Platinum-group element and sulphide mineralogy in ultramafic complexes at western Andriamena,Madagascar

Abstract

Small (<2 km diameter), ultramafic intrusive complexes in the Andriamena region of Madagascar carry high concentrations of platinum-group elements (PGE). The PGE mineralisation at Andriamena is unusual in that it is not obviously associated with abundant sulphide or chromitite layers or pods, as in many layered complexes and Alaskan-type intrusions. Mineralogical investigation of rocks from one complex, designated UM2, has revealed a complex array of Pt and Pd-bearing platinum-group minerals (PGM) that tend to be concentrated interstitially between olivine crystals in dunite units. These PGM are dominated by PGE arsenides and stibioarsenides, accompanied by a small population of Pd oxides that occur in unit close to the surface and appear to have formed by supergene alteration. The PGM population displays a notable contrast between Pt-bearing and Pd-bearing minerals. Palladium-bearing PGM are always found entirely within or at least partially enclosed by Fe–Ni–Cu sulphides. In contrast, Pt-bearing PGM are either found on the margins of Fe–Ni–Cu sulphides or more commonly, they are entirely isolated from sulphides in hydrosilicates or as inclusions in olivine. The discovery of these sperrylite (PtAs₂), irarsite (IrAsS) and hollingworthite (RhAsS) inclusions in olivine implies that development of these minerals took place at magmatic temperatures, coincident with the growth of olivine, not in late-stage interstitial fluids. This leads to a new magmatic model for the development of the mineralisation. In this model, PGE and As are initially concentrated into a sulphide liquid in a deeper magma chamber and intruded as a slurry of silicate liquid–olivine–sulphide liquid into the dyke-like Lavatrafo intrusions. The high As content of the sulphide promotes early crystallisation of Pt, Ir and Rh arsenide and sulpharsenide PGM, some of which became detached from the sulphide and became incorporated into growing olivine crystals. Palladium arseno-antimonide PGM follow at a later stage. Early crystallisation of Pt and Pd-bearing PGM leaves the sulphide liquid depleted in these elements. Migration of the fractionated sulphide liquid inwards and downwards towards the centre of the intrusion produces a zone of Cu-rich sulphides that is PGE-poor. Such a strong spatial separation PGE from base metal sulphides is uncommon in magmatic PGE deposits and this is suggested to arise from the As-rich nature of the early sulphide liquid which promoted the formation of PGM earlier than would have occurred in an equivalent As-poor system.