ALTERATION MINERALOGY AT THE ARCHEAN FIVE MILE LAKE VOLCANIC-ASSOCIATED MINERAL PROSPECT IN NORTHEASTERN MINNESOTA

Detailed alteration mineral assemblage mapping has identified an extensive 2.7 billion year old semi-conformable quartz + epidote ± plagioclase ± actinolite ± chlorite alteration zone at the Five Mile Lake volcanic-hosted massive sulfide (VMS) prospect in the Vermilion District of NE Minnesota. This alteration zone is cross-cut by later disconformable sericite and sericite + chlorite alteration zones. At the present time, there are two possible interpretations regarding the genesis of the semi-conformable alteration zone: a) formation at relatively low temperatures (<250°C) from spillitization associated with cool, downwelling seawater; b) formation at high temperatures (>350°C) from calcium-dumping associated with the lower parts of a convective geothermal system. The disconformable sericite and sericite + chlorite alteration zones likely represent hydrothermal upflow zones in close proximity to synvolcanic faults. Microprobe analyses of feldspars, epidote-group minerals (epidote, clinozoisite, zoisite), chlorite, actinolite, and white micas from the Five Mile Lake alteration zones have recently been completed and our data is currently being evaluated. Our results will be compared to electron microprobe analyses of similar mineral species from hydrothermal alteration zones associated with volcanic-hosted massive sulfide deposits (VHMS) in the region of Noranda Quebec, Canada. This comparision will be used to assess the types of ancient hydrothermal processes that occurred in association with VHMS-type mineralization at Five Mile Lake prospect.