Unnamed (Bruce Hills)

Prospect, Inactive

Commodities and mineralogy

Main commodities Cu; Mo
Other commodities Ag; Pb; Zn
Ore minerals chalcopyrite; galena; hematite; malachite; molybdenite; molybdite; pyrrhotite; sphalerite
Gangue minerals chlorite; k-feldspar; limonite; montmorillonite; quartz

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale D-2
Latitude 58.983
Longitude -136.341
Nearby scientific data Find additional scientific data near this location
Location and accuracy
The deposit is in the Bruce Hills between Wachusett Inlet and Burroughs Glacier. It occupies parts of sections 23, 24, 25, and 26, T. 33 S., R. 54 E., of the Copper River Meridian. The location given is the approximate midpoint of the outcrop of the deposit on the ridge between the inlet and glacier, at an elevation of about 2300 feet. The deposit is mainly in the Mt. Fairweather D-2 quadrangle but it extends easterly into the D-1 quadranagle.
The deposit includes two occurrences investigated in detail by the U.S. Bureau of Mines (Kimball and others, 1978, figs. C62 & 63).

Geologic setting

Geologic description

The deposit is in foliated granodiorite that is part of a batholithic mass that underlies much of the northeast part of the National Park. The intrusion, of Cretaceous age, has invaded a shale-carbonate sequence of Paleozoic age now mainly converted to marble and schist (Brew and others, 1978). The deposit is about two miles west of the main batholithic contact. Occasional masses of hornfels (roof pendants?) included in the granitic rock suggest that the deposit could lie fairly close to the roof of the intrusion. A strong fault which strikes about N 30-40 E seems to form the southeast limit of the deposit (MacKevett and others, 1971, fig. 10). Andesite dikes that strike east-northeast cut through the deposit.
The main ore minerals, chalcopyrite and molybdenite, occur in quartz and quartz-feldspar veinlets, as disseminations and as fracture coatings. Galena, pyrrhotite, and sphalerite occur locally. Locally the sulfide minerals have been oxidized to secondary malachite, molybdite, and limonite. The deposit is in heavily iron-stained granodiorite.
The deposit appears to be low grade, on an average, but has the potential for the occurrence of substantial local zones of higher grade. The average grade is about 0.05 percent copper and less than 100 ppm molybdenum. Parts of the deposit, however, contain about 2000 ppm copper and 200 ppm molybdenum, and selected veins contain as much as 3400 ppm copper and 1000 ppm molybdenum. The area is partly soil covered.
Geologic map unit (-136.342878327095, 58.9826064235766)
Mineral deposit model Porphyry copper-molybdenum similar to porphyry copper (Cox and Singer, 1986; model 17).
Mineral deposit model number 17
Age of mineralization Tertiary.
Alteration of deposit K-feldspar occurs as a secondary mineral in and near quartz veins, and the host granodiorite is anomalously rich in K-feldspar, particularly south and east of the deposit (Kimball and others, 1978, p. C306). Feldspathization was noted elsewhere in the area. Montmorillonite is developed locally. The deposit itself is propylitically altered, with chlorite replacing primary mafic minerals.

Production and reserves

Workings or exploration
The occurrence of copper and molybdenum in the Bruce Hills was first reported by Rossman (1963, B 1121-K). Seven claims were staked in 1967 and the deposit was blanketed by 55 claims located in 1969. At the time of the U.S. Bureau of Mines investigation in 1977 (Kimball and others, 1978, p. C297) all claims were inactive. Mineralization is exposed on the surface and possibly in shallow hand-dug trenches. The average grade of the deposit is about 0.05 percent copper and 75 ppm molybdenum. An exposure at about 1750 to 1950 on the northeast side of the deposit, possibly the one originally noted by Moerlein (1971), appears to be somewhat richer. Samples collected along a northeast line about 330 feet long averaged 0.17 percent copper and 0.026 percent molybdenum (Kimball and others, 1978, p. C300-305). The values are close to Moerlein's estimate of 0.2 percent copper and 0.017 percent molybdenum. The U.S. Geological Survey sampled the southwest part of the deposit because, at the time of their investigation, the ridge and north areas were snow covered; maximum values reported were 3000 ppm copper and 1000 ppm molybdenum (MacKevett and others, 1971, p. 48-50). The USGS also conducted a soil survey.
Both USGS and USBM investigators report higher-grade type samples with values to 4000 ppm Cu and 2000 ppm molybdenum.
Indication of production None
Reserve estimates A resource exists in the mid 0.0X percent range for copper and low 0.0X range for molybdenum; local zones are of higher grade.

Additional comments

The Bruce Hills prospect is about 8.5 miles west of the Nunatak molybdenum deposit (MF036) and only about 2 miles north of the Wachusetts copper-molybdenum vein (MF060).
The site is in Glacier Bay National Park and Preserve.

References