Grotto

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 55.198
Longitude -131.733
Nearby scientific data Find additional scientific data near this location
Location and accuracy 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.

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).
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.
Geologic map unit (-131.73467876614, 55.1976528432424)
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 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).
Indication of production Undetermined

References