Grotto

Site Name

Primary: Grotto

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Commodity

Primary: Copper

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Location

State: Alaska
District: Ketchikan

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Land Status

Not available

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Prospect
Operation Type: Unknown
Years of Production:
Organization:
Significant:

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Physiography

Not available

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Mineral Deposit Model

Model Name: Polymetallic veins

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Orebody

Not available

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Structure

Not available

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Alterations

Alteration Type: L
Alteration Text: Probably local silicification, carbonatization, pyritization, and introduction of hydrothermal hematite.

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Rocks

Not available

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Analytical Data

Not available

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Materials

Ore: Pyrite
Ore: Chalcopyrite
Gangue: Quartz

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Comments

Comment (Reference): Primary Reference = Maas and others, 1995

Comment (Workings): Workings / Exploration = The Grotto prospect was explored in the early 1900s by a shaft and about 550 feet of drifts and crosscuts. The owners at that time reported 11% Cu across a 5-foot -wide mineralized zone (Brooks, 1902, p. 71).

Comment (Deposit): Model Name = Polymetallic veins (Cox and Singer, 1986; model 22c)

Comment (Geology): Age = Late Triassic or younger.

Comment (Geology): Geologic Description = Southern Gravina Island is underlain by an assemblage of undivided Silurian or Ordovician metamorphosed bedded and intrusive rocks; a stock and associated dikes of Silurian trondhjemite that cuts the metamorphic assemblage; and a sequence of Upper Triassic carbonate, clastic, rhyolitic, and basaltic strata that unconformably overlies the older rocks (Berg, 1973, 1982; Berg and others, 1988). The rocks are complexly folded and are cut by high-angle faults and by low-angle thrust faults. In many places, the Triassic rhyolite and the rocks beneath it are permeated by microscopic particles of hydrothermal hematite, giving them a pink, purple, or red hue (Berg, 1973, p. 14).? Brooks (1902, p. 70-71) describes a deposit on the Grotto claim as mineralized breccia in shear zones that cut the foliation of greenschist country rock, which also is mineralized, especially near the shear zones. Brooks does not identify the ore or gangue minerals. The owners at the time of Brooks' investigation reported an average of 11% Cu across 5 feet of a 25-foot-wide mineralized zone. The high copper assay suggests that at least one of the ore minerals is chalcopyrite.? Wright and Wright (1908, p. 140) describe the Grotto deposit only as a thick vein that has been developed by about 550 feet of drifts and crosscuts. The vein apparently follows a fault that cuts greenschist, conglomerate, and rhyolite.? Maas and others' (1995, p. 227) description of the mineral deposits in the Seal Cove area probably applies in general to the Grotto prospect. They report that chalcopyrite occurs as vein fillings, disseminations, and in fault breccias west and northwest of Seal Cove. The mineralized breccias have a siliceous matrix. Small quartz-barite veins with galena and sphalerite have been found west of Seal Cove, and on the northeast slopes of Punch Hill. The rocks at or near the Grotto prospect are cut by high-angle faults that strike north-northeast and northwest (Maas and others, (1995, fig 58).? Maas and others (1995, p. 227) report that copper mineralization on southern Gravina Island generally is associated with faulting. The deposits are mainly in meta-andesite (greenschist) and trondhjemite, but also in the overlying Triassic strata. The deposits are chiefly chalcopyrite- and pyrite-bearing quartz fissure veins, but the sulfide minerals also occur as disseminations in the metavolcanic rocks, in silicified zones in the trondhjemite, and as clasts or pods in silicified or carbonatized breccia. The character and setting of the deposits suggest that they mainly are polymetallic veins of Late Triassic or younger age.

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References

Reference (Deposit): Berg, H.C., 1982, The Alaska Mineral Resource Assessment Program; guide to information about the geology and mineral resources of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Circular 855, 24 p.

Reference (Deposit): Wright, F.E., and Wright, C.W., 1908, The Ketchikan and Wrangell mining districts, Alaska: U.S. Geological Survey Bulletin 347, 210 p.

Reference (Deposit): Brooks, A.H., 1902, Preliminary report on the Ketchikan mining district, Alaska, with an introductory sketch of the geology of southeastern Alaska: U.S. Geological Survey Professional Paper 1, 120 p.

Reference (Deposit): Berg, H.C., 1973, Geology of Gravina Island Alaska: U.S. Geological Survey Bulletin 1373, 41 p.

Reference (Deposit): Elliott, R.L., Berg, H.C., and Karl, Susan, 1978, map and table describing metalliferous and selected nonmetalliferous mineral deposits, Ketchikan and Prince Rupert quadrangles, Alaska: U.S. Geological Survey Open-file Report, 78-73-B,17 p., scale 1:250,000.

Reference (Deposit): Maas, K.M., Bittenbender, P E., and Still, J.C., 1995, Mineral investigations in the Ketchikan mining district, southeastern Alaska: U.S. Bureau of Mines Open-File Report 11-95, 606 p.

Reference (Deposit): Berg, H.C., Elliott, R.L., and Koch, R.D., 1988, Geologic map of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Mineral Investigations Series Map MF-1807,27 p., scale 1:250,000.

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