Midnight Mountain

Prospect, Inactive

Commodities and mineralogy

Main commodities Au
Ore minerals gold
Gangue minerals arsenopyrite; quartz

Geographic location

Quadrangle map, 1:250,000-scale BN
Quadrangle map, 1:63,360-scale D-6
Latitude 65.76
Longitude -164.59
Nearby scientific data Find additional scientific data near this location
Location and accuracy Midnight Mountain is a prominent upland reaching an elevation of 2,720 feet in the southeast part of the Bendeleben D-6 quadrangle. It is located on the continental divide which separates the drainages of the Serpentine River (Schlitz Creek) and Kougarok River (Taylor Creek) in this area. This location is on the south flank of Midnight Mountain at an elevation of about 2,100 feet.

Geologic setting

Geologic description

Midnight Mountain is composed of Lower Paleozoic metasedimentary rocks on its northwest side and polydeformed pelitic schist of possible Precambrian age elsewhere (Hudson, 1984; Till and others, 1986). Surface material in the prospect area is frost-riven rubble; outcrops are restricted to a few places at higher elevations on Midnight Mountain. White quartz veinlets and stringers are ubiquitous in the metasedimentary rocks of this area. On the south flank of the mountain, quartz vein fragments are associated with rust-stained frost boils and some contain arsenopyrite. Here, soil and selected rock samples collected over a length of about 2,000 feet between elevations of 2,000 and 2,100 feet contain highly anomalous amounts of gold and arsenic (Hudson, 1984). Iron oxide-stained soil samples, commonly with quartz vein fragments, contain up to 725 ppb Au and greater than 1,000 ppm As. Quartz vein fragments, some with visible arsenopyrite, contain up to 765 ppb gold and greater than 1,000 ppm arsenic. This gold-bearing area seems to be mostly in altered and quartz-veined polydeformed metapelitic schist on the south side of Midnight Mountain. Bedrock exposures of this mineralization have not been observed. Lode gold deposits in the upper Kougarok River area, including this prospect on Midnight Mountain, may be associated with emplacement and crystallization of the Oonatut Granite Complex. The Midnight Mountain prospect is 4 to 4.5 miles south of outcrops of the Oonatut Granite Complex (Hudson, 1979). Regional gravity data show that the Oonatut Granite is extensively distributed in the subsurface (Barnes and Hudson, 1977); it probably extends to the south under the Midnight Mountain area (McDermott, 1983). The Oonatut Granite is part of the western Seward Peninsula tin granite suite (Hudson and Arth, 1983). K/Ar ages for the Oonatut Granite Complex are about 70 my (Hudson, 1979, p. 24).
Geologic map unit (-164.592707224226, 65.7592810645)
Mineral deposit model Gold-bearing quartz veins and schist
Age of mineralization The epigenetic mineralization here is probably mid- to Late Cretaceous in age. Lode gold deposits in the upper Kougarok River area, including this prospect on Midnight Mountain, may be associated with emplacement and crystallization of the Oonatut Granite Complex. The Midnight Mountain prospect is 4 to 4.5 miles south of outcrops of the Oonatut Granite Complex (Hudson, 1979). Regional gravity data show that the Oonatut Granite is extensively distributed in the subsurface (Barnes and Hudson, 1977) and that it probably estends to the south under the Midnight Mountain area (M. McDermott, unpublished data, 1983). This large exposed granite complex is part of the western Seward Peninsula tin granite suite (Hudson and Arth, 1983). K/Ar ages for the Oonatut Granite Complex are about 70 my. Alternatively, gold-bearing mineralization here may be similar in age to some lode gold deposits of southern Seward Peninsula. The southern Seward Peninsula lode gold deposits formed as a result of mid-Cretaceous metamorphism (Apodoca, 1994; Ford, 1993, Ford and Snee, 1996; Goldfarb and others, 1997) that accompanied regional extension (Miller and Hudson, 1991) and crustal melting (Hudson, 1994). This higher temperature metamorphism was superimposed on high pressure/low temperature metamorphic rocks of the region.
Alteration of deposit The area contains abundant small quartz veins, veinlets, and stringers in pelitic metasedimentary rocks. Iron oxide-staining of frost-riven soils is well-developed in the area of anomalous gold and arsenic samples.

Production and reserves

Workings or exploration There are no workings on this prospect.
Indication of production None

References

MRDS Number 10307267

References

Apodoca, L.E., 1994, Genesis of lode gold deposits of the Rock Creek area, Nome mining district, Seward Peninsula, Alaska: Boulder, Colorado, University of Colorado, Ph.D. dissertation, 208 p.
Ford, R.C., 1993, Geology, geochemistry, and age of gold lodes at Bluff and Mt. Distin, Seward Peninsula, Alaska: Golden, Colorado School of Mines, Ph.D. dissertation, 302 p.
Ford, R.C., and Snee, L.W., 1996, 40Ar/39Ar thermochronology of white mica from the Nome district, Alaska--The first ages of lode sources to placer gold deposits in the Seward Peninsula: Economic Geology, v. 91, p. 213-220.
Goldfarb, R.J., Miller, L.D., Leach, D.L., and Snee, L.W, 1997, Gold deposits in metamorphic rocks in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 151-190.
Hudson, T.L., 1984, Tin systems of Seward Peninsula, Alaska: Anchorage, Anaconda Minerals Company internal report, 51 p. (Report held by Cook Inlet Region Inc., Anchorage, Alaska)
Hudson, T.L., 1994, Crustal melting events in Alaska, in Plafker, G., and Berg, H. C., eds., The Geology of Alaska: Geological Society of America, DNAG, The Geology of North America, Vol. G-1, p. 657-670.
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., 1983, Seward Peninsula reconnaissance 1982 geophysical report: Anchorage, Anaconda Minerals Company internal report, 29 p. (Report held by Cook Inlet Region, Inc., Anchorage, Alaska.)
Reporters Travis L. Hudson (Applied Geology)
Last report date 3/15/1999