LITHOGEOCHEMICAL CHARACTERISTICS OF THE VOLCANIC SYSTEM HOSTING THE FLAMBEAU CU-ZN-AU DEPOSIT, NORTHWESTERN WISCONSIN

The Flambeau Cu-Zn-Au deposit is a volcanogenic massive sulfide (VMS) deposit that formed during volcanism associated with the collision of the Pembine-Wausau terrane along the southern margin of the Superior craton during the Penokean orogeny. Previous geochronological studies have constrained the timing of this event, but the lithogeochemical characteristics of volcanic rocks are poorly understood. Lithogeochemical research on the volcanic rocks of the Pembine-Wausau terrane are inhibited by thick Quaternary glacial deposits and the absence of mineral exploration and mining activities in Wisconsin for the past 20+ years. Despite these difficulties, the geochemical characteristics of these volcanic rocks are important to document as they can provide insight to the petrogenesis of arc magmatism/collision and the magmatic/tectonic controls on VMS mineralization during ocean-continent collision.

This study describes the lithogeochemistry and chemostratigraphy of the volcanic rocks hosting the Flambeau Cu-Zn-Au deposit, Wisconsin. This deposit was the only VMS deposit mined in the Penokean orogen. Least altered rocks in the Flambeau stratigraphy consist of a felsic quartz-augen schist and a “metadacite”. When not obscured by foliation or alteration, volcanic textures indicate that both host rocks are likely either primary pyroclastic deposits or their reworked equivalents. Zr/Ti and Nb/Y ratios reveal a bimodal distribution of rock types between andesites and rhyolites. Chlorite schists, interpreted to represent mafic volcanic rocks, are rare. All rocks have arc-like characteristics on primitive-mantle normalized plots with light REE enrichment and negative Nb and Ti anomalies. Andalusite-bearing hydrothermally altered units have had most of their primary volcanic texture destroyed. However, their Zr/Ti and Nb/Y ratios and other geochemical characteristics are most similar to the andesitic rocks within the stratigraphy. When compared to strata hosting other VMS deposits, there are unique differences that indicates a diverse petrogenetic history for the Pembine-Wausau terrane. This may have implications for the apparent Cu:Zn variation across the orogen.