Distribution of sulphides and PGE within the porphyritic websterite zone of the Munni Munni Complex, Western Australia

Abstract

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 t...