TECHNIQUES AND METHODOLOGY USED IN A MINERALOGICAL AND OSMIUM ISOTOPESTUDY OF PLATINUM-GROUP MINERALS FROM ALLUVIAL DEPOSITS IN COLOMBIA, CALIFORNIA, OREGON AND ALASKA

Platinum-group minerals (PGM) from placer deposits in Colombia, Califo rnia, Oregon and Alaska were investigated with the electron microprobe proton microprobe (mu-PIXE) and ion probe to analyse their major and trace element contents and Os-187/Os-186 isotopic ratios. Most of the grains in the samples investigated proved to be essentially homogeneou s alloys of Pt-Fe and Os-Ir-Ru although a few of them contained inclus ions of other PGM such as cooperite and laurite. Detailed analyses wer e undertaken on the Os-Ir-Ru alloy phases. The Os-187/Os-186 isotope r atios fell into a range from 1.005 to 1.156 and are consistent with da ta published on PGM from other placer deposits from these regions. The ratios, together with the trace element data (and in particular the l ow rhenium content) determined by ion probe and mu-PIXE, indicate that crustal osmium was not incorporated in the grains and that no signifi cant evolution of the Os-187/Os-186 ratios occurred during their histo ry. These data, along with mineralogical and textural evidence, are co nsistent with a mantle origin for the grains through ultramafic intrus ions, although the data do not entirely rule out alternative interpret ations.