Mineral resources of the proposed additions to the Scapegoat Wilderness, Powell and Lewis and Clark Counties, Montana

Abstract

The proposed additions to the Scapegoat Wilderness include the Silver King-Falls Creek and Arrastra-Stonewall units. They cover about 84 square miles (218 km 2 ) south and southeast of the wilderness in the Helena and Lewis and Clark National Forests, northwest Montana. Their mineral resources were evaluated as part of a study by the U.S. Forest Service to determine their suitability for addition to the wilderness. The additions have copper resources, which may also contain some silver, some lead, and possibly zinc and gold. Stone and sand and gravel deposits are present, but deposits of equal or better quality are available closer to major markets. The Silver King-Falls Creek addition may have a potential for the discovery of oil and gas, but detailed studies, including seismic surveys and drilling, would be required to adequately determine the potential. The additions have a low potential for geothermal resources. The geology is structurally complex and consists of sedimentary rocks that range from Precambrian Y to Holocene in age. Precambrian rocks are locally intruded by sills of intermediate composition. The mineral resource evaluation is based ongpological, geochemical, economic, and geophysical studies. A total of 480 rock samples and 120 stream-sediment samples were collected during geologic mapping; 86 lode samples and two placer samples were collected during the examination of mines and prospects. Results from these samples delineate areas that are anomalously high in base metals, precious metals, and mercury. The geophysical investigations indicate small magnetic and gravity anomalies, which mostly appear to be unrelated to mineralized areas. More than 60 recorded mining claims have been located in theadditions. Except for a small amount of copper shipped from StonewallMountain, no record of mineral production exists for the additions.Gold has been mined from placer deposits near the southern boundary ofthe Arrastra-Stonewall unit, and small amounts of copper have beenextracted from deposits near the northern boundary. A copper deposit in Cotter basin, adjacent to the northern boundary of the Arrastra-Stonewall unit, was being explored and developed by industry during the present investigations, and minor production has resulted. This deposit, a carbonate vein system, is estimated to contain 1 million tons (910,000 t) of paramarginal resource averaging0.85 percent copper and 0.2 ounce byproduct silver per ton (6.9 g/t) with selected areas of higher grade that may be presently economic. Similar copper deposits may occur in this carbonate vein system in the northern and eastern parts of the Arrastra-Stonewall unit. Smaller and lower grade copper deposits occur near the mouth of Byrnes Creek and near the head of Alice Creek, in the Silver King-Falls Creek unit.