PETROLOGY AND CU-NI-PGE MINERALIZATION OF THE BOVINE IGNEOUS COMPLEX, BARAGA COUNTY, NORTHERN MICHIGAN

The Bovine Igneous Complex (BIC) is a small (1x0.5km) basin-shaped mafic/ultramafic intrusion emplaced into Paleoproterozoic sedimentary rocks of the Baraga Basin. Although isotopic dating is lacking, the BIC intrusion was likely emplaced during the early magmatic stage of the 1.1 Ga Midcontinent Rift, given its similarities to other rift-related intrusions.

The intrusion has undergone exploration drilling for Cu-Ni-PGE by Kennecott since 1995. For this study, two drill cores (08BIC044 and BIC01-01) were investigated for their petrographic attributes, mineral compositions, and whole rock geochemistry. A detailed (1:6,000) geologic map of the BIC was also produced.

Field mapping, core logging, and petrography have found that the BIC can be subdivided into three main zones: a lower ultramafic zone, an upper ultramafic zone, and a gabbro zone, each of which can be further subdivided by cumulate mineralogy. The lower ultramafic zone is comprised of a 145-m-thick, basal feldspathic wehrlite unit (Ol ortho to mesocumulate) overlain by a 50m thick feldspathic olivine clinopyroxenite (Cpx+Ol mesocumulate). The base of the upper ultramafic zone is marked by the reappearance of a feldspathic wehrlite (Ol mesocumulate) that is almost 300 m thick. This olivine cumulate then gives way up section to a cumulate progression of Cpx+Ol (180m)→Cpx+Ox±Ol (15m) → Pl+Cpx+Ox (>60m=gabbro zone). The cumulus regression evident at the lower and upper ultramafic zone contact and the heterogeneous nature of the basal upper ultramafic zone strongly implies that this contact demarks two major magma emplacement events. Further evidence of two emplacement episodes comes from cryptic variations of mg# in olivine and augite, which show trends that are consistent with two cycles of emplacement followed by fractional crystallization.

Trace element geochemistry implies that the two magma pulses came from a similar parental magma. Analyses of fine wehrlite at the basal contact were used to estimate a parental composition of a high-Ti, low-Al primitive olivine tholeiite (mg#~68). Chalcophile element geochemistry indicates the occurrence of three distinct sulfide saturation events. The lower two mineralization events occurred by S contamination, whereas the upper mineralization is a PGE reef-type caused by fractional crystallization.