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). The Friday prospect was probably staked for copper in the early 1900s (U. S. Bureau of Mines, 1977); it was mapped, and the prospect briefly examined, by the U.S. Geological Survey in 1969 (Elliott and others, 1978). Elliott and others describe the deposit as pyrite- and chalcopyrite-bearing quartz-carbonate-barite? veins in breccia zones in strongly iron-stained metamorphosed volcanic, sedimentary and intrusive country rocks. Berg (1973, pl. 1) maps the rocks at and near this prospect as Triassic basal conglomerate containing clasts of the underlying trondhjemite and metamorphic assemblage. The mineralized breccia zones probably are hosted by this conglomerate, rather than by the metamorphic unit described by Elliott and others. The country rocks at the Friday prospect are cut by a high-angle fault that strikes NE, parallel to the SE shoreline of Nehenta Bay (Berg, 1973, pl. 1). Several occurrences of barite-bearing fissure veins are in Triassic sedimentary rocks on the islands at the mouth of Nehenta Bay. The characteristics and setting of these deposits suggest that they are polymetallic veins of Late Triassic or younger age. Workings visible in 1969 were a few small pits and short tunnels. |
