Brady Glacier nickel-copper deposit

Prospect, Inactive

Commodities and mineralogy

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale C-3
Latitude 58.5533
Longitude 136.9275
Nearby scientific data Find additional scientific data near this location
Location and accuracy The location is approximately the discovery nunatak of the deposit at an elevation of about 3300-feet on Brady Glacier. Except for exposures on nunataks, the deposit is covered by ice. It extends for about 0.4 miles northeast of the location point and about 0.6 miles west-southwest of the location point. The deposit may extend further west , possibly northwest, but the possible extensions were not drilled because of ice thickness. The location of the coordinates is the approximate common corner of sections 23, 24, 25, 26, T. 38 S., R. 51 E., of the Copper River Meridian. (See figure 3, Himmelberg and Loney, 1981, for this location relative to the discovery nunataks, contact of the intrusion, drill holes and cross-sections of the deposit.)

Geologic setting

Geologic description

The Brady Glacier nickel-copper deposit is near the local floor of the Crillon-LaPerouse layered intrusion near an east-pointing embayment of the contact of the intrusive (Himmelberg and Loney, 1981, figs. 2 and 3, p. 4-5; Cornwall, 1971, p. 79-82). The mafic complex was intruded into hornblende schist interfingered with biotite schist. The schistose host rocks were interpreted as Alexander complex by Brew and others (1978, p. B12-13). Berg and others (1972) and Jones and others (1978) have placed the rocks in Wrangellia or Chugach terranes. The Tertiary age of the mafic intrusions (see below) is more consistent with the Chugach-Wrangellia interpretation.
The Crillon-LaPerouse host intrusion is the largest of four layered mafic-ultramafic plutons in the Fairweather Range in Glacier Bay National Park and Preserve (Brew and others, 1978). The intrusion has an exposed thickness of about 6000 feet, and consists mainly of interlayered olivine gabbro and norite. Thin layers of ultramafic rock occur throughout the section, but are most abundant near the base. Layering and other sedimentary-like features suggest the body formed mostly by cumulus processes.
The Crillon-LaPerouse pluton, and the other layered plutons of the National Park, are probably underlain by a dike-like feature of north-northwest trend. The structure is indicated by the +30 mGal contour (Barnes, in Brew and others, 1978, p. B51-69, esp. p. 67 and figure B4; Barnes and Watts, 1977). An underlying dike-like feature is also suggested by magnetics (Griscom, in Brew and others, 1978, p. B22-37, figure B1). It appears to be thickest underneath the Crillon-LaPerouse pluton. Its probable existence is consistent with the trend of significant cumulate-type ore deposits along the Fairweather Range.
The proposed underlying dike is a megadike like the Great Dyke of Rhodesia (Zimbabwe); the overlying layered intrusions are like the major chromite-bearing mafic-ultramafic funnels that form the upper part of the Great Dyke (Worst, 1960; Bichan, 1969).
In the Crillon-LaPerouse body itself, the predominantly mafic host rocks of the deposit consist of magnesian augite and bronzite, plagioclase (An81-63) and olivine (Fo71-86), also accessory chromite, ilmenite, magnetite, and graphite. Local phases are ultramafic in composition.
The Brady Glacier nickel-copper deposit consists mainly of stratigraphically continuous disseminated-sulfide zones locally more than 400 feet thick that contain as much as 10 percent sulfide minerals. Massive sulfide zones of up to 10 feet in thickness occur locally, especially near the contacts of gabbroic and ultramafic cumulates.
The dominant ore sulfides are hexagonal and monoclinic pyrrhotite, pentlandite and chalcopyrite. Altaite, cubanite and niccolite occur as minor components of the primary ore. Bornite, mackinawite, and violarite appear to have formed by secondary reactions between the primary sulfide phases (Czamanske and others, 1977, p. 14; Czamanske and others, 1981, p. 2001-2010; Himmelberg and Loney , 1981). Thicknesses where Ni + Cu equal or are greater than 0.5 percent exceed 100 feet are common. In drill hole NUC-3, in an apparent keel-like zone, apparent ore thickness exceeds 400 feet (Himmelberg and Loney, 1981, fig. 5).
The deposit contains relatively low concentrations of PGEs (Pd, Pt, and Rh). The total PGE content of disseminated or average ore is about 0.18 ppm. Massive sulfide zones contain about 1 ppm total PGEs.
Abundant carbon (now graphite), possibly derived from the intruded sedimentary rocks, kept low-valent sulfur stable and allowed the formation of a stable immiscible ore-sulfide phase that separated from the magma (Czamanske and others, 1977).
The deposit is Tertiary in age. A mid-Tertiary age of about 30 Ma is consistent with new data reported by Goldfarb (1997) and with Ar-Ar dates reported by Himmelberg and Loney in 1981 (p. 5) The deposit appears to thicken and become richer to the west, suggesting that resources identified to date by drilling are probably minimal.
Geologic map unit (-136.929388615875, 58.5529265409585)
Mineral deposit model Disseminated to massive sulfide deposit formed from immiscible sulfide fluid injected into cumulus layers of silicate minerals. Similar to Stillwater Ni-Cu and Duluth Cu-Ni-PGE (Cox and Singer, 1986; models 1 and 5a). The deposits are synorogenic to mid-Tertiary tectonic activity (Foley and others, 1997, p. 441-443).
Mineral deposit model number 1, 5a
Age of mineralization Probably mid-Tertiary (Goldfarb, 1997; Himmelberg and Loney ,1981).

Production and reserves

Workings or exploration
The deposit was discovered in a helicopter exploration conducted by Fremont Exploration Co. in 1958. Fremont covered the anticipated area of the deposit with 224 claims. Fremont drilled a total of 32 core holes in the nunataks or through the ice before turning the project over to Newmont Exploration, Ltd. Newmont drilled 14 additional holes and conducted extensive metallurgical tests on the core from the property. An additional 36 holes were drilled after 1961; drill hole total is 82 holes (Kimball and others, 1978, p. C99-101).
Twenty claims covering the core of the deposit and, supported by drill discovery data, were patented in 1965. Later Newmont joint ventured the deposit with Cities Service and Union Pacific Resources. Although Richard Ellett of Newmont (1975) considered the deposit the largest nickel-copper deposit in the United States, its location within Glacier Bay National Park and Monument precludes its development. The deposit is now owned by the University of Alaska.
Newmont and partners also did considerable project planning. The project could be developed from a ten-mile-long tunnel driven from the Boussole Bay area.
An ice-depth survey suggests a deep canyon exists about a mile or two north of the deposit (Watts and England, 1976). This canyon may limit potential of the deposit, but it does not affect currently estimated resources.
Indication of production None
Reserve estimates Drill-indicated reserves are about 100 million tons of rock containing 0.5 percent nickel and 0.3 percent copper. PGEs average about 0.18 ppm and exceed 1 ppm in massive sulfide units and in flotation concentrates. About 250,000 ounces of PGEs exist in the drilled Ni-Cu resource area (Czamanske and others, 1981).

Additional comments

Brady Glacier is the largest or among the largest of nickel-copper deposits in the United States (Ellett, 1975). It also has a substantial resource of PGEs.
Extensive metallurgical work done by Newmont and followed up by Czamanski and others (1981) show that resources are only partly recoverable. Only about 1/2 of the estimated PGE resource is recoverable using the techniques tested. Distinct phases of PGEs have not been identified, but some are liberated after regrinding of the bulk flotation concentrates, and are potentially recoverable by ultrafine gravity or electrodynamic separators. Nickel recovery is about 80 percent. At low nickel concentrations, a considerable amount of the nickel is in the silicate phase and is not recoverable.
The work done suggests that recoveries in an industrial-scale operation could be maximized. Inasmuch as the deposit is not yet limited to the west, any increase in reserves could contribute towards process development and scale of operation.
Patented claims at the site are now owned by the University of Alaska; they are in Glacier Bay National Park and Preserve.

References

MRDS Number A013136; M046921

References

Bichan, R., 1969, Chromite seams in the Hartley complex of the Great Dyke of Rhodesia: Economic Geology Monograph 4, p. 95-113.
Czamanske, G.K., and others, 1977, The Brady Glacier Ni-Cu deposit, southeastern Alaska [abs.]: Geological Association of Canada, Program with abstracts, v. 2, 1977, Annual Meeting, Vancouver, p. 14.
Ellett, R.D., 1975, Statement and discussion--Adverse effects of proposed legislation upon Alaska nickel mining, in The regulation of mining activities within areas of the National Park System: U.S. Congressional Senate Committee hearing before the Committee on Internal and Insular Affairs, Oct. 7, 1975, U.S. Congress, 94th, 1st Session, p. 311-316
Foley, J.Y., Light, T.D., Nelson, S.W., and Harris, R.A., 1997, Mineral occurrences associated with mafic-ultramafic and related alkaline complexes in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 396-449.
Jones, D.L., Silberling, N.L., and Hillhouse, John, 1978, Wrangellia, a displaced terrane in northwestern North America: Canadian Journal of Earth Sciences, v. 14, p. 2365-2477.
Watts, R.D., and England, A.W., 1976, Radio-echo soundings of temperate glaciers: Journal of Glaciology, v. 17, no. 75, p. 39-48.
Worst, B.G., 1960, The great dyke of Southern Rhodesia: Southern Rhodesia Geological Survey Bulletin 47, 237 p., 9 sheets.
Reporters C.C. Hawley (Hawley Resource Group)
Last report date 4/4/1999