Caribou Dome

Prospect, Active

Alternative names

Denali Copper

Commodities and mineralogy

Main commodities Cu
Other commodities Ag; Zn
Ore minerals azurite; bornite; chalcocite; chalcopyrite; malachite; native copper; pyrite; sphalerite
Gangue minerals calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale HE
Quadrangle map, 1:63,360-scale A-1
Latitude 63.1385
Longitude -147.1444
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Caribou Dome prospect is at an elevation of about 4,500 feet in the Clearwater Mountains on the northwest side of the pass between Windy Creek and the South Fork of Pass Creek. The location is at the massive sulfide outcrop known as Lense 6 or Number 6 and is near the center of section 34, T. 20 S., R. 3 E., of the Fairbanks Meridian.

Geologic setting

Geologic description

The Caribou Dome 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 up to 80 degrees northwest and strike northeast. About a mile northwest of the prospect, the Nikolai Greenstone is intruded by a strongly fractionated pluton dated by K-Ar methods at 130 to 143 Ma (Smith, 1981).
The prospect consists of delicately-layered to massive beds of chalcopyrite and pyrite in black argillaceous limestone and black calcareous argillite (Stevens, 1971, 2008). The massive, dominantly chalcopyrite beds are up to 12 inches thick and assay up to 12 percent copper. Pyrite commonly occurs as framboids. Minor bornite, chalcocite, and native copper, and sparse sphalerite occur locally. Gossan overlies the sulfide bodies and contains malachite, azurite, chalcocite, and minor chalcopyrite. A prominent gossan consisting of a jet-black earthy residue with relict bedding overlies the highest-grade portions of the sulfide horizons (D. L. Stevens, personal observation). The rocks in this region were regionally metamorphosed to prehnite-pumpellyite facies (Stevens, 1971, 2008), but the very fine-grained sulfide minerals in the deposit were not recrystallized, as shown by chalcopyrite grains as small as 1 micron. 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 wide. Movement along this fault has developed drag-folds with amplitudes of up to 50 feet in the steeply-dipping sulfide horizon. 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 occurred 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. One of the holes of the 2009 drilling had a 7-foot intercept in a new zone of mineralization; the intercept averaged 1.83 percent copper. A 6.8 foot interval in the other hole averaged 0.51 percent copper in what was probably the extension of one of the mineralized horizons. The prospect remains open at depth and along strike both to the northeast and southwest on at least four of the known ore horizons. The 2010 trenching revealed numerous intervals with greater than 0.5 percent copper, including a one meter interval that contained 8.92 percent copper and many intervals 2 to 7 meters in length that contained over 2 percent copper and up to 27.6 grams of silver per tonne. Seven of the 2011 drill holes cut mineralization (Caribou King Resources, 2011). The best intercept was 3.4 meters with 3.36 percent copper and 5.54 grams of silver per tonne.
This sulfide prospect is interpreted to have formed in a reducing or euxinic marine basin with abundant organic matter and sulfate-reducing bacteria (Stevens, 1971, 2008). 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, 2008). These values confirm the biogenic reduction of sea water 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.
Coventry Resources Ltd.’s 2016 program at the sediment-hosted Caribou Dome copper project included 22 drill holes for a total of 6,520 m. Near-surface, high-grade mineralization was extended for greater than 120 meters along strike and to greater than 300 meters depth to the northeast. Coventry better defined the extents of very-high-grade, shallow mineralization and identified an area for a potential starter open-pit mine (Athey and Werdon, 2017).
Geologic map unit (-147.146658141413, 63.1380540522079)
Mineral deposit model Basaltic Cu (Cox and Singer, 1986; model 23).
Mineral deposit model number 23
Age of mineralization Syngenetic in Late Triassic rocks (Stevens, 1971; Wilson, and others, 1998).
Alteration of deposit There is no hydrothermal alteration associated with the sulfide mineralization (Stevens, 1971; Roberts and Stevens, 2015).

Production and reserves

Workings or exploration
The Caribou Dome prospect (formerly the Denali Copper prospect) was discovered by M. A. Kaufman in 1963 while mapping for the State of Alaska Division of Mines and Minerals (Stevens, 2008). It subsequently was 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 (Stevens, 2008). 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 extension. 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 1969 and 1970 as part of the underground exploration program. In 1999, Ahtna Resources Ltd. optioned the property; their work included geochemical and geophysical surveys, three core holes, detailed geologic mapping, and additional metallurgical testing.
The Caribou Dome prospect was acquired by Caribou Copper Resources Ltd. (formerly YOW Capital Management) in 2008 (Caribou Copper Resources, Ltd., 2010). They drilled two holes from the surface in 2009 that totaled 2,038 feet (Caribou Copper Resources Ltd., 2009). In the summer of 2010, they dug 9 trenches and collected numerous samples (Caribou Copper Resources, Ltd., 2010b). In 2011, Caribou King Copper drilled 9 holes from 7 locations to test mineralization found in the 2010 trenching.
In 2014 geologic mapping and sampling was completed for Hatcher Resources Inc. Over 5 days, 18 rock and 27 stream sediment samples were collected on the Caribou Dome prospect (Roberts and Stevens, 2015).
Coventry Resources completed a three-dimensional induced polarization (3DIP) geophysical survey and diamond drilling at Caribou Dome in 2015. The 3DIP survey covered 1500 meters along strike of mineralization and found a strong, continuous 500 meter long anomaly coinciding with known mineralization (Coventry Resources, 2015a). Over ten drill holes were completed in order to verify historic drill results and to target under-explored targets. Initial drill assay highlights include 10.1 meters with 7.1 percent copper and 12.2 meters with 3.2 percent copper (Coventry Resources, 2015b).
Coventry Resources Ltd.’s 2016 program at the sediment-hosted Caribou Dome copper project included 22 drill holes for a total of 6,520 m. Near-surface, high-grade mineralization was extended for greater than 120 meters along strike and to greater than 300 meters depth to the northeast. Coventry better defined the extents of very-high-grade, shallow mineralization and identified an area for a potential starter open-pit mine. Coventry delineated multiple new high-priority induced-potential targets over 7 kilometers of strike. Soil sampling defined a new, 5-kilometer-long, copper-in-soil anomaly at the Senator prospect, located 11 kilometers from the Caribou Dome deposit, and follow-up rock-chip sampling revealed sediment-hosted copper mineralization with assays up to 12.1 percent copper. Coventry also conducted a metallurgy test for Lense 4/5/6 and Lense 7/8, which indicates greater than 99 percent recoveries; concentrates grading greater than 27.4 percent Cu were achieved. An economic assessment and scoping study are scheduled for completion in early 2017 (Athey and Werdon, 2017).
Indication of production None
Reserve estimates R.H. Seraphim (unpublished report, 1970, cited in Stevens, 2008) estimated the drill-indicated reserves of the main horizon as 550,000 tons that contain an average of 5.84 percent copper, 0.3 ounce of silver per ton, and a trace of gold.

References

MRDS Number A011307

References

Reporters D.L. Stevens (Stevens Exploration Management Corporation); D.J. Grybeck (Contractor, USGS); N.V. King (Alaska Earth Sciences); M.B. Werdon (DGGS)
Last report date 8/26/2017