Mineral resources of the Rincon wilderness study area, Pima County, Arizona

Abstract

The Rincon wilderness study area comprises about 254 km 2 (98 mi 2 ) of the Rincon Mountains 15-30 km (10-20 mi) east of Tucson, Arizona. The area lies within the Coronado National Forest and forms a belt around the north, east, and south sides of the Saguaro National Monument (fig. 1). A mineral resource survey was made of the area in 1977 by the U.S. Geological Survey and the U.S. Bureau of Mines. This study indicates that the potential for finding metallic or nonmetallic mineral deposits, petroleum or coal or geothermal energy in the study area is low. This appraisal is based on geologic and geochemical investigations and on examination of all mineralized prospects. A geologic map was made of the area and its immediate surrounding. Chemical and spectrographic analyses were made of 1.30 stream-sediment samples and of 143 rock samples. The locations of mining claims were compiled and samples from most of the claims were assayed. There has been no recorded mineral production from within the study area. The geology of the Rincon Mountains is structurally complex. The core of the Rincon Mountains, lying mainly within the Saguaro National Monument, is underlain by igneous and metamorphic rocks chiefly of Precambrian and Tertiary ages. The study area lies on the flanks of the mountains in a terrain of sedimentary, metamorphic, and igneous rocks mainly of Precambrian through mid-Tertiary age. The rocks of this area are cut by many faults, with low-angle or bedding-parallel faults abundant. the distribution of these faulted rocks around the flanks of the Rincon Mountains is the result of a doming in the core area, the sliding of rock masses from the high part of the dome down its flanks, and to the effects of erosion. The potential for economic metallic mineral deposits is considered low because the known areas of anomalous concentrations of metals are small and the concentration of metals weak and erratic. The surface signs of mineralization are largely restricted to 4 locales in which some prospects exist (fig. 1): (1) east of Colossal Cave, southwest or the Rincon Mountains, (2) north of Happy Valley, east of the mountains, (3) between Roble and Youtcy Canyons, northeast of the mountains, and (4) near Italian Trap north of the mountains. Very little primary sulfide mineralization is present and signs of alteration are restricted to narrow zones along faults and fractures. The geochemical anomalies are weak. The sites at which anomalous values of copper, molybdenum, or silver were obtained are mainly controlled by faults. Nonmetallic mineral resources occur in deposits that are too small and too remote to be economically attractive. Sand and gravel deposits in the main drainages are similar to larger deposits that occur closer to nearby highways and cities. Limestone and marble present in some of the metamorphic rocks are too impure and too broken for use as dimension stone. Use of this marble as decorative rock is limited by remote markets. Limestone suitable for making cement is not likely to be found in large quantities within the Rincon wilderness study area. Closely spaced fractures in the granitic rocks makes them of little value for building stone.