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
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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
Published Date
2003-05
Publisher
University of Minnesota Duluth
Type
Technical Report
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.
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This item is a duplicate of the item at this URL: http://hdl.handle.net/11299/187084
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Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811-1142 and Department of Geology, University of Wisconsin Oshkosh, 800 Algoma Boulevard, Oshkosh, Wisconsin 54901-8649. Polished thin section preparation was funded by a Minerals Coordinating Committee
Research Grant (State of Minnesota) to George Hudak (University of Wisconsin Oshkosh), Steve
Hauck (Natural Resources Research Institute) and John Heine (Natural Resources Research
Institute), and by a Permanent University Trust Fund Grant from the Natural Resources Research
Institute, University of Minnesota- Duluth to John Heine and George Hudak. Microprobe
analysis was funded by an Undergraduate Student Faculty Collaborative Research Grant from
the University of Wisconsin Oshkosh to Stephanie Hocker (University of Wisconsin Oshkosh)
and George Hudak, and by a Geological Society of America Research Grant to Stephanie
Hocker.
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Hocker, Stephanie M; Hudak, George J; Heine, John J. (2003). 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. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/190420.
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