Sulfur isotope compositions in the weathering profile of magmatic Ni-Cu deposits in SW Australia

The Nova-Bollinger Ni-Cu sulfide ore deposit is hosted in layered mafic and ultramafic intrusive rocks of the Mesoproterozoic Albany-Fraser zone, located about 160 km east-northeast of Norseman, Western Australia. Nova and Bollinger are two adjacent but spatially distinct orebodies with a combined pre-mining resource estimate of 13.1 million tonnes (Mt) with about 2 % Ni, 0.8 % Cu, and 0.1 % Co (IGO Ltd., unpublished data, 2018) ¹. Significant challenges are posed in exploring for magmatic Ni-Cu deposits that are buried under post-mineral cover. For example, electromagnetic and gravity surveys identify numerous targets but are unable to distinguish economic mineralization. Previously, it was suggested that the regular pattern of S isotope compositions (δ³⁴S_CDT) of surficial sulfate in lakes and groundwaters in southern Australia provides an ideal baseline against which to search for anomalous δ³⁴S_CDT values associated with base-metal or gold mineralization ². In the absence of lakes and readily accessible groundwaters in prospective areas, soils and rocks make a convenient sampling medium. Here, we investigated the exploration potential of δ³⁴S_CDT of the trace sulfur content of unconsolidated surface sediments, saprolite, and bedrock samples above Nova and two nearby sub-economic prospects, Griffin and Chimera. The δ³⁴S_CDT values likely reflect a two end-member system, with values ranging from -5.8 at depth to 21.4 ‰ near the surface, showing little dependence on lithology. Values in samples closer to the surface are similar to modern seawater sulfate that has a globally homogenous δ³⁴S_CDT value of 21.0 ± 0.2 ‰ ³, whereas at depth, values approach typical mantle S isotopic compositions of 0 ± 2 ‰ ⁴. In support of this, rocks at Nova have a δ³⁴S_CDT of around 0 ‰ and regional metagabbro are between -2 and 4 ‰ ⁵. On a regional scale, in both Western Australia (Yilgarn Block) and South Australia, the δ³⁴S_CDT values of surficial gypsum have a regular pattern over distances of up to 1000 km, with the highest values (~ 21 ‰) near coastlines decreasing to δ³⁴S_CDT values of ~ 14 ‰ further inland ². This is suggested to be predominantly the result of the delivery of salts to the Australian landscape as aerosols, with volatile biogenic S compounds of mostly marine origin (δ³⁴S_CDT of ~ 1 ‰) that proportionately increase in importance further inland resulting in decreasing  δ³⁴S_CDT values ². Located approximately 200 km inland, δ³⁴S_CDT results in samples within 10 metres of the surface at Nova, Griffin, and Chimera are in agreement with this and range from 12.6 to 20.4 ‰. Given that near-surface δ³⁴S_CDT values above Nova, Griffin, and Chimera appear to be mostly related to seawater-derived sulfate with minimal magmatic influence, δ³⁴S_CDT shows little potential as a field sampling technique to vector for deposits buried under post-mineral cover. However, at depth, δ³⁴S_CDT shows a clear relationship between the mixing of seawater sulfate and magmatic S weathering into the environment, indicating that analysis of S isotopes of otherwise apparently barren cores has utility in mineral exploration.