Petrology and Cu-Ni-PGE Mineralization of the Bovine Igneous ...

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22 Modal Rock Types in Core 06BIC007.....................................................................46 23 Modal Mineralogy of Core 06BIC007......................................................................47 24 Mineral Habits in Core 06BIC007............................................................................48 25 Photomicrograph of Chill Zone and Feldspathic Wehrlite Unit...............................49 26 Photomicrograph of Feldspathic Olivine Clinopyroxenite Unit and Oxide Gabbro Zone..........................................................................................................................50 27 Photomicrograph of Diabase Dike and Alteration Characteristics...........................51 28 Photomicrograph of Amphibole and Apatite Habit..................................................52 29 Cryptic Variation versus Stratigraphic Height.........................................................55 30 Cumulate Stratigraphy..............................................................................................61 31 Differentiation Path of Possible Parent Magma........................................................64 32 Cryptic Variation Diagram and Cumulate Stratigraphy...........................................65 33 Emplacement and Crystallization Model..................................................................70 34 Trace Element Spider Diagram.................................................................................73 35 Rare Earth Element Spider Diagram.........................................................................73 36 20% Olivine PELE Fractional Crystallization Diagrams.........................................77 37 15% Olivine PELE Fractional Crystallization Diagrams.........................................78 38 Chalcophile Element Abundance versus Stratigraphic Height.................................81 viii
1.0 Introduction The failed rifting of the North American craton approximately 1.1 billion years ago led to extensive igneous activity in the Lake Superior region and now forms the province called the Midcontinent Rift (MCR). Various geologic, geophysical, geochemical, and geochronologic studies of MCR rocks in the Lake Superior region have revealed that the rift evolved by several stages of magmatism and tectonism. Mafic to felsic magmatism over a 23 plus Ma period led to the extrusion of subaerial lavas as well as the emplacement of multiple subvolcanic intrusions of various sizes. Four stages of magmatism are generally recognized (Miller and Vervoort, 1996; Nicholson et al. 1997; Miller and Severson, 2002; Vervoort et al., 2007; Heaman et al., 2007): an early magmatic stage of rapid and diverse volcanism and intrusion between 1112 and 1107 Ma, a latent magmatic stage of diminished felsic-only magmatism between 1107 and 1102 Ma, a main magmatic stage creating the largest volume of igneous rocks in the MCR between 1102 and 1094 Ma, and a late magmatic stage of waning volcanic activity intermixed with detrital sedimentation from 1094 to 1086 Ma. Many intrusions related to the early stage of magmatism tend to be small ultramafic bodies that show significant sulfide mineralization enriched in nickel, copper, and precious metals (Hollings et al., 2007). The Bovine Igneous Complex (BIC), the focus of this study, is such an intrusion. The BIC, located 8 kilometers southeast of the town of L’Anse, Michigan, was emplaced into Paleoproterozoic sulfidic, argillic sedimentary rocks of the Baraga Basin during the early magmatic stage of the MCR (Fig. 1). Based on reconnaissance bedrock mapping and exploratory drilling by Kennecott, the BIC is a roughly bowl-shaped, layered mafic/ultramafic 1
Page 1 and 2: Petrology and Cu-Ni-PGE Mineralizat
Page 3 and 4: Acknowledgements I would like to th
Page 5 and 6: While this cumulate stratigraphy of
Page 7 and 8: 4.6.1 Little BIC...................
Page 9: Figures 1 Geologic Map of Southwest
Page 13 and 14: 08BIC044 • Figure 2. Geology of t
Page 15 and 16: 1.1.2 Baraga Group Collision of an
Page 17 and 18: lithosphere and the lower crust (Ni
Page 19 and 20: 1.1.4 Bovine Igneous Complex The (B
Page 21 and 22: and a sulfur isotope study is under
Page 23 and 24: those goals will be to characterize
Page 25 and 26: 2.1 Core Logging and Sampling Three
Page 27 and 28: exporting data layers from ArcMap a
Page 29 and 30: Figure 7. Modal rock type classific
Page 31 and 32: were chosen from each rock type whi
Page 33 and 34: outcrops are very coarse-grained an
Page 36 and 37: Figure 12. Bedrock geologic map of
Page 38 and 39: The feldspathic wehrlite is in turn
Page 40 and 41: unit to subprismatic at the top (Fi
Page 42 and 43: Figure 15. Stratigraphic variation
Page 44 and 45: 3.2.2 08BIC044 This 731m long hole
Page 46 and 47: The lower feldspathic wehrlite unit
Page 48 and 49: the dramatic increase in clinopyrox
Page 51 and 52: Figure 17. Generalized lithostratig
Page 53 and 54: Figure 20. Stratigraphic variation
Page 55 and 56: Figure 21. Lithostratigraphy of dri
Page 57 and 58: Figure 23. Modal mineralogy of plag
Page 59 and 60: A B Figure 25. A.) Photomicrograph
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A B Figure 27. A.) Photomicrograph
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3.3 Mineral Chemistry and Cryptic L
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Figure 29. Stratigraphic variation
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All values in parts per million (pp
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4.0 Discussion The petrographic, mi
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the lower feldspathic wehrlite unit
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would expect from the fractional cr
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Cumulate Stratigraphy Pl+Cpx+Ox Cpx
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orthocumulate zone similarly increa
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upper slate of the Michigamme forma
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C. Emplacement of upper ultramafic
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Figure 34. Normalization spider dia
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aluminum but higher titanium, iron,
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B. QFM C. QFM-2 Figure 36. Mineral
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Figure 37. Mineral paragenesis calc
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Figure 38. Plot of Cu, Ni, Pd, and
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wehrlite followed by a feldspathic
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Johnson, R.C., 2007, Petrography an
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Appendix A Petrographic Description
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Appendix A-1 Bulk Rock Petrography
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044-414.7 fwer med crs to crs wehrl
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Appendix A-2 SILICATE MINERAL PETRO
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044-497.5 fwer med to med crs calci
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Appendix B Mineral Chemical Data Py
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Spectrum 3 53.26 0.94 2.43 7.6 0 15
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Average 86.1 50.1 8.1 41.8 SD 0.37
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Spectrum 3 54.15 0.84 1.64 5.3 0 16
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Spectrum 5 55.36 1.56 5.96 0 19.97
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Ol.2 40.99 16.16 0 42.84 100.0 82.5
Page 119:
Spectrum 6 53.95 0.95 2.41 6.92 17.
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Petrology and Cu-Ni-PGE Mineralization of the Bovine Igneous ...

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