Potato Mountain

Prospects, Inactive

Alternative names

Quartz Porphry dike
Red Fox
Eureka
Daisy
Iron Creek valley
Big and Little Potato Mountain divide
and Iron Creek-Oakland Creek divide

Commodities and mineralogy

Main commodities Sn
Ore minerals cassiterite
Gangue minerals quartz; sulfide minerals; tourmaline

Geographic location

Quadrangle map, 1:250,000-scale TE
Quadrangle map, 1:63,360-scale C-6
Latitude 65.664
Longitude -167.554
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Quartz Porphyry dike has some significance compared to the other minor prospects of the Potato Mountain area (Red Fox, upper Buck Creek valley, Eureka, Daisy, Iron Creek valley, Big and Little Potato Mountain divide, and Iron Creek-Oakland Creek divide, Mulligan, 1965, p. 69-70) because of recent diamond-drilling here (Meyers, 1990). It is located on the ridgecrest separating the headwaters of Peluk Creek (a north tributary to West Fork of Buck Creek) and the west tributary to Red Fox Creek. The elevation of the prospect area is approximately 1,050 feet. This locality was not identified separately by Cobb and Sainsbury (1972) but historical references for the general area (Potato Mountain) are sumarized by Cobb (1975) under the name 'Potato Mtn.'.

Geologic setting

Geologic description

The Quartz Porphyry dike area is part of the thermally metamorphosed upland of Potato Mountain. The protolith for the hornfels is a sedimentary sequence of laminated to thinly bedded mudstone, siltstone, sandstone and some impure limestone that is of unknown but probable Paleozoic age. These rocks have a slaty cleavage but are everywhere thermally recrystallized. The most prominent felsic dike of the general area is exposed here (Mulligan, 1965, figure 23). It is a light-gray porphyritic granite with medium- to coarse-grained quartz and feldspar phenocrysts in a very fine-grained groundmass. The groundmass and the feldspar phenocrysts are locally replaced by quartz, tourmaline, and sericite (Hudson, 1983).
The presence of a granite stock at depth is suggested by the large area of hornfels and by gravity data. A gravity profile and model across the Potato Mountain upland (McDermott, 1982; Hudson and Reed, 1997, figure 5B) indicate that a buried granite stock has an irregular, perhaps block-faulted upper surface at a depth of about 1,300 feet. Significant block faulting is also suggested by discontinuities in magnetic data for the area (McDermott, 1983).
Although old prospect pits are present in this area, Mulligan (1965, p. 69-70) could not confirm the presence of significant tin. Kennecott (Meyers, 1990) identified this area as an intensely altered zone spatially associated with the northeast-trending quartz porphyry dike. Diamond-drill hole PMD-2 was drilled vertically 998 feet here to test the altered zone. According to Meyers (1990), this hole (collar elevation of 1,100 feet) encountered hornfels from 0 to 180 feet; weakly argillized feldspar porphyry from 180 to 186 feet; hornfels from 186 to 268 feet; moderately argillized feldspar porphyry (268 to 274 feet), hornfels with possible skarn and probable intense tourmalinization from 274 to 974 feet; and greenish sulfide-rich clay with clasts of green-brown garnet or tourmalinized rock from 974 to 998 feet. The hole could not be continued through the clay-rich material and was terminated at 998 feet.
Tin values were weakly to moderately anomalous; 30 to 300 ppm with a high of 400 ppm throughout PMD-2. Boron was only locally high uphole but below 750 feet was generally in the 1000 to 2000 ppm range. Arsenic was generally in the 20 to 50 ppm range with local spikes to greater than 1 per cent; arsenic levels seemed to increase below 750 feet. Fluorine did increase downhole, from a 700 to 900 ppm range to 1,500 to 2,500 ppm below 500 feet. Silver variation was similar to fluorine; 0.1 to 0.2 ppm above 500 feet and 0.5 to 0.6 ppm below.
Other minor prospects in the Potato Mountain upland (including Red Fox, upper Buck Creek valley, Eureka, Daisy, Iron Creek valley, Big and Little Potato Mountain divide, and Iron Creek-Oakland Creek divide, Mulligan, 1965) are minor mineral occurrences, such as local quartz veining, and commonly the location of old prospecting pits. Subsequent work (Mulligan, 1965) did not identify them as significant.
Geologic map unit (-167.556759346476, 65.6632289106058)
Mineral deposit model Altered hornfels with weak tin metallization and related hydrothermal alteration. Potential tin vein (model 15b) or tin greisen (model 15c) of Cox and Singer (1986) at depth.
Age of mineralization Late Cretaceous; the tin metallization at Potato Mountain is inferred to be similar in age to other tin sytems of western Seward Peninsula. These tin systems are interpreted to be linked to the evolution of associated granite intrusions that are 70 to 80 my old (Hudson and Arth, 1983, p. 769).
Alteration of deposit Local argillic alteration and probably local weak to moderate tourmaline replacement.

Production and reserves

Workings or exploration Local surface pits and one 998 foot vertical diamond-drill hole
Indication of production None

References

References

Hudson, T.L., 1983, Interim report on the Potato Mountain tin system: Anchorage, Alaska, Anaconda Minerals Company internal report (Report held by Cook Inlet Region, Inc., Anchorage, Alaska).
Hudson, T.L., and Arth, J. G., 1983, Tin granites of Seward Peninsula, Alaska: Geological Society of America Bulletin, v. 94, p. 768-790.
McDermott, M.M., 1982, Gravity profiles of Black Mountain and Potato Mountain, Seward Peninsula: Anchorage, Alaska, Anaconda Minerals Company internal report (Report held by Cook Inlet Region, Inc., Anchorage, Alaska).
McDermott, M.M., 1983, Investigation of the magnetic contact aureoles of the Khotol and Black Mountain granites, Alaska: Anchorage, Alaska, Anaconda Minerals Company internal report (Report held by Cook Inlet Region, Inc., Anchorage, Alaska).
Meyers, W.C., 1990, Report on 1990 exploration activities at the Potato Mountain tin prospect, Teller AMS sheet, Seward Peninsula, Alaska: Anchorage, Alaska, Kennecott Exploration Inc. internal report.
Reporters Travis L. Hudson (Applied Geology)
Last report date 5/10/1998