SULFUR ISOTOPIC STUDIES OF CU-NI SULFIDE MINERALIZATION ASSOCIATED WITH THE BIC INTRUSION, BARAGA COUNTY, MICHIGAN

The BIC Cu-Ni prospect is associated with the Marquette-Baraga dike swarm in the Upper Peninsula of Michigan. Host rocks for sulfide mineralization include olivine melagabbro and feldspathic wehrlite. Sulfide minerals occur interstitially to silicate minerals forming a net-texture. Massive sulfide mineralization occurs sporadically near the base of the intrusion. Sulfide assemblages are composed of pyrrhotite, chalcopyrite-cubanite, pentlandite and minor pyrite. Country rocks to the intrusion include various Paleoproterozoic units, including sulfidic-graphitic pelitic rocks of the Michigamme Formation.

Sulfur isotopic studies of the BIC prospect were initiated to evaluate the potential importance of country rock-derived sulfur in the mineralization process. Prior studies of other deposits in the Marquette-Baraga dike swarm have failed to show that country rock sulfur was assimilated, and leaves open the possibility that concentration of mantle sulfides in the conduit environment may have been the principle mechanism of ore concentration. δ³⁴ S values of sulfide minerals in the BIC intrusion fall in a narrow range between -0.41 and 1.23 ‰. In contrast, sulfide minerals from the Michigamme Formation show elevated δ³⁴ S values between 5 and 8‰. Our data indicated that less than 10% of sulfide from the Michigamme Formation whose δ³⁴ S are in the range mentioned above could be incorporated in the sulfide mineralization. There are two alternatives that may be proposed: 1) Mantle-derived sulfur of ~0‰ can be sufficiently concentrated to result in ore-grade accumulation of sulfide minerals or 2) Country rocks with δ³⁴ S values close to 0‰ exist in the stratigraphic succession and were not sampled. Additional studies of country rock sulfide are in progress and are required before alternative 2) can be eliminated.