DISTRIBUTION OF SULFIDES AND PGE WITHIN THE PORPHYRITIC WEBSTERITE ZONE OF THE MUNNI MUNNI COMPLEX, WESTERN-AUSTRALIA

The Munni Munni Complex is a boat-shaped layered intrusion, divided into a lower ultramafic 'keel', up to 1850 m thick, overlain by a gabbroic sequence at least 3000 m thick. The top of the ultramafic series is marked by a 20-80 m thick porphyritic websterite zone (PWZ), that extends laterally to form a 200 m thick layer of pyroxenite along the walls of the intrusion. Platinum-group element (PGE)-enriched disseminated sulphides occur within the top 20 m of the PWZ unit along its entire exposed strike length, with maximum total PGE grades up to 8 ppm over 500 mm widths. The PWZ marks a pronounced discontinuity in mineral composition trends within the intrusion, between uniform relatively high Mg# and Cr-rich pyroxenes in the main part of the ultramafic series, to gabbroic cumulates showing a monotonous trend of declining Mg# due to closed system fractionation in a large chamber. The Cr2O3 content of cumulus augite shows a dramatic drop from about 0.2% to less than 0.05% over less than 4 m near the top of the PWZ. Disseminated cumulus sulphides are abundant within an interval from 5 m above the gabbro contact to 15 m below it. Two sulphide components are present. The first is a relatively PGE-poor component, characterized by coincidence of peak Cu, Ni, S and PGE abundances. The second is a variable composition component, strongly PGE enriched at the base and depleted at the top, characterized by systematic offsets between the peak PGE and Cu abundances. The 'offset' sulphides always occur above the 'coincident' sulphides, and their first appearance corresponds to the sudden drop in Cr2O3 content of clinopyroxene within the PWZ. The coincident sulphides are interpreted to have formed by batch equilibration of sulphide liquid droplets during the magma mixing event that produced the PWZ. The offset sulphides formed subsequently by fractional segregation and gravity settling of sulphide liquid droplets from a stagnant hybrid melt layer dominated by the gabbroic component. The observed lateral distribution of PGE-rich sulphides in the PWZ is inconsistent with derivation of PGE from trapped liquid in underlying cumulates and provides strong evidence for the existence of primary PGE-enriched cumulus sulphide liquid.