Synsedimentary, Diagenetic, and Metamorphic Pyrite, Pyrrhotite, and Marcasite at the Homestake BIF-Hosted Gold Deposit, South Dakota, USA: Insights on Au-As Ore Genesis from Textural and LA-ICP-MS Trace Element Studies

The Paleoproterozoic Homestake deposit, northern Black Hills, South Dakota, is the largest banded iron-formation (BIF)-hosted gold deposit in the world and one of the largest single gold deposits globally (~1,300 t Au mined at an average grade of 8.4 g/t). The origin of the deposit has remained in dispute from its discovery, with views broadly falling into two groups: (1) the gold and associated elements were externally sourced and deposited in a suitable structural and/or chemical trap (e.g., iron formation), and (2) gold was indigenous to the host iron formation, from which it was remobilized into dilatant structural zones during deformation and metamorphism. In recent times, most workers have favored the former model, appealing to the broad synchroneity of Paleoproterozoic metamorphism, felsic magmatic activity, and gold event timing.

LA-ICP-MS analysis of synsedimentary to early diagenetic sulfides at Homestake and surrounding area indicates that the Au in the orebodies may have originally had a significant syngenetic component. In particular, LA-ICP-MS mapping of multistage pyrites from carbonaceous shale in the upper Poorman Formation shows that Au and a suite of trace elements (Co, Ni, Cu, Zn, Se, Mo, Ag, Sb, Te, Au, Hg, Tl, Pb, and Bi), similar to those in diagenetic pyrite from around the globe, are contained in anhedral, sponge-textured cores and nodules surrounded by relatively trace element barren, Au-poor, euhedral pyrite rims. Furthermore, interelement ratios (e.g., Co/Ni, Ni/Ag) of the trace element-rich “spongy” and nodular pyrites are also similar to those of typical diagenetic pyrite. LA-ICP-MS imaging of pyrrhotite in the same rocks reveals multistage growth in this mineral as well, with the earlier generation having higher concentrations of Ag, Sb, Pb, Tl, and Bi than its presumed metamorphic counterpart. Imaging of marcasite in the shales of the upper Poorman Formation demonstrates this mineral’s high abundance of W and Tl relative to all generations of pyrite and pyrrhotite. Mass-balance calculations indicate that the volume of upper Poorman Formation in the mine area (approx. 12 km³) could potentially yield ~4,500 t of Au, a value greater than the total mined resource by more than a factor of three.

These geochemical data and calculations suggest that a significant portion of the gold at Homestake may have been sourced from the relatively thick carbonaceous and sulfidic black shale facies in the stratigraphic footwall to the host iron formation, in a manner similar to other sediment-hosted gold districts.