McMillan

Mine, Inactive

Alternative names

Columbia
Ophir

Commodities and mineralogy

Main commodities Au
Other commodities Ag; Pb
Ore minerals arsenopyrite; galena; gold; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale SR
Quadrangle map, 1:63,360-scale C-7
Latitude 60.5936
Longitude -149.5908
Nearby scientific data Find additional scientific data near this location
Location and accuracy The mine is located in the SW1/4 section 23, T. 6 N., R. 2 W., of the Seward Meridian, at elevations of between 3,200 feet and 3,400 feet. It is situated on the south side of the divide south of Slate Creek, approximately 1.25 miles west of the Seward Highway. This is location 57 of Cobb and Tysdal (1980), location 37 of MacKevett and Holloway (1977), and location S-249 of Jansons and others (1984). This location is accurate to within a quarter of a mile.

Geologic setting

Geologic description

The deposit at the McMillan mine consists of auriferous, sulfide-bearing quartz veins in sheared graywacke and slate of the Valdez Group of Late Cretaceous age (Tysdal, 1978 [MF-880-A]). These quartz veins are 3 inches to 2 feet wide ard are exposed in at least four locations in a half-mile-wide belt near the top and along the southeast flank of the divide south of Slate Creek.
The adit is at 3,300 feet elevation. A caved trench is located a quarter of a mile southwest of the adit at an elevation of 3,400 feet. A second trench is located 500 feet west of the adit at an elevation of 3,200 feet.
Three veins are exposed in the McMillan adit. The best exposed ore is at the intersection of two veins approximately 75 feet from the portal. One vein, which strikes north and dips 75E, contains considerable galena, sphalerite, and visible gold throughout a thickness ranging from 1 to 2 feet over a 6-foot strike length. This vein assayed 0.739 ounce of gold per ton and 0.55 ounce of silver per ton. The second vein, which strikes N20W and dips 55E, averages 3 inches thick and contains arsenopyrite, galena, and considerable free gold. his vein assayed as much as 5.04 ounces of gold per ton and 1.2 ounces of silver per ton. Both veins are terminated at their southern ends by a fault that strikes N40E and dips 80SE (Hoekzema and Sherman, 1983). Gold production is estimated at 769 ounces (Cobb and Tysdal, 1980).
These veins are similar to those in a 1.0- to 1.5-mile-wide belt that parallels the Gilpatrick dike and extends from south of Slate Creek to Frenchy Creek.
Geologic map unit (-149.592957119286, 60.5930314566718)
Mineral deposit model Low-sulfide Au-quartz veins (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Cretaceous, or younger; veins cut rocks of the Valdez Group of Late Cretaceous age.

Production and reserves

Workings or exploration
Underground workings consist of a 170-foot-long adit at 3,300 feet elevation. It strikes N55W and terminates in a 60-foot-long crosscut that strikes N55E. A caved trench is 0.25 mile southwest of the adit at 3,400 feet elevation, and a second trench is 500 feet west of the adit at an elevation of 3,200 feet. There are also several prospect pits (Jansons and others, 1984). A small prospect mill was installed sometime between 1910 and 1917, and several tons of ore were milled at it (Tuck, 1933).
The U.S. Bureau of Mines collected 18 samples between 1979 and 1980 and mapped the workings (Jansons, 1981). A grab sample from the N20W vein in the adit assayed 5.04 ounces per ton. The best chip sample, also from the N20W vein in the adit, assayed 1.485 ounces of gold per ton. Of the 18 samples, three had no detectable gold, and the remaining samples contained 0.05 ppm to 14 ppm gold (Hoekzema and Sherman, 1983).
Indication of production Yes; small
Reserve estimates The U.S. Bureau of Mines inferred reserves of 250 tons of ore having an estimated grade of 0.2558 ounce of gold per ton (Hoekzema and Sherman, 1983).
Production notes A small mill was installed sometime between 1910 and 1917, and several tons of ore were milled by it (Tuck, 1933). The total gold production is estimated at 769 ounces (Cobb and Tysdal, 1980).

References