Grizzly Bear

Prospect, Active

Alternative names

Grizzly Butte

Commodities and mineralogy

Main commodities Au; Cu
Other commodities Ag; W
Ore minerals bornite; chalcopyrite; magnetite; pyrite; pyrrhotite

Geographic location

Quadrangle map, 1:250,000-scale TK
Quadrangle map, 1:63,360-scale D-2
Latitude 62.9749
Longitude -147.873
Nearby scientific data Find additional scientific data near this location
Location and accuracy The center of the Grizzly Butte prospect is at an elevation of about 4,000 feet about 1 mile northwest of VABM 5608 'Watana' and about 15 miles southwest of the bridge on the Denali Highway over the Susitna River near the community of Denali. The prospect is near the northeast corner of section 35, T. 22 S., R. 2 W. The location is accurate.

Geologic setting

Geologic description

The rocks in the area are mainly Triassic and/or Jurassic basalt flows with minor argillite, limestone, and tuff, intruded by gabbroic sills (Csejtey and others, 1978 [geologic map]; Full Metal Minerals, 2011). These rocks are intruded by an intrusive complex that consists of plugs and dikes of hornblende diorite and a porphyritic diorite to syenite stock. The intrusive complex is exposed for about 1.2 square kilometers but extends under surficial material to the west; the aeromagnetic data suggest it is much larger. The intrusive complex and surrounding rocks are cut by trachyte, felsic, diorite, and andesite dikes. In their regional mapping, Csejtey, Nelson and others (1978 [geologic map]) did not map an intrusive at this prospect although Tertiary granodiorite and a Cretaceous or Tertiary granite are several miles away.
As described by Csejtey and others (1978 [mineral deposits]), a sulfide-bearing felsic dike cuts Triassic metabasalt at the Grizzly Butte prospect. The sulfide minerals are bornite, chalcopyrite, pyrite, and pyrrhotite. Low-grade sulfide veins and disseminated sulfides also occur in silicified metabasalt. A U.S. Geological Survey sample of mineralized volcanic rock contained 0.5 part per million (ppm) gold; a sample of felsic intrusive rock contained 3 ppm silver and 350 ppm copper (Miller and others, 1978). A U.S. Bureau of Mines grab sample of limonitic metabasalt contains 5.17 percent copper, 0.4 parts per million gold, and 550 parts per million tungsten (Kurtak and others, 1992).
Cities Service Minerals Company held 96 claims over the prospect in 1972 and 1973 and Northland Mines held 18 claims in 1974. Kennecott Exploration staked claims in 1998, carried out rock and stream-sediment geochemical surveys, and flew an aeromagnetic survey over the area. Full Metals Minerals optioned the property from Kennecott in 2010 and staked additional claims (Full Metal Minerals, 2011). They began drilling in August 2010.
A zone of strong potassic alteration about 1,000 by 2,000 meters in area is centered on the intrusive complex (Full Metals Minerals, 2011). Disseminated and fracture-controlled chalcopyrite and bornite(?) coincide with the southwest portion of the altered zone; to the northeast, magnetite-biotite-flooded stockworks are more common. One or more zones of sericitic alteration are along the east side of the potassic alteration and epidote-magnetite alteration is along the west side. Pyrite, pyrrhotite, and magnetite occur along fracture zones in the rocks around the intrusive; sparse disseminated chalcopyrite is widespread in the diorite-syenite intrusive. The mineralization is accompanied by strong geochemical soil anomalies in gold, silver, and copper. Rock samples contain up to 17,900 ppm copper 1,680 ppm molybdenum, and 2,660 parts per billion gold.
Geologic map unit (-147.875252905546, 62.9744450720514)
Mineral deposit model Unclear.
Age of mineralization Probably related to an intrusive complex of uncertain age; possibly Cretaceous or Tertiary.
Alteration of deposit A zone of strong potassic alteration about 1,000 by 2,000 meters in size is centered on the mineralized intrusive complex. One or more zones of sericitic alteration are along the west side of the potassic alteration and epidote-magnetite alteration is along the west side.

Production and reserves

Workings or exploration Surface sampling by U.S. Geological Survey and U.S. Bureau of Mines (Miller and others, 1977; Kurtak and others, 1992). Cities Service Minerals Company held 96 claims in 1972-73 and Northland Mines held 18 claims in 1974. Kennecott Exploration staked claims in 1998, carried out rock and stream-sediment geochemical surveys, and flew an aeromagnetic survey over the area. Full Metals Minerals optioned the property from Kennecott in 2010 and staked additional claims (Full Metal Minerals, 2011). They began drilling in August 2010.
Indication of production None
Reserve estimates None.
Production notes None.

Additional comments

MAS/MILS number 20760051.

References

MRDS Number A012166

References

Full Metal Minerals, 2011, The Grizzly Butte project, Alaska Range, Talkeetna Mountains, AK: http://www.fullmetalminerals.com/s/GrizzlyButte.asp (as of February 16, 2011).
Reporters R.K. Rogers (Contractor, USGS); D.J. Grybeck (Contractor, USGS)
Last report date 2/28/2011