Investigation of platinum-group minerals (PGM) from Pindos chromitites (Greece) using hydroseparation concentrates

Abstract

The Pindos ophiolite complex, located in northern Greece, hosts small podiform chromitites characterized by very low platinum group element (PGE) grades. PGE (excluding Os) analyses from four chromitite samples, collected for this study, are below 400 ppb. Consequently, we used the technique of hydroseparation to concentrate PGM from Pindos chromitites. More specifically, we investigated two separate composite samples from Pefki and Milia chromitites.

The Pefki concentrate contains 67 PGM that include secondary Ru-bearing minerals, ruarsite (RuAsS), laurite (Ru,Os)S2, irarsite (Ir,Ru,Rh,Pt)AsS, alloys of Os–Ir–Ru, hollingworthite (Rh,Pt,Pd)AsS, paolovite (Pd2Sn), braggite (Pd,Pt,Ni)S, sperrylite (PtAs2) and four unnamed PGM, electrum (Au,Ag) and native silver. The investigation of the Milia concentrate yields fifty one grains of PGM including primary laurite, Os–Ir alloys, erlichmanite, secondary Ru-bearing minerals, irarsite, Ru-alloys, ruarsite and Ru-based metals sulphides. PGM occur as both single and polyphase particles in both concentrates. The bulk of mineralization for grains between 5 and 15 μm is finer-grained in Milia (64.7%) than in Pefki (41.8%). The latter concentrate hosts considerably more altered PGM grains than the former.

The hydroseparation process has recovered significantly more, as well as novel, PGM grains than the in situ mineralogical examination of single chromitite samples from the neighboring Korydallos occurrence. 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 in Pefki. The latter possibility is suggested by the presence of secondary PGM, which may be related to the different alteration processes that affect the Pefki and Milia chromitites. In summary, this study provides significant information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing of these minerals.