Platinum-group-element minerals (‘osmiridium’) from Waratah Bay, Victoria, Australia, and a review of other reported Victorian occurrences

Abstract

Minerals consisting of the platinum-group elements iridium, osmium and ruthenium (IPGE) are widely dispersed in alluvial deposits derived from ultramafic igneous complexes originating in ophiolite belts. Rock types, such as cumulate dunite–pyroxenite–harzburgite, and their serpentinite derivatives are especially fertile sources of IPGE minerals, which are colloquially termed ‘osmiridium’. IPGE minerals have been recorded from five localities in Victoria—beach sands at Waratah Bay, and stream concentrates from Turtons Creek, Stockyard Creek and Horseshoe Creek in South Gippsland. A grain of IPGE was recently found in a gully near the Dolodrook River further north. These occurrences are all poorly documented by comparison with the northwest Tasmanian deposits, and only Waratah Bay has provided enough material to enable full characterisation. Microprobe data show that all four minerals currently defined in the Os–Ru–Ir system occur at Waratah Bay, including rutheniridosmine. Gold and chromite are also present in the alluvial samples from Waratah Bay and Turtons Creek. Attempts to correlate compositions of alluvial chromite with those from likely proximal serpentinite source rocks proved to be inconclusive due mainly to inadequate representative sampling. However, the chromite compositions are broadly characteristic of those found in the early Paleozoic ultramafic ophiolite complexes in northwestern Tasmania, in which IPGE minerals and chromite are abundant. Regional aeromagnetic trends across Bass Strait suggest that the Victorian IPGE occurrences represent disaggregated and metamorphosed extensions of the Tasmanian sequences.