Abstract 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.

Abstract Epithermal precious-metal and mercury deposits are present in the Sonoma and Clear Lake volcanic fields of central California and several hot springs in the Clear Lake volcanic field are presently depositing mercury and gold. The deposits and hot springs are associated with late Miocene to Holocene volcanic centers developed above a zone of thin crust and hot asthenosphere termed a slab window (Dickinson and Snyder, 1979, Benz and others, 1992) as the end of Pacific plate subduction was marked by the passage of the Mendocino triple junction along the California coast. Mercury deposition is actively occurring at the Sulphur Bank mercury mine, but no precious metals are present there because the geothermal system is vapor-dominated. In the water dominated geothermal systems at Wilbur Springs (Peters, 1990, Donnelly and others, 1993) and springs near the Cherry Hill gold deposit, both cinnabar and gold are being deposited (Pearcy and Petersen, 1990). Transport of mercury and gold is in a fluid which also contains high concentrations of petroleum and associated methane and CO2 derived from thermal degradation of organic matter in sedimentary rocks (Peabody, 1989). Chemical and isotopic analysis of oxygen and deuterium of the hot springs indicate that three types of fluid are present: moderate chloride, isotopically heavy, evolved formation fluid equilibrated with oceanic sedimentary rocks; evolved meteoric water; and isotopically light meteoric water (Peters, 1990,1991, Sherlock and Jowett, 1992, and Donnelly-Nolan and others, 1993). High concentrations of Hg, As, Sb, Au, and Ag occur in precipitates from hot springs composed dominantly of the isotopically heavy fluid, but not in the moderate-temperature, oxidized springs that are mixtures of these two fluid types (Peters, 1990, Donnelly-Nolan and others, 1993). The McLaughlin gold deposit (initial reserves of 2.9 million oz of gold) is economically the most important deposit in the Clear Lake and Sonoma volcanic fields. This precious metal-mercury hydrothermal system developed within and adjacent to andesitic vents and dikes emplaced along the Stony Creek fault zone (Lehrman, 1986). Gold occurs in opal, chalcedony, and quartz veins, and the highest gold values typically occur in amber to brown opal containing petroleum. Gold occurs in several sites within the petroleum-bearing opal: as a filling of 2050 micron- diameter oval voids representing large fluid inclusions; as 2-4 micron size crystals that coalesce to form dendrites of gold along primary vein banding; and in syneresis cracks which cut the vein banding. Oxide phases of Ga, In, Sn, and Ni are present within the petroleum-bearing opal. The isotopically heavy McLaughlin ore fluid plots in the field of andesitic magma volatiles (Hedenquist and Aoki, 1990, Giggenbach, 1987) and evolved formation waters (Sherlock and Jowett, 1992) suggesting that these two components are present Andesitic vents and dikes at the McLaughlin gold deposit suggest that a larger intrusion underlies the area and provided the heat source for the hydrothermal system. Andesitic vents along the Stony Creek fault provided a conduit for volatiles degassing from the intrusion to become entrained within the hydrothermal fluid composed of gas-oil-field water derived from the Great Valley sequence. The McLaughlin gold deposit reflects the complex interaction of three types of fluid each transporting a different elemental suite: evolved gas-oil field formation water transporting petroleum, Ga, In, Sn, Ni, and Hg; andesitic magmatic fluid transporting Au, Ag, Hg, Sb, and As; and near-surface meteoric water. Prospective areas for precious metal hot-spring deposits occur in the volcanic-structural environment above the thin crust and hot asthenosphere within the slab window in the Coast Ranges and parts of the Great Valley sequence where blind thrusts and associated faults are intruded by Pliocene to Holocene intrusive rocks. Mercury deposits with little or no gold content form along major structures from gas-oil field fluids with little or no magmatic component in the fluid and contain petroleum, Ni, Ga, In, Sn, and other transition elements. Epithermal gold deposits contain a significant magmatic component characterized by Au, Ag, As, Sb, and Hg as well as a gas-oil- field fluid component characterized by petroleum and transition metals. Both deposit types may occur along the same structures.

Abstract This tour begins at the Red Lion Hotel in Santa Rosa and ends in Healdsburg. Load into vans at the Red Lion Hotel parking lot. The road log begins at the Mark West-River Road exit from Highway 101, about 10.5 to the north of the Red Lion Hotel. The route for the trip is shown in Figure 1.

Abstract The Calistoga Mining District, is one of three districts in the northern California Coast Ranges, where epithermal precious metals deposits have been economically mined. The district produced over 1.5 million ounces of silver (with lesser amounts of gold, copper and lead) intermittently over a 76 year period. The Palisade and Silverado mines were the two producers of the district. Precious metals enrichment is associated with northeast-striking, en echelon quartz + chalcedony + adularia vein systems, hosted by flows and pyroclastic rocks of die Tertiary Sonoma Volcanics. Basement rocks are part of the Mesozoic and early Cenozoic Franciscan Complex, composed largely of tectonized marine sediments and dismembered ophiolite. Silver dominates gold by 74:1, and occurs in various sulfides, sulfosalts and selenides. Gold occurs in its native state, but is rarely macroscopic. In addition, this system is highly enriched in Cu, Sb, Zn, Pb, As and Ba, and contains anomalous concentrations of Se, Cd, Hg, Te, Ga, Tl, Mo and Bi. Veins formed at +200 m depths, along dilational segments of normeast-striking (sinistral) conjugate Reidel shears, associated with a zone of San Andreas-style dextral wrench faulting. The zone of alteration and vein propagation is restricted to a corridor 1.5 km wide by 14.5 km long. This corridor appears to be structurally related to a local dilatiuaal jog in a 305°-striking dextral shear zone. The orientation of the optimal plane of extension was approximately 196° (right hand rule), dipping 74°. Because the volcanic pile has deformed as a homogeneous medium, a comparison of 595 vein and shear joint attitudes, taken in the Silverado and Palisade vein systems, provides a means of determining finite strain ellipsoid axes for vein formation. Based on these data, the orientation of Z is calculated to be 202°, plunging 18° and X at 106°, plunging 16°. Because of scatter in the data, the above axes are assigned error limits of ±15°. These values do not take into account the possibility of post-mineral rotation. Fluid inclusion geothermometry indicates that this was a boiling system, with temperatures averaging 212° and 249° for the Palisade and Silverado veins, respectively. The mineralizing fluids were low salinity (-1.0 wt. % NaCl equivalent), NaCl-dominated, and possibly enriched in CO2. They are believed to be evolved meteoric waters, chemically and isotopically similar to those of the Geysers and McLaughlin systems. Metals were presumably transported as bisulfide and (to a lesser extent) chloride complexes. Precipitation was triggered by CO2 and H2S partitioning in response to fault-induced increases in vertical permeability.