TRANSITIONAL EPITHERMAL-TO-TYPICAL VOLCANOGENIC MASSIVE SULFIDE DEPOSITS IN THE EARLY PROTEROZOIC PENOKEAN OROGEN, NORTHERN WISCONSIN

Seventeen volcanogenic massive sulfide (VMS) deposits and occurrences have been discovered in the Early Proterozoic Pembine-Wausau terrane of northern Wisconsin, a 250 km long belt of dominantly metavolcanic and lesser metasedimentary rocks accreted to the southern margin of the Superior Craton. The VMS deposits occur in three districts, each a center of bimodal calc-alkaline felsic-dominated volcanic rocks with lesser mafic flows, tuffs, iron formation, and tuffaceous metasediments. The deposits vary from Cu-Au (Bend and Flambeau) to Zn-Cu (Pelican, Hawk, and Crandon) to Zn-Pb-Cu (Lynne and Horse Shoe). The Cu-Au-rich deposits are concentrated at the western end of the volcanic terrane, Zn-Cu deposits at the eastern end, and the Zn-Pb-Cu in the central part.

Host rocks for the Pelican, Lynne, Bend, and Flambeau VMS deposits are chemically similar calc-alkaline dacite-rhyodacite and are enriched in light-REE (La/Yb=5-25) and Th (3-18 ppm). Pb isotope data from the deposits and Nd isotopic analysis of felsic rocks (e_Nd(T)=0 to -4) from the Pelican deposit suggest the presence of a significant enriched component derived from older Archean age continental crust. These data and the felsic-dominated nature of the VMS-hosting sequences suggest formation in an extensional continental arc or incipient back-arc tectonic setting. Furthermore, the compositional similarity of the host rocks for the different VMS deposits suggests that factors other than source rock composition controlled the metal content of these deposits. The anomalously high Au, As, Sb, Bi, and Te content, high-sulfidation mineral assemblages, strong quartz-sericite to aluminous alteration, and heavy-REE mobility in the Cu-Au-rich VMS deposits resemble epithermal-like VMS deposits such as those from the Okinawa trough. Thus, the VMS deposits in the western portion of the Pembine-Wausau terrane record a transition between an epithermal environment and the more typical VMS environment of deposits to the east.