Orange Mountain

Prospect, Inactive

Alternative names


Commodities and mineralogy

Main commodities Ag; Au
Ore minerals gold; pyrite; tetrahedrite
Gangue minerals barite; quartz

Geographic location

Quadrangle map, 1:250,000-scale PM
Quadrangle map, 1:63,360-scale A-2
Latitude 55.209
Longitude -160.621
Nearby scientific data Find additional scientific data near this location
Location and accuracy 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.

Geologic setting

Geologic description

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.
Geologic map unit (-160.623057106151, 55.2081669219277)
Mineral deposit model Epithermal gold veins
Age of mineralization Eocene or younger.
Alteration of deposit The alteration consists of as pervasive argillization and pyritization of andesite tuffs and silicification of felsic tuffs.

Production and reserves

Workings or exploration
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.
Redstar Gold Corp. conducted two phases of exploration in 2016 on their Unga project, which includes the Shumagin (PM064), Apollo (PM079; PM084), Centennial, Orange Mountain (PM067), Zachary Bay (PM054), Amethyst (PM092), and Aquila (PM086) epithermal gold-silver prospects. The Shumagin trend parallels the historically mined Apollo–Sitka vein system (PM079; PM084; PM076), which between 1886 and 1922 produced approximately 150,000 ounces of gold at a grade of approximately 0.292 ounce of gold per ton (Athey and Werdon, 2017).
In June 2016, Redstar Gold Corp.’s Phase I surface program included classifying alteration assemblages and conducting other geologic work to identify drill targets. From October through November, Phase II’s drilling program tested the down-dip and along-strike expansion potential of high-grade vein/breccia mineralization within the Shumigan Gold Zone (PM064). A total of 1,505 meters were drilled in 7 holes spaced over ~750 meters of strike length. All drill holes intersected the target structure, which includes multi-generational phreatomagmatic breccias, hydrothermal breccias, and late breccias and veins with colloform-crustiform- to cockade-textured quartz-adularia-carbonate (± rhodochrosite, ± green clay). Select intervals from drill holes 16SH019 and 16SH020 include 0.9 meter at 14.95 grams of gold per tonne and 1.15 meters at 11.3 grams of gold per tonne; release of full assay results is planned for late January 2017 (Athey and Werdon, 2017).
Indication of production None


MRDS Number 10308138


Ellis, W.T., and Apel, R.A., 1990, Unga/Alaska Peninsula 1990 final report: Battle Mountain Exploration Company report, 49 p., 29 map sheets at various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.)
Ellis, W.T., and Randolph, D., 1991, Unga project 1991 final report: Battle Mountain Exploration Company report, 36 p. 13 map sheets at various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.)
Peterson, R.J., Handverger, P., Rosenkrans, D., Bartels, E, and Woolston, D., 1983, Shumagin Islands precious metal exploration program southwest Alaska: UNC Teton Exploration Drilling Company report, 61 p. (Report held by the Aleut Corporation, Anchorage, Alaska.)
Peterson, R.J., Lemmers, J., Handverger, P., Gallagher, J., Pilcher, R., East, J., Macleod, T., Bartels. E., 1982, Geology and precious metals potential Unga, Popof, and Korovin Islands, Shumagin Group, Aleutian Chain, Alaska: UNC Teton Exploration Drilling Company report, 127 p., 5 map sheets, various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.)
Reporters S.H. Pilcher; M.B. Werdon (DGGS)
Last report date 8/26/2017