Re-Os isotopic constraints on the source of platinum-group minerals (PGMs) from the Vestrev pyrope-rich garnet placer deposit, Bohemian Massif

Mineralogical studies of the heavy fraction from a Holocene pyrope-rich garnet placer deposit at Vestřev (Krkonoše Piedmont Basin, Bohemian Massif) have identified the presence of very rare grains of platinum group minerals (PGM). Pt-Fe alloy grains are accompanied by Os-Ir-Ru minerals (native osmium, iridium, and ruthenium) with inclusions of Pt-Fe alloy and hongshiite (PtCu). This mineral assemblage is typical for several mantle settings including ophiolites. The chemistry of the Os-Ir-Ru minerals shows an enrichment of the PGM in Ru, which is typical of ophiolites. The grain morphology of PGM and pyrope-rich garnet (mostly rounded with numerous euhedral/subhedral grains) does not exclude a common source. In-situ laser-ablation MC-ICP-MS was used to measure the Re-Os isotopic compositions of single Os-rich grains, which show heterogeneous subchondritic Os isotopic compositions (¹⁸⁷Os/¹⁸⁸Os=0.12082-0.12505±0.00003). This precludes their low-temperature origin and indicates derivation of platinum-group elements (PGEs) essentially from mantle-derived rocks without a significant contribution of crustal Os. The mantle model age (T_MA) and Re-depletion model age (T_RD) model ages range from ~0.4 to ~1.0Ga and most likely reflect a long history of melt depletion that affected the mantle sources of PGM.