Platinum-Group Element mineralisation in sulfide-poor komatiites - a case study from Mt. Clifford, Western Australia

An unusual occurrence of "offset reef' style platinum-group element (PGE) mineralisation has been discovered at Mount Clifford, Agnew-Wiluna Belt, Western Australia. Ten samples have been studied from a diamond drill-core intersecting the 1000 m thick Mount Clifford dunite body consisting of strongly serpentinised olivine adcumulates and accessory chromite. Sulfides are minor and include pyrrhotite, pentlandite, and millerite. High-precision whole-rock ICP-MS data show that the PGE mineralization extends over tens of meters, whereas the lateral extent is unknown. Palladium and Pt abundances are closely associated with base-metal sulfides but show offset distribution patterns with maximum concentrations associated with the relatively sulfide-poor base of a disseminated sulfide-bearing layer. Whole-rock concentrations range up to 4100 ppb Pd and 920 ppb Pt. Iridium, Rh, and Ru follow the Pd-Pt trend but are strongly depleted (Ir similar to 0.4 ppb, Ru similar to 0.5 ppb, Rh similar to 0.1 ppb).

The PGE mineralization zone is underlain by an interval of strong Ni depletion within olivine mesocumulates that is moreover characterised by an unusual negative correlation between Ni and the Mg number (Mg/(Mg+Fe)) of the rock. This relationship is interpreted to be the result of an equilibration of olivine of variable forsterite content with a relatively large mass of magmatic sulfide, of which, however, there is no direct evidence in the Mount Clifford dunite. The Ni depletion is accompanied by strong depletion in Ir, Ru and Rh.

Sixty-six anhedral platinum-group minerals (PGM) were identified using electron-microprobe analysis. The PGM are randomly hosted in serpentine, are typically smaller than 5 pm, and can be categorized into three groups: Pd-antimonides, Pt-dominated PGM, and Pt-bearing Ni-antimonides. Mineral compositions and textures indicate that the PGM were formed at a late stage during hydration of the host rock.

The association of the PGE zone with a reversal in the Mg/Fe ratio of the host cumulates, and the distinctive offset relationship of multiple Pt, Pd and Cu-S peaks, are suggestive of a primary magmatic origin, but this is inconsistent with current models for "offset reef PGE concentrations which involve the onset of sulfide liquid saturation from a previously sulfide-saturated, PGE-undepleted magma. The association of enriched Pt and Pd with highly depleted Ni and IPGEs in immediately underlying cumulates is highly unusual. Deposition of Pt and Pd from hydrothermal or late deuteric fluids migrating through the crystal pile remains as the most likely explanation but leaves critical features unexplained. This unusual zone of enrichment may be related to the presence of a nearby but so far undiscovered body of differentiated magmatic sulfide liquid.