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The distribution and behaviour of platinum in soils of the Tulameen ultramafic complex, southern British Columbia : applications to geochemical exploration for chromite-associated platinum deposits Cook, Stephen John


Exploration for chromitite-associated Pt deposits is hampered by a poor understanding of the distribution and behaviour of Pt in the surficial environment. This study investigates Pt content, residence sites and PGE mineralogy of soils developed on till and colluvium above the Tulameen ultramafic complex in southern British Columbia. Seventy-six soil profiles, as well as sediments, bogs and waters were sampled above the dunite core of the Tulameen complex, within which Pt occcurrences consist of massive-to-discontinuous segregations of platinic chromitite. Pt content of the -212 um fraction of soils and sediments was determined by fire assay-inductively coupled plasma spectroscopy. Samples from fourteen selected profiles were then examined in detail to determine Pt mineralogy and its distribution between different size, density and magnetic fractions. Pt concentrations in the -212 um fraction of C horizon soils range from 2 to 885 ppb and are closely related to soil dunite content, as estimated from MgO content and verified by XRD mineralogy. Dunite colluvium (mean: 24.2% MgO), locally-derived dunitic till (mean: 16.5% MgO) and exotic non-dunitic till (mean: 5.7% MgO) have median Pt concentrations of 88 ppb, 36 ppb and 8 ppb, respectively. This trend is evident in all size and density fractions. Pt content of heavy mineral (SG > 3.3) fractions is 10-2Ox greater than in light mineral fractions. Pt is most abundant in the heavy magnetic fraction from non-dunitic tills and dunitic tills remote from known mineralization, but the proportion of Pt in the heavy non-magnetic fraction increases with increasing proximity to mineralization. SEM and microprobe studies of heavy fractions from C horizons identified Pt-Fe-Cu alloys as free grains, and as inclusions in Mg-silicates and chromites. Chromite occurs as Mg-Cr-rich anhedral fragments and as Fe-rich euhedral to subhedral crystals. The latter, relatively more important in the magnetic fraction, are interpreted as Pt-poor grains disseminated throughout the dunite whereas fragments are relatively more important in the non-magnetic fraction and are interpreted as remnants of Pt-bearing massive chromitite segregations. The abundance of chromite fragments in soils near chromitite segregations accounts for the high Pt content of the non-magnetic heavy fractions of these soils. The -270 mesh fraction or the magnetic heavy fraction of C horizon soils would be the most suitable sample media for reconnaissance geochemical sampling. However, the greater contrast, more limited dispersion and Mg-Cr-rich chromite association of the non-magnetic heavy fraction make it a more suitable media for detailed geochemical sampling.

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