Electron Microprobe Analysis of Alteration Mineralogy at the Archean Five Mile Lake Volcanic Associated Massive Sulfide Mineral Prospect in the Vermilion District of Northeastern Minnesota

Abstract

Alteration mineral assemblage mapping at the Five Mile Lake Prospect in the Vermilion District of northeastern Minnesota has identified two distinct types of alteration zones within 2.7 billion year-old volcanic and volcaniclastic rocks associated with volcanic-hosted massive sulfide (VHMS) mineralization (Hudak et al., in press; Odette et al., 2001a, 2001b; Peterson, 2001). Regional semi-conformable alteration zones are composed of various proportions of quartz + epidote ± amphibole ± chlorite ± plagioclase feldspar. These regional, semiconformable alteration zones are locally crosscut by several relatively narrow, northeasttrending disconformable alteration zones composed of fine-grained chlorite and/or sericite that are closely associated with synvolcanic fault zones. Electron microprobe analyses of the various alteration mineral phases (epidote group minerals, chlorite, amphibole, white mica, and feldspar) have been conducted in an effort to better understand the hydrothermal processes associated with the development of the semiconformable and disconformable alteration zones at the Five Mile Lake Prospect. These analyses indicate that: a) epidote group minerals range in composition from zoisite/clinozoisite to pistacite; b) chlorite is dominantly ripidolite; c) amphibole is primarily actinolite and ferroactinolite, with magnesio-hornblende and ferro-hornblende also present; d) sericite is finegrained muscovite; and e) feldspar is albite. This mineral chemistry suggests the presence of a complex, long-lived hydrothermal system that evolved from seafloor-proximal (hundreds of meters) to deeper subseafloor environments (~1-3 km) as the volcanic rocks were buried by rapid, dominantly effusive mafic to intermediate volcanism and associated sedimentation. Alteration mineral chemistry at the Five Mile Lake Prospect is remarkably similar to that from the Noranda VHMS mining camp of Canada as well as other VHMS orebodies. This mineral chemistry, combined with favorable volcanology and numerous untested geophysical targets, suggest that the Five Mile Lake Prospect, as well as the uppermost several hundred meters of the Lower Member of the Ely Greenstone, have excellent exploration potential for VHMS mineral deposits.