Mineral resources of the Bristol/Granite Mountains Wilderness Study Area, San Bernardino County, California

Abstract

At request of Bureau of Land Management, 45,000 acres of Bristol/Granite Wilderness Study Area (CDCA-256) were studied by U. S. Geological Survey and U. S. Bureau of Mines. In this report, area studied is referred to as the wilderness study area, or simply the study The wilderness study area is located 25 mi southeast of Baker, California. Two iron deposits total about 110,000 tons at 47 percent iron but are not large enough to constitute resources. The potential for undiscovered iron resources is low. There is low resource potential for undiscovered tungsten, silver, gold, lead, zinc, limestone marble, perlite, and oil and gas. Sand and gravel resources are present but more accessible supplies are abundantly available outside of study area. Character and Setting The Bristol/Granite Wilderness Study Area (CDCA-256) lies in Mojave Desert, near Baker, Calif. The study area occupies approximately 45,000 acres and includes rugged Granite Mountains and flanking lowlands on north and west sides of range (fig. 1). The Granite Mountains are composed of Mesozoic (63 to 240 million years before present (Ma); see Geologic Time Chart, appendix), plutonic, and metaplutonic rocks, with small pendants of calcareous Paleozoic (240 to 570 Ma) strata (fig. 2). Tertiary (2 to 63 Ma) igneous dikes, volcanic rocks, coarse sedimentary debris, and Quaternary (2 Ma to present) alluvium are present in study area around flanks of range. There are no active mines in study area. Mineral Resource Potential Skarns (formed where calcium-rich rocks have been metamorphosed) are favorable hosts for iron and tungsten deposits. Small deposits of magnetitehematite contain about 110,000 tons of ore averaging 47 percent iron at Iron Victory prospect and 10 tons at 43 percent iron at Comanehe mine (fig. 2). Neither deposit is considered large enough to constitute a resource. The potential for undiscovered iron resources in study area is low. Streamsediment samples contain tungsten, molybdenum, and mineral scheelite (indicative of tungsten mineral occurrences in skarn rocks). The study area has a low potential for undiscovered tungsten resources. An occurrence of silver at Silver Lode mine adjacent to wilderness study area is associated with sulfides and hydrothermally altered granite along fracture zones within a Cretaceous (about 75 Ma) pluton. Altered granite of same pluton exposed within study area adjacent to Silver Lode mine has low potential for undiscovered hydrothermal silver and associated lead, zinc, and gold resources. The study area has low mineral potential for undiscovered epithermal gold, limestone marble, perlite, and oil and gas resources. Sand and gravel are