Alpha Ridge

Prospect, Probably inactive

Commodities and mineralogy

Main commodities Au
Other commodities Ag; As; Pb; Sb; Zn
Ore minerals arsenopyrite; galena; gold; sphalerite; stibnite
Gangue minerals albite; ankerite; quartz

Geographic location

Quadrangle map, 1:250,000-scale NM
Quadrangle map, 1:63,360-scale C-2
Latitude 64.6267
Longitude -165.5198
Nearby scientific data Find additional scientific data near this location
Location and accuracy The prospect is on the ridge between Alpha Creek (in the Nome C-1 quadrangle) and Sledge Creek, west tributaries to Snake River. Mineralization extends along the ridge for at least 1,000 feet between elevations of about 800 and 1,050 feet. The location is at the most intensely mineralized part of the prospect at an elevation of about 1,000 feet on the ridge and in the NW1/4 section 17, T. 10 S., R. 34 W., Kateel River Meridian. It is accurate within about 500 feet.

Geologic setting

Geologic description

The Alpha Ridge prospect is an old prospect but its location was first reported by Sainsbury and others (1972 [OFR 72-321]). It was explored by Kennecott Exploration Company from 1990 to 1993. A series of soil samples collected in a reconnaissance survey were anomalous in arsenic, gold, and other elements. The reconnaissance survey was followed up with a detailed soil geochemical survey and trenching; about 1,000 feet of trenches were excavated with a trackhoe.
Mineralization at the site is associated with highly albitized, arsenic-rich rocks and with massive quartz veins; the mineralization appears to be crudely stratabound (C.C. Hawley, Cindy Buxton, and D.L. Olson, written communication, 1992). The most intense mineralization is in an area about 300 by 300 feet on the crest of the ridge immediately below a saddle at an elevation of 1,040 feet. This saddle coincides with a high-angle fault of nearly east-west strike. To the east this fault trends toward the head of the placer gold deposit on Alpha Creek (NM196) in the Nome C-1 quadrangle.
Maximum gold content found in the trench samples was 2,200 ppb; numerous samples contained more than 500 ppb gold and more than 10,000 ppm arsenic. Lead and zinc are also locally strongly enriched. The highest lead and zinc contents are associated with a massive, west-northwest-striking quartz vein that is exposed in a trench and in rubble about 300 feet north of the fault-controlled saddle. Lead values in galena and possibly as an oxidized mineral, are as high as 5,000 ppm in rock samples. Zinc, inferred to originally have been in sphalerite, is as high as 580 ppm. Antimony is consistently anomalous in the trench samples. The highest antimony value, 370 ppm, coincides with a gold value of 2,900 ppb.
Although the main area of mineralization is below the strong fault which underlies the saddle at the south end of the prospect, areas of albitized rock with quartz float exist at least 1,000 feet north of the area of most intense mineralization. Grab samples from outlying areas locally contain more than 9,000 ppb gold. The auriferous area on Alpha Ridge almost certainly contributed placer gold to Sledge Creek (NM169), as well as to the Alpha Creek placer (NM196) in the Nome C-1 quadrangle.
The prospect is near the crest of a major antiform, and country rock is the calcareous metaturbidite schist unit of Bundtzen and others (1994).
Geologic map unit (-165.52240694454, 64.6259321761544)
Mineral deposit model Low-sulfide Au-quartz veins? (Cox and Singer, 1986; model 36a?).
Mineral deposit model number 36a?
Age of mineralization Mid-Cretaceous; postdates regional metmorphism and may be similar in age to low-sulfide Au-quartz veins of the Nome district (see, for example, the Divide prospect, NM058).
Alteration of deposit Albitization associated with the introduction of arsenopyrite and other metallic minerals in schist.

Production and reserves

Workings or exploration About 1000 feet of trenches was excavated in 1991. An extensive soil geochemistry survey, part of a grid that extended from Monument Creek to Bangor Creek, was also completed between 1991 and 1993 by Kennecott Exploration Company. The prospect was probably originally located before 1920.
Indication of production None