Abstract

The Geordie Lake intrusion is a small body of sulfide-bearing mafic rocks that has intruded contemporaneous syenites of the Coldwell alkaline complex, northwestern Ontario. The intrusion consists of alternating layers of troctolite containing dendritic olivines and ophitic gabbros. Cumulate rocks are absent and the rocks are interpreted to be a series of rapidly cooled intrusions of tholeiitic basaltic magma. Compositional data are presented for olivine, plagioclase, alkali feldspar, clinopyroxene, amphibole, magnetite, and ilmenite. These data, together with whole-rock major and trace element analyses, demonstrate that the Geordie Lake magma was relatively evolved at the time of emplacement. The formation of secondary amphiboles and biotite together with low equilibration temperatures calculated for coexisting Fe-Ti oxides demonstrates that extensive deuteric subsolidus reequilibration has occurred.Sulfides consist principally of disseminated chalcopyrite associated with minor bornite, pyrite, millerite, cobaltite, siegenite, sphalerite, and galena. Tellurides (hessite, melonite, altaite) and platinum-group minerals are associated primarily with disseminated chalcopyrite and are rare in late-stage massive chalcopyrite. The platinum-group minerals are dominantly palladium tellurides and arsenides. Rare sperrylite is the only platinum mineral present.Textural relationships suggest that rapid cooling of the magma and crystallization of olivine and magnetite induced the early separation of an immiscible sulfide melt. This melt remained as a liquid until the silicate host rocks had cooled to subsolidus temperatures. During the final stages of crystallization of the silicate magma the sulfide melt and residual water-rich fluids were trapped together within pore spaces. Crystallization of Ti-poor magnetite, tellurides, and platinum-group minerals from the sulfide melt occurred at low temperatures (<550 degrees C). The fugacities of sulfur and tellurium prevailing during crystallization of the sulfide melt are estimated from predominance diagrams. Pyrrhotite, pentlandite, and Ir-, Os-, and Rh-based minerals are absent from the sulfide assemblage. The predominance of Pd and Cu tellurides and sulfides, coupled with low Pt(Pt + Pd) and high Cu/(Cu + Ni) ratios, suggests that the Geordie Lake platinum-group minerals-sulfide assemblage crystallized from a relatively evolved sulfide melt associated with an evolved tholeiitic magma.

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