Mineral resources of the Lincoln Back Country Area, Powell and Lewis and Clark Counties, Montana

A mineral survey of the Lincoln Back Country Area was made by the U.S. Geological Survey and the U.S. Bureau of Mines during the summer of 1970; the U.S. Geological Survey made an aeromagnetic survey in 1966 and a gravity survey in 1970. The area, about 375 square miles, lies about 45 miles northwest of Helena and about 50 miles northeast of Missoula, Montana. Sedimentary rocks of Precambrian (Belt Supergroup), Cambrian, Devonian, and Mississippian ages crop out in the area. The Precambrian rocks have been intruded by diorite sills of Precambrian age, sills of uncertain age of monzonite to andesite composition, and aplite dikes of probable Cretaceous age. ln addition there are small exposures of tuff of Tertiary age. The rocks, except for the tuff, have been broken by thrust and normal faults and are folded in places.