Eureka

Prospect, Probably inactive

Commodities and mineralogy

Main commodities Au
Other commodities As; Cu
Ore minerals arsenopyrite; chalcopyrite; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale PA
Quadrangle map, 1:63,360-scale D-4
Latitude 56.98135
Longitude -135.15023
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Eureka prospect is about 0.4 mile southwest of Arguello Island at the head of Silver Bay. The prospect is about 0.6 mile west-southwest of the center of section 30, T. 56 S., R. 65 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 history of the Eureka prospect which was briefly described by Becker (1897) and Knopf (1912). Bittenbender and others (1999) mapped an 85-foot adit on the property and there is a second, caved adit nearby. A quartz vein near a slate-graywacke contact is exposed for about 40 feet in the adit. The vein is about 2 inches thick. A sample across it contained 27.5 parts per million (ppm) gold, 3.2 ppm silver, and 4,060 parts per million arsenic. Becker (1897) noted that chalcopyrite is present and that the pyrite-quartz vein here is in slate near dikes and brecciated masses of diorite (which is probably graywacke in modern terminology).
Geologic map unit (-135.151994190319, 56.9809839909708)
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 rocks.

Production and reserves

Workings or exploration The property has a 85-foot adit and another of unknown length that is caved.
Indication of production Undetermined
Reserve estimates None.
Production notes Unknown; possibly a small amount.

Additional comments

MAS number: 0021160009.

References