Anomalous Reef-type platinum-group element mineralisation in the Wiluna domain, Agnew-Wiluna Greenstone Belt, Western Australia

Abstract

The ultramafic horizons of the Wiluna domain (Agnew-Wiluna Greenstone Belt, Western Australia) form two main tabular and sheet-like fractionated units, which generally comprise a 5-10m-thick basal pyroxenite layer, a 50-100m-thick meso-to-orthocumulate-textured peridotite horizon, which is overlain by a core of wehrlite, ranging 10-50m in thickness, grading upwards into oikocrystic pyroxenites (30-50m-thick) and upper gabbroic margins (30-50m-thick). Primary textures are exceptionally well preserved: pegmatoidal and dendritic (harrisitic) olivine domains are common in all lithologies, whereas spinifex-textured horizons and flowtop breccias are absent. The fractionation sequence and the geochemical profile of the two ultramafic units are almost indistinguishable, suggesting that the belts may have been structurally duplicated. The ultramafic units display a steady trend towards increasing Fe content up stratigraphy, above a narrow basal zone showing a reversed trend. Calculated Ni contents of cumulus olivines indicate that the magma remained sulfur-undersaturated up to about the middle of the unit, where fractionation of a small proportion of sulfide (R>1000) occurred and depleted the upper fractionated levels in Ni. The presence of various anomalous PGE-enriched horizons (SPt-Pd= 200-300 ppb) in the fractionated pyroxenite layer directly overlying the basal ortho-mesocumulate-textured unit indicates the occurrence of localised weak reef-style PGE mineralisation. Reef-style PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich disseminated Fe-Cu sulfide mineralisation zones within thick differentiated intrusions (e.g. Platinova reef in the Skaergaard intrusion) rather than to other zones in komatiite-hosted systems.