|Quadrangle map, 1:250,000-scale||KC|
|Quadrangle map, 1:63,360-scale||A-6|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||
The Concord group of prospects is at an elevation of about 300 feet near the south foot of Punch Hill . The prospects are in section 20, T. 71 S., R. 91 E., of the Copper River Meridian, about 0.8-1.0 mile north-northwest of the mouth of the unnamed creek at the north entrance to Dall Bay. The site corresponds to loc. 120 in Elliott and others (1978), and to loc. 312 in Maas and others (1995). The location is accurate within 0.1 mile.Also see Additional comments.
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. 72-73) describes the deposits on the Concord group of claims as sulfide-bearing quartz-barite-carbonate veins at
greenschist-pegmatite contacts. The veins contain chalcopyrite, sphalerite, probably pyrite, and low Ag and Au values. The Sunrise vein is 12-18 inches thick, strikes N75W and dips 80 SW, and was traced in outcrop for several hundred feet. The vein contained a conspicuous mass of chalcopyrite ore, which was said to carry values of $72 per ton, chiefly in Cu, and a little Ag and Au.
Maas and others' (1995, p. 229) best samples of the Concord deposit were: 1.4% Cu in a 6-foot sample; 0.44% Cu in a 16-foot sample; and 0.24% Cu in a 38-foot sample. Workings that could be identified at the time of their investigation included 2 adits, one 11.5 feet long and one caved; a 20-foot flooded shaft; and several trenches.Maas and others (1995, p. 227) description of the mineral deposits in the area north of Dall Bay probably applies in general to the Concord group of deposits. They report that chalcopyrite is associated with sheared, silicified zones, chiefly in trondhjemite. Small barite veins are also found in the area. About 0.6 mile north of Dall Bay, chalcopyrite occurs as clasts or pods in a siderite-hematite matrix in altered trondhjemite. One deposit that Maas and others assign to the Concord group is on a NE-striking, high-angle fault that forms the contact between the trondhjemite and the Silurian-Ordovician metamorphic rocks (Maas and others, 1995, loc. 312-1 on fig. 58). The characteristics and setting of the deposits suggest that they mainly are polymetallic veins of Late Triassic or younger age.
|Geologic map unit||(-131.743678048444, 55.1736532926529)|
|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.|
|Workings or exploration||
Early assays of a conspicuous mass of chalcopyrite ore were said to carry values of $72 per ton, chiefly in Cu, and a little Ag and Au (Brooks, 1902).Maas and others' (1995, p. 229) best samples of the Concord deposit were: 1.4% Cu in a 6-foot sample; 0.44% Cu in a 16-foot sample; and 0.24% Cu in a 38-foot sample. Workings that could be identified at the time of their investigation included 2 adits, one 11.5 feet long and one caved; a 20-foot flooded shaft; and several trenches.
|Indication of production||Undetermined|
Additional commentsEarly reports describing the Concord group refer to it as the Apex group; the Concord no. 2, 3, and 4 (group); and the Blue Jay, Old Man, and Sunrise claims (Cobb and Elliott, 1980, p. 144-146).
Berg, H.C., 1982, The Alaska Mineral Resource Assessment Program; guide to information about the geology and mineral resources of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Circular 855, 24 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.
Cobb, E.H., and Elliott, R.L., 1980, Summaries of data on and lists of references to metallic and selected nonmetallic mineral deposits in the Ketchikan and Prince Rupert quadrangles, Alaska: U.S. Geological Survey Open-File Report 80-1053, 157 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.
|Reporters||H.C. Berg, USGS|
|Last report date||7/5/1999|