GEOCHEMICAL CHARACTERIZATION OF THE BACK FORTY VOLCANOGENIC MASSIVE SULFIDE DEPOSIT IN MENOMINEE COUNTY, MICHIGAN
The host rocks of sulfide mineralization are chloritized and sericitized rhyolite breccias and pyroclastic crystal tuff, often interlayered with fine tuffaceous sediments and intruded by late dacitic quartz feldspar porphyries. Three chemically distinct varieties of rhyolitic rocks have been identified based on trace element characteristics out of which two are found to host the sulfide mineralization. With moderate Zr/Y ratios, intermediate HFSE concentrations and moderately fractionated REE patterns, the host rock rhyolites represent calc-alkaline chemical affinities.
Massive sulfide ore horizons at Back Forty include pyrite, sphalerite, chalcopyrite and galena with variable amounts of Au and Ag and minor amounts of pyrrhotite and arsenopyrite. The upper levels of the deposit, close to the surface, are characterized by two oxidized iron-rich gossans which consist principally of hematite, goethite with significant amounts of Au, Ag and minor amounts of electrum, bornite, argentite and other minerals. In general, the relatively higher abundances of Au and Ag in Back Forty, when compared with other VMS deposits, indicate higher concentrations of Au and Ag in the mineralizing fluid or processes of metallic redistribution by late hydrothermal solutions.