Sources of placer platinum in Yukon: provenance study from detrital minerals

Abstract

Source rocks for the platinum group minerals (PGM), historically reported in a number of Yukon placers, remain either unknown or poorly understood. A study of heavy-mineral samples from five creeks draining bedrock in west and central Yukon was undertaken to confirm the presence of placer platinum, to determine which mafic–ultramafic rock is the source of PGM in Kluane area, southern Yukon, and to explain platinum occurrences in Canadian and Florence creeks, central Yukon, where no known mafic–ultramafic rocks are present. Diverse composition of chromian spinel and clinopyroxenes from three creeks in the Kluane area indicate several sources of ultramafic rocks, including fragments of Alpine-type peridotites formed in back-arc basin and mid-ocean-ridge settings, and a source rock for zoned zinc-rich chromites of unknown origin. The Kluane ultramafic sills are the most likely source of PGM in this area. The heavy-mineral sample from Canadian Creek returned one PGM grain, no chromite, and abundant ilmenite and titanomagnetite. A group of chromium-rich magnesian ilmenites (∼4 wt.% MgO) closely match the composition of ilmenites from continental mafic intrusions produced during continental rift magmatism. This supports the continental rifting event recently proposed for this part of Yukon and indicates the economic potential of the Canadian Creek platinum occurrence. Composition of spinel from Florence Creek sample indicates an Alaskan-type intrusion as the source of PGM.