PARAGENESIS OF COBALT AND NICKEL IN THE PROTEROZOIC SHEEP CREEK MASSIVE SULFIDE DEPOSIT, CENTRAL MONTANA

Sheep Creek, located in central Montana, is a stratiform Cu-(Co-Ni-Ag) massive sulfide deposit within the Newland Formation of the Proterozoic Belt Supergroup. The deposit consists of at least three separate mineralized zones, here referred to as the Lower, Middle, and Upper Sulfide Zones (LSZ, MSZ, USZ). The USZ contains an inferred resource of 7 Mt at 2.4% Cu, 0.1% Co and 12.3 gpt Ag. The LSZ and MSZ are currently being evaluated. Whereas previous work has described the geologic setting of the deposit, this study is focusing on the paragenesis of the ore, with special attention to the mineralogical residence of cobalt and nickel.

Pyrite, chalcopyrite and tennantite are the main sulfide minerals at Sheep Creek. The LSZ has abundant dolomite and local accumulations of Ca-phosphate minerals, and has elevated Au. The USZ is rich in barite with some Ba-rich feldspar, and has elevated Ag and Pb. Co and Ni are present in all zones, and occur in two main forms: 1) in solid solution within porous, fine-grained pyrite; and 2) as discrete Co-Ni minerals. Fine-grained pyrite, locally containing several wt % each of Co, Ni, and As, formed either by amalgamation of pyrite framboids or by replacement of marcasite. Marcasite is locally preserved, but is not as good a host to Co-Ni as is pyrite. The formation of marcasite suggests locally acidic conditions, which may have been less conducive to co-precipitation of Co and Ni. Some samples show porous pyrite overgrown by coarser-grained, non-porous pyrite. The latter form of pyrite contains very little Co and Ni, but is associated with chalcopyrite, tennantite, galena, bornite, siegenite (Co,Ni)₃S₄, and trace amounts of cobaltite and bismuthinite. Siegenite, previously unrecognized at Sheep Creek, has been identified in drill core and polished sections from the LSZ and USZ, and is probably also present in the MSZ.

Based on textural relations, we believe Co-Ni associated with fine-grained pyrite is syngenetic or early diagenetic in origin. Siegenite, closely associated with tennantite, formed in a later stage of diagenesis in which (Co,Ni,As)-rich pyrite was recrystallized and partially replaced by interaction with higher temperature, Cu-rich fluids.