The distribution of platinum-group elements, nickel, copper, and gold in the Muskox layered intrusion, Northwest Territories, Canada

Abstract

The Muskox intrusion was emplaced into a rift in the northwestern Canadian Shield at about 1270 Ma. This rifting event was associated with extensive flood basalt magmatism (Coppermine River basalt) and is attributed to the development of a mantle plume centered some 400 km north of the intrusion. Nickel-copper sulfides occur at the margins of the intrusion. The geologic setting of these sulfides is similar to that of a number of Ni-Cu sulfide deposits, such as those found in the Noril'sk-Talnakh area, the Duluth Complex, the Insizwa Complex, and the Cape Smith belt. Some of the marginal sulfides at Muskox are similar in composition to the Cu-PGE-rich Noril'sk-Talnakh sulfides and contain 16 percent Cu and up to 200ppm Pt + Pd + Au. However, most of the Muskox marginal sulfides were found to contain 3 to 10 percent Ni + Cu and intermediate levels of platinum-group elements (PGE) (1-10 ppm) similar to the Minnimax deposit of the Duluth Complex. The PGE content of the sulfides increases from the margins to the interior of the intrusion. This increase could be due to an increase in the ratio of silicate to sulfide liquid (R factor) during the crystallization of the intrusion. The high percentage of delta 34 S per mil of the sulfides indicates that the S in the sulfides was derived from the country rocks. Therefore, the amount of sulfide liquid that formed could have been controlled by the availability of S, which in turn was controlled by the distance from the contact with the country rock. Disseminated sulfides occur in association with a chromite-rich orthopyroxenite in the layered series of the Muskox intrusion. These sulfides have a strata-bound character similar to classic PGE reefs, such as the Merensky reef of the Bushveld intrusion. The sulfides from the Muskox reef unit contain only 1 to 10 ppm PGE, an order of magnitude less than sulfides from classic PGE reefs. The lower PGE content of the Muskox reef sulfides is attributed to a lower R factor; approximately 1,000 at Muskox versus greater than 10,000 for classical reefs. The sulfide-bearing rocks have higher Cu/Ni, Pd/Ir, Pd/Pt, and Rh/Ir ratios than sulfide-poor rocks. This is because the sulfides are enriched in Cu, Pd, and Rh to a greater degree than in Ni, Pt, and Ir. The difference in the degree of enrichment for these metals could be the result of Ni, Pt, and Ir having lower partition coefficients in sulfide liquid than Cu, Pd, and Rh; contamination of the Muskox magma with a partial melt of country rock; or a combination of these factors.