CHARACTERIZATION OF COUNTRY-ROCK HOSTED SULFIDE TEXTURES WITHIN THE VICINITY OF THE EAGLE NI-CU-PGE DEPOSIT, UPPER PENINSULA, MICHIGAN

The Eagle deposit, hosted within the Baraga Basin on Michigan’s Upper Peninsula, is a high-grade, conduit style, magmatic Ni-Cu-PGE deposit considered to be related to early stage, Midcontinent Rift magmatism. The Baraga Basin is a structural basin largely comprised of ~1.85 Ga Michigamme Formation metapelites, metagraywacke, and iron formation, conformably overlying lesser amounts of basal Goodrich Quartzite. Underlying the basin is the ~2.7 Ga granite-greenstone terrain of the Northern Complex which is comprised largely of granite, granitic-gneiss, and greenstone. Within the vicinity of the Eagle deposit, the Michigamme and Goodrich Quartzite contain up to 10 and 5 percent sulfide respectively, while sulfide in the basement rocks is disseminated and rare.

Multiple authors have suggested contamination of mafic magma by country rock derived sulfur is a critical factor in the genesis of magmatic-sulfide ore deposits. While the mechanisms by which country rock sulfur is incorporated into magma are not well understood, evidence from case studies support claims that externally-derived sulfur contributes to sulfide mineralization. Over the last decade, studies conducted on the Eagle deposit have unearthed evidence suggesting that a significant quantity of ore-forming sulfur was derived from Baraga Basin metasediments and the underlying Archean basement.

In this study, we characterize several modes of sulfide occurrence within the country rocks of the Eagle deposit using a combination of reflected and refracted light petrography and SEM w/ EDS. Along with enhancing descriptions of previously recognized textures, we report several new sulfide textures and comment on their origins. While typical sulfide assemblages consist of pyrite, pyrrhotite and minor chalcopyrite, trace amounts of other sulfides also exist. Sulfides range from disseminated, very fine-grained pyrite to massive, pyrrhotite-pyrite bands and lenses usually indicating hydrothermal influence. Other observations include occurrences of sulfide zonation, complex emplacement structures, and epitaxial overgrowths. The exceptional textural variety of sulfides preserved in these rocks record a complex diagenetic and metamorphic history further complicated by mobilization and re-emplacement by later hydrothermal fluids.