Sulfur Isotope and Mineralogical Studies of Ni-Cu Sulfide Mineralization in the Bovine Igneous Complex Intrusion, Baraga Basin, Northern Michigan

Abstract

The Bovine Igneous Complex (referred to as the BIC intrusion) occurs within the Marquette-Baraga dike swarm in the Upper Peninsula of Michigan. The intrusion is likely related to the ~1.1 Ga Midcontinent Rift system based on chemical similarities with the nearby Eagle mine and other Midcontinent Rift-related picrites, the lack of deformation fabric, and well-developed hornfels that overprints Penokean deformation fabrics. However, there is currently no age date for the BIC intrusion to confirm that it is a rift-related intrusion. The BIC is a relatively small layered intrusion with dimensions of ~1,100 × 400 × 732 m thick. Units include feldspathic wehrlite/olivine melagabbros, feldspathic olivine clinopyroxenite, and gabbro. The Little BIC, a satellite intrusion located ~100 m from the main intrusion, is primarily feldspathic wehrlite. Ni-Cu sulfide mineralization occurs in both intrusions, with a higher relative proportion in the Little BIC intrusion. Sulfide assemblages are comprised predominantly of pyrrhotite, chalcopyrite, and pentlandite; textural types vary from disseminated through globular to semimassive net-textured and locally massive. The δ 34S values of sulfide minerals in the BIC intrusion range from −6.0 to +1.5‰, with ~ 80% of the values between 0 and 1.5‰. The Proterozoic Michigamme Formation country rocks show δ 34S values between 3.0 and 9.8‰. The δ 34S values of the semimassive sulfides in the Little BIC intrusion are similar to those in country rock (3.8−6.7‰), whereas the massive sulfide values are similar to those of the sulfide minerals in the BIC intrusion. Although many values are near those of mantle-derived sulfur, the presence of strongly negative values in both intrusions indicates that crustally derived sulfur was involved in the mineralization process. In addition the distinctly positive values of disseminated sulfides in the Little BIC intrusion indicate that a large fraction of the sulfur involved in the mineralization was of country-rock origin and that at least two pulses of sulfide-saturated liquids were involved in its genesis. The Δ33S values for sulfide minerals in the BIC and the Little BIC intrusions range from −0.33 to +0.35‰, indicating that a component of Archean sulfur was also involved in the mineralization. Olivine Fo and Ni contents are consistent with rock types being related via fractional crystallization, but this process cannot account for the wide range of sulfide zones found throughout both intrusions. Multiple pulses of magma, several sulfide saturated and with distinct sulfur isotope compositions, were involved in the formation of the BIC and the Little BIC intrusions. Both sulfide concentration and isotopic composition suggest that sulfide saturation was promoted via the addition of country rock-derived sulfur.