Mineral resources of the North Fork Owyhee River Wilderness Study Area, Owyhee County, Idaho

Abstract

The North Fork Owyhee River Wilderness Study Area (ID-016-040), within the southern Owyhee Mountains in southwest Idaho, encompasses 49,470 acres, of which the U.S. Geological Survey and the U.S. Bureau of Mines were asked to 39,745 acres. Hereafter, the terms study and refer only to the smaller acreage. Field work for this report was carried out between 1984 and 1985. No mines or prospects are located in the area, although a few borrow pits exist just outside its southern boundary. The only identified resource is blocky basalt suitable for use as building or decorative stone. The area has a low resource potential for gold, silver, zinc, and uranium. The potential for petroleum and natural gas and geothermal energy is unknown. Character and Setting The North Fork Owyhee River Wilderness Study Area is located in the southern Owyhee Mountains in southwest Idaho, about 75 mi southwest of Boise (fig. 1). The area is situated along a stream-dissected plateau with canyons as deep as 400 ft. The area is underlain by a thick pile of Miocene (about 5 to 24 million years before present) volcanic flows and minor interbedded sedimentary rocks. A thick rhyolite ashflow tuff sheet is exposed throughout most of the area; the tuff is overlain in the southern part of the area by another rhyolite ash-flow tuff and a younger sequence of thin basalt flows and interbedded sedimentary rocks. The older ash-flow tuff fills a collapse caldera formed due to the emplacement of the tuff. Several small-displacement normal faults of various trends cut the rocks. The rhyolite ash-flow tuffs are also intensely fractured locally. There are no known mines, prospects, claims, or mineral leases in the area or its immediate vicinity. Several small borrow pits for road construction material exist along the southern boundary (fig. 2). Identified Resources The only identified resource in the area is blocky basalt suitable for use as building or decorative stone. The thin, blocky slabs of basalt occur over an area of several square miles in the southern part of the area. Mineral Resource Potential Gold, silver, zinc, and uranium have minor resource potential in the wilderness area (fig. 2). The potential for geothermal energy and petroleum and natural gas is unknown. Two mining districts occur in the region near the wilderness area: the South Mountain (silver-leadzinc) and the Silver City (gold-silver) districts. Mineralization in the former takes the form of silverlead replacement veins and silver-lead-zinc skarn within regionally metamorphosed rocks. The Silver City district, in particular the Delamar mine area, is characterized by epithermal mineralization along Miocene normal faults and fractures within a contemporaneous, inferred silicic ring complex. Anomalous concentrations of antimony, arsenic, copper, mercury, selenium, and zinc are associated with the gold-silver deposits. Relatively high concentrations of arsenic, molybdenum, silver, and zinc have been detected in samples collected in the wilderness area. A similar geologic environment (caldera-related silicic volcanic rocks and contemporaneous faults and fractures) and trace-element anomalies similar to those in the Silver City district suggest that