CHARACTERISTICS OF ORE AND ALTERATION MINERAL ASSEMBLAGES AT THE FLAMBEAU VOLCANOGENIC MASSIVE SULFIDE DEPOSIT, NORTHWESTERN WISCONSIN

The Paleoproterozoic Flambeau Cu-Zn-Au volcanogenic massive sulfide (VMS) deposit is located within the Pembine-Wausau Terrane of the Penokean Orogeny near the town of Ladysmith, Wisconsin. Of the many VMS deposits in the Penokean Orogen, the Flambeau is the only one to have been mined. Mining began in 1992 and lasted until 1997 when extraction of the supergene enriched cap was completed. The high-grade copper ore body produced nearly 1.8 million tons of ore before the open pit was completely refilled and the site was reclaimed. The geology of the Flambeau deposit is characterized by massive to semi-massive Cu-Zn-Pb sulfides hosted in an alteration zone consisting of chlorite-andalusite-biotite schists. Before metamorphism, these rocks were formed in a submarine hydrothermal system and their compositions can provide insight into the mechanisms of gold enrichment at the Flambeau mine.

This study focuses on the trace mineral abundances and mineralogical variations of the ore and alteration zone at the Flambeau deposit. Samples were acquired from drill core stored at the Wisconsin Geologic and Natural History Survey core repository. There are two main types of primary ore: a massive pyrite-chalcopyrite dominated assemblage and a weakly banded sphalerite-pyrite-galena dominated assemblage. Using scanning electron microscopy, trace minerals identified in the ore zone include tellurides (hessite, altaite, tsumoite), electrum, sulfosalts, arsenopyrite, acanthite, bismuthinite, and an unnamed tungsten mineral. These variations are important in determining the temperature and composition of the hydrothermal fluids since these trace minerals form under specific hydrothermal conditions. Preliminary data suggests that there may have been magmatic fluids present in the seawater-dominated hydrothermal system as many of these trace minerals are not commonly associated with traditional VMS deposits. This interpretation is supported by geochemical characteristics of the alteration assemblages. Mass balance calculations suggest a sericite-silica dominated assemblage consistent with argillic alteration. Based on these observations, the Flambeau deposit is likely an example of a hybrid VMS-epithermal system.