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 CHUR1300Ma 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 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. There is no detectable mass-independent fractionation of δ33S in the Nova-Bollinger ores. 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 CHUR1300 Ma and record contamination of a primitive mantle melt by up to 30% lower continental crustal material. 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.