The Island Mountain sulfide deposit is in the southwest corner of Trinity County about 90 miles north of Ukiah and 30 miles east of Garberville. It was discovered about 1897 but not worked until 1915, shortly after the completion of the nearby Northwestern Pacific Railroad. From 1915 until 1930, 132,000 tons of ore were mined and yielded 9 million pounds of copper, 144,000 ounces of silver, and 8,600 ounces of gold. An estimated 158,000 tons of ore remain.
The deposit is a lenticular massive sulfide body consisting predominantly of pyrite, chalcopyrite, and pyrrhotite, with smaller amounts of magnetite, arsenopyrite, galena, and bornite. The gold and silver are present in the sulfides either in solid solution .or as admixtures. The ore contained an average of 1.09 ounces of silver and .065 ounces of gold per ton. Country rock consists of graywacke, shale, glaucophane schist, and chert. Greenstone and andesite are present.
Aubury, L. E., 1908, Island Mountain Consolidated Copper Mine: California Min. Bur. Bull. 50, pp. 148-150.
Logan, C. A., 1926, Trinity County, Island Mountain Consolidated Copper Mine: California Min. Bur. Rept. 22, pp. 14-15.
Stinson, M.C., 1957, Geology of the Island Mountain copper mine, Trinity County: California Jour. Mines and Geology, vol. 53, pp. 9Â·33.