Mexican (Treadwell Mines group)

Mine, Inactive

Commodities and mineralogy

Main commodities Au
Other commodities Ag; As; Cu; Mo; Pb; Sb; W; Zn
Ore minerals arsenic; chalcopyrite; galena; gold; magnetite; molybdenite; orpiment; pyrite; pyrrhotite; realgar; scheelite; sphalerite; tetrahedrite
Gangue minerals albite; calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale JU
Quadrangle map, 1:63,360-scale B-2
Latitude 58.2654
Longitude -134.3698
Nearby scientific data Find additional scientific data near this location
Location and accuracy This mine is on the northeast shore of Douglas Island on the southwest shore of Gastineau Channel, 1/4 mile southeast of Douglas. It is near the southwest corner of SE1/4 section 36, T. 41 S., R. 67 E. of the Copper River Meridian. The location is accurate.

Geologic setting

Geologic description

The Mexican mine is part of the Treadwell Mines group that includes the Treadwell (JU199), 700-Foot (JU200), and the Ready Bullion (JU206) mines. Between 1885 and 1923, the Treadwell group collectively mined 28 million tons of ore that produced 3.2 million ounces of gold and over 180,000 ounces of silver (Redman and others, 1989). Of that total, the Mexican Mine and Ready Bullion Mine each accounted for approximately 500,000 ounces of gold (Light and others, 1989). There are approximately 75 miles of underground workings among the 4 mines; all but 3,360 feet are now (2001) flooded. There is a large glory hole at the Treadwell Mine and 2 smaller glory holes at the Mexican Mine. Placer gold was discovered in Ready Bullion Creek in 1880 and in 1881, the Ready Bullion, Mexican, Treadwell and Mexican lodes were discovered. The Treadwell Mine began operation in 1882, and the Mexican Mine began work in 1886. The Treadwell and Mexican mines were consolidated in 1891 and major development at the Ready Bullion mine began in 1894. Work began at the 700-Foot mine, which was midway between the Treadwell mine and the Mexican mine, in 1897. The Treadwell, 700-Foot, and Mexican mines were connected to the Ready Bullion Mine by a tunnel in 1904, but a concrete bulkhead was placed in the tunnel in 1913 (Stone and Stone, 1980). By the end of 1916, the mines were being worked as much as 2,700 feet below the surface. In 1917, a cave-in at the 700-Foot shaft flooded the Treadwell, 700-Foot, and Mexican mines. Only the Ready Bullion survived and remained in operation until 1923 when lean ore caused the mine to cease operations (Redman and others, 1989).
The deposit is in thick, altered albite-diorite sills (Spencer, 1905). The sills have intruded northwest-trending, black phyllite that forms the footwall of the diorite; a greenstone sill forms the hanging wall. The albite-diorite sills are generally conformable to the foliation, pinch and swell along strike, and are up to 290 feet thick and 3,800 feet long (Spencer, 1906). Base and precious metals are disseminated in the altered albite-diorite and in reticulated veins and stringers of calcite and quartz that vary from a few millimeters to a few centimeters in thickness. The veins and stringers constitute almost 20 percent of the albite diorite (Light and others, 1989). The highest gold concentrations tend to be associated with the highest density of quartz and calcite veinlets. Pyrite is the most abundant sulfide, with minor native gold, chalcopyrite, galena, sphalerite, molybdenite, native arsenic, realgar and orpiment. The gold is irregularly distributed and is independent of pyrite distribution. From 60 to 75 percent of the gold was free-milling and over the life of the Treadwell Mine group it is estimated that overall gold recovery averaged almost 90 percent (Light and others, 1989; Redman and others, 1989).
The rocks in the general area are Upper Jurassic or Cretaceous marine argillite and graywacke, interbedded with andesite or basalt (Brew and Ford, 1985). The bedded rocks are regionally metamorphosed to prehnite-pumpellyite or greenschist grade, and cut by diorite or gabbro dikes and sills.
Geologic map unit (-134.371567393272, 58.2650713631792)
Mineral deposit model Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization The age of mineralization in the Berners Bay district is about 55 Ma, the same as the other mesothermal gold-quartz-vein deposits in the Juneau Gold Belt (Goldfarb and others, 1997).
Alteration of deposit The albite-diorite sills are altered. The alteration assemblage includes albite, calcite, and quartz along with lesser amounts of biotite, chlorite, epidote, hornblende, magnetite, pyrite, pyrrhotite, rutile, sericite, and zoisite (Spencer, 1905).

Production and reserves

Workings or exploration There are approximately 75 miles of underground workings among the 4 mines of the Treadwell Group; all but 3,360 feet are now (2001) flooded. There is a large glory hole at the Treadwell Mine and 2 smaller glory holes at the Mexican Mine. Placer gold was discovered in Ready Bullion Creek in 1880 and in 1881, the Ready Bullion, Mexican, Treadwell and Mexican lodes were discovered. By the end of 1916 the mines were being worked as much as 2,700 feet below the surface. In 1917, a cave-in at the 700-Foot shaft flooded the Treadwell, 700-Foot, and Mexican mines. Only the Ready Bullion survived and remained in operation until 1923, when lean ore caused the mine to cease operations (Redman and others, 1989). In the early 1980s Occidental Minerals and Barrick Resources conducted exploratory drilling in the Treadwell area. From 1986 through 1997 Echo Bay Mines explored the area, including deep drilling beneath Gastineau Channel in 1990. Their deep drilling confirmed that the Treadwell vein system continues below the deepest known workings (Miller and others, 1992; L. Miller, personal communication, 2001; Swainbank and others, 1991).
Indication of production Yes; large
Reserve estimates From 1986 through 1997 Echo Bay Mines explored the area, including deep drilling beneath Gastineau Channel in 1990. Their deep drilling confirmed that the Treadwell vein system continues below the deepest known workings (Miller and others, 1992; L. Miller, personal commun., 2001; Swainbank and others, 1991). At the time the Treadwell mines closed, there were about 3 million tons of proven reserves, including broken ore in stopes, pillars, and deep ore under development (Redman and others, 1989).
Production notes Between 1885 and 1923, the Treadwell Group collectively mined 28 million tons of ore that produced 3.2 million ounces of gold and over 180,000 ounces of silver. Of that total, the Mexican Mine and Ready Bullion Mine each accounted for approximately 500,000 ounces of gold. There were at least 300 stamps in use at the Treadwell, nearly 200 stamps at the 700-Foot and Mexican mines, and 150 stamps at the Ready Bullion Mine. A cyanide plant serviced all four mines by 1910.

References

MRDS Number A012060

References

Goldfarb, R.J., Miller, L.D., Leach, D.L., and Snee, L.W, 1997, Gold deposits in metamorphic rocks in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 151-190.
Miller, L.D., Goldfarb, R.J., Gehrels, G,E., and Snee, L.W., 1994, Genetic links among fluid cycling, vein formation, regional deformation, and plutonism in the Juneau gold belt, southeastern Alaska: Geology, v. 22, p. 203-206.
Stone, D., and Stone, B., 1980, Hard rock gold--The story of the great mines that were the heartbeat of Juneau: Vanguard Press Inc., 86 p.
Reporters J.C. Barnett and L.D. Miller (Juneau, Alaska )
Last report date 12/15/2001