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.

Abstract A late Pliocene and Pleistocene hot-spring deposit consisting of sinter terraces underlain by veins is the site of the McLaughlin Mine in northern California. The deposit is localized along the contact between hanging wall mudstone of the Upper Jurassic Knoxville Formation (basal formation of the Upper Jurassic and Cretaceous Great Valley sequence) and serpentinized ultramafic and mafic rocks of the Middle Jurassic Coast Range ophiolite in the footwall. The Stony Creek fault separates the two units, and dips moderately northeasterly. Pliocene basaltic andesite and volcaniclastic rocks unconformably overlie the sedimentary and ophiolitic rocks in the mine area. These rocks intruded as sills to shallow depths or erupted after phreatomagmatic explosions from small volcanic centers. Magmatism was also localized along the Stony Creek fault. The volcanic rocks are part of the older phase of the Clear Lake Volcanics, which formed in a Pliocene to Holocene volcanic field that lay mostly to the west and northwest. The deposit consists of two epithermal orebodies, the north and south orebodies, which are connected by a narrow 2one of mineralization. The oldest hydrothermal event recognized in the deposit was adularization of the hanging wall mudstone and silicification of the footwall serpentinite. Alteration formed a roughly tabular body that is discontinuous along the Stony Creek fault. Basaltic andesite sills, plugs, and flows are propylitized and locally argillized. Precious metals are present in opal, chalcedony, and quartz veins that cut brittle rocks, including altered rocks, basaltic andesites, and tabular lithons present in the footwall serpentinite. The south

Abstract The discovery of gold mineralization at the Manhattan mercury mine, Napa and Yolo counties, California, in 1978 was the result of applying a new exploration ore deposit model in a region previously unprospected for gold. This discovery, the McLaughlin gold mine, owned and operated by the Homestake Mining Company, is a classic mercury-bearing, epithermal hot springs gold deposit currently producing approximately 7,776,000 g (250,000 troy oz) of gold annually. The exploration program that led to this discovery was based on a genetic model for such deposits. The model was developed using basic knowledge of ore-forming processes combined with careful field observations made during examination of many known precious metal districts in the western United States. Theoretically, hot spring systems could host gold deposits, and indeed, gold in these environments had been recognized for years. However, at the time of the McLaughlin discovery, it was not known to occur in sufficient quantities to inspire geologists to explore hot spring systems specifically for gold. Field observations recognized the presence of hot spring deposits and mercury mineralization associated with precious metal districts, as at Bodie, California, and this association prompted the development of a hypothetical model. Subsequent exploration activities were directed toward the examination of mercury mines and prospects as potential targets for gold deposits. The model envisioned hot spring deposits (sinter) containing mercury mineralization at the surface, with anomalous gold values grading downward into significant ore-grade gold values. The initial examination of the Manhattan mercury mine recognized the features represented in the hypothetical model, and sampling detected anomalous to ore-grade gold mineralization at the surface. An aggressive exploration program resulted, with advancement of the project through Homestake’s staged exploration program to full production. This program consisted of surface mapping and sampling followed by three-dimensional testing in the target exploration and deposit development stages of exploration. Exploration activities during the four and one-half year period prior to transfer of the deposit to Homestake’s operations group included drilling of 409 holes, underground drilling and crosscutting, engineering studies, metallurgical test work, environmental base line studies, governmental and public relations programs, and economic analyses to produce a conceptual feasibility study justifying the project for transfer in June 1982 to the operations group for design, construction, and ultimately, production. The steps that led to the McLaughlin discovery are outlined below, as are the results of the various phases of exploration.

The basal Great Valley sequence in Napa and southern Lake Counties, California, is a mappable chaotic unit composed largely of ophiolitic debris. Serpentinite flows and breccias, mafic breccias and associated finer-grained clastic rocks, and blocks of extrusive greenstone, mafic breccia, chert, bedded and unbedded clastic sedimentary rocks, phyllites, actinolitic greenschists, and hornblende amphibolites are mixed with Great Valley sequence mudstone and serpentinous mudstone. The chaotic unit extends along strike for at least 50 km. Cross-sections indicate that it extends for at least 20 km across strike and is up to 1 km thick. It is involved in complex folds caused by imbricate thrust faulting. The unit lies directly above the serpentinite that represents the Coast Range Ophiolite within the study area and below the well-bedded Great Valley sequence of Upper Jurassic and Cretaceous age. Its lower contact is enigmatic but is probably depositional; the upper contact is sheared and gradational. Locally the unit represents the entire Tithonian Stage. Ophiolitic detritus in the lower Great Valley sequence is also found elsewhere in the Northern California Coast Ranges—near the Geysers, in Rice Valley, near Wilbur Springs, along the Bartlett Springs Road near Walker Ridge, near Cooks Springs, and at Crowfoot Point west of Paskenta. Other accumulations of ophiolitic debris are inter-layered in the Great Valley sequence at various stratigraphic levels in and near the study area. This detritus takes four forms, which may be mixed together: (1) sedimentary serpentinite debris flows; (2) mafic breccias; (3) basaltic sandstones; and (4) polymict, polymorphous chaotic units with blocks-in-matrix texture, like the rocks in Napa County described here. Widespread detrital textures and the common occurrence of spaced, rather than penetrative, shear foliation in its matrix demonstrate that the chaotic unit in Napa County is not a tectonic melange, and I interpret it to be an amalgam of olistostromes. These ophiolitic olistostromes are a facies distinct from the overlying turbidites. Thus, the basal Great Valley sequence in this area is composed of two different rock types: very proximal ophiolitic debris flows, and substantially more distal subsea-fan rocks derived from a volcanic arc. Ophiolites may form at mid-ocean ridges, in back-arc or forearc basins, or in island arcs. Ophiolitic detritus may be eroded and deposited on ophiolitic basement in any environment in which the oceanic crust is deformed. The geology of surrounding terranes and the petrologic features of the ophiolitic basement below the Great Valley sequence suggest that the basement was formed in a back-arc basin. The stratigraphy of the chaotic rocks that overlie the basement in Napa County suggests that they were deposited on deeply eroded basement in a technically active forearc basin. Large volumes of rock stuffed under the hanging-wall slab after the onset of subduction may have uplifted the forearc basin and subjected its basement to erosion. A wave of uplift may have passed across the basin, so that debris shed from eroding oceanic basement was deposited directly on freshly exposed harzburgite tectonite. Some blocks may have been carried completely across the forearc basin and into the trench, and incorporated into the Franciscan melange wedge, which is also rich in ophiolitic blocks. The change from back-arc to forearc basin was probably caused by collisional tectonics and the establishment of a new subduction zone off the western coast of California during the Late Jurassic Nevadan orogeny. Stratigraphic relationships in and above the Coast Range Ophiolite are unusual through much of the Northern Coast Ranges. Nearly complete ophiolites are the exception rather than the rule, and in many areas only serpentinite is present. In some areas, ophiolitic debris different from that described here overlies the serpentinite. In other areas, arc-derived submarine fan rocks of the Great Valley sequence directly overlie serpentinized harzburgite tectonite. The relationships described here suggest that many of these contacts are not tectonic, and that the Coast Range Ophiolite does not owe its fragmentary nature to tectonic dismemberment. Rather, it is likely that the ophiolitic basement below the Great Valley sequence was deeply eroded during Mesozoic time. Many of the contacts throughout the Coast Ranges along which sedimentary rocks overlie serpentinite—which must represent deep layers of the oceanic crust or the upper mantle—are in the main nonconformities.