Chromite and platinum-group element mineralization in ultramafic rocks of the speik complex, eastern Alps

摘要

Variably serpentinized ultramafic rocks of the Speik COmplex in teh Austroalpine basement of the Eastern Alps are host to minor concentrations of chromian spinel. Harzburgites and pyroxenites represent highly depleted residual upper mantle; ultramafic and mafic cumulates are also present. A number of textural and chemical types of chromite are described, including magnesian (Hochgrossen) and ferric (Kraubath, Pernegg) varieties, with Cr numbers ranging from 55 to 95. Besides a variety of base metal minerals and silicates, chromitites host complex inclusion assemblages of platinum-group minerals, including sulfides, sulfarsenides, arsenides, antimonides and alloys of all six platinum-group elements (PGE). Chondrite-normalized PGE distributions display a high variability between different chromite types, with Ru usually dominating, and variable Pt/Ir values. Similarly, ~(187)Os/~(188)Os values of chromitites vary considerably. This is exlained via mixing of crustal and mantle components during partial melting of upper mantle in a mantle wedge above subducted oceanic crust, and subsequent redistribution during polyphase metamorphism.