Isotopic investigations of the Nova-Bollinger Ni–Cu–Co deposit in the Fraser Zone, Albany-Fraser Orogen, Western Australia

Abstract

The Fraser Zone of the Albany-Fraser Orogen of Western Australia contains an extensive suite of mafic and mafic–ultramafic intrusions. This includes the Nova-Bollinger intrusions (ca 1300 Ma), which host economic Ni-sulfide mineralisation, a prime example of magmatic sulfide liquids emplaced into the deep crust under ambient granulite facies conditions. Sulfur, Sr and Sm–Nd isotopes, and trace elements have been used to evaluate the role of crustal contamination of the parental magmas to Nova-Bollinger and other sulfide-bearing mafic–ultramafic bodies within the Fraser gabbro suite. All ore types have δ³⁴S values between +1.6 and +8.0‰ VCDT with a positively skewed distribution and a median value of +2.3‰ VCDT, anomalous relative to uncontaminated mantle values near 0‰ VCDT. No significant Δ³³S anomalism was observed. The initial ⁸⁷Sr/⁸⁶Sr and ¹⁴⁷Sm/¹⁴³Nd ratios of most of the Nova-Bollinger igneous rocks have an age of formation of 1300 Ma, which is similar to those of CHUR_1300Ma and record limited contamination by lower continental crustal material. The country rocks of the Snowys Dam Formation, locally enriched in iron and sulfur, are likely candidates as crustal contaminants. These metasedimentary sulfides show similar ranges in values for both δ³⁴S and Δ³³S to the ores, but their Sr isotopic range is greater than observed in the host intrusions at Nova-Bollinger and in most of the other sampled intrusions. Bollinger globular sulfides with ɛSr values of 63.1 and 144.5 and ɛNd values of −5.7 and −1.8 are suggestive of localised crustal contamination from adjacent wall rock marbles, but otherwise high ɛSr country rocks do not appear to have been dominant contaminants. Bulk assimilation of the country rocks, as modelled in both silicate and sulfide portions of the system, of up to 30% contamination of a primitive mantle melt by siliceous country gneisses, would have been sufficient to produce the degree of crustal contamination recorded in the Nova-Bollinger ores, and more regionally within the Fraser gabbros. Beyond indications that S was crustally derived, it is not possible to identify a specific source, but Snowys Dam sediments are the most likely. There is no compelling evidence for Archean S in the ores, unsurprisingly given the mid-Proterozoic age. It is likely that much of the contamination of the magma and generation of sulfide liquid took place during lateral flow through linked sill complexes within the deeply buried Snowys Dam metasediments, prior to the emplacement of the magmas to form the orebodies.

KEY POINTS

1. Nova-Bollinger Ni–Cu–Co sulfide ores have δ³⁴S values between +1.6 and +8.0‰ VCDT, with a median value of +2.3‰ VCDT, anomalous relative to uncontaminated mantle values but similar to the range in the country rock metasediments. Values are consistent with equilibration of sulfide xenomelts at R factors of 200–700.

2. There is no detectable mass-independent fractionation of δ³³S in the Nova-Bollinger ores.

3. The initial ⁸⁷Sr/⁸⁶Sr and ¹⁴⁷Sm/¹⁴³Nd ratios of most of the Nova-Bollinger igneous rocks at the age of formation of 1300 Ma are similar to those of CHUR_1300 Ma and record contamination of a primitive mantle melt by up to 30% lower continental crustal material.

4. Unusual globular ores at Bollinger have anomalous ɛSr indicating localised contamination from adjacent wall rock marble; however, this contamination is not pervasive through the orebody. It is likely that much of the contamination of the magma and generation of sulfide liquid took place during lateral flow through linked sill complexes within the deeply buried Snowys Dam metasediments, prior to the emplacement of the magmas to form the orebodies.

Keywords:

• magmatic sulfide ores
• nickel deposits
• Albany-Fraser Orogen
• Proterozoic
• Nova-Bollinger
• sulfur isotopes
• Sr isotopes
• Nd isotopes

Acknowledgements

We acknowledge the contributions of David Hammond, Sebastian Staude, Steve Rennick, Paul Polito and Tony Crawford. Mike Verrall assisted with the Tornado XRF mapping system. Sue Golding from the University of Queensland Stable Isotope Geochemistry Laboratory is thanked for assistance with S isotopic analyses; we thank David Bruce from the University of Adelaide MAS for performing the Sr and Sm–Nd isotopic analysis. Roland Maas from the University of Melbourne is thanked for analytical support and insightful discussions. Catherine Spaggiari and Hugh Smithies provided helpful advice on the regional context. Will Smith and an anonymous reviewer provided constructive reviews that considerable improved the final product.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Full data for S isotope analyses and whole rock trace-element analyses collected in this study are archived and available for free download on the CSIRO Data Access Portal at https://doiorg/10.25919/rcst-9d44.

Additional information

Funding

We thank Independence Group NL for financial support and the Nova mine geological staff for extensive access to drill core, underground exposures and assay data.