Mineral resources of the Pinnacles Wilderness Contiguous Wilderness Study Area, Monterey and San Benito counties, California

Abstract

In 1984 and 1985, at request of Bureau of Land Management, U.S. Geological Survey and U.S. Bureau of Mines conducted surveys to assess mineral resources (known) and mineral resource potential (undiscovered) of 2,ZOO acres of 5,838acre Pinnacles Wilderness Contiguous Wilderness Study Area (CA-040-303) in southern Gabilan Range near Hollister, Calif.. In this report, area studied is referred to as the wilderness study area, or simply the study No active mines, prospects, or identified mineral or energy resources were identified in study area. Two areas were determined to have low potential for mineral resources. One of these areas has low potential for gold, and silver resources. The other has low potential for diatomite and oil and gas resources. Character and Setting The study area consists of five parcels with a total area of 2,200 acres located about 30 mi south of Hollister, Calif, (fig. 1). These parcels are adjacent to and contiguous with Pinnacles National Monument. The study area is located in southern Gabilan Range, part of Coast Ranges physiographic province. The oldest rocks in study area are Cretaceous (144-66 million years before present, Ma) granitic rocks, which are cut by a graben (downdropped fault block) that preserves Miocene (24-5 Ma) volcanic rocks (fig. 2) (see appendix for geologic time chart). Overlying volcanics are Miocene marine and continental sedimentary rocks, including units formed by erosion of older igneous rocks. Although there has been some mining activity in area in past, there is no present mining or exploration activity. Identified Resources and Mineral Resource Potential There are no active mines, prospects, or identified mineral or energy resources in study area. Two areas have mineral or energy resource potential (fig. 2). That part of study area that overlies source area of volcanic rocks, represented by a swarm of rhyolitic dikes, has low potential for gold and silver in hot-spring deposits. Those areas underlain by Miocene sedimentary rocks have low potential for diatomite in shale deposits and oil and gas resources.