Platinum enrichment processes in ultramafic massifs of the Uralian Platinum belt, Russia - evidence for mineralization at the magmatic to hydrothermal transition

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Chromite and native Pt are usually considered to be early high temperature phases of primitive basaltic melts. Consequently, when chromite and Pt-Fe alloy are found enriched within primitive magmatic cumulates, it is assumed that these phases were enriched at magmatic temperature.The chromite-Pt enclaves in the massifs of the Uralian Platinum belt show many features that argue against a purely magmatic origin. Compared with the respective liquidus phases in basaltic melts, chromite and Pt-Fe alloys in the Uralian massifs can be orders of magnitude more coarse-grained. Hydrous silicates are commonplace, and chromite carries many generations of late hydrous phases within euhedral cavities that follow the crystal symmetry of chromite. The question is whether the fluid phases were derived directly from the parental melts of the cumulates or whether they reflect a random overprint. Primitive island arc melts - the potential parents to the ultramafic Uralian massifs - contain ~ 4 wt.% H2O. That H2O concentration is sufficient to render derivative melts and their cumulates saturated with a hydrous fluid phase. The working hypothesis is whether and to what extent magmatic chromite and Pt-Fe microliths can be mobilized and concentrated by intercumulus melts and fluids segregating from the crystallizing plutonic roots of island arc ankaramites.An important aspect to understand the enrichment processes is to date the mineralization. All platiniferous ultramafic massifs of the Uralian Platinum Belt are aligned parallel to the old suture marking the former Silurian convergent plate margin in the Ural mountains. Hence, if they outline a fossil volcanic arc, they should all return similar emplacements ages. The dating method of choice is the 190Pt-186Os decay scheme, tested here on Pt-Fe alloys from the Kondyor massif. If all platiniferous intrusions of the UPB return Pt mineralization ages identical within error, a genetic link must have existed between emplacement and cooling histories of the Uralian massifs and their Pt mineralization.