). Massive sulphides and separates yield a Model 3 isochron age of 1891 ± 37 Ma, similar to the magmatic U–Pb zircon and baddeleyite intrusion ages of 1844 ± 3 Ma and 1846 ± 5 Ma, respectively (Page & Hoatson, 2000), suggesting that these samples have probably remained part of an isotopically closed system. A Model 3 isochron was selected because of the small sample size (McIntyre et al., 1966). Troctolite with matrix-disseminated sulphides (40% sulphide vol.) possesses extremely enriched Os isotopic signatures (Os = +450 to +470), whereas those from weakly mineralized troctolite and peridotite range are significantly less radiogenic (Os = +60 to +369). Troctolite, olivine gabbro and peridotite with matrix and disseminated sulphide yield a Model 3 isochron (selected because of the small sample size; McIntyre et al., 1966) of 1893 ± 57 Ma (Fig. 9), within error of the magmatic U–Pb zircon age, suggesting that these samples have remained part of an isotopically closed system. All Re–Os isotopic data from the Sally Malay intrusion do not form an isochron, but lie between the 1845 Ma chondritic and enriched reference lines (Fig. 10).
The Os isotopic dataset from the McIntosh intrusion is too small for an isochron regression to be made. The three data points define a limited initial Os range (Os = 38–87; Fig. 9) intermediate between those observed for the Panton and Sally Malay intrusions.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPALAEOPROTEROZOIC LAYERED...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Evidence for sulphide saturation
Evidence for sulphide saturation in the Panton, Sally Malay and McIntosh intrusions (Sun et al., 1991
; Sun & Hoatson, 2000) includes the following: (1) high background levels of sulphur (typically >500 ppm), especially when the sulphur content of a rock is recalculated to remove the effect of cumulus phases to 100% trapped melt fraction (Sun & Hoatson, 2000); (2) high PGE abundances in chilled margin (e.g. Sun et al., 1991); (3) presence of trace sulphides in all samples. As all magmas have probably attained sulphide saturation, then in these reduced magmatic systems, excess sulphur will be present within an immiscible sulphide melt. PGE, including Os, and Re will be partitioned into this immiscible sulphide component.
Petrogenesis of the intrusions
Table 6 summarizes the geochemical features of the Panton, Sally Malay and McIntosh intrusions. All the intrusions display similar major and trace element abundances and initial Nd isotopic compositions, and tholeiitic to subalkaline magmatic geochemical trends. Other geochemical features include higher LILE/HFSE ratios than MORB (but comparable with OIB and CFB), and chondrite-normalized LREE enrichment and flat HREE abundances. The observed trace element geochemistry is indicative of both enriched-mantle sources (similar to OIB and CFB sources) and/or mantle melts that have assimilated upper-crustal material. However, many portions of the intrusions possess a negative Nb anomaly, which is suggestive of assimilation of upper-crustal material. When viewed in a [Th/Sm]MN vs [Nb/Th]MN diagram, data from all intrusions lie on a mixing curve between a basalt or picrite derived from typical depleted mantle and upper crust (Fig. 7), implying that crustal contamination was an important process in the petrogenesis of these intrusions.
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The initial Nd isotopic composition for all samples is significantly lower than that of depleted mantle of the same age (Nd = 4·5; Michard et al., 1985), but is comparable with that of enriched magmas such as OIB (e.g. Stille et al., 1983; Saunders et al., 1988). In contrast to the relatively constant trace element and Nd isotopic data, a wide range in Os isotopic compositions is observed for the three intrusions: low (Os = +11) for the Panton intrusion; low to intermediate (Os = +38 to 87) for the McIntosh intrusion; and intermediate to extremely radiogenic (Os = +69 to 1300) for the Sally Malay intrusion. Thus, crustal contamination has clearly been an important petrogenetic process for the McIntosh and Sally Malay intrusions. The Os isotopic signatures observed in the Panton intrusion are consistent with either crustal contamination or an enriched source with isotopic characteristics lying within a range typical of enriched-mantle reservoirs, similar to the source of OIB. This includes enriched Os isotopic signatures associated with plume-derived magmas, including Gorgona (Walker et al., 1991), Noril’sk–Talnakh (Walker et al., 1994), Karoo (Ellam et al., 1992), Keweenawan basalts (Shirey, 1997) and OIB, which range up to Os = +12 (Shirey, 1997). Such a mantle source could also contribute to the enriched trace element and Nd and Os isotopic characteristics observed in the McIntosh and Sally Malay intrusions.
Two-component and AFC mixing models
Two-component and AFC mixing models were used to examine potential source regions for the intrusions. Only small amounts of contamination are permissible to retain the generally primitive characteristics, particularly high MgO and low SiO2, of the intrusions, typically <10% for both isotopic systems. The crustal contamination processes were assessed using both trace element and Os and Nd isotopic data. Typically, Nd and Os isotopes are preferred over trace element data for petrogenetic modelling as they provide more useful discriminants between different petrogenetic histories (Carlson, 1991).
Modelling with trace element data
Figure 7 shows primitive mantle-normalized trace element data from each of the intrusions in a [Th/Sm]MN vs [Nb/Th]MN diagram. All trace element data from all intrusions lie on a mixing line between typical Archean crust and a MORB magma and a picrite, with a maximum degree of contamination of 20% and 10%, respectively.
Modelling with Nd and Os isotopic data
To develop the isotopic compositions of the intrusion a variety of end-members for a mantle-derived magma were selected, including (1) uncontaminated tholeiitic depleted mantle-derived basalt, (2) picrite and (3) enriched lithospheric mantle-derived melt. The modelling parameters for the magma types above are shown in Table 7.
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For an upper-crustal composition, average upper-crustal compositions (Taylor & McLennan, 1995) were employed because of the paucity of Sm–Nd isotopic data for sediments from the Central Zone in the Halls Creek Orogen. Interaction between a mantle-derived magma and lower crust is extremely difficult to model, owing to the paucity of Nd and Os isotopic composition for an Archaean or Palaeoproterozoic lower crust. Furthermore, isotopic and trace element analyses from younger lower crust show great variability (e.g. Taylor & McLennan, 1995; Frick et al., 1996; Saal et al., 1998). Thus, two extreme end-members were employed: (1) unradiogenic Os and radiogenic Nd isotopic compositions similar to those observed in lower-crustal xenoliths (e.g. Rudnick & Fountain, 1995; Saal et al., 1998); (2) radiogenic Os and unradiogenic Nd isotopic signatures as observed in lower-crustal sections, such as the Lewisian (Weaver & Tarney, 1984; Frick et al., 1996). The parameters for the three crust types employed include the following:
- upper crust: Nd 26 ppm, Sm/Nd = 0·213, Os 0·05 ppb, Re/Os = 20 (Molzahn et al., 1995; Taylor & McLennan, 1995);
- Type 1 lower crust (Lewisian-like): Nd 18 ppm, Sm/Nd = 0·183, Os 0·05 ppb, Re/Os = 50 (Weaver & Tarney, 1984; Frick et al., 1996);
- Type 2 lower crust (based on the average composition of a range of lower-crustal xenoliths): Nd 15 ppm, Sm/Nd= 0·240, Os 0·1 ppb and Re/Os = 2 (Rudnick & Fountain, 1995; Saal et al., 1998).
Both the upper- and lower-crustal end-members used in the modelling were developed by using a chondritic initial 187Os/188Os ratio with an upper- and lower-crustal Re/Os ratio (Saal et al., 1998; Shirey & Walker, 1998) and a depleted mantle Nd isotopic signature and lower-crustal Sm/Nd ratio (Taylor & McLennan, 1995) with ages ranging from 3·6 Ga to 1·856–1·84 Ma; 3·6 Ga was used as an upper bound based on the oldest zircons observed in the Halls Creek Orogen (Page & Sun, 1991). Table 8 shows the isotopic composition of the upper and lower crust thus formed.
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Figure 11 shows the crustal contamination curves in Os–Nd space for each of the mantle-derived magmas with the upper-crustal and Type 1 lower-crustal contaminant at ages of 3·6, 3·3, 3·0, 2·7 and 2·4 Ga. Figure 12 shows the amount of contamination required by the Nd and Os isotopic systems. Ancient upper and Type 1 lower crusts (3·6 Ga) produce models that better fit the isotopic composition of the intrusions at smaller degrees of assimilation. If the models are valid then tectonic models for the development of the Halls Creek Orogen must include and explain the presence of ancient crust.
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On the basis of Os and Nd isotopic two-component mixing, a picritic parental melt was the only viable parental magma for the Panton intrusion for both Type 1 lower crust and upper crust. However, this clearly contrasts with the basaltic parental magma inferred from olivine and spinel compositions in ultramafic cumulates (Sun & Hoatson, 2000). Similarly, for the McIntosh intrusion, a picritic parental magma is preferred, again in contradiction to the estimated parental magma composition determined by Sun & Hoatson (2000) using mineral chemistry. Finally, the extremely radiogenic Os isotopic signatures of the Sally Malay intrusion can be explained only by a basaltic parental magma from an enriched-mantle source or an asthenospheric depleted mantle-like source, possessing low Os concentrations. This is in agreement with the estimated parental magma of Sun & Hoatson (2000). Modelled Type 2 lower crust possesses relatively unenriched Os isotopic and enriched Nd isotopic compositions. As a result of these compositions, extremely large amounts of Type 2 lower-crustal assimilation are required for all magma types based on the Nd isotopic composition alone, typically >30% (see Table 8). Assimilation of this much material is unlikely, hence Type 2 lower crust is not considered to be a viable contaminant.
AFC processes could have reproduced the Os and Nd isotopic characteristics of the three intrusions, but only if two different parental magmas were involved (Fig. 13). For example, a picritic parental melt is a viable parental magma for both the Panton and McIntosh intrusions with upper crust being the contaminant during AFC. For the Sally Malay intrusion, however, which possesses exceptionally radiogenic Os isotopic compositions, only a basaltic melt was a viable parental magma regardless of whether the contaminant is upper or Type 1 lower crust. However, this AFC modelling was undertaken using a DOs = 7, which assumes that the magma was sulphide undersaturated. AFC processes are thought by some workers (e.g. Sun et al., 1989; Lambert et al., 1994) to drive magmas to sulphide saturation, in addition to the fact that these magmas were probably sulphide saturated (Sun & Hoatson, 2000), thus a larger DOs of 30 000 (Lambert et al., 1998b; Fig. 13) would be more appropriate for these systems. However, with such a high DOs, typical of a sulphide-saturated magma, the Os and Nd isotopic compositions of the Panton, Sally Malay and McIntosh intrusions could not be reproduced, regardless of the parental magma. Similar to two-component mixing, modelled Type 2 lower crust again requires an unrealistically large amount of crustal assimilation for all intrusions.
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Implications of crustal contamination modelling for the behaviour of chalcophile isotope systems in sulphide-saturated magmas
Theoretical models
As discussed previously, crustal contamination will probably result in a magma attaining sulphide saturation, with the resulting formation of an immiscible sulphide phase. This immiscible sulphide phase will partition Re and Os, significantly depleting these elements from the sulphide-saturated silicate magma. In contrast, in sulphide-undersaturated magmas, Re and Os abundances will remain near constant, as both elements are incompatible.
Using bulk two-component mixing models is inappropriate for chalcophile isotope systems in sulphide-saturated magmas for a number of reasons. First, the Os and Re concentrations of both the parental magmas and contaminants are often poorly constrained, and will also vary markedly during the petrogenesis of the intrusions, thus estimating the concentration of Os and Re at any time in a sulphide-saturated magma will be extremely difficult. Second, the extent of crustal contamination and the effect on the Os isotopic system can be masked by other physical processes. For example, in extremely turbulent magmatic systems (high R-factor magma systems) vigorous mixing between host magma and the crustally contaminated sulphides can reduce the apparent initial Os isotopic composition of the sulphides to near-chondritic values (Lambert et al., 1998b). Finally, the assimilation process may be selective, with only the Os isotopic systematics of the intrusions affected (e.g. selective assimilation of crustal sulphides via devolatilization, e.g. Lambert et al., 1998b; Ripley et al., 1998), similar to the model proposed by Lesher & Campbell (1993) for the genesis of komatiite-hosted Ni deposits at Kambalda. Molzahn et al. (1993) noted that the soluble (volatile) component of crustal rocks contains higher radiogenic Os concentrations than the bulk crustal rocks. Thus, digestion of radiogenic crust via devolatilization may produce extremely enriched Os isotopic composition in the sulphide component, but retain the same original trace element signature and Nd isotopic values present in the parental magma before assimilation. Further work is required to fully realize the mechanics of crustal contamination, in particular how this process affects systems with two phases (silicate and sulphide melt) and also the effect of variable contaminants (i.e. S-rich compared with S-poor crustal rocks).
Implications for the Os isotopic compositions on the sulphide-saturated intrusions of the Halls Creek Orogen
On the basis of trace element and Nd isotopic data, all the intrusions in the Halls Creek Orogen have assimilated crustal material. However, there is a large variation in the amount of contamination indicated by the Nd isotopic system compared with that indicated by the Os isotopic system in each of the intrusions. Furthermore, although the intrusions occupy a small temporal and spatial region, the AFC models that provide the best fit to the observed data indicate that each intrusion was sourced from a very different parental magma compared with the neighbouring intrusions. This is due to the effects of sulphide saturation on the Re–Os isotopic system in each of the intrusions.
Different amounts of both upper-crustal and Type 1 lower-crustal contamination are indicated by two-component modelling based on the Nd and Os isotopic systems (see Fig. 11). Typically, far larger amounts of crustal contamination are required by two-component mixing models based on Os isotopic data compared with those based on Nd isotopic data. This is due to the factors outlined above, namely: (1) selective assimilation of Os-rich volatile components; (2) the difficulty of estimating the Os concentration in sulphide-saturated magmas at a particular stage; (3) R-factor equilibration. In particular, low R-factors (typically R < 200) in the Sally Malay intrusion (Sproule et al., 1999) will allow limited equilibration between the immiscible sulphide proto-ore and host magma and retention of enriched crust-like Os isotopic compositions. In contrast, within the Panton intrusion, high R-factors (up to 50 000: Sproule et al., 2000) allow extensive equilibration between the host magma and the crustally contaminated sulphides, reducing the apparent initial Os isotopic composition of the sulphides to near-chondritic values. The McIntosh intrusion may represent an example of intermediate R-factor (200–800), between that of the Sally Malay and Panton intrusions. Similarly, these processes would also be responsible for the different parental magmas indicated for each of the intrusions from two-component mixing and AFC modelling that contradict the clearly basaltic parental magma reported by Sun & Hoatson (2000) for all intrusions based on olivine and chromite compositions. The low R-factors in the Sally Malay intrusion (Sproule et al., 1999) will result in retention of crustal-like high Os isotopic signatures, which will require a basaltic parental magma in two-component mixing. In contrast, in the Panton and McIntosh intrusion, higher R-factors will produce more chondritic values, which implies a picritic parental magma in two-component mixing.
In summary, two-component and AFC mixing models do not successfully explain both the Nd and Os isotopic compositions of the intrusions. In particular, for the McIntosh and Panton intrusions an Os-rich parental magma is preferred, in contrast to the apparent tholeiitic parental magma suggested by olivine and spinel compositions (Sun & Hoatson, 2000). The lack of convergence in modelling of the two isotopic systems is the result of the host magmas attaining sulphide saturation, thereby causing decoupling of the Nd and Os isotopic systems as Os is preferentially hosted in immiscible sulphide and Nd in the silicate component. An ancient crustal (3·6 Ga) contaminant is preferred, as this will reduce the amount of contamination required. However, parental magmas derived from an enriched-mantle source will also reduce the amount of crustal assimilation required and/or potentially allow for a younger crustal assimilant.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPALAEOPROTEROZOIC LAYERED...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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- The trace element and Os and Nd isotope geochemistry of the Panton intrusion is consistent with either a magma derived from an enriched-mantle source or a crustally contaminated magma derived from a more depleted mantle source.
- The Sally Malay and McIntosh intrusions display trace element and isotopic compositions consistent with crustal contamination of a mantle-derived magma. Crustal contamination would mask the trace element and Nd and Os isotopic characteristics of the original parental magma.
- Models involving an upper-crustal or Lewisian-type lower-crustal contaminant (3·6 Ga) are preferred; however, models that use a parental magma derived from an enriched-mantle source region give reasonable results, involve low degree of contamination, and reduce the required age of the upper crust.
- Sulphide saturation of the magmas results in decoupling of the Nd and Os isotopic systems, and prevents accurate estimation of the Os concentration of the parental magma. Thus, two-component and AFC-type modelling based on a chalcophile isotopic system, e.g. the Os isotopic system, and a lithophile isotopic system, e.g. the Nd isotopic system, is inappropriate.
- Contamination by ancient upper crust or Lewisian-type lower crust is required to account for the radiogenic Os and unradiogenic Nd isotopic compositions of the intrusions and must be considered in any tectonic model for the Halls Creek Orogen.
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ACKNOWLEDGEMENTS |
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Financial and logistical support and permission to publish were kindly provided by Helix Resources NL. Access to diamond drill core from the Sally Malay intrusion was kindly provided by Normandy Exploration Limited. This research was part of a Ph.D. study by R. A. Sproule funded by an Australian Postgraduate Award (Industry). Thanks are due to A. Donaghy, L. Frick, S. Graham, D. Korke, R. Maas and J. McBride for their assistance during sample collection and acquisition of analytical data, and for their constructive comments. The authors wish to thank the reviewer Professor R. C. Price for his extremely insightful comments, which substantially improved this paper.
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FOOTNOTES |
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*Corresponding author. Present address: Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, Willet Green Miller Centre, Ramsey Lake Road, Sudbury, Ontario, P3E 5B6, Canada. E-mail: rsproule{at}nickel.laurentian.ca
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