Bowieite

Abstract: The Fiield Platinum Province, New South Wales, Australia, contains Alaskan-type mafic-ultramafic complexes with notable PGE mineralization. This mineralization has evolved in several stages ranging from high- to low-temperature hydromagmatic, with weathering and latertic concentration followed by erosion and placer accumulation in alluvial channels. This last stage has furnished the largest quantity of platinum from the area. PGM nuggets were collected in a corridor, south and southeast of Fifield, joining the leads exploited in the past and continuing northwards. The following PGM association was observed in the nuggets: isoferroplatinum, native osmium, osmiridium, iridosmine, laurite and bowieite. Isoferroplatinum is the matrix mineral in the nuggets, while (It, Os, Pt) alloys occur as exsolutions. The nuggets sometimes carry inclusions of euhedral chromite. Two sulphidic phases-laurite and bowieite-are rare and occur embedded as trapped early formed crystals in isoferroplatinum. This PGM association is entirely different, when compared to the PGM in pegmatoid clinopyroxenites (“P-units”) described previously (Johan et al., 1989), both as to the nature of the mineral species and to the chemistry of isoferroplatinum common to both occurrences. This proves conclusively that the source for the alluvial nuggets must be of a different nature from the presently established hard rock mineralization. The co-existence of iridosmine and osmiridium exsolutions in isoferroplatinum proves a high temperature origin for the nuggets, and excludes a low temperature origin through in situ overgrowth from aqueous solutions in either the laterite or the erosive profile. The presence of chromite inclusions of a certain diagnostic composition indicates that the original host rock for the nuggets was a chromitite within dunite. The chemistry of the coexisting alloy phases provides a temperature estimate of about 800 °C, which is compatible with Alaskan complexes in Alaska and the Urals. The textural study of the nuggets shows different rates of cooling from nugget to nugget, with many textures relating to heterogeneous nucleation in metallurgical processes.