Big Joe

Prospect, Undetermined

Commodities and mineralogy

Main commodities Cu
Ore minerals chalcopyrite; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale A-6
Latitude 55.192
Longitude -131.741
Nearby scientific data Find additional scientific data near this location
Location and accuracy The approximate location of the Big Joe prospect is at an elevation of about 1050 feet on the northeast ridge of Punch Hill. The site is in section 17, T. 77 S., R. 91 E., of the Copper River Meridian. It corresponds to loc. 115 in Elliott and others (1978). The location is probably accurate within 0.1 mile.

Geologic setting

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).
Wright and Wright (1908, p. 140) describe the Big Joe deposit as a well-defined quartz fissure vein that strikes N-S, carries pyrite and some chalcopyrite, and can be traced for more than 3000 feet in chlorite schist.
Maas and others' (1995, p. 227) description of the mineral deposits in the Seal Cove area probably applies in general to the Big Joe 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. In addition to the N-S fault that probably localizes the Big Joe vein, the rocks near the prospect also are cut by a high-angle fault that strikes NW (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.
Geologic map unit (-131.74267848593, 55.1916528551189)
Mineral deposit model Polymetallic veins (Cox and Singer, 1986; model 22c)
Mineral deposit model number 22c
Age of mineralization Late Triassic or younger.
Alteration of deposit Probably local silicification, carbonatization, pyritization, and introduction of hydrothermal hematite.

Production and reserves

Workings or exploration The deposit was explored in the early 1900s by an open cut (Brooks, 1902, p. 70).
Indication of production Undetermined