gems, olivine, peridot, San Carlos, Arizona, Apache

At the request of the Council of the San Carlos Apache Tribe the U. S. Geological Survey entered into a cooperative agreement calling for a brief reconnaissance study to determine, as far as practical, the mineral potential of the San Carlos Indian Reservation. Five months of field work was done during the winter and spring of 1952-53. About 30 percent of the reservation is covered by alluvial deposits of late Tertiary, Pleistocene, and Recent age, and another 60 percent is covered by volcanic rocks of Tertiary and perhaps Pleistocene age. These rocks are younger than the major epochs of metallization in southeastern Arizona. The remainder of the area is underlain by pre-Cambrian and Paleozoic sedimentary rocks, pre-Cambrian granite, and pre-Devonian diabase. The reservation is located in two physiographic provinces: the northern part is in the Colorado Plateaus province and the southern part is in the Basin and Range province. The boundary between the provinces generally follows the southern rim of the Natanes Plateau. The mountain ranges south of the Natanes Plateau are, in general, tilted fault-block mountains composed largely of southwestward-dipping pre-Cambrian and Paleozoic sedimentary rocks. The northern part of the reservation is dominated by the Natanes Plateau. West of Blue River, the plateau is composed of generally flat-lying pre-Cambrian and Paleozoic rocks overlain by Tertiary volcanic flows. East of Blue River, the older rocks disappear under the Tertiary volcanic rock and reappear at one place only between Arsenic Tubs and Park Creek.