District to Camp Controls on the Genesis of Komatiite-Hosted Nickel Sulfide Deposits, Agnew-Wiluna Greenstone Belt, Western Australia: Insights from the Multiple Sulfur Isotopes

Abstract

The Agnew-Wiluna greenstone belt of Western Australia is the largest komatiite-hosted nickel sulfide belt in the world and contains two world-class Ni-Cu-(PGE) deposits and a host of smaller deposits. This study focuses on the broader scale geology of this greenstone belt in order to understand the key controls on the genesis of the komatiite-hosted Ni-Cu-(PGE) deposits, with specific focus on camp to district controls. We apply multiple sulfur isotopes to this geologic framework and conclude not only that the addition of crustal sulfur is a prerequisite for ore genesis in komatiite systems, but above all that the sulfur required to generate world-class deposits is most likely derived from barren volcanic massive sulfide lenses, which are spatially and genetically associated with felsic volcanic and volcaniclastic sequences that were emplaced coevally with large komatiitic sills and channelized lava flows. Multiple sulfur isotope data can be utilized in exploration at the deposit to district scales. At the deposit scale, the spatial pattern of mass-independent S isotope values (Δ 33 S) provides crucial insight into the identification of proximal high-grade and high-tenor ores in mineralized systems. In fact, sulfur data reflect the assimilation process that occurred upon komatiite emplacement, whereby hot turbulent magma thermomechanically eroded and assimilated exhalative sulfides spatially located close to vent with negative to near zero Δ 33 S values, whereas less turbulent flows interacted with distal sulfidic shales having Δ 33 S values above 0 per mil. Accordingly, the spatial variation of multiple sulfur isotope values in magmatic sulfides and associated host rocks may be utilized as a vector towards high-grade ores of poorly known systems. At the district scale, rather than ascertaining what controls the distribution of komatiite-hosted Ni-Cu-(PGE) deposits, the appropriate question to ask is what controls the distribution of country rock sulfides, considering that exhalative sulfides may be crucial to ore genesis in komatiite systems. We propose that felsic lava domes unambiguously mark their vents and can be directly mapped or inferred from gravity data. This work provides the first step in identifying district-scale control on komatiite-hosted Ni-Cu-(PGE) deposits. This is the scale that has high impact on exploration for new komatiite-hosted nickel sulfide belts globally.