Latitude: 55°9'50"N
Longitude: 131°43'58"W
Location: The prospects at and near Dall Bay that are described in this record lie in a quarter-mile-wide area that extends northwestward from tidewater at the north mouth of the bay, inland for about 0.55 mile, to an elevation of about 150 feet. The map site is in section 29, T. 77 S., R. 91 E., of the Copper River Meridian. It corresponds to loc. 129 in Elliott and others (1978), and to loc. 314 (1-6) in Maas and others (1995). It also represents Maas and others' loc. 313 (1-3), about a half-mile to the northwest. The location is accurate within 0.1 mile.
Geology: The country rocks in the area of these prospects consist of an assemblage of undivided Silurian or Ordovician metamorphosed bedded and intrusive rocks that are intruded by stocks and dikes of Silurian trondhjemite (Berg, 1973; Berg and others, 1988), The rocks are cut by a complex system of high-angle faults that mainly strike NE and NW. In many places, they also are permeated by finely disseminated hydrothermal hematite, giving them (especially the trondhjemite) a pink or red hue commonly mistaken for potassium feldspar. According to Elliott and others (1978), who summarize Brooks' (1902) description of the prospects at Dall Bay, the principal deposit in the early 1900s was a chalcopyrite-bearing quartz vein in greenschist and pegmatite. The owners at that time reported an average grade of $6 Au/ton (Au at $20.67/ounce), 11% Cu, and low Ag values; and had explored the deposit by two shafts. A reconnaissance examination of the area in 1968 by the USGS showed disseminated pyrite and chalcopyrite, and pyrite- and chalcopyrite-bearing quartz-carbonate-barite veins in breccia zones in iron- and copper-stained metamorphic rocks (samples 68Bg752-754, in Koch and Elliott, 1978 [OFR 78-156A]). As described by Maas and others (1995, p. p. 227, 229; loc. 314) the deposits on the north side of Dall Bay consist of chalcopyrite in one or more sheared, silicified, zones in altered trondhjemite. Their best sample was a 75-foot width of mineralized rock containing 1.4% Cu. Workings, dating to the early 1900s, included two shafts, one 60 feet deep and a flooded one 30 feet deep; and several surface trenches, pits, and opencuts. These deposits were explored in 1956 by by private interests, who drilled 14 holes totalling about 3900 feet (Maas and others, 1995, p. 228). Mineralization was found in eight holes drilled along about 600 feet of strike length. Assays ranged from 0.38% Cu to 1.1% Cu over intervals of 5 to 46 feet. In the area about a half-mile northwest of Dall Bay (the 'Dall Bay area' of Maas and others, 1995, p. 227; loc. 313-1), chalcopyrite occurs as clasts or pods in a siderite-hematite-matrix breccia in altered trondhjemite. Maas and others' best sample was a 27-foot width of otherwise unspecified mineralized rock containing 1.1% Cu and 1.8 ppm Au. Workings, probably dating to the early 1900s, included a caved adit, and several trenches and opencuts. About 1000 feet north of Dall Bay, a pit and trenches expose mineralized breccia that averaged 1.1% Cu over 28 feet, and 0.7% Cu over 23 feet (Maas and others, 1995, p. 228; loc. 313-2,3). Select samples contained up to 2.5% Cu. The characteristics and setting of the deposits in the Dall Bay area indicate that they mainly are polymetallic veins. Their similarity to deposits in the Seal Cove area (see, for example, KC 121) suggest that they probably are Late Triassic or younger in age.
Workings: The deposits on the north side of Dall Bay were explored in the early 1900s by two shafts, 30 and 60 feet deep, and by several trenches, pits, and opencuts (Brooks, 1902, p. 71-72). The owners at that time reported a pay shoot 3 feet wide containing an average of 11% Cu, $6 in gold/ton (Au at $20.67/ounce), and a trace of silver. The deposits were explored in 1956 by private interests, who drilled 14 holes totalling about 3900 feet (Maas and others, 1995, p. 228). Mineralization was found in 8 holes drilled along about 600 feet of strike length. Assays ranged from 0.38%-1.1% Cu over intervals of 5 to 46 feet. Sampling by Maas and others (1995, p. 229) showed a 75-foot width of otherwise unspecified mineralized rock containing 1.4% Cu. In the area up to a half-mile north of Dall Bay, workings, probably dating to the early 1900s, included a caved adit and several trenches and opencuts (Maas and others, 1995, p. 227; loc. 313). Maas and others' samples at one site showed a 27-foot width of otherwise unspecified mineralized rock containing 1.1% Cu and 1.8 ppm Au. At another site, their samples of mineralized breccia averaged 1.1% Cu over 28 feet and 0.7% Cu over 23 feet. Select samples contained up to 2.5% Cu.
Age: Probably Late Triassic or younger.
Alteration: Local silicification, pyritization, and carbonatization, and widespread permeation of country rocks by finely disseminated hydrothermal hematite.
Commodities (Major) - Ag, Au, Cu; (Minor) - Barite
Development Status: Undetermined.
Deposit Model: Polymetallic veins (Cox and Singer, 1986; model 22c)
References
Berg, H.C., 1973, Geology of Gravina Island, Alaska: U.S. Geological Survey Bulletin 1373, 41 p. 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 I-1807, 27 p., scale 1:250,000. 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. Elliott, R.L., Berg, H.C., and Karl, S.M.,1978, Map and table describing metalliferous and selected non-metalliferous mineral deposits in the Ketchikan and Prince Rupert quadrangles, Alaska: U.S. Geological Survey Open-File Report 78-73B, 17 p., 1 sheet, scale 1:250,000. Koch, R.D., and Elliott, R.L., 1978, Analyses of rock samples from Ketchikan quadrangle, southeastern Alaska: U.S. Geological Survey Open-File Report 78-156-A, 162 p., 1 sheet, scale l:250,000. 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.
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