Platinum-group mineral characterisation in concentrates from low-grade PGE chromitites from the Vourinos ophiolite complex, northern Greece

Abstract

The Vourinos ophiolite complex, located in northern Greece, hosts various chromite deposits characterised by very low platinum-group element (PGE) grades. Total PGE (excluding Os) concentrations in the fourteen chromitite samples collected for use in this study varied from 200 to 300 ppb.Previous reports on the platinum-group minerals (PGM) from the Vourinos chromitites, obtained data by in-situ investigation on polished sections. Consequently, we used the technique of hydroseparation to study the PGM from the concentrates in the Vourinos chromitites. More specifically, we investigated two separate composite samples from Voidolakkos and Xerolivado chromitites. The Voidolakkos concentrate sample contains 74 PGM that include: laurite (Ru, Os)S2; irarsite (Ir, Ru, Rh, Pt)AsS; erlichmanite (Os, Ru)S2; ruthenium pentlandite; iridium (Ir(Os)); osmium (Os(Ir, Ru, Pt)); secondary phases composed of Ru, Os and Cu; alloys of Ir–Fe; Rh– and Ru–Ni–Fe alloys; and Os–Ir–Fe alloys. The investigation of Xerolivado concentrate sample yield 45 grains of PGM, including laurite, irarsite, erlichmanite, minor other PGE sulphides, Os–Ir–Ru alloys, iridium and secondary phases of Ru–Os alloys. PGM occur as both single and polyphase particles in both samples. The bulk of mineralisation in Voidolakkos is dominated by a finer variety (<10 μm) of PGM than the Xerolivado sample (mainly <20 μm). The former occurrence hosts considerably more altered PGM grains, less laurite and a larger variety of PGM than the latter, whereas Os–Ir alloys are present in almost equal amounts in both samples. The hydroseparation process has recovered significantly more, as well as novel, PGM grains than were known from previous in-situ mineralogical examination of single chromitite samples. Although, most of the PGM occur as free particles and in-situ textural information is lost, single grain textural evidence is observed. The mineralogical and grain size differences between the two samples may reflect styles of mineralization and indicate significant remobilization of PGE. The latter possibility is suggested by the presence of secondary PGM, which may be related to the different alteration processes that affect the Voidolakkos and Xerolivado chromitites. In summary, this study provides significant new information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing of these minerals.