Sulfide-poor platinum-group mineralization in komatiitic systems; Boston Creek flow, layered basaltic komatiite, Abitibi Belt, Ontario

Abstract

Geochemical studies of platinum-group elements (PGE), Au, Ag, and other metals (S, Se, As, Sb, Cu, Ni, Cr, Co, V, Ti) and electron microprobe studies of them have provided new constraints on the character and origin of the recently discovered sulfide-poor PGE mineralization in the Boston Creek flow layered basaltic komatiite of the Abitibi belt. High PGE levels characterize a 2-m-thick interval overlapping the contact of the clinopyroxenite and gabbro layers (50 ppb Pt, 75 ppb Pd, 10 ppb Au), the basal part of the clinopyroxenite layer (20 ppb Pt, 33 ppb Pd, 49 ppb Au), and the basal part of the peridotite layer (3 ppb Ir, 15 ppb Pd). The PGE-rich clinopyroxenite and gabbro layers contain grains of Pd, Pt, and Rh bismuthotellurides, antimonides and arsenides, and gold and silver minerals, which occur in spatial association with sparsely disseminated interstitial chalcopyrite, skeletal magnetite-ilmenite, and secondary amphibole alteration of chalcopyrite and magnetite-ilmenite. These rocks are spatially associated with zones of sheared amphibolite that are relatively poor in PGE and rich in Ni, Co, S, and Se. Minor anorthosite patches, dikes, and discontinuous layers immediately up section of the mineralized gabbro are relatively poor in PGE, especially Ir (< or =0.02 ppb), and Ni, Cu, Co, and S. The PGE-enriched peridotite is slightly richer in Cr and Ni but poorer in Co, Ag, S, and Se compared to the overlying peridotite.The textural and geochemical characteristics indicate a sulfide control in the origin of the PGE mineralization, but they contradict an important role for settling of segregated sulfide, Cl-rich late-stage magmatic volatiles, or hydrothermal fluid activity during metamorphism. Alternatively, they suggest PGE mineralization by in situ segregation of sulfide accompanying the crystallization of Fe-Ti oxide from upwardly mobile, volatile-charged interstitial melt that evolved in the clinopyroxenite and basal gabbro and subsequently formed the anorthosite. Nickel, Co, Se, S, Ag, and Cu were selectively mobilized relative to the PGE in the mineralized rocks by hydrothermal fluids and reconcentrated in zones of shear deformation and brittle failure during greenschist facies contact metamorphism. Iridium and Pd probably concentrated in the basal peridotite by coprecipitation of an alloy-sulfide phase during the fractionation of olivine + or - chromite. Exploration criteria potentially useful in the search for economic concentrations of PGE in geologically similar environments of the Abitibi belt and elsewhere may include magnetite-ilmenite-rich clinopyroxenite and gabbro within thick layered mafic-ultramafic flows and especially intrusions; and Cu, V, Fe, and Ti fractionation trends and anomalies.