The Geysers-Clear Lake area, located about 150 km north of San Francisco, is mainly underlain by Jurassic and Cretaceous rocks of the Franciscan assemblage (composed dominantly of a melange of gray wacke and argillite with lesser amounts of altered mafic igneous rocks, radiolarian chert, serpentine, limestone, and very minor blocks of blueschist, eclogite, and amphibolite), the Great Valley sequence (mainly siltstone and argillite in the Clear Lake region), and associated ophiolitic rocks that accumulated in marine settings, and that later were deformed at an obliquely convergent subduction margin (McLaughlin, 1977,1981; McLaughlin and Ohlin, 1984; Thompson, 1989). Later strike-slip movement on northward-propagating faults of the San Andreas transform system, some of which pass through The Geysers-Clear Lake area, cut and offset the thrust sheets, and shut down subduction at the latitude of Clear Lake about 3 Ma (Atwater, 1970; Blake and others, 1978; Dickinson and Snyder, 1979). Simultaneously, behind a northward-migrating triple junction, there also has been a northward migration of volcanic centers in the Coast Ranges of California above a window of anomalously shallow asthenosphere, with the most recent volcanic activity focused in the Clear Lake region (Dickinson and Snyder, 1979; Hearn et al., 1981; McLaughlin, 1981; Fox et al., 1985).
Figures & Tables
Active Geothermal Systems and Gold-Mercury Deposits in the Sonoma-Clear Lake Volcanic Fields, California
Since the discovery of gold and silver in the northern part of the Napa Valley in 1858, ore deposits and geothermal systems have drawn a variety of geologists to study one of the few areas in the United States where hot springs are actively depositing gold and mercury. The geothermal systems and very young precious-metal and mercury deposits occur in two adjacent volcanic fields, the older Sonoma volcanic field and the younger Clear Lake volcanic field. In the eastern foothills of the Napa Valley, precious metal deposits hosted by the Sonoma volcanic field produced only a small amount of gold and silver. The fertile soil and good drainage of the volcanic rocks in this area gave way to vineyards and wineries and the mines were closed and abandoned. The younger Clear Lake volcanic field has gone through several cycles of mineral and geothermal development The hot springs in the volcanic field were developed initially for their supposed medicinal benefits although many of the springs contained toxic levels of mercury. Mercury and sulfur were mined from several of the deposits present throughout the volcanic field and spectacular samples containing plumes of native gold within cobbles of cinnabar were discovered in the Sulphur Creek District. In spite of the known association of gold and mercury, mercury mining dominated the mineral development within the volcanic field until the mid-1940's. Development of The Geysers for geothermal power in 1960 began a new phase of economic development, and geothermal power production has continued to be important in the western part of the volcanic field. The most recent mineral development was the discovery of the McLaughlin gold deposit in 1978 at the site of the old Manhattan Mercury Mine. Since that time exploration has continued for auditional epithermal precious-metal deposits but without success.
This guidebook provides an overview of the geothermal systems and ore deposits in the Sonoma and Clear Lake volcanic fields. Several research papers in this guidebook provide important new concepts and data on the ore deposits, geothermal systems, and volcanic rocks within the two volcanic fields from the perspective of geologists, geochemists, geophysicists, and petrologists. In addition, a paper by Fraser Goff and Cathy Janik provides the ftrst comprehensive field guide to the geothermal features within the Clear Lake volcanic field. This field conference and guidebook should provide the basis for new research and a better understanding of the processes that have contributed to the formation of the ore deposits and geothermal systems in the Clear Lake and Sonoma volcanic fields.