CONTROLS ON THE EMPLACEMENT AND GENESIS OF THE MKD5 AND SARAH'S FIND NI-CU-PGE KOMATIITE-HOSTED DEPOSITS, MOUNT KEITH, AGNEW-WILUNA GREENSTONE BELT, WESTERN AUSTRALIA

The Mount Keith Ultramafic unit (MKU) of Western Australia is one of the most highly nickel sulfide-endowed komatiite belts in the world. The internal architecture of the MKU 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 internal architecture of the MKU and the transitional contacts between the various komatiite units, coupled with the significant lateral variation between the Mount Keith Ultramafic Complex and Sarah's Find, suggests that the MKU has experienced variations in magma flux during its emplacement and assembly history. The nickel sulfide ore typology and style vary greatly along the strike extension of the MKU. Basal massive nickel sulfide mineralization generally occurs in the thinner meso- and orthocumulate-textured units, whereas stratabound disseminated nickel sulfide mineralization is hosted in the adcumulate-textured pods. We hypothesize that the formation of basal massive and stratabound nickel sulfide deposits is due to the localized coalescence of entrained sulfide blebs. In fact, the very low PGE content of the initial liquid of the MKU suggests that the initial magma pulse that penetrated through the dacite host-rock had already equilibrated with sulfides at depth and/or carried entrained immiscible sulfide blebs. We argue that upon emplacement, the intruding magma experienced a significant thermal shock at the contact with water-saturated volcaniclastic breccias. The sudden chilling would have increased the viscosity of the magma, possibly to the point where it was no longer able to sustain the suspension of the immiscible sulfide liquid. As a result, the sulfide blebs coalesced and formed the basal massive sulfide nickel sulfide mineralization at the base of the sill (i.e. Sarah's Find). A prolonged episode of focussed high volume magma flow within the sill resulted in the emplacement of thick, lens-shaped accumulations of olivine adcumulate. Local variations in intensive parameters other than crustal assimilation (e.g. T, fO2, fS2) may be principally responsible for sulfide-saturation and controlled the local distribution of stratabound disseminated nickel sulfide mineralization, generally localized within the core of the thicker dunite lenses.