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).This occurrence consists of stratiform layers, up to 1.6 feet thick, of massive pyrite and sphalerite in Upper Triassic rhyolite at its depositional contact with overlying Upper Triassic carbonaceous slate (H.C. Berg, unpublished field data, 1968; Maas and others, 1995, p. 227). The contact strikes west and dips steeply to the north, and is truncated by a NW-striking, high-angle fault along the southwest shore of Bostwick Inlet (Berg, 1973). Samples collected by Maas and others (1995, p. 229) across about 10 feet of mineralized rock contained up to 1.6% Zn and small amounts of Ag. The characteristics and setting of this occurrence indicate that it is a stratiform, volcanogenic massive-sulfide deposit of Late Triassic age. As such, it is the southernmost known occurrence of a belt of Upper Triassic, stratabound, volcanogenic massive-sulfide deposits in southeastern Alaska (Berg, 1981).