Irishman Mine, Fairbanks North Star Borough, Alaska, USAi

Regional Level Types
Irishman Mine	Mine
Fairbanks North Star Borough	Borough
Alaska	State
USA	Country

Geologic Description = The Irishman deposit is one of two structurally controlled, auriferous vein-fault deposits at the Grant mine; these deposits are hosted in polymetamorphic schist and quartzite. The O'Dea is the other vein-fault system and is described under the Grant gold mine (FB058). From December 1980 to June 1981, Bundtzen and Kline (1981), spent 25 days mapping the underground workings at the Grant mine, and the following is a summary of their findings. At the Irishman vein, late-stage, silica injection was accompanied by free gold, arsenopyrite, lead-antimony sulfosalts, and rare scheelite. Ore fluids were emplaced along a fault zone trending sinuously N. 20-40 E. and dipping steeply to the southeast. On the 150-foot and 200-foot levels, the vein dips from 58 to 72 degrees, but flattens out to 15 to 35 degrees in raises above the 100-foot level where it intersects incompetent graphite-muscovite schist. Vein widths vary from 3 to more than 22 inches and average about 13 inches wide on the 100-foot and 150-foot levels. By 1982, underground exploration and development had shown that the Irishman vein was more than 400 feet long and had not yet bottomed out at depth. The southwest end of the system probably continues past the known ore body, but the northeast end is cut by a north-northwest-striking fracture system. The first recorded exploration occurred in 1929, when lode gold mineralization was found at the bottom of an old shaft that was sunk for placer prospecting. The shaft was sunk through muck and gravel to bedrock thatdipped east at a steep angle (Hill, 1933, p. 150). The original shaft was continued in bedrock to a depth of 240 feet, with 280 feet of drifts on the 200-foot and 240-foot levels. The 5- to 6-foot-wide quartz vein was said to strike N. 40 E. and dip 65 E. (Hill, 1933). Before 1931, approximately 500 to 600 tons of ore was milled from this vein. Exploration and development at the mine has been intermittent since the 1920's. Prior to 1950, about 6,000 tons of ore were mined from the Irishman vein (Eakins and others, 1985, p. 16). From 1979 to 1981, Tri-Con Mining, operator for Silverado Gold Mines, Ltd., at the Grant gold mine, engaged in an aggressive exploration and development program on the Irishman, O'Dea, and other gold-bearing shear zones on the Grant property (Eakins and others, 1985, p. 16). Underground workings consisted of 3,600 feet of drifts, crosscuts and raises on and above the 200-foot level of the O'Dea zone (Bundtzen and Kline, 1981). During 1984, a $1.9 million exploration program was completed at the Grant mine (Eakins and others, 1985, p. 16). The drilling program defined an ore body on the O'Dea zone that extends 4,000 feet on strike and is as much as 1,000 feet deep. In 1985, the Grant mine recovered 1,563 ounces of gold and 1,178 ounces of silver from 5,036 tons of ore (Bundtzen and others, 1986, p. 26). During the seven weeks of mine production in 1989, the mill processed 8,000 tons of material that yielded 732 ounces of gold and 138 ounces of silver. From December 1987 to February 1989 when the mine closed, the Grant mine produced a total of 9,454 ounces of gold and 3,658 ounces of silver from 96,555 tons of ore at an average grade of 0.098 ounce of gold per ton (Bundtzen and others, 1990, p. 36-37). The gold ranges from 744 to 776 fine (Glover, 1950).