Mineral resources of the Rincon Wilderness Study Area, Harney County, Oregon

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At the request of the U.S. Bureau of Land Management, the U.S. Geological Survey and the U.S. Bureau of Mines conducted field studies of 20,500 acres of the Rincon Wilderness Study Area. The Rincon Wilderness Study Area (OR-002-082) is located between Callow Valley and the Pueblo Mountains in southeastern Oregon. In this report, the area studied is referred to as the "wilderness study area," or simply the "study area." Field work was conducted by the U.S. Geological Survey during 1986 and 1987, and by the U.S. Bureau of Mines during 1986, to evaluate the identified mineral resources (known) and the mineral resource potential (undiscovered) of the study area. No mineral resources were identified in the study area. However, the study indicates moderate potential for silver resources in a rhyolite ash-flow tuff exposed near the central part of the study area and high potential for sand and gravel resources in lake shoreline deposits along the northwest boundary of the study area. The entire study area has low potential for geothermal resources and no potential for oil and gas. Character and Setting The Rincon Wilderness Study Area is located along Callow Rim approximately 90 mi south of Burns, Oregon, and 10 mi west of Fields, Oregon (fig. 1). Callow Rim is a fault scarp that rises 1,000 to 1,900 ft above Callow Valley and forms the entire lenglh of the impressive 60-miManuscript approved for publication, June 22, 1988. long easl valley wall. A large displacement faull at Ihe base of Ihe escarpment is Ihe dominanl geologic structure in the region and forms Ihe west margin of Ihe 30by 90mi, north-trending Sleens-Pueblo Mountains fault block. The area east of Ihe rim is characterized by a gently southwesl-tilied dip slope cul by several west-flowing creeks (fig. 1). Middle Miocene-age basalt flows form the 1,600-flhigh Callow Rim escarpment and are Ihe oldesl rocks exposed in Ihe sludy area. The middle Miocene ranges from 11.2 lo 16.6 million years before presenl, or Ma; see appendixes for geologic lime chart. A sequence of andesile flows, luffaceous sedimenlary rocks, and ash-flow luffs overlies Ihe basall flows. The straligraphic section is capped by younger andesile flows of lale Miocene and Pliocene age (Walker and Repenning, 1965). The western part of the study area extends into Callow Valley, a broad, irregular-shaped graben. Pleistocene shoreline deposits and Holocene dunes are exposed in the valley. Identified Resources No mines, claims, or prospects were found, and no mineral or energy resources were identified within or adjacent to the study area. Mineral Resource Potential A rhyolite ash-flow tuff near the central part of the study area (fig. 2) has moderate potential for silver resources. One unaltered sample of this tuff contains Mineral Resources of the Rincon Wilderness Study Area, Harney County, Oregon E1