Edgecumbe Exploration

Prospect, Probably inactive

Alternative names

Bonanza Nos. 21 & 22

Commodities and mineralogy

Main commodities Au
Other commodities As; Hg
Ore minerals arsenopyrite; gold; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale PA
Quadrangle map, 1:63,360-scale D-4
Latitude 56.99053
Longitude -135.15336
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Edgecumbe Exploration prospect is about 0.5 mile northwest of Arguello Island at the head of Silver Bay. The prospect is about 0.6 mile northeast of the center of section 25, T. 56 S., R. 64 E.

Geologic setting

Geologic description

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 1940s.
Little is known about the early history of this property; it was held by the Edgecumbe Exploration Company in 1930 and they continued to hold the property into the 1940s (Bittenbender and others, 1999). A mill was erected in 1941 to process ore from this and other properties in the area. There is no record of production but there may have been some. The workings include a 120-foot adit with a winze, and a raise to the surface.
The deposit consists of a gold-quartz vein in Sitka Graywacke (Bittenbender and others, 1999). The vein consists mainly of milky white quartz with some bands of ribbon quartz. The quartz contains disseminated pyrite and arsenopyrite and bands of these sulfides also occur adjacent to the vein next to the wallrock. The vein pinches and swells, reaching a maximum thickness of about 5 feet; it pinches out to the southeast and is truncated on the northwest by a fault (Bittenbender and others, 1999, fig. 15). The highest values for gold in samples collected by Bittenbender and others, was 1,810 parts per billion (ppb) in a select sample and 1,170 ppb across 0.7 feet. Samples from the vein typically have elevated arsenic and mercury values.
Geologic map unit (-135.15512424958, 56.9901649977884)
Mineral deposit model Low-sulfide, gold-quartz vein (Cox and Singer, 1986; model 36a).
Mineral deposit model number 36a
Age of mineralization Cretaceous or younger based on the age of the host rock.

Production and reserves

Workings or exploration Little is known about the early history of this property. It was held by the Edgecumbe Exploration Company in 1930 and they continued to hold the property into the 1940s. A mill was erected in 1941 to process ore from this and other properties in the area. The workings include a 120-foot adit with a winze and a raise to the surface.
Indication of production Yes; small
Reserve estimates None.
Production notes A mill was erected in 1941 to process ore from this and other properties in the area. There is no record of production but there may have been some.

References

References

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.
DeArmond, R.N., 1997, Haleys and Silver Bay, in Around and About Alaska: Sitka Sentinel, series of 29 Articles, April to October, 1997.
Reporters Donald J. Grybeck (U.S. Geological Survey)
Last report date 2-Jan-05