Mineralogy and Formation of a Platinum Placer Deposit, Southern South Island, New Zealand

Placer platinum group minerals (PGM), with subordinate detrital gold, have been concentrated in Pleistocene coastal sands near the present Foveaux Strait shoreline. This study characterises the PGM in what is New Zealand's only identified PGM deposit. This PGM deposit is immediately downstream (<2 km) of historic and modern placer gold mines that contain only minor PGM in the same sedimentary sequence. The PGM placer deposit is dominated by Pt–Fe alloys, with subordinate sperrylite, Ru–Ir–Os alloy, and cooperite‐braggite solid solution. The PGM placer includes abundant 100–200 µm equant particles, many of which are angular or subrounded and have external crystalline forms, and their internal grains (20–50 µm) were not deformed by sedimentary transport. Flakes are a minor component in the PGM placer, in contrast to PGM in the upstream gold mines that had predomiantly flaky morphology. Flakes of Pt–Fe and Ru–Ir–Os alloys have undergone substantial internal deformation, with original coarse grains (~50 µm) overprinted by finer (5–10 µm) variably annealed grains and subgrains, and flake rims consist of micron‐scale annealed grains. The PGM placer is interpreted to be a residual deposit after most gold and some PGM flakes floated on, or were suspended in, tidal and/or fluvial flood waters.