COMPOSITIONAL VARIATIONS IN CU-NI-PGE SULFIDES OF THE DUNKA ROAD DEPOSIT, DULUTH COMPLEX, MINNESOTA - THE IMPORTANCE OF COMBINED ASSIMILATION AND MAGMATIC PROCESSES

The Dunka Road deposit is one of ten occurrences of Cu-Ni sulfides bea ring platinum-group elements (PGE) on the northwestern margin of the D uluth Complex, in Minnesota. Mineralization has been linked to contami nation of the host troctolitic magma through assimilation of argillace ous rocks from the Virginia Formation. On the basis of texture and com position, the sulfide mineralization is divided into five types: 1) no rite-hosted disseminated sulfides, 2) troctolite-hosted disseminated s ulfides, 3) PGE-rich disseminated sulfide horizons, 4) pyrrhotite-rich massive sulfides, and 5) chalcopyrite-rich disseminated sulfides. The norite-hosted sulfides exhibit features suggestive of the magma's sub stantial contamination, such as high proportions of pyrrhotite and ars enide minerals, and high mean values of S/Se (9,700) and delta(34)S (1 1.2 parts per thousand). 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 mo derate degrees of contamination, as indicated by their intermediate me an values of S/Se (4,600) and delta(34)S (7.8 parts per thousand). The PGE-rich sulfide horizons show little sign of contamination, and have mantle-like mean values of S/Se (2,600) and delta(34)S (2.1 parts per thousand). Their very high PGE contents suggest that they formed at e levated R factors. The pyrrhotite-rich massive sulfides and associated chalcopyrite-rich disseminated sulfides have relatively high mean val ues of S/Se (8,000) and delta(34)S (10.2 parts per thousand), indicati ve of significant contamination. The former are interpreted to represe nt a cumulate of monosulfide solid-solution (mss), whereas the chalcop yrite-rich sulfides represent the fractionated sulfide liquid. A gener al increase in the degree of contamination is observed toward the base of the intrusion, associated with a decrease in R factor and metal co ncentration of the sulfides. This likely results from the introduction of partial melt from the metasedimentary country-rocks, which was coo ler than the mafic magma and led to the early crystallization of the s ulfide liquid.