Origins of Nd–Sr–Pb isotopic variations in single scheelite grains from Archaean gold deposits, Western Australia

Abstract

Accessory gangue scheelite (CaWO₄) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique.

Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial ⁸⁷Sr/⁸⁶Sr and ε_Nd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have ⁸⁷Sr/⁸⁶Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone.

The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that ⁸⁷Sr/⁸⁶Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite.

A positive correlation between Na and REE suggests that REE³⁺ are accommodated by the coupled substitution REE³⁺+Na⁺=2 Ca²⁺ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (<m) to medium (<km) scale processes involving mixing between “fresh” batches of hydrothermal fluid with fluids that had already been involved in extensive wall-rock alteration.

The very high-ε_Nd values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high Sm/Nd ratios. However, tiny (<20 μm) grains of secondary hydroxyl-bastnäsite were found within micro-fractures of one scheelite grain containing an extremely high-ε_Nd signature. The hydroxyl-bastnäsite probably formed during recent REE redistribution within the scheelite as a result of meteoric fluid circulation. The scale of this cryptic low-temperature alteration is sufficient to explain the anomalously high-ε_Ndi values observed in scheelite from Western Australia.

Introduction

Archaean lode gold deposits account for ∼60% of the world's gold production (Keays, 1987). These deposits, usually hosted by greenstone sequences, are associated with crustal-scale structures, and occur late in the evolution of the cratons, post-dating the major metamorphic and magmatic activity. Extensive discussions regarding the genesis of these deposits have focused on their relationships to metamorphic and magmatic events, and the nature and source of the ore-forming fluids and solutes (e.g., Groves et al., 1998, Kerrich, 1989). Radiogenic isotopes in hydrothermal minerals have been used extensively to study these relationships. In this paper, we investigate the causes for isotopic inhomogeneities and anomalous ε_Ndi signatures in scheelite from lode gold deposits in Western Australia, and show how small-scale inhomogeneity can affect the interpretation of isotopic data of hydrothermal minerals.

Scheelite, CaWO₄, is a common accessory mineral in hydrothermal Au-deposits, and its precipitation either predates or is contemporaneous with that of Au (Uspensky et al., 1998). The crystal structure of scheelite can accommodate high concentrations of rare earth element (REE; >1 wt.% REE₂O₃; Brugger et al., 1998), Sr (>1 wt.% Sr; Anglin et al., 1987), and Pb (up to 450 ppm; Brugger, unpublished data). Scheelite is one of the few minerals characterised by high Sm/Nd ratios and is therefore potentially useful for Sm/Nd dating. Furthermore, the high Nd and Sr contents of scheelite make it a useful tracer of fluid chemistry in ore-forming environments.

Cottrant (1981) was the first author to use the REE patterns of scheelite to infer the source of ore-forming fluids. As scheelite does not incorporate significant amounts of Rb, its Sr isotopic signature reflects that of the parent fluid if the system has remained closed. Regional and mine-scale variations in the isotopic composition of scheelite from Au-deposits in the Yilgarn Craton (Western Australia; Müller et al., 1991, Ghaderi, 1998) and in the Zimbabwe Craton (Darbyshire et al., 1996) were interpreted as resulting from mixing of Sr extracted from the greenstones and the underlying felsic basement by circulating hydrothermal fluids.

Sm/Nd dating of mineralisation was first used by Fryer and Taylor (1984) and has since been applied to uraninite (e.g., Maas, 1989) and fluorite (e.g., Chesley et al., 1991). Bell et al. (1989) reported the first Sm/Nd ages for scheelite from Au deposits in the Late Archaean Abitibi Greenstone Belt. However, the scheelite ages obtained by these authors are much younger than the dates given by other minerals, and their interpretation remains controversial (Anglin et al., 1996). In contrast, several studies have reported Sm/Nd dates of scheelite that are consistent with available geochronological data and cross-cutting relationships (Au-W-veins and auriferous quartz reefs of the Zimbabwe craton, Darbyshire et al., 1996, Oberthür et al., 2000; late Archaean Au-deposits of the Norseman area, Western Australia, Ghaderi, 1998).

More recent studies have demonstrated the potential of multi-tracer isotopic studies of scheelite for a better understanding of poly-phase mineralisation. Using Sr, Nd and Pb isotopes, Eichhorn et al. (1997) distinguished one stage of mineralisation and three stages of subsequent metamorphism in the stratabound scheelite mineralisation of Felbertal (Austria). By combining Sm/Nd, U/Pb and Re/Os isotope data, Frei et al. (1998) were able to distinguish a Late Archaean mineralisation and an Early Proterozoic tectono-thermal event that disturbed the U/Pb system but not the Re/Os and Sm/Nd systems in Au-deposits of the Harare–Shamva Greenstone Belt (Zimbabwe).

Scheelite from the Mt. Charlotte Mine (Western Australia) shows highly variable REE patterns (Sylvester and Ghaderi, 1997). Type I patterns are characterised by a strong enrichment in middle REE (MREE) and Eu anomalies are lacking or slightly positive. By contrast, Type II patterns are flat or depleted in the MREE and have large positive Eu-anomalies. Both types can coexist in the same grain and may alternate in fine-scale oscillatory zoning patterns Brugger et al., 2000a, Brugger et al., 2000b. The Sm/Nd isotopic study of Mt. Charlotte scheelite yielded a 2772±82 Ma age from slightly scattered data (Kent et al., 1996a). This age is older than that of the host rocks and was considered to result from the mixing of Nd from distinct sources. Assuming an age for mineralisation equal to the Ar/Ar age of 2602 Ma obtained by Kent and McDougall (1995) for hydrothermal muscovites, Kent et al. (1996a) calculated a large range in initial ε_{Nd,2602 Ma} values for their scheelite samples (ε_{Nd,2602 Ma}=+3 to +9). Scheelite ε_{Nd,2602 Ma} values >+5 cannot be reconciled with any known contemporary source of Nd in the Yilgarn craton. In order to explain such high-ε_Nd values, Kent et al. (1996a) proposed an Nd source in the surrounding komatiites, despite the fact that komatiites are a poor source of Nd and that no similar (primary) isotopic composition has been recorded in these rocks at that age (McCulloch and Bennett, 1994). Nevertheless, the anomalously high-ε_{Nd,2602 Ma} signature was later confirmed by Ghaderi (1998). By contrast, initial ε_{Nd,2630 Ma} values of scheelite from gold deposits in the Norseman area, located 160 km south of Kalgoorlie, are consistent with that of the surrounding rocks, and the Sm/Nd data yield a “realistic” isochron age at 2630±31 Ma (Ghaderi et al., 1999).

The coexistence of distinct REE patterns and their spatial distribution in Mt. Charlotte scheelite have been used to explore the dynamics of scheelite crystallisation and the trace element evolution of the parental hydrothermal solutions (Brugger et al., 2000b). In an earlier work, Ghaderi (1998) proposed a model that explains the different types of REE patterns in scheelite as a result of different coupled substitutions involving either Na⁺ or a vacancy to charge balance the incorporation of REE³⁺ ions. In this model, distinct end-member REE patterns are related to precipitation from fluids with distinct origins. In contrast, Brugger et al. (2000b) suggested a model involving fractional crystallisation of scheelite in a periodically recharged hydrothermal system.

In this paper, we report Nd–Sr–Pb isotopic data for micro-sampled areas within single scheelite grains characterised by either Type I or Type II REE patterns. This isotopic study aims to examine whether the two types of REE patterns can be traced to distinct sources and/or fluid pathways, in an attempt to constrain genetic models for scheelite and Au deposition. The isotopic data are complemented by elemental analyses of Na and light REE (LREE), which may help decipher the mechanism(s) responsible for REE substitution in scheelite, and hence distinguish between the fluid mixing and fractional crystallisation models for the origin of the REE inhomogeneity. In addition, in order to investigate the extent and origin of the anomalously high initial ε_Nd signature found by Kent et al. (1996a) and Ghaderi (1998) in some Mt. Charlotte scheelites, we studied the distribution of REE in one scheelite containing the “high-ε_Nd” signature.