HYDROTHERMAL ALTERATION AND MINERALIZATION AMONG THE GOLD ZONES OF THE BACK FORTY VOLCANOGENIC MASSIVE SULFIDE DEPOSIT

The Back Forty Volcanogenic Massive Sulfide (VMS) deposit, located along the Menominee River in the Upper Peninsula of Michigan, is the second largest deposit in the early Proterozoic Penokean Volcanic Belt. The VMS mineralization, which is hosted by felsic volcanic rocks, is characterized as Kuroko-style and consists of massive, semi-massive and stringer sulfide mineralization. The dominant ore in the VMS deposit is pyrite, a gangue mineral. Valuable ores in the deposit consist of sphalerite, chalcopyrite and galena. Adjacent to the main massive sulfide mineralization, there are three designated “gold zones” that have proven preferential to precious metal mineralization of gold and silver. These precious metals have been remobilized from the main area of mineralization. The three gold zones, which are named the Porphyry Margin Zone (PM), 90 Zone and Near Surface Zone (NS), have markedly different presentations in hand sample. PM Zone deposits are found in a quartz-feldspar porphyry, while 90 Zone and NS Zone deposits are found in a rhyolite crystal tuff host rock. The 90 Zone host rock is so intensely altered by chlorite, though, that it can be referred to as a chlorite crystal tuff. The objectives for this research are three-fold. The first objective is to describe the mineralization and alteration in representative samples from each zone in thin section. The second objective is to attempt to identify similarities, if they exist, and point out differences in the mineralogy and alteration among the zones. The overall objective is to determine if there are any characteristics to suggest that the gold mineralization in the zones represent a single mineralizing event or multiple, separate pulses of mineralization. At this point, it appears that vast differences in chlorite appearance and distribution may signal a different mineralizing event for the PM Zone than in the 90 Zone and NS Zone.