GEOCHEMISTRY AND PETROGRAPHY OF THE STRATA HOSTING THE FLAMBEAU CU-ZN-AU DEPOSIT: REVISITING WISCONSIN’S ONLY PAST-PRODUCING VOLCANOGENIC MASSIVE SULFIDE MINE

Volcanogenic massive sulfide (VMS) deposits are polymetallic mineral deposits and are the source of many important base and precious metals (e.g., Cu, Zn, Pb, Au, Ag). The size and abundance of VMS deposits in Wisconsin indicates potential for the state to be a world-class mining district. These VMS deposits are hosted within the Wisconsin Magmatic Terrane, a Paleoproterozoic juvenile arc sequence within the Penokean Orogeny. The Flambeau deposit was the only one of many economical VMS deposits to be mined. After extracting only the supergene-enriched cap to the ore, the mine was closed and site was rehabilitated. Research on the region essentially ceased along with exploration and mining activity. The primary objective of this study is to revisit the Flambeau deposit and describe the geology, geochemistry, and petrography of the strata hosting the deposit in light of almost 20 years of advances in the fields of geochemistry, economic geology, and the tectonic evolution of the region.

With very little outcrop in the area surrounding the past-producing mine, relogging and sampling of historic drill cores was done at the core repository owned by the Wisconsin Geological and Natural History Survey. Major and trace element geochemistry were analyzed at the Materials Science Center at UW-Eau Claire. In addition to examining the regional geology of Rusk County, chemostratigraphic sections through the Flambeau deposit were developed and the chemical evolution of the volcanic system that hosts the ore body was described. The Flambeau deposit is deposited as a series of stacked ore lenses with zones of footwall assemblages of strongly altered quartz-sericite-pyrite-sphalerite schist. Hangingwall assemblages are more diverse but contain metamorphosed altered mineral assemblages, such as porphyroblastic andalusite, biotite, and garnet, throughout the strata. This indicates that the hydrothermal system that formed the deposit continued to evolve after the deposition of the ore body and the subsequent deposition of intermediate pyroclastic assemblages. The geochemical data presented in this manuscript provides valuable constraints on the petrogenesis of the volcanic assemblages in this part of the Penokean Orogeny.