Compositional variations in Cu-Ni-PGE sulfides of the Dunka Road Deposit, Duluth Complex, Minnesota; the importance of combined assimilation and magmatic processes

Abstract

The Dunka Road deposit is one of ten occurences of Cu-Ni sulfides bearing platinum-group elements (PGE) on the northwestern margin of the Duluth Complex, in Minnesota. Mineralization has been linked to contamination of the host troctolitic magma through assimilation of argillaceous rocks from the Virginia Formation. On the basis of texture and composition, the sulfide mineralization is divided into five types: 1) norite-hosted disseminated sulfides, 2) troctolite-hosted disseminated sulfides, 3) PGE-rich disseminated sulfide horizons, 4) pyrrhotite-rich massive sulfides, and 5) chalcopyrite-rich disseminated sulfides. The norite-hosted sulfides exhibit featues suggestive of the magma's substantial contamination, such as high proportions of pyrrhotite and arsenide minerals, and high mean values of S/Se (9,700) and δ34S (ll.2%₀). They are also generally metal-poor, implying that the sulfides interacted with a relatively low volume of silicate melt (i.e., low R factor). The troctolite-hosted sulfides formed at moderate degrees of contamination, as indicated by their intermediate mean values of S/Se (4,600) and δ34S (7.8%₀). The PGE-rich sulfide horizons show little sign of contamination, and have mantle-like mean values of S/Se (2,600) and δ34S (2.1%o). Their very high PGE contents suggest that they formed at elevated R factors. The pyrrhotite-rich massive sulfides and associated chalcopyrite-rich disseminated sulfides have relatively high mean values of S/Se (8,000) and δ34S (10.2%o), indicative of significant contamination. The former are interpreted to represent a cumulate of monosulfide solid-solution (mss), whereas the chalcopyrite-rich sulfides represent the fractionated sulfide liquid. A general increase in the degree of contamination is observed toward the base of the intrusion, associated with a decrease in R factor and metal concentration of the sulfides. This likely results from the introduction of partial melt from the metasedimentary country-rocks, which was cooler than the mafic magma and led to the early crystallization of the sulfide liquid.