CONTRASTING HYDROTHERMAL BEHAVIOR OF PLATINUM-GROUP ELEMENTS OF IR AND PD SUB-GROUPS AS EXEMPLIFIED BY PLATINUM-GROUP MINERALS IN GREAT SERPENTINITE BELT, EASTERN AUSTRALIA

Platinum-group minerals occur in various microtextural settings in pod iform chromitite of the Great Serpentinite Belt of eastern Australia. Phases of the Ir sub-group of the platinum-group elements (IPGE) are p resent either as inclusions in or interstitially to chromite and are u naffected by (or only altered in situ during) hydrothermal processes. By contrast, almost all phases of the Pd sub-group (PPGE) are located in a 'secondary' textural setting-in fractures in chromite or between fragments of brecciated chromite. The different settings indicate that the IPGE and PPGE mineralization episodes were associated with magmat ic and hydrothermal stages, respectively. The presence of an included PPGE mineral in a hydrothermally PPGE-mineralized chromitite sample su ggests that the hydrothermal PPGE minerals were at least partly derive d by remobilization of magmatic PPGE in the chromitite pod. These obse rvations indicate that PPGE are more mobile than IPGE under hydrotherm al conditions and point to the potential for hydrothermal Pt and Pd mi neralization in favourable geological environments.