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Sulfur isotope and mineralogical studies of Ni-Cu sulfide mineralization in the Bovine igneous complex intrusion, Baraga Basin, northern Michigan

Kellie A. Donoghue, Edward M. Ripley and Chusi Li
Sulfur isotope and mineralogical studies of Ni-Cu sulfide mineralization in the Bovine igneous complex intrusion, Baraga Basin, northern Michigan
Economic Geology and the Bulletin of the Society of Economic Geologists (April 2014) 109 (2): 325-341

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 approximately 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 approximately 1,100 X 400 X 732 m thick. Units include feldspathic wehrlite/olivine melagabbros, feldspathic olivine clinopyroxenite, and gabbro. The Little BIC, a satellite intrusion located approximately 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 delta (super 34) S values of sulfide minerals in the BIC intrusion range from ?6.0 to +1.5ppm, with approximately 80% of the values between 0 and 1.5ppm. The Proterozoic Michigamme Formation country rocks show delta (super 34) S values between 3.0 and 9.8ppm. The delta (super 34) S values of the semimassive sulfides in the Little BIC intrusion are similar to those in country rock (3.8?6.7ppm), 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 Delta (super 33) S values for sulfide minerals in the BIC and the Little BIC intrusions range from -0.33 to +0.35ppm, 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.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 109
Serial Issue: 2
Title: Sulfur isotope and mineralogical studies of Ni-Cu sulfide mineralization in the Bovine igneous complex intrusion, Baraga Basin, northern Michigan
Affiliation: Indiana University, Department of Geological Sciences, BGloomington, IN, United States
Pages: 325-341
Published: 201404
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 49
Accession Number: 2014-004508
Categories: Economic geology, geology of ore depositsIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 4 tables, geol. sketch maps
N48°00'00" - N49°00'00", W89°00'00" - W88°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201404
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