PLATINUM-GROUP ELEMENT MINERALIZATION IN CHROMITE-RICH ROCKS OF BRAGANCA MASSIF, NORTHERN PORTUGAL

Results are presented of a mineralogical and geochemical investigation of the platinum-group element (PGE) mineralization of schlieren-type podiform chromite deposits in the Braganca massif of northern Portugal. The relative effects of primary fractionation between the PGE and a late serpentinization alteration overprint have been distinguished. The PGE are grouped on the basis of mineralogy into a Ru-Os-Ir group (laurite), an Ir-Rh-Pt group (minerals of the irarsite-hollingworthite-platarsite series and sperrylite) and a Pd-Pt group (alloys. sulphide, antimonide and potarite). The six PGE are fractionated during primary chromite crystallization. The Ru-Os-Ir group is the first to start forming minerals and is followed by the Ir-Rh-Pt group. The fractionation is evidenced by positive chondrite-normalized slopes for some of the samples and the presence of Pt-As-bearing minerals at the edges of chromite grains but not in their centres. Pd is not part of the fractionation trend of the Ru-Os-Ir and Ir-Rh-Pt groups and instead is incorporated, together with some of the Pt content, into the structure of pentlandite during its initial crystallization. Alteration of the samples during serpentinization has led to the breakdown of pentlandite to heazlewoodite and magnetite in some sulphide grains and the liberation of Pd together with that Pt which was present in the base-metal sulphide. This secondary assemblage is associated with Pd-sulphide species or, on decreasing S activity, with Pt- and Pd-bearing alloys. Associated with decreasing S activity is a laurite alteration trend, identified as an increase in the Ru/(Ru+Os+Ir) ratio of the mineral grains, reflecting a loss of Ir and Os during serpentinization. Desulphurization of the Ir-Rh-Pt group can also lead to the formation of Pt-Ir alloy. Platinum-group mineral grains located within chromite typically have polygonal outlines, giving a euhedral appearance. This is thought to be the result of recrystallization of the enclosing chromite grains during tectonic emplacement of the ultrabasic assemblage.