MASSIVE SULFIDES OF THE MICHIGAMME FORMATION IN THE VICINITY OF THE EAGLE NI-CU-PGE DEPOSIT, UPPER MICHIGAN

The Eagle Ni-Cu-PGE sulfide deposit, located in the Baraga Basin of northern Michigan, is associated with picritic magmatism that characterizes the early stages (~1107 Ma) in the development of the Midcontinent Rift System. Pyrrhotite, chalcopyrite and pentlandite occur in disseminated, net-textured, and massive mineralization within a magma conduit system hosted in Proterozoic metasedimentary rocks.

Recent drilling has detected four types of massive sulfide hosted in black slates of the Michigamme Formation country rock. Type 1 consists of massive sulfide similar in mineral assemblage to that found in intrusive rocks. Type 2 consists of sulfide intermixed with quartz, with localized soft-sediment folding. Type 3 consists of discrete pods of massive pyrrhotite, whereas pods of chalcopyrite constitute Type 4. There are no vein networks associated with Type 3 and 4 sulfides. Type 1 sulfides show sulfur isotope values of 1.2 to 3.9‰, a range nearly identical to that reported by Ding et al. (2012) for intrusive-hosted massive mineralization. Type 1 mineralization is also characterized by higher PGE contents (e.g., 693 ppb Pt, 1200 ppb Pd) than either Types 2 or 3 mineralization. Class 2 sulfides show d ³⁴S values between 7.5 and 11.8‰, and sharp sulfide-slate contacts with no noticeable alteration mineralogy. Class 3 sulfides display d ³⁴S values between 5.4 and 10.3‰, and show acicular muscovite forming along the sulfide-slate boundary. Class 4 has d ³⁴S values between 1.6 and 2.9‰, and relatively high Pd values. Acicular Mg-rich chlorite is found around the sulfide.

Sulfur isotopic data suggest that Types 1 and 4 sulfides are derived from the magmatic system, with low d ³⁴S values reflecting isotope exchange between mantle- and country rock-derived sulfur in the conduit system. Furthermore, the spatial proximity and metal contents of Types 1 and 4 suggest that they may be related via fractional crystallization of a sulfide liquid. Type 3 pyrrhotite represents recrystallized pyrite with S isotope values that reflect the reduction of sulfate to sulfide in the sedimentary environment. Type 2 sulfides formed from hydrothermal fluids that obtained sulfur from sedimentary sulfides in the Michigamme formation. Hydrothermal fluid flow was initiated in response to increased temperatures in the vicinity of the magma conduits.