Platinum-group element distribution in komatiitic and tholeiitic volcanic rocks from Munro Township, Ontario

Abstract

Platinum, Pd, Ir, Au, MgO, Cr, S, Ni, Co, Cu, and Zn were determined in volcanic rocks from Pyke Hill, Fred's flow, and Theo's flow in Munro Township, Ontario, to establish the fractionation trends of the platinum-group elements in these rocks and to evaluate the relevant processes controlling such fractionation. Distinctive and relatively consistent fractionation trends are characteristic of Pt, Pd, and Ir. Platinum and Pd are usually concentrated in the upper units of flows (spinifex-textured rocks) relative to the lower olivine and pyroxene-rich cumulates, whereas the reverse is true for Ir. Theo's flow does not show these patterns in that Pt, Pd, and Ir behave in a similar fashion. The platinum-group element fractionation trends formed during the crystallization of these flows suggest that sulfide immiscibility has an insignificant role. It is concluded that, in general, Ir must be coprecipitated with olivine or pyroxene whereas Pt and Pd are rejected from the structures of these minerals. The Ir ferromagnesian association probably does not involve a substitution of Ir in the mineral structure of olivine or pyroxene.