Copper Zone Prospect, Petersburg District, Wrangell-Petersburg Borough, Alaska, USA
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Geology: The rocks in the prospect area are part of a belt of Mesozoic or Paleozoic sedimentary and volcanic rocks that have been metamorphosed to Tertiary or Cretaceous schist and gneiss The belt is about 1 1/2 mile wide and strikes northwest (Brew, 1997; George and Wyckoff, 1973). The metamorphic rocks are bounded on the east by a thick, regionally extensive, 60 to 70 Ma tonalite sill and on the west by a 90 Ma granodiorite pluton (Brew, 1997 Still and others, 2002). On the north side of Groundhog Basin, the metamorphic rocks are intruded by a 16.3 Ma biotite 'tin' granite pluton about 1,000 by 2,0000 feet in size. The granite is probably the source of the numerous rhyolite dikes and sills that extend from it and the mineralization in the area (Newberry and Brew, 1989). The earliest work reported on this prospect was done by El Paso Natural Gas Company in 1972; they collected 1,044 rock samples and drilled nine holes (George and Wyckoff, 1973; Still and others, 2002) . They defined a mineralized area about 125 feet by 160 feet in size. Still and others (2002) report that the area is notable for rhyolite sills up to 60 feet thick that generally follow the layering in gneiss and schist. Disseminated chalcopyrite occurs along fractures that parallel and crosscut the foliation of the metamorphic rocks. Some lenses and bands of massive chalcopyrite and pyrrhotite up to 0.4 feet thick also occur along the fractures. Chip samples up to 0.7 feet long across the fractures contained up to 4,580 parts per billion gold, 19.65 ounces of silver per ton, 8.1 percent copper, 1.71 percent lead, 2.65 percent zinc, and 1,728 parts per million tin. Only one of the nine holes drilled by El Paso Natural Gas Company intersected mineralization; the best intercept was 70 feet that averaged 0.11 percent copper. The deposit is similar to the mineralization at the nearby Groundhog Basin prospect (PE040) and may be a continuation of it.
Workings: The earliest work reported on this prospect was by El Paso Natural Gas Company in 1972; they collected 1,044 rock samples and drilled nine holes. The prospect was later sampled by Still and others (2002) as part of a BLM mineral assessment.
Age: 16.3 Ma based on a probable genetic tie to a nearby, zinnwaldite 'tin' granite (Newberry and Brew, 1989).
Alteration: None noted specifically.
Commodities (Major) - Ag, Au, Cu, Pb, Sn, Zn
Development Status: None
Deposit Model: Banded Ag-Cu-Sn-Pb-Zn tabular replacement bodies, veins, and stringers.
5 entries listed. 5 valid minerals.
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Brew, D.A., 1997, Reconnaissance geologic map of the Petersburg C-1 Quadrangle, Southeastern Alaska: U.S. Geological Survey Open-File Report 97-156-H, 23 p., 1 sheet, scale 1:63,360. George, R.H., and Wyckoff, B.S., 1973, Whistlepig mineral exploration program, Alaska, Final report 1972 (with attached diamond drill hole logs and analyses): Unpublished El Paso National Gas Company report 109 p. 12 sheets (available at the Bureau of Land Management, Minerals Information Center, Juneau Alaska). Newberry, R.J., and Brew, D.A., 1989, Epigenetic hydrothermal origin of the Groundhog Basin-Glacier Basin silver-tin-lead-zinc deposits, southeastern Alaska: U.S. Geological Survey Bulletin 1903, p. 113-121.3 Still, J.C., Bittenbender, P.E., Bean, K.W., and Gensler, E.G., 2002, Mineral assessment of the Stikine area, central southeast Alaska: Bureau of Land Management Technical Report 51, 560 p.