The formation age of platinum-group minerals (PGM) in placer deposits has traditionally been difficult to constrain. We have applied the Pt-Os and Re-Os isotope systems to this problem by analyzing a suite of PGM from a placer deposit in southeastern Borneo that are derived, by mechanical processes, from chromitites of the Meratus ophiolite. Published subduction and emplacement ages and biostratigraphy of pelagic sediments of the ophiolite sequence define a minimum age for genesis at a spreading ridge. However, igneous components of the ophiolite have previously been undateable. Alluvial PGM grains (n = 260) from the Pontyn River, which drains the Meratus Mountains, were analyzed by laser ablation-multicollector-inductively-coupled mass spectrometry (LA-MC-ICPMS). Re-Os data do not show any isochronous relationship. Despite a significant range in 187Os/188Os (0.122–0.141), 187Re/188Os values show a very narrow range (0.000005–0.002980). In contrast, the PGM have a wide range in both 186Os/188Os (0.119801–0.120315) and 190Pt/188Os (<0.00001–1.493430), yielding a precise Pt-Os isochron age of 197.8 ± 8.1 Ma (2σ). This age fits well with published age constraints for this ophiolite and we argue that it dates the crystallization of the PGM. Previous studies have shown that the Pontyn PGM are derived from ophiolitic chromitite; therefore, the PGM Pt-Os isochron age also provides the first absolute age constraint for the genesis of igneous rocks of the Meratus ophiolite. These results highlight the potential of the Pt-Os geochronometer as a tool for dating the crystallization age of PGM found in placer deposits, for dating primary platinum mineralization in general, and for use in ophiolite geochronology.

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