Duchess

Mine, Inactive

Commodities and mineralogy

Main commodities Cu; Pb; Zn
Ore minerals arsenopyrite; chalcopyrite; cubanite; galena; pyrite; pyrrhotite; sphalerite
Gangue minerals calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale SR
Quadrangle map, 1:63,360-scale A-3
Latitude 60.0213
Longitude -147.9194
Nearby scientific data Find additional scientific data near this location
Location and accuracy The mine is located a half-mile east of Horseshoe Bay at an elevation of 380 to 470 feet. This is in the NW1/4 section 9, T. 2 S., R. 9 E., of the Seward Meridian. This is location 258 of Cobb and Tysdal (1980) and location S-4 of Jansons and others (1984). This location is accurate to within 300 feet; a mine is shown at the location on the 1:63,360-scale, A-3 topographic map.

Geologic setting

Geologic description

The mineralized zone at the Duchess mine trends N 30 E and is parallel to the bedding of the slate-graywacke host rock that dips steeply northwest (Stejer, 1956). Nelson and others (1985) have mapped the bedrock in this area as Orca Group of early Tertiary age. The mine is on the northwest limb of a shallowly-dipping, northeast-plunging antiform (Stejer, 1956).
The mine workings consist of two major levels with portals at elevations of 380 feet and 470 feet and a 10-foot prospect adit at an elevation of 630 feet. The underground workings, which were developed from 1905 to 1918, aggregate about 3,000 feet of drifts and crosscuts and explore the mineralized zone for a distance of 500 feet along strike (Kurtak and Jeske, 1986). Lenses of massive sulfides occur throughout the workings. They range from a few inches to as much as 60 feet wide and from less than one foot to at least 490 feet long. The mineralization typically consists of a disseminated halo of pyrite surrounding massive pyrite, with some chalcopyrite (Stejer, 1956). Other minor sulfides present include cubanite, sphalerite, pyrrhotite, arsenopyrite, and galena (Kurtak and Jeske, 1986). The footwall of the massive sulfide lenses is in many places a slate bed; the highest grade of copper mineralization is near the footwall (Kurtak and Jeske 1986). Within or near the massive sulfide lenses, the country rock shows varied degrees of silicification, sericitization, and kaolinization (Stejer, 1956). Sixteen samples collected from the 400-foot level contained trace to 1.48 percent copper, 0.1 to 0.8 percent zinc, trace to 0.02 ounce of gold per ton, and 0.4 to 0.7 ounce of silver per ton (Stejer, 1956).
Geologic map unit (-147.921415491759, 60.0206494293376)
Mineral deposit model Besshi massive sulfide (Cox and Singer, 1986; model 24b)
Mineral deposit model number 24b
Age of mineralization Tertiary or younger; the mineralization is in rocks of the Orca Group of Tertiary age.
Alteration of deposit The wallrock alteration of the deposit at the Duchess mine consists of silicification, sericitization, and kaolinization. The alteration is zoned; silicification near the hanging wall grades into argillic and sericitic alteration toward the footwall (Stejer, 1956).

Production and reserves

Workings or exploration The mine workings consist of two major levels with portals at elevations of 380 and 470 feet and a 10-foot prospect adit at an elevation of 630 feet. The underground workings, which aggregate about 3,000 feet of drifts and crosscuts, explore the mineralized zone for a distance of 500 feet along strike (Stejer, 1956). The workings were initially developed in 1905 with all of the production occurring in 1906. From 1907 to 1916 about 2,850 feet of drift and crosscuts were driven in an attempt to delineate additional ore bodies. Sixteen samples collected from the 400-foot level contained trace to 1.48 percent copper, 0.1 to 0.8 percent zinc, trace to 0.02 ounce of gold per ton and 0.4 to 0.7 ounce of silver per ton (Stejer, 1956).
Indication of production Yes; small
Reserve estimates Chugach Native Corporation, the current property owner, estimates a reserve of 2,700,000 tons with 3.8 percent copper (Rogers and Hoyt, 1999).
Production notes The deposit was initially staked in 1899, and by 1905 as much as 300 feet of exploration adits had been driven. All of the production from the Duchess mine occured in 1906. Production totalled 215,000 pounds of copper from 2,850 tons of ore (Rogers and Hoyt, 1999).

References

References

Rogers, R., and Hoyt, M., 1999, Mineral prospects on Chugach Alaska Corporation lands: Chugach Alaska Corporation in-house report, 10 p., 1 table, 10 maps.
Smith, S.S., 1917, The mining industry in the Territory of Alaska during the calendar year 1916: U.S. Bureau of Mines Bulletin 153, 89 p.
Reporters Jeff A. Huber and Carol S. Huber (Anchorage)
Last report date 10/15/2001