GEOCHEMICAL CHARACTERIZATION OF THE BACK FORTY VOLCANOGENIC MASSIVE SULFIDE DEPOSIT IN MENOMINEE COUNTY, MICHIGAN

The Back Forty volcanogenic massive sulfide (VMS) deposit, located in Menominee County, Michigan is a Paleoproterozoic (1874 ± 4 Ma) polymetallic ore deposit which formed during the Penokean Orogeny. Back Forty belongs to a large group of VMS deposits (e.g. Crandon, Flambeau) located in the Ladysmith-Rhinelander greenstone belt which is primarily composed of island arc volcanic and intrusive rocks. The nature of mineralization at Back Forty is similar to the bimodal felsic-dominated Kuroko-type VMS deposit model and the ore minerals are primarily hosted in rhyolitic rocks which appear to be related to deeper plutons of granite and tonalite with adjacent mixtures of mafic volcanic rocks along with tuffaceous and argillaceous sedimentary sequences. The mineralization consists of massive, semi-massive and stringer sulfide zones along with sulfide-poor Au and Ag enriched zones. It has been estimated that there are 15.13 million measured and indicated tonnes of ore minerals with average grades of 2.03 ppm Au, 24.28 ppm Ag, 3.06% Zn, 0.33% Cu and 0.22% Pb.

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.