Denali Copper; Pass Creek; Caribou Dome Prospect, Valdez Creek Mining District, Matanuska-Susitna Borough, Alaska, USAi

Regional Level Types
Denali Copper; Pass Creek; Caribou Dome Prospect	Prospect
Valdez Creek Mining District	Mining District
Matanuska-Susitna Borough	Borough
Alaska	State
USA	Country

Location: The Denali Copper prospect is at an elevation of about 4,600 feet in the Clearwater Mountains, on the northwest side of the northwesternmost of the two passes between Windy Creek and the South Fork of Pass Creek. It is in sec. 34, T. 20 S., R. 3 E., of the Fairbanks Meridian. There is a short dirt airstrip located in this pass, and a dirt road connects the airstrip with a small camp at an elevation of about 4,200 feet. There is also trail access from the Denali Highway. The camp is located on the southwest bank of a gulch which traverses the mineralized area. The map site is that of the upper of two adit portals and is accurate to within several hundred feet.
Geology: The Denali Copper prospect is near the upper contact of the Nikolai Greenstone, a thick sequence of Upper Triassic marine andesite and basalt, and subordinate clastic and carbonate units (Stevens, 1971, Wilson, and others, 1998). At the prospect, the strata dip at up to 80 degrees northwest and strike northeast. About a mile northwest of the prospect, the strata are cut by a strongly fractionated pluton dated by K-Ar methods at 130 to 143 m.y. (Smith, 1981). The deposit consists of delicately-bedded to massive chalcopyrite and pyrite beds in black argillaceous limestone and black calcareous argillite. The massive, dominantly chalcopyrite beds are up to 12 inches thick and assay up to 12% copper. Pyrite commonly occurs as framboids. Minor bornite, chalcocite, and native copper occur locally, as well as sparse sphalerite. Gossans overlie the sulfide bodies and contain malachite, azurite, chalcocite, and minor chalcopyrite. A unique gossan material consisting of a jet-black earthy residue with relic bedding overlies the highest-grade portions of the sulfide horizons (D. L. Stevens, personal observation). Several major northeast-trending, strike-slip faults traverse the area. The most important occurs in the footwall just south of the main sulfide horizon; it is marked by a zone of fault gouge 20 to 50 feet in width. Movement along this fault has induced drag-folding of the steeply-dipping sulfide horizon, producing fold amplitudes of up to 50 feet. Northwest dipping thrust faults were encountered underground on the 4630-level adit. These faults offset the main horizon about 30 feet (Stevens, 1971). Regional mapping also shows a major southeast-dipping thrust fault that strikes northeast. The main sulfide horizon, on which most of the exploration effort has been made, is up to 400 feet long and 30 feet wide; it extends at least 1,000 feet below the surface outcrop as confirmed by drilling. The sulfide horizons are characterized by 'pinching and swelling' along strike as well as down dip. As one horizon 'pinches', other horizons may 'swell'. The deposit remains open at depth and along strike both to the northeast and southwest on at least four known horizons. Their steep dip makes additions to the reserves expensive. The rocks in this region were regionally metamorphosed to prehnite-pumpellyite-quartz facies (Stevens, 1971), but the very fine-grained sulfide minerals in the deposit were not recrystallized, as shown by chalcopyrite grains as small as 1 micron. This sulfide deposit is interpreted to have formed in a reducing or euxinic marine basin with abundant organic matter and sulfate reducing bacteria (Stevens, 1971). Sulfur isotope ratio analyses of the chalcopyrite and pyrite averaged -28.35 permil with a standard deviation range of only 1.01 permil (Stevens, 1971). These values strongly confirm the biogenic reduction of the seawater sulfate to produce the sulfide ion and suggest the possibility of a closed system. The copper was probably derived by weathering of the subaerial copper-rich volcanic rocks adjacent to the marine basin.
Workings: The deposit was discovered by M. A. Kaufman in 1963 while mapping for the State of Alaska Division of Mines and Minerals. It was subsequently staked by prospectors working for Leo Mark Anthony. From 1964 through 1968, exploration consisted of trenching, geologic mapping, geochemical and geophysical surveys, and diamond core drilling. In 1969, a 1,400-foot-long adit was driven from a portal at an elevation of 4,630 feet. This adit was driven to intersect and follow the drill-indicated trend of the widest known part of the deposit, which was informally called the main horizon. A crosscut was driven into the hanging wall approximately orthogonal to the main ore horizon to provide drill stations to intersect the down-dip extent of the main horizon. Slightly inclined percussion drill holes from this adit tested the adjacent areas out to about 100 feet. Surface diamond drilling tested the other four or five known ore horizons. In 1970, an 1,800 foot long, minus-15-degree spiral decline was driven to provide drill stations for deep intercepts of the mineralized horizon, and to obtain bulk samples of the deposit. The main horizon has been tested by drilling as deep as 1,000 feet below outcrop. Drill testing of the other ore horizons has rarely been deeper than 300 feet. Bulk samples for metallurgical testing were collected in both 1969 & 1970 as part of each underground exploration program. Three diamond core holes were drilled during the summer of 1999.
Age: This stratiform deposit is Late Triassic in age.
Alteration: There is no hydrothermal alteration associated with the sulfide mineralization.
Production: There has been no production.
Reserves: The drill-indicated reserves of the main horizon were calculated to be 550,000 tons containing 5.84% copper, 0.3 ounces of silver per ton, and just a trace of gold (unpublished report by R. H. Seraphim, 1970). The other ore horizons have not been explored sufficiently to be included in reserve and resource calculations. The fine-grained nature of the sulfide minerals causes metallurgical complications which have largely been resolved by technological progress since the work done in the early 1970's.

Commodities (Major) - Cu; (Minor) - Ag, Zn
Development Status: None
Deposit Model: Basaltic Cu (Cox and Singer 1986; model 23)

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Link to USGS - Alaska:	HE191

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References