Unnamed (Cobblestone River)

Occurrence, Probably inactive

Commodities and mineralogy

Main commodities Be; Th; U
Ore minerals beryl; unknown thorium- and uranium-bearing minerals
Gangue minerals feldspar; garnet; quartz; tourmaline

Geographic location

Quadrangle map, 1:250,000-scale NM
Quadrangle map, 1:63,360-scale D-1
Latitude 64.9806
Longitude -165.464
Nearby scientific data Find additional scientific data near this location
Location and accuracy The occurrence is on a steep ridge about 1 mile east of the Cobblestone River and about 3.3 miles north of Mosquito Pass. The occurrence is locality 114 of Hummel (1975). It is in the SE1/4 section 12, T. 6 S., R. 34 W., Kateel River Meridian at an elevation of about 2,200 feet. The occurrence is a pegmatite; it probably is located within about 1,000 feet, but may be representative of one or more pegmatite bodies that occur near this location.

Geologic setting

Geologic description

This granite pegmatite is an isolated occurrence north of the Thompson Creek orthogneiss, and its age is uncertain. Granite pegmatites are abundant in the Kigluaik Mountains, and others may be present near this occurrence. The pegmatites are noticeably radioactive; radioactivity measured on the ground with a scintillometer is as much as 500 counts per second or 3 to 5 times common background (Hawley and Associates, 1978, Section IV). Tourmaline and garnet are common accessory minerals, and the more radioactive pegmatites commonly contain smoky quartz. Beryl occurs in some of the pegmatite, including a body described by Moffit (1913, p. 25) about 1 mile west of the mouth of North Star Creek (NM046).
Some granite pegmatites are within the Thompson Creek orthogneiss or appear to be spatially associated with it, particularly on its south or hanging wall side (Hummel, 1962 [MF248]; Till, 1980). The Thompson Creek orthogneiss has been dated as latest Proterozoic (555 Ma, Amato and Wright, 1998) and some pegmatites may also be this age. However, metasedimentary rocks of the Kigluaik Mountains underwent granulite facies metamorphism and partial melting in the mid-Cretaceous, and some pegmatites are crosscutting to local structure and mid-Cretaceous in age (Throckmorton and Hummel, 1979; Till, 1983; Miller and Hudson, 1991; Hudson, 1994; Till and Dumoulin, 1994; Amato and others, 1994; Amato and Wright, 1997, 1998).
The host rocks to this pegmatite are upper amphibolite facies metasedimentary rocks that are probably derived from Precambrian protolith (Sainsbury, 1972; Bunker and others, 1979; Till and Dumoulin, 1994). They are thought to have undergone regional high-pressure metamorphism along with many other rocks of Seward Peninsula in the Late Jurassic or Early Cretaceous (Sainsbury, Coleman, and Kachadoorian, 1970; Forbes and others, 1984; Thurston, 1985; Patrick, 1988; Patrick and Evans, 1989; Armstrong and others, 1986; Hannula and McWilliams, 1995). Higher temperature metamorphism overprinted these rocks in conjunction with regional extension, crustal melting, and magmatism in the mid-Cretaceous (Throckmorton and Hummel, 1979; Till, 1983; Evans and Patrick, 1987; Leiberman, 1988; Patrick and Leiberman, 1988; Miller and Hudson, 1991; Miller and others, 1992; Dumitru and others, 1995; Hannula and others, 1995; Hudson and Arth, 1983; Hudson, 1994; Amato and others, 1994; Amato and Wright, 1997, 1998). Uplift of the higher temperature metamorphic rocks took place in the mid- to Late Cretaceous and in the Eocene (Calvert, 1992; Dumitru and others, 1995).
Geologic map unit (-165.466636692931, 64.9798407053686)
Mineral deposit model Simple granite pegmatite with rudimentary zoning.
Age of mineralization Late Proterozoic or mid-Cretaceous; either the age of the Late Proterozoic Thompson Creek orthogneiss or mid-Cretaceous granulite facies metamorphism.

Production and reserves

Workings or exploration Reconnaissance uranium exploration including airborne radiometrics, stream sediment surveys, and ground traverses have been completed in the Kigluaik Mountains.
Indication of production None


MRDS Number 10307911


Armstrong, R.L., Harakal, J.E., Forbes, R.B., Evans, B.W., and Thurston, S.P., 1986, Rb-Sr and K-Ar study of metamorphic rocks of the Seward Peninsula and southern Brooks Range, Alaska, in Evans, B.W., and Brown, E.H., eds., Blueschists and eclogites: Geological Society of America Memoir 164, p. 184-203.
Calvert, A.T., 1992, Structural evolution and thermochronology of the Kigluaik Mountains, Seward Peninsula, Alaska: Stanford, Calif., Stanford University, M.Sc. thesis, 50 p.
Evans, B.W. and Patrick, B.E., 1987, Phengite 3-T in high pressure metamorphosed granitic orthogneisses, Seward Peninsula, Alaska: Canadian Mineralogist, v. 25, part 1, p. 141-158.
Hannula, K.A., and McWilliams, M.O., 1995, Reconsideration of the age of blueschist facies metamorphism on the Seward Peninusla, Alaska, based on phengite 40Ar/39Ar results: Journal of Metamorphic Geology, v. 13, p. 125-139.
Hannula, K.A., Miller, E.L., Dumitru, T.A., Lee, Jeffrey, and Rubin, C.M., 1995, Structural and metamorphic relations in the southwest Seward Peninsula, Alaska; Crustal extension and the unroofing of blueschists: Geological Society of America Bulletin, v. 107, p. 536-553.
Hawley, C.C., and Associates, 1978, Uranium evaluation of the Seward-Selawik area, Alaska: Department of Energy, Grand Junction, Colo., Open-File Report GJBX-105(78), 91 p.
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.
Leiberman, J.E., 1988, Metamorphic and structural studies of the Kigluaik Mountains, western Alaska: Seattle, University of Washington, Ph.D. dissertation, 191 p.
Miller, E.L., Calvert, A.T., and Little, T.A., 1992, Strain-collapsed metamorphic isograds in a sillimanite gneiss dome, Seward Peninsula, Alaska: Geology, v. 20, p. 487-490.
Patrick, B.E., 1988, Synmetamorphic structural evolution of the Seward Peninsula blueschist terrane, Alaska: Journal of Structural Geology, v. 10, p. 555-565.
Patrick, B.E., and Evans B.W., 1989, Metamorphic evolution of the Seward Peninsula blueschist terrane: Journal of Petrology, v. 30, p. 531-555.
Patrick, B.E., and Leiberman, J.E., 1988, Thermal overprint on blueschists of the Seward Peninsula, the Lepontine in Alaska: Geology, v. 16, p. 1100-1103.
Thurston, S.P., 1985, Structure, petrology, and metamorphic history of the Nome Group blueschist terrane, Salmon Lake area, Seward Peninsula, Alaska: Geological Society of America Bulletin, v. 96, p. 600-617.
Till, A.B., 1980, Crystalline rocks of the Kigluaik Mountains, Seward Peninsula, Alaska: Seattle, University of Washington, M.Sc. thesis, 97 p.
Till, A.B., 1983, Granulite, peridotite, and blueschist--Precambrian to Mesozoic history of Seward Peninsula: Alaska Geological Society Journal, Proceedings of the 1982 Symposium on Western Alaska Resources and Geology, p. 59-65.
Reporters C.C. Hawley and Travis L. Hudson
Last report date 10/22/1999