Abstract

An unusual occurrence of sulfur-poor platinum group element (PGE) mineralization has been discovered at Mount Clifford in the Agnew-Wiluna belt, Western Australia, within the lower part of a 1,000-m-thick komatiitic dunite body. The host body consists primarily of strongly serpentinized olivine adcumulates with accessory chromite.

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 4,100 ppb Pd and 920 ppb Pt. Iridium, Rh, and Ru follow the trends in Pd and Pt but are strongly depleted (Ir ~0.4 ppb, Ru ~0.5 ppb, Rh ~0.1 ppb). Platinum group minerals (PGM) were located in essentially sulfide free serpentine 2 m below the peak in sulfide abundance and can be categorized into three groups: Pd antimonides, Pt-dominated PGM, and Pt-bearing Ni antimonides.

The PGE mineralization zone is underlain by an interval of strong Ni depletion within olivine mesocumulates that is characterized by an unusual negative correlation between Ni and the Mg number 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. However, there is no direct evidence of this sulfide mass anywhere in the entire Mount Clifford dunite body. The Ni depletion is accompanied by strong depletion in IPGE (Ir, Ru, and Rh).

The association of the PGE zone with a reversal in the Mg/Fe ratio of the host cumulates and a distinctive offset relationship of multiple Pt, Pd, and Cu-S peaks is suggestive of a primary magmatic origin. However, this is inconsistent with current models for offset-reef PGE concentrations which involve the onset of sulfide liquid saturation from a previously sulfur undersaturated, PGE-undepleted magma. The association of enriched Pt and Pd with highly depleted Ni and IPGE in immediately underlying cumulates conflicts with this model. We suggest a combination of processes: early sulfide segregation at low R values producing Ni and IPGE depleted olivine, partial redissolution and removal of most of this sulfide component during a major replenishment event, and subsequent overprint by a migrating Pt-Pd-Cu–rich fluid.

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