Cobalt-copper deposits of the Blackbird district, Lemhi County, Idaho

Abstract

The Blackbird district is in east-central Idaho, about 20 miles west-southwest of Salmon. The area is one of deeply weathered, flat-topped upland surfaces cut by several steep-walled valleys, which are tributary to the canyon of Panther Creek. Most of the area has a heavy vegetative cover and outcrops are relatively scarce except in the walls of the steeper valleys. The rocks of the district consist mostly of metamorphosed sedimentary rocks of the pre-Cambrian Yellowjacket formation (Belt series); a part of the Cretaceous Idaho batholith cuts across the northern part of the district, and acid porphyry dikes and metamorphosed basic rocks cut the Yellowjacket rocks. Structurally, the sedimentary rocks are divided by faults into three, roughly north-south blocks. The center one (Blackbird structural block) appears to have been more tightly squeezed than the others into relatively tight folds, with the development of widespread schistosity (flow cleavage). The rocks of the two outside blocks are in more open folds. In general they are nonschistose, except for the north end of the western block, where there are schistose rocks cut by several north-dipping thrust faults. The northern parts of the central and western blocks contain considerable garnet, chloritoid, and cordierite. The Blackbird structural block is cut by a number of mineralized shear zones. Those dipping moderately northeast and striking northwest, and those dipping steeply and striking north and northeast; appear to be most important. The mineralized rock contains chalcopyrite, cobaltite, pyrite, and pyrrhotite into a gangue of quartz, biotite, tourmaline, ankerite, and muscovite; the deposits were formed mostly by replacement of the shear zones. In addition, the block is cut by north-dipping thrust faults of west to northwest strike, and a number of high-angle faults. The district was first prospected about 1893; considerable developmont was done at the Brown Bear mine in 1899-1902, at the Haynes-Stellite in l917-1920, and at the Uncle Sam mine in 1938-1941, During World War II, the U.S. Bureau of Mines explored in the district with bulldozer and diamond drill., and the U.S. Geological Survey mapped the district and logged the drill cores. The Howe Sound Co. also did some diamond drilling in the district, and in 1945 the Calera Mining Co., Blackbird Division (subsidiary of Howe Sound Co.), started underground development at the Calera adit. Although the district has had very little production to the present, it is believed that a large tonnage of copper-cobalt ore exists in the district which should permit mining to be continued over a long period. The report contains brief descriptions of all the accessible workings in the district, of which the most important are Calera, Brown Bear, Uncle Sam, and Hawkeye mines. In the Calera adit, about 1,700 feet of the mineralized zone, ranging in width from 3 feet to 40 feet and averaging about 15 feet; have been explored (August 1946); the zone lies on a wide northwest-striking shear zone dipping moderately ( 60° ±) northeast. The Brown Bear adit is in a wide, mineralized, north-south shear zone in which are higher-grade pods plunging 25° to 35° north. The Uncle Sam mine explores a relatively narrow north-south shear zone in which are two or three north-plunging ore shoots. The Hawkeye mine is in a broad zone of mineralized schist in which are several north-plunging lenses of ore.