The Bovine Igneous Complex (BIC), located 8 kilometers southeast of the town of L’anse, Michigan, is a small basin-shaped mafic/ultramafic intrusion emplaced in the southwestern part of the Paleoproterozoic Baraga Basin. Although age dating of the BIC intrusion has so far been unsuccessful, the intrusion was very likely emplaced during the early magmatic stage of Midcontinent Rift development, given its similarities to other mineralized early stage intrusions, such as Tamarack and Eagle. Targeted by Kennecott as a Cu-Ni-PGE prospect, the intrusion has undergone extensive exploration drilling since 1995. To date, the intrusion has been found to be only weakly to moderately mineralized with Cu-Ni-PGE sulfides. Metal tenors provided by initial drilling averaged less than .5% Cu and Ni, and less than 350 ppb Pt and Pd (Rossell, 2008). Preliminary evaluation of field mapping, core logging and geochemical data by Rossell (2008) interpreted the intrusion to be a layered ultramafic/mafic body with an igneous stratigraphy composed of a basal wehrlite, overlain by a clinopyroxenite, and capped by a gabbro.
This study was undertaken to further refine the igneous stratigraphy of the BIC, specifically its phase layering and cryptic variation, toward the goal of better understanding its emplacement, crystallization, and mineralization history. For this study, two drill cores (BIC01-01 and 08BIC-044) that profile the BIC were investigated for their petrographic attributes, cryptic mineral compositions, and whole rock lithochemistry. In addition, a detailed (1:6,000) re-mapping of the BIC was conducted.
This study has found that the lithostratigraphy of the main intrusion is generally similar to that found by Rossell (2008), but when mineral modes and textures are factored in, the cumulate stratigraphy of the BIC is found to progresses from an olivine cumulate with intercumulus augite and plagioclase (feldspathic wehrlite unit), to an augite+olivine cumulate with intercumulus plagioclase (feldspathic olivine clinopyroxenite unit), to an augite+oxide±olivine cumulate with intercumulus plagioclase (oxide clinopyroxenite unit), and finally, a plagioclase+augite+oxide cumulate (oxide gabbro unit) at the top. Complimenting this cumulate phase layering is a smooth cryptic variation of upward decreasing mg# in olivine and augite composition. While this cumulate stratigraphy of Ol #1; Ol + Aug #1; Ol + Aug + Ox #1; Aug + Pl + Ox – Ol #1; Aug + Pl + Ox is evident in the field exposures and in the BIC01-01 drill core, a lower ultramafic zone was discovered in the longer 08BIC-044 core. This lower ultramafic zone is composed of feldspathic wehrlite unit (Ocp cumulate) and overlying feldspathic olivine clinopyroxenite unit (COp cumulate). Although generally similar to the same units composing the lower part of the main intrusion (called the upper ultramafic zone), there are subtle differences in mode, texture and composition that indicate that the lower ultramafic zone is a separate intrusive component of the BIC.
The emplacement model is proposes that the system begins with the injection of a small pulse of magma through the Archean gneisses and into the base of the Michigamme Formation. This small pulse fractionally crystallizes from the base up, a feldspathic wehrlite followed by a feldspathic olivine clinopyroxenite. The conduit is then reopened and a second larger magma pulse is intruded above the first pulse and fractionally crystallizes producing the cumulate stratigraphy seen in outcrop and drill core BIC01-01. Due to the collinear nature of trace element and rare earth geochemistry from the lower ultramafic, upper ultramafic, and gabbro zones it is concluded that the two magma pulses came from the same parent magma source. This parent magma was determined to be a high magnesium low aluminum tholeiitic basalt with an Mg# between 68 and 70. These estimates were derived by the manipulation of geochemistry obtained from a sample located in the basal chill. Chalcophile element geochemistry suggests the BIC underwent three episodes of sulfur saturation. The first event occurred as the initial pulse, which formed the lower ultramafic zone, was injected into the surrounding country rock. Next, as the second pulse of magma injected above the first, the magma became oversaturated leading to a spike in metal tenors at the base of the upper ultramafic zone. The BIC reached sulfide saturation for a third time in passive event caused by progressive fractional crystallization.
University of Minnesota M.S. thesis. July 2011. Major: Geological sciences. Advisor: James D. Miller. 1 computer file (PDF); viii, 109 pages, appendices A-B.
Foley, Daniel Jay.
Petrology and Cu-Ni-PGE mineralization of the Bovine Igneous Complex, Baraga County, Northern Michigan..
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