SULFIDE MINERALOGY OF THE BIC LAYERED INTRUSION, BARAGA BASIN, NORTHERN MICHIGAN

The BIC intrusion occurs within the Marquette-Baraga dike swarm in the Upper Peninsula of Michigan. The intrusion is related to the ~1.1 Ga Midcontinent Rift System; the layered intrusion is relatively small at ~1100m X 400m x 620 m thick. Units include feldspathic wehrlite, clinopyroxenite and gabbro. Disseminated sulfides (3 volume percent in the main intrusive body, up to 10 volume % in Little BIC) occur throughout the layered sequence. δ³⁴S values range from -0.41 to 1.23 ‰ and are within the range considered normal for sulfur of upper mantle origin. Detailed petrographic examination has been undertaken to evaluate the variations in sulfide assemblages throughout the intrusion.

In the basal feldspathic wehrlite sulfide minerals occur interstitially to olivine and pyroxene, and tend to be associated with ilmenite or amphibole. The assemblages are richer in Cu than many magmatic Ni-Cu sulfide occurrences that are associated with mafic to ultramafic rocks. Mineral percentages in both the wehrlite and clinopyroxenite are: chalcopyrite, 50-70%; pentlandite, 20-30%; pyrrhotite, 15-50%; cubanite is locally abundant and pyrite may constitute up to 5 volume % of the assemblage. Pyrrhotite and pentlandite commonly show partial alteration to oxide minerals and chalcopyrite also occurs as stringers and veinlets. The clinopyroxenite unit contains a similar sulfide assemblage, but bornite is also present in amounts up to 5 volume %. Sulfides in the gabbro contain pyrrhotite (20-50%), but are still rich in chalcopyrite (20-30%) and pentlandite (5-25%). The relative abundance of Cu-rich minerals in the BIC intrusion is consistent with a sulfide system whose composition was controlled by mass effects and processes such as fractionation of parental magma. The Cu-rich nature of the sulfide assemblage is not a result of fractional crystallization of sulfide liquid, but resulted from the strong partitioning of Cu, as well as Ni, into a small mass of available sulfide liquid. The amount of sulfide in the BIC intrusion is above the expected “cotectic” proportion and leaves open the possibility that despite the mantle-like δ³⁴S values, addition of crustal sulfur was the prime cause of sulfide saturation in the magma.