Silicate inclusions in isoferroplatinum: Constraints on the origin of platinum mineralization in podiform chromitites

Abstract

The origin of large platinum-group mineral (PGM) nuggets inside chromitite schlieren in some Ural-Alaskan type ultramafic complexes has been assigned to orthomagmatic processes. A promising phenomenon which may provide new insights into the processes responsible for PGE mineralization formation, or at least place constraints on existing models, are the multiphase inclusions hosted in PGM which have been previously found in platinum nuggets from alluvial deposits. For the first time, this study examines multiphase inclusions hosted in isoferroplatinum nuggets from lode chromitite schlieren in almost unaltered dunite from the Matysken Ural-Alaskan type complex (Koryak Highlands, Far East Russia). These multiphase inclusions are comprised of diopside, hydrous silicates, apatite, plagioclase, K-feldspar, silica and other minerals that are distinctly different from the host dunite mineralogy. Taken together with similar mineral assemblages in Cr-spinel-hosted inclusions, these inclusions cannot be crystallization products of a mafic/ultramafic melt, but instead require an alternative explanation. We critically evaluate different genetic models and conclude that a range of fluid-assisted metamorphic processes should be involved in producing massive PGM within chromitite schlieren in dunites.