Mineral Resource Assessment of Selected Areas in Clark and Nye Counties, Nevada, Edited by Steve Ludington U.S. Geological Survey Scientific Investigations Report 2006-5197 Prepared in cooperation with the Nevada Bureau of Mines and Geology and the University of Nevada, Las Vegas Chapter J. Mineral Resource Potential of the River Mountains Area of Critical Environmental Concern, Clark County, Nevada By Steve Ludington, Stephen B. Castor, Brett T. McLaurin, and Kathryn S. Flynn Summary and Conclusions The River Mountains Area of Critical Environmental Concern (ACEC) contains occurrences of altered and mineral- ized volcanic rocks that are similar to epithermal precious- metal deposits. An area in the southern part of the ACEC, near Railroad Pass and Boulder City, Nevada, has moderate potential for the occurrence of undiscovered epithermal pre- cious-metal deposits. The results of historical exploration and geochemical studies made for this report indicate that the exposed occurrences do not have high enough precious-metal concentrations to encourage development. Until the 1960s, there was extensive mining of manga- nese at the north end of the ACEC. These deposits are not now viable, and there is only low potential for the occurrence of undiscovered sedimentary manganese deposits. Potentially commercial perlite occurs in the northeast part of the ACEC. There are current mining claims staked for perlite, and a small area has high potential for the occurrence of perlite deposits. There is no potential for the occurrence of other deposits of locatable or leasable minerals. The River Mountains ACEC has areas of high, moder- ate, and low potential for the occurrence of crushed-stone aggregate deposits, and areas of high and low potential for the occurrence of sand and gravel aggregate deposits. Introduction This report was prepared for the U.S. Bureau of Land Management (BLM) to provide information for land plan- ning and management, and, specifically, to determine min- eral resource potential in accordance with regulations at 43 CFR 2310, which governs the withdrawal of public lands. The Clark County Conservation of Public Land and Natu- ral Resources Act of 2002 temporarily withdraws the lands described herein from mineral entry, pending final approval of an application for permanent withdrawal by the BLM. This report provides information about mineral resource potential of these lands. The River Mountains ACEC was studied in the field to confirm descriptions of the geology that were gleaned from the scientific literature. Numerous samples were collected and analyzed. Definitions of mineral resource potential and certainty levels are given in appendix 1, and are similar to those out- lined by Goudarzi (1984). Lands Involved The River Mountains ACEC is southeast of Las Vegas, directly east of the city of Henderson and northwest of Boul- der City. It is reached from city streets and paved roads that surround it, as well as by hiking and bicycle trails that enter the area from the Bootleg Canyon road, at the southeast corner of the ACEC. A loop trail, under development, will skirt the south and west sides of the ACEC (http://www.rivermountain- strail.com). A legal description of these lands is included in appendix 2. Physiographic Description The River Mountains consist of a series of low mountains whose ridgelines trend north-northwest, parallel to normal faults in the area. Elevations range from about 750 m at the foot of the mountains to a little more than 1,100 m on the highest peaks. Geologic Setting The River Mountains are in the central Basin and Range Physiographic Province, generally characterized by north- trending mountain ranges and intervening valleys, and lie between the Sevier orogenic belt and the Colorado Plateaus. The area underwent large-magnitude extension in the middle to late Miocene, and it lies directly north of the Colorado River extensional corridor (Deubendorfer and others, 1998). The rocks exposed in the River Mountains form part of an