Reconstructing Paleoproterozoic Volcanism in Northwestern Wisconsin : Geochemistry of the Flambeau Cu-Zn-Au Mine

Abstract

The Flambeau Cu-Zn deposit in northwestern Wisconsin, USA, is one of several volcanogenic massive sulfide (VMS) deposits in the state but remains the only one that has been mined because of unfavorable state mining legislation. The supergene enriched zone of the ore body was mined from 1993-97 producing 1.9 million tons of ore (4.1% Cu, 1.0% Zn, 1.7% g/t Au, 31 g/t Ag). Now fully reclaimed, further studies can only be conducted by examining the hypogene geology through drill cores that are found at the Wisconsin Geological History Survey (WGHS) in Mt. Horeb, WI. The Flambeau mine is a part of the Wisconsin Magmatic Terranes composed of plutonic, volcanic, and sedimentary rocks that were accreted during a collision between the Pembine-Wausau terrane and the Superior craton during the Paleoproterozoic Penokean orogeny. This tectonic setting has since been overprinted by many regional metamorphism events. The host rocks of the Flambeau are a part of the Marshfield domain consisting of both mafic and felsic pyroclastic flows suggesting an island-arc basin assemblage. The supergene enrichment zone was formed through weathering and redistribution of metal in the orebody, sequentially removing Zn and Pb from the ore to concentrate Cu and Au. The geology of the Flambeau deposit has presented unusual hydrothermal mineral assemblages suggestive of atypical sulfide mineralization for normal seawater hydrothermal systems. This project aims to assess the magmatic and tectonic affinity of the least-altered host rocks to provide an insight to the petrogenesis of the Flambeau volcanic system and control on VMS mineralization during the ocean-continent collision.