Grotto

Prospect in Alaska, United States with commodity Copper

Geologic information

Identification information

Deposit ID 10001597
MRDS ID A012357
Record type Site
Current site name Grotto

Geographic coordinates

Geographic coordinates: -131.73483, 55.1978 (WGS84)
Relative position The Grotto prospect is at an elevation of about 250 feet, approximately 0.5 mile north-northwest of the west head of Seal Cove. The site is in section 7, T. 77 S., R. 91 E., of the Copper River Meridian. It corresponds to loc. 109 in Elliott and others (1978). The location is accurate within about 0.1 mile.
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Geographic areas

Country State
United States Alaska

Commodities

Commodity Importance
Copper Primary

Materials information

Materials Type of material
Chalcopyrite Ore
Pyrite Ore
Quartz Gangue

Alteration

  • (Local) Probably local silicification, carbonatization, pyritization, and introduction of hydrothermal hematite.

Mineral occurrence model information

Model code 85
USGS model code 22c
Deposit model name Polymetallic veins
Mark3 model number 46

Host and associated rocks

  • Host or associated Host
    Rock type Metamorphic Rock > Schist > Greenschist

Nearby scientific data

(1) -131.73483, 55.1978

Comments on the geologic information

  • 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.
  • Age = Late Triassic or younger.

Economic information

Economic information about the deposit and operations

Development status Prospect
Commodity type Metallic

Mining district

District name Ketchikan

Comments on the workings information

  • 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).

Reference information

Links to other databases

Agency Database name Acronym Record ID Notes
USGS Mineral Resources Data System MRDS A012357
USGS Alaska Resource Data File ARDF KC112

Bibliographic references

  • 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.

  • Deposit

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

  • Deposit

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

  • 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.

  • 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.

  • 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.

  • 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.

Comments on the references

  • Primary Reference = Maas and others, 1995

General comments

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

Reporter information

Type Date Name Affiliation Comment
Reporter 05-JUL-99 H.C. Berg U.S. Geological Survey