SULFIDES IN THE MUSKOX INTRUSION

Abstract

The Muskox intrusion is a layered, dominantly ultramafic pluton, in northern Canada. Sulfide phases belonging to the Ni–Cu–Fe–S system are concentrated along its margins and in chromite-rich horizons within its central layered series. They are sparsely disseminated through the rest of the intrusion, except in a core zone, which is almost barren of sulfides.The primary sulfide assemblages in the layered series range from nickel-rich at the base, through copper-rich at intermediate levels, to iron-rich near the roof. Nickel:copper ratios in the sulfides vary sympathetically with those of the host silicates. A similar but inverted sequence of sulfide assemblages occurs in the marginal zone. This and textural evidence indicate that the disseminated sulfides in the main body of the intrusion formed by segregation of sulfides from inter-cumulous magma and that they crystallized in equilibrium with their respective host silicates. The ultimate origin of the sulfur is not known. Sulfides in the upper part of the layered series and upper border zone appear to have formed by late replacement of the host rocks at temperatures below 675 °C.The concentration of sulfides along the intrusive contacts, and in adjacent wallrocks, developed during the primary cooling cycle by a process of desulfidization of the Muskox magma. Sulfide transfer in these locations was controlled primarily by the temperature gradient, not by gravity settling of a separate sulphide fluid.During subsequent serpentinization of the intrusion, the introduced fluid phase became progressively more oxygen depleted. The effect was most pronounced in the central regions, where primary sulfides ultimately decomposed to form native metals. Sulfur so released migrated as H₂S to more peripheral parts of the intrusion, where it entered into various reactions with primary sulfides, oxides, and silicates.Separate processes thus operated to achieve desulfidization and desulfurization at magmatic and submagmatic temperatures, respectively, of the main body of the intrusion and to favor development of sulfide-enriched zones towards its margins.Sulfides in both the Muskox feeder dyke and the diabase dykes of the area are disseminated relatively uniformly through these rocks. These sulfides appear to have crystallized in situ and show no obvious zoning relationships.