Orange Mountain Prospect, Aleutians East Borough, Alaska, USAi

Regional Level Types
Orange Mountain Prospect	Prospect
Aleutians East Borough	Borough
Alaska	State
USA	Country

Location: This prospect is on southeastern Unga Island approximately 4,000 feet southwest of Red Mountain (Wilson and others, 1988, locality 29). The map site is at an elevation of 1,060 feet, at the top of a hill marked by a large color anomaly. Orange Mountain is informally named for the color anomaly; it is not named on the topographic map. The location is accurate.
Geology: This prospect, marked by a 2.7-square-mile color anomaly, is the largest single area of alteration on Unga Island. It is in rocks mapped as the late Eocene to early Oligocene Popof volcanic rocks (Wilson and others, 1995). From 1979 to 1983, it was explored first by Resource Associates of Alaska and then by UNC Teton Exploration Drilling Company (Peterson and others, 1983). The prospect consists of a gently-dipping, interbedded sequence of silica-replaced felsic tuffs and argillically altered andesite tuffs and flows. The alteration extends over a vertical range of at least 500 feet (Ellis and Apel, 1990). Numerous shears, faults, fracture zones, and breccias result in structural complexities (Peterson and others, 1983). The silica-replaced tuffs contain 5 percent disseminated pyrite. They also contain veins, disseminations, and pods of barite, and veinlets and pods of quartz. In brecciated areas, the quartz and barite veins locally occur together, and some of the quartz is replaced by barite. The argillically altered andesite contains 25 percent pyrite as veinlets and disseminations. Tetrahedrite has also been reported. There is a positive correlation between precious metal values and the silicified rock; the highest values are associated with shears and breccias. The highest precious metal values obtained from surface samples were 2.85 ppm gold and 26 ppm silver; their mean value was 0.045 ppm gold and 0.4 ppm silver (Peterson and others, 1983). Detectable and weakly anomalous gold values occur in drill holes in sections 100 or more feet thick (115 feet in OM-2 and 370 feet in OM-3). The highest subsurface precious metal values were 0.41 ppm gold and 3.4 ppm silver. Mean values of base metals were at 40 ppm copper, 20 ppm lead, 20 ppm zinc, and 50 ppm arsenic. The entire area is enriched in mercury, and values of as much as 107 ppm of mercury occur in silicified tuffs. The mean mercury value is 1.6 ppm. Thallium, tellurium, antimony, and bismuth were also detected. In 1990 Battle Mountain Exploration Company mapped the site and collected approximately 117 rock chip samples. Their mapping indicated that the silicified rock forms a cap overlying argillically altered tuffs containing as much as 70 percent pyrite (Ellis and Apel, 1990). Gold values in quartz veins cutting the silicified tuffs ranged from 0.05 to 0.25 ppm.
Workings: During the period 1979 to 1983 Resource Associates of Alaska and UNC Teton Exploration Drilling Company conducted 8.5 miles of grid mapping, collected and analyzed 750 rock and soil samples, conducted 1.4 miles of VLF-EM survey, and drilled 3 core holes for a total of 2,453 feet. The highest metal values of surface samples were 2.85 ppm gold and 26 ppm silver. The mean values were 0.045 ppm gold and 0.4 ppm silver. The highest subsurface values were 0.41 ppm gold and 3.4 ppm silver. In 1990 Battle Mountain Exploration mapped the site in detail and collected approximately 150 rock samples. A cluster of low-level gold anomalies was outlined at the top of Orange Mountain. Individual quartz veinlets assayed 0.05 to 0.25 ppm gold.
Age: Eocene or younger.
Alteration: The alteration consists of as pervasive argillization and pyritization of andesite tuffs and silicification of felsic tuffs.

Commodities (Major) - Ag, Au
Development Status: None
Deposit Model: Epithermal gold veins

Other Databases

Link to USGS - Alaska:	PM067

Other Regions, Features and Areas containing this locality

References