Abstract

The Katiniq nickel deposit is an example of an association of magmatic Fe-Ni-Cu sulfides with komatiitic volcanism, in the Cape Smith-Wakeham Bay foldbelt in northern Quebec. The deposit is unusual because of its Proterozoic age in contrast to the Archcan ages of most other deposits of this type, and is of interest in view of the relatively low degree of deformation and metamorphism to which it has been subjected.Nickel sulfide mineralization occurs within the Katiniq sill, an olivine-rich subvolcanic intrusive lens up to 150 m thick. The sill consists of a number of olivine-rich intrusive units, each characterized by a concentric zonation in olivine content from a maximum in the core to minimums at the margins. This zonation is attributed to a mechanism whereby the sill behaved as an open channel, forming part of a subvolcanic feeder system. Magma flowing through the channel carried a progressively higher proportion of intratelluric olivine phenocrysts with time, while crystallization of earlier, less olivine-rich material occurred inward from the margins. Flowage eventually became choked off as the channel became clogged with olivine.Sulfide mineralization occurs dominantly at or close to the base of the sill and shows a marked correspondence with irregularities in the footwall topography. Typical intersections consist of massive ore at the base, succeeded upward by net-textured ore and disseminated sulfides. These observations indicate a primary magmatic origin for the sulfides.The average composition of the liquid portion of the magma-olivine suspension intruded into the sill was that of a pyroxenitic komatiite, having an MgO content of about 19 percent. The chemical variations within the volcanic rocks overlying the sill are consistent with the derivation of these rocks from a liquid of this composition by low-pressure fractionation of olivine.The mean Ni/Cu ratio of the Katiniq sulfides is 4.3, close to that predicted for a sulfide liquid in equilibrium with a magma of the above composition. Massive sulfides may show significantly higher Ni/Cu ratios than net-textured and disseminated types, a discrepancy which is attributed to hydrothermal remobilization of Cu.Concentrations of precious metals, particularly Pt and Pd, are unusually high in the Katiniq sulfides; this may be partially due to higher concentrations of these elements in the parent silicate magma but is thought to be largely the consequence of equilibration of the sulfide liquid with very large volumes of silicate magma. Accumulation of immiscible sulfide liquid droplets in the sill probably occurred over a period of time when large volumes of silicate magma were flowing through the sill and subsequently on toward the surface.

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