Controls on the emplacement and genesis of the MKD5 and Sarah's Find Ni-Cu-PGE deposits, Mount Keith, Agnew-Wiluna Greenstone Belt, Western Australia

The Mount Keith (MKD5) nickel sulfide deposit is one of the largest komatiite-hosted nickel sulfide deposits in the world; it is hosted by a distinctive spinifex-free, cumulate-rich, ultramafic horizon/unit termed the Mount Keith Ultramafic (MKU). The Mount Keith Ultramafic shows significant variation along its lateral extent. The internal architecture is made up of adcumulate-textured pods and lenses, which are flanked by thinner meso- and orthocumulate-textured units, overlain by pyroxenitic and gabbroic horizons. The lateral and vertical changes in the geometry and internal architecture reflect variations in the lithological association and emplacement conditions along the strike extent of the belt. The chilled margins of the Mount Keith Ultramafic unit contain similar to 1,200 ppm Ni. Olivine cumulates average similar to 2,500-3,500 ppm Ni, with few exceptions ( Ni > 4,500 ppm) reflecting occurrence of minor nickel sulfides, whereas pyroxenites and gabbros generally contain, respectively, similar to 1,500-2,000 and similar to 100-1,000 ppm Ni. Olivine cumulates generally contain low Cr concentrations (< 2,500 ppm Cr), with the rare presence of chromite-rich intervals containing anomalously high values (> 5,000 ppm Cr). The internal stratigraphy of the Mount Keith Ultramafic coalesced and formed the basal massive sulfide nickel sulfide mineralization at the base of the sill (i.e., Sarah's Find). Prolonged focused high volume magma flow within the sill resulted in the emplacement of a thick, lens-shaped accumulation of olivine adcumulate. Local variations in intensive parameters other than crustal assimilation (e.g., T, fO(2), fS(2)) may be principally responsible for sulfide supersaturation and controlled the local distribution of stratabound disseminated nickel sulfide mineralization (e.g., MKD5 Ni Deposit), generally localized within the core of the thicker dunite lenses.