Are Placer Platinum Nuggets Formed During Lateritization? The Verdict from the Owendale Alaskan-Uralian Complex in Australia Is an Emphatic No!

Abstract

Abstract The thick laterite developed over platinum group element (PGE)-bearing ultramafic rocks of the Owendale Alaskan-Uralian Complex in New South Wales, Australia, provides an ideal environment in which to address the question of whether Pt-Fe nuggets are formed during lateritization. This is an important issue to settle because Pt-Fe nuggets in alluvial placers and eluvial deposits associated with the Alaskan-Uralian complexes were the world’s major source of Pt prior to the commencement of Pt production from the Bushveld Complex and continue to produce a significant amount of Pt. Some of the Owendale laterites have high Pt but low Cu contents, while others have both high Pt and high Cu contents. Heavy mineral concentrates were prepared from about 1 kg of drill chips from both types of laterites. Only five of the 61 samples processed contained any platinum group minerals (PGMs) greater than 5 μm in size, even though many of the samples contained more than 1 g/t Pt. The largest PGM found was about 100 μm long, and the majority were <20 μm. The bulk of the PGMs recovered were zoned PGMs consisting of a core of isoferroplatinum mantled by tetraferroplatinum with an outer rim of tulameenite; many of these zoned PGMs are encased in hematite grains that often have high Pt contents and appear to be pseudomorphs after the PGMs. The textural evidence indicates that at least half of the primary PGMs in the ultramafic protoliths to the laterites were destroyed during weathering and that the liberated PGEs could have been available for the formation of PGM nuggets. However, despite the large amount of PGEs liberated during the destruction of the primary PGMs, no evidence was found for the neogenic growth of PGE nuggets. Rather, the Pt liberated during the destruction of the PGMs appears to have only traveled distances of micrometers to tens of centimeters to form Pt nanoparticles or Pt oxides or to be absorbed/adsorbed by the Fe oxide hosts.