PLATINUM GROUP ELEMENT GEOCHEMISTRY OF THE KARMUTSEN FLOOD BASALT PROVINCE, WRANGELLIA TERRANE, VANCOUVER ISLAND

As one part of a long-term research project on the nature, origin and evolution of the Triassic Wrangellia flood basalts in B.C., Yukon and Alaska, we have initiated a detailed study of the platinum group element (PGE) geochemistry of Karmutsen tholeiitic basalts and picrites on Vancouver Island. Concentrations and relative abundances of the different PGE in basalts are sensitive indicators of the relative S-saturation state of magmas, noble metal enrichment, and melting processes in the mantle. The Karmutsen Formation on Vancouver Island is perhaps the thickest accreted section of an oceanic plateau worldwide, exposing up to 6000 m of basal sediment-sill complexes, basaltic to picritic pillow lavas, pillow breccia, and thick, massive basalt flows. PGE (Ir, Ru, Rh, Pt, Pd, Au) concentrations were determined by the NiS-fire assay method for 25 samples (6-19 wt% MgO) from three different areas on Vancouver Island (Mt. Arrowsmith, Schoen Lake, Karmutsen Range). The incompatible element-depleted high-MgO basalts and picrites (11-19 wt% MgO) from the Karmutsen Range have relatively low Pd (8-14 ppb), Pd/Pt (0.9-1.3), Pd/Ru (4.7-6.5), and Cu (83-102 ppm). They show positive correlations between olivine phenocryst contents, whole rock MgO values and PGE concentrations, suggesting relative compatibility of the PGE in olivine (as alloys?) during crystallization. In contrast, the incompatible element-enriched low-MgO tholeiites (6-9 wt% MgO) are characterized by relatively high Pd (15-26 ppb), Pd/Pt (2-4.7), Pd/Ru (22-77), and Cu (115-217 ppm). Pd concentrations and Pd/Pt in the tholeiitic basalts are among the highest observed to date in oceanic plateaus (Ontong Java, Kerguelen) and islands (Iceland), continental rifted margins (East Greenland), and continental flood basalts (Noril'sk, Siberia). Importantly, there is no geochemical evidence that any of the sampled basalts and picrites from Vancouver Island underwent sulfide-saturated fractionation (e.g. Cu/Pd = 10,350 ± 2400 [1sd] for all samples), which indicates that they were erupted with their full complements of metals. Future work will extend the PGE database to include Wrangellia flood basalts in Yukon and Alaska and will provide a complete overview of the noble metal systematics across this entire accreted large igneous province.