Green Lake Prospect, Chicagof District, Sitka Borough, Alaska, USA
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Location: The Green Lake prospect is about 0.1 mile southeast of the outlet of Green Lake and about 0.1 mile northeast of the center of section 29, T. 56 S., R. 65 E.
Geology: This deposit is one of many of similar deposits (PA002 to PA018) scattered over an area of about 6 square miles at the head of Silver Bay (Bittenbender and others, 1999). The deposits are gold-quartz veins with sparse sulfides, usually only pyrite and arsenopyrite. Samples that have been analyzed with modern methods usually show anomalous arsenic even if arsenopyrite is not identified in the rocks, and several parts per million mercury. The veins are often parallel to the bedding of the host rock which is graywacke and argillite of the Sitka Graywacke of Cretaceous age (Loney and others, 1975). Many of the so-called veins in the early literature are actually fault zones with lenses of quartz or concentrations of quartz stringers along the fault zone. Prospecting began in the area in 1871. The Stewart Mine (PA012) was located in 1872 and it was the first lode-gold mine in Alaska. The Silver Bay area has been prospected intermittently to the present but the veins are relatively small and most are low grade. The area has produced relatively little gold, many of the properties have not been active since before 1900, and there has been no production since the early 1940's. Little is known about the Green Lake prospect beyond what Roehm (1938 [PE 116-2]) reported. The prospect was discovered in 1912 and was explored soon after by a few surface cuts and a short tunnel. It was dormant until 1928 when it was restaked and work was started on a tunnel. The work continued until at least 1938 when Roehm visited the property. The rocks in the vicinity are graywacke and slate that strike about N50W and dip steeply south. There are two parallel 'veins' on the property; they are about 300 feet apart and parallel the bedding of the host rock. However, from the description, the 'veins' probably are fault zones with quartz stringers. The longest tunnel, the one Roehm mapped, is 324 feet long with a then-caved 65-foot crosscut. A fault zone ('vein') with quartz stringers was cut in about the middle of the tunnel. Adjacent to this 'vein', disseminated pyrite and calcite extend a few feet into the wallrock and some of the quartz stringers as well. The fault zone is silicified with small veinlets of quartz. A channel sample across 6 feet of the mineralized zone contained 0.02 ounce of gold per ton. Roehm's map shows quartz stringers and sulfide mineralization at a short crosscut at the end of the tunnel but he provides no details. Roehm also sampled a large trench on one of the veins on the surface above the tunnel; a channel across 5 feet of mineralization contained $0.50 in gold (0.014 ounce of gold per ton.). Free gold can be seen in samples from some of the trenches. There is no record of further work.
Workings: The prospect has been explored by three tunnels; the longest and most recent is 324 feet long. There are also several trenches.
Age: Cretaceous or younger based on the age of the host rock.
Alteration: The fault is silicified with quartz stringers; disseminated pyrite and calcite and quartz stringers extend into the graywacke host rock for a few feet from the fault.
Production: Probably none.
Commodities (Major) - Au
Development Status: Undetermined.
Deposit Model: Low-sulfide, gold-quartz vein (Cox and Singer, 1986; model 36a).
4 entries listed. 4 valid minerals.
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Bittenbender, P., Still, J.C., Maas, K., and McDonald, M., Jr., 1999, Mineral resources of the Chichagof and Baranof Islands area, southeast Alaska: Bureau of Land Management, BLM-Alaska Technical Report 19, 222 p. Loney, R.A., Brew, D.A., Muffler, L.J. B., and Pomeroy, J.S., 1975, Reconnaissance geology of Chichagof, Baranof, and Kruzof Islands, southeastern Alaska: U.S. Geological Survey Professional Paper 792, 105 p. Nelson, G.E., 1931, Report on the Green Lake Group and the Apollo Group: Unpublished report, 3 p. (On file at the Juneau Mineral Center, U.S. Bureau of Land Management, Juneau, Alaska.) Roehm, J.C, 1938, Preliminary report of Green Lake Group of claims, Silver Bay, Sitka Precinct, Alaska: Alaska Division of Geological and Geophysical Surveys, Property Examination PE 116-2, 2 p.