Makushin Valley

Prospect, Inactive

Commodities and mineralogy

Main commodities Ag; Au
Other commodities As; Hg
Ore minerals gold; pyrite
Gangue minerals chlorite; epidote; quartz

Geographic location

Quadrangle map, 1:250,000-scale UN
Quadrangle map, 1:63,360-scale C-2
Latitude 53.9238
Longitude -166.6633
Nearby scientific data Find additional scientific data near this location
Location and accuracy North side of Makushin River Valley, about 5,000 feet (1.5 km) inland from coast at Broad Bay. Prospect extends over a distance of about 1 km along and below ridge.

Geologic setting

Geologic description

Propylitically altered porphyritic andesite and andesitic volcaniclastic rocks of the Unalaska Formation (see Drewes and others, 1961) contain quartz veins ranging from 1 inch (3 cm) to 2 feet (60 cm) in thickness. The veins are erratic (in distribution?) and discontinuous and trend N 50-60ΓΈ W (Simpson, 1986). Quartz veins are exposed on the ridge abd in the 'Makushin Vein' at 1,500' (450 m) elevation and in steep slopes on the north side of the valley. These veins consist of milky white quartz, trace pyrite and anomalous gold and silver. The lower elevation 'Makushin Vein' ranges between 4 and 7 feet (1.2 and 2.1 m) thick and is traceable for 1,200 feet (360 m). It trends N 70 W and dips steeply to the southwest. The Makushin Vein is composed of quartz that varies from gray to white and except for one observed 3 inch (8 cm) wide sulfide and visible gold-bearing zone, is sulfide free. The sulfide-bearing zone is located in approximately the center of the vein and contains very fine-grained sulfides and gold (0.1 mm) in a matrix of chlorite and epidote (Simpson, 1986). Later work by Battle Mountain Exploration Co. found the Makushin Vein bifurcates into two main structural trends running E-W and NW-SE. East-west trending veins are composed of massive, milky quartz that has been brecciated and then healed by multiple void-filling bands of cockscomb quartz. Northwest-southeast trending 'veins' generally have dense zones of quartz 'crackle-breccia.' Other nearby veins were examined by BMEC and typically were moderately anomalous in gold.
Geologic map unit (-166.665189053205, 53.9229347956625)
Mineral deposit model Epithermal gold vein
Mineral deposit model number 25
Age of mineralization Late Tertiary or younger
Alteration of deposit Wall rocks are propylitically altered volcaniclastic rocks that show no wall-rock alteration as a result of veining. Propylitic alteration is thought to predate mineralization and is probably not associated with quartz veining and gold mineralization.

Production and reserves

Workings or exploration Extensive sampling and mapping by Kennecott-Alaska in 1985 (Simpson, 1986) and BMEC in 1990 (Randolph, 1991). Veins along the ridge contained anomalous gold and silver but were too widely spaced and discontinuous to be of interest (Simpson, 1986). Gold grades derived from chip samples across the Makushin Vein system range up to 0.48 oz/t (16 ppm) and 3.12 oz/ton (107 ppm) silver, although Simpson (1986) reported 0.1 to 1.0 ppm gold is more typical. The zone having visible sulfides and gold yielded one sample of 8.575 oz/t (294 ppm) gold and 85.87 oz/t (2,944 ppm) silver (Simpson, 1986). Fluid inclusion studies suggest a formation temperature of 230 to 250 degrees C; however, the system apparently did not undergo boiling (Randolph, 1991). BMEC sampling indicated NW-SE trend yields gold values consistently below 500 ppb while E-W trend ranged from 570 ppb to 3.34 oz/t (115 ppm). Arsenic and mercury values are high, whereas base metal values are low.
Indication of production None
Reserve estimates Based on a body 450 by 60 by 2m, reserves of 110,000 tons suggested by Simpson (1986). No grade given.

References

MRDS Number A106003

References

Randolph, D.B., 1991, Unalaska project, 1990 final report: Battle Mountain Exploration Company, Alaska District, 62 p., 5 appendices, 15 plates, various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.)
Simpson, D.F., 1986, Aleutian Islands project, 1985 final report: Kennecott Alaska Exploration Company, 54 p. (Report held by the Aleut Native Corporation, Anchorage, Alaska.)
Reporters F.H. Wilson
Last report date 5/6/1996