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GSA Annual Meeting, November 5-8, 2001

Paper No. 0
Presentation Time: 1:30 PM-5:30 PM

EVALUATION OF HYDROTHERMAL ALTERATION MINERAL ASSEMBLAGES AND THEIR RELATIONSHIP TO VMS-STYLE MINERALIZATION IN THE FIVE MILE LAKE AREA OF THE ARCHEAN VERMILION GREENSTONE BELT, NORTHEASTERN MINNESOTA


ODETTE, Jason D.1, HUDAK, George J.2, SUSZEK, Thomas1 and HAUCK, Steve A.3, (1)Department of Geology, Univ of Wisconsin Oshkosh, 800 Algoma Blvd, Oshkosh, WI 54901, (2)Geology, Univ of Wisconsin Oshkosh, 800 Algoma Blvd, Oshkosh, WI 54901, (3)Economic Geology Group, Nat Rscs Rsch Institute, University of Minnesota - Duluth, Duluth, MN 55811, odettj61@uwosh.edu

The Five Mile Lake volcanic-associated massive sulfide (VMS) prospect occurs 15 miles southwest of Ely, Minnesota, within mafic Archean metavolcanic and metasedimentary rocks. Investigation of the mineralogical, chemical, and spatial characteristics of the hydrothermal alteration in mafic pillow lava, diabase dikes, and felsic rocks revealed five distinct hydrothermal alteration mineral assemblages: 1) Least altered (LA); 2) Quartz + albite + epidote ± chlorite (QAE); 3) Fe-chlorite ± actinolite (CA); 4) Quartz + albite + actinolite + epidote (QAA); and 5) Sericite + quartz (SQ).

The QAE and QAA are regionally extensive and represent semiconformable synvolcanic alteration zones. Mass balance analysis using constant Al2O3 and bestfit (based on Al2O3, TiO2, Zr, Nb) isocons (Grant, 1986) shows that relative to least altered rocks, QAE rocks show gains in SiO2 and Na2O and losses in CaO, Fe2O3, FeO, MgO, MnO, K2O, Cu and Zn, reflecting regional silicification and spilitization followed by later epidotization in the deeper parts of a VMS-style hydrothermal system. The QAA has a similar early history, but local cross-cutting zones of QAA may represent rocks altered within upflow zones.

Mass balance analysis of CA pillows shows enrichment in MgO, Fe2O3, FeO, and Zn and depletion in SiO2, K2O, and CaO, reflecting mixing of cooler downwelling Mg-rich seawater, and hotter upwelling Fe-Zn-rich evolved hydrothermal fluids. Locally, CA-filled amygdules occur within the QAE pillow cores, and these rocks are adjacent to thin (<1m thick) mineralized exhalite horizons.

Locally, diabase dikes are altered to epidosite. Mass balance analysis indicates that the epidosite zones are enriched in CaO, and depleted in Na2O, K2O, MnO, Fe2O3, FeO, Zn, and Cu relative to the least altered diabase. These epidosite zones represent local high water:rock ratio alteration within lower semi-conformable alteration zones associated with base metal leaching.

Crosscutting SQ alteration zones occur in northeastern trending zones located in the central and east-central parts of the prospect. SQ zones are closely associated with synvolcanic fault zones, and are believed to represent hydrothermal upflow zones of evolved hydrothermal fluid. These SQ zones are closely associated with Zn-Cu stringer-type VMS mineralization at the prospect.

Session No. 173--Booth# 28

Economic Geology (Posters)
Thursday, 8 November 2001: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 33, No. 6, October 2001, p.2490
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