The spectrum of ore deposit types, their alteration and volcanic setting in the Penokean volcanic Belt, Great Lakes Region, USA.

摘要

The Paleoproterozoic Penokean volcanic belt hosts one of the most important VMS districts worldwide. Despite the significant mineral endowment of the region, scientific understanding of the mineral deposits is limited, resulting from a relative lack of exploration and production coupled with extensive glacial deposits that cover much of the region. The current thesis presents new drill core observations, geochemistry, and petrography to supplement previous research on the most important mineral deposits in the region. A synthesis of this data provides an update to the understanding of the tectonic-volcanic setting, alteration styles, and mineralization processes for mineral deposits across the Penokean volcanic belt.;The results of the present study reveal that most of the Penokean sulfide deposits are classical VMS deposits despite a wide range of alteration and mineralization characteristics. The deposits form a bimodal distribution of Cu-type and Zn-Cu type massive sulfide deposits and, with the exception of the Lynne deposit, most of the significant deposits contain low concentrations of lead. In addition, three of the deposits, namely Back Forty, Bend, and Crandon, have elevated gold contents. The Reef Au-Cu deposit, located in the Wausau volcanic complex is a gold-rich sulfide deposit that is characterized by strongly deformed and recrystallized rocks of an unknown mafic protolith. This deposit is unique amongst the VMS deposits in the belt and appears to have formed within a distinct geodynamic environment.;The styles of hydrothermal alteration vary between deposits, with white-mica-chlorite-quartz assemblages being the most prevalent. Aluminous alteration represented by andalusite-biotite-white mica schists has been noted at Flambeau and calc-silicate mineral assemblages are present at Lynne, Ritchie Creek, and Reef. Calc-silicate mineral associations have also been observed at the Pelican River and Spirit deposits, suggesting that the volcanic host rocks were originally interbedded with limestone or carbonate-altered. Regional metamorphism varies from lower greenschist to amphibolite grade and has obscured relationships in some deposits.;All major VMS deposits of the Penokean volcanic belt occur within felsic-dominated volcanic successions and are hosted by vent-proximal volcanic facies associations. For example, the Back Forty deposit is hosted within a felsic succession comprising coherent rhyolite units and associated volcanic breccias that is at least 1200 meters thick. Some of the smaller deposits recognized, including Ritchie Creek and Bend, are characterized by a thick succession of fine-grained, bedded volcaniclastic deposits that are interpreted to represent distally derived volcanic material. Mafic-dominated host rock successions are uncommon in the Penokean volcanic belt and the Horseshoe prospect represents the only known deposit that occurs within a dominantly basaltic volcanic succession.;The present study indicates that the Penokean VMS district is best classified as a bimodal-felsic lithostratigraphic type. Chemically, the felsic volcanic rocks hosting the VMS deposits appear to have been generated from a highly evolved source which may have included partial melting of continental crust. These data indicate that the Penokean VMS deposits formed within a rifted continental margin arc that formed along the southern margin of the Superior Craton. The data presented here provides an updated volcanological, geochemical, and genetic prospective for the formation of VMS deposits across the Penokean volcanic belt and can be used to guide future exploration and research in the region.