Geology and Lithogeochemistry of the Paleoproterozoic Flambeau Cu-Zn-Au Volcanogenic Massive Sulfide Deposit, Wisconsin, USA

Abstract

Abstract The Paleoproterozoic Flambeau Cu-Zn-Au deposit in northwestern Wisconsin is the only volcanogenic massive sulfide (VMS) deposit to reach production in the Pembine-Wausau terrane of the Penokean orogen, despite the discovery of more than 150 million tonnes of mineralization within 14 deposits since the 1970s. Renewed exploration interest in the region and need for critical minerals has highlighted the importance of the VMS deposits in the region and the need for an improved understanding of their geologic setting. This study examined and sampled 50 drill cores from the Flambeau deposit stored at the Wisconsin state core repository to determine the volcanic, hydrothermal, and tectonic setting of the deposit. The least altered units at the deposit are metadacite and quartz-augen schists that are interpreted to be rapidly emplaced felsic to intermediate crystal-rich tuffs and lapilli tuffs. There are also chlorite ± garnet schists that are interpreted to be mafic sills and/or small flows. The main altered unit is biotite ± andalusite ± white-mica schist that contains porphyroblasts of biotite and/or andalusite that replace the intermediate volcaniclastic units. The ore horizon contains massive sulfide, metachert, and semimassive mineralization within a quartz–white-mica schist. Stratigraphic relationships and chemostratigraphy structurally above and below the ore horizon indicate that the strata are overturned. Despite structural and metamorphic destruction of microtextural evidence, evidence for lateral mineralizing fluid flow in rapidly emplaced, permeable volcanic units, interlayered impermeable to semipermeable units, and macrotextural evidence for lit-par-lit replacement of tuffaceous layers indicate that the Flambeau ores are replacement-style mineralization. Trace element systematics reveal that mafic rock and intermediate rocks are calc-alkalic with low Ti/V values, characteristic of arc environments. Felsic rocks are calc-alkalic, depleted high field strength element (HFSE), FI- to FII-type rhyolites that are also consistent with an arc environment. These lithogeochemical characteristics are similar to those of modern mature oceanic-arc settings. This setting contrasts with other host rocks at nearby deposits (e.g., Lynne, Back Forty) that have petrogenetic evidence for the presence of older Archean basement (i.e., continental crust). Therefore, the geology of the Flambeau deposit highlights the variability of VMS-forming environments within the Penokean orogen and has important local and regional implications for other VMS deposits globally.