SULFUR ISOTOPIC EVIDENCE FOR DISTINCT SOURCES OF SULFUR IN THE BIC AND LITTLE BIC INTRUSIONS, BARAGA BASIN, MICHIGAN

The BIC intrusion is located in the Marquette-Baraga dike swarm in the Upper Peninsula of Michigan. The intrusion is part of the Midcontinent Rift System, and is ~ 43 km west of Rio Tinto’s Eagle Ni-Cu deposit. Although BIC is situated in a similar geologic setting, it is characterized by a distinctly different geometry. BIC is a funnel shaped, layered intrusion containing a basal feldspathic wehrlite underlying a clinopyroxenite and an upper gabbro unit. All three layers show moderate to extreme alteration. Disseminated and massive sulfides are distributed throughout the layered intrusion with the majority of the massive sulfides located at the contacts between the feldspathic wehrlite and the metasedimentary hornfels, and the feldspathic wehrlite and clinopyroxenite. A satellite intrusion, known as Little BIC, is located a few hundred meters west of the main intrusion but does not display the same layered pattern. Little BIC is predominately a feldspathic wehrlite and contains both semimassive and massive sulfides.

Sulfide mineralogy of both intrusions includes chalcopyrite, pyrrhotite and pentlandite. Chalcopyrite is dominant, locally constituting 70 % of the sulfide assemblage. Sulfur isotopic studies were performed on the intrusive rocks and the sulfur-rich rocks of the surrounding Michigamme Formation to aid in determining the origin of the sulfur in the mineralization. Sulfide minerals in the BIC intrusion show δ³⁴S values in the narrow range of -.2 to 1.4‰. Sulfide minerals in the Michigamme Formation have δ³⁴S values between 5and 8 ‰. δ³⁴S values of the semimassive sulfides in Little BIC are similar to those of country rock (5 to 6 ‰) whereas the massive sulfide values are similar to those of the BIC intrusion. The distinct difference between δ³⁴S values of sulfide types in Little BIC indicates that at least two pulses of sulfide-saturated liquids were involved in its genesis. The high δ³⁴S values of semimassive sulfides indicate that country rock contamination occurred and leaves open the possibility that early Ni-sequestration by sulfide liquid may have lead to ore formation in the conduit system. The mineralogy and δ³⁴S values of sulfide assemblages in the BIC intrusion support the premise that fractional crystallization in a staging chamber controlled their formation