Abstract The Franciscan complex of California provides the ideal field laboratory to examine the rock record of subduction. This field trip guide describes a two-day field trip of the 2013 Geological Society of America Cordilleran Section Meeting. The field stops include a stop along the Panoche Road in the southern Diablo Range, and four in the San Francisco Bay region: one on the southern margin of the California State University East Bay campus in the Hayward Hills; one, with additional optional stops, at El Cerrito quarry north of Berkeley; one at Ring Mountain on Tiburon Peninsula; and the final stop at Rodeo Cove of the Marin Headlands. The geology seen at these stops provides insight into subduction initiation processes, subduction accretion and subduction erosion, accretion of ocean plate stratigraphy, mélange evolution, multiple burial-exposure cycles, exhumation of high-pressure metamorphic rocks, localization of subduction megathrust slip, large-scale subduction complex architecture, and spreading ridge deformation. The field trip stops will captivate visitors with their scenic beauty as well as their interesting geology.

Abstract Manifestations of a major thermal anomaly in the Geysers-Clear Lake area of northern California include the late Pliocene to Holocene Clear Lake Volcanics, The Geysers geothermal field, abundant thermal springs, and epithermal mercury and gold mineralization. The epithermal mineralization and thermal springs typically occur along high-angle faults within the broad San Andreas transform fault system that forms the western boundary of the North American plate in this area. The young volcanic rocks overlie Mesozoic marine rocks of the Great Valley sequence which have been thrust above the coeval Franciscan Complex and penecontemporaneously dropped back down along low-angle detachment faults. Many of the waters of the region are non-meteoric as defined by their isotopic signature. One type of isotppically shifted water emerges from or near Great Valley sequence rocks and is the most chloride rich. It is interpreted to be evolved connate in origin. A second type, evolved meteoric water has moderate chloride contents, high boron contents, and high B/Cl ratios and is found locally in Franciscan rocks, notably at the Sulphur Bank mercury mine where it probably results from near-closed-system, repeated boiling of meteoric water in host rocks that also contribute organic components to the water. At the Sulphur Bank mine fracturing of otherwise impermeable Franciscan rocks by faulting has created a localized zone of permeability in which thermal water boils repeatedly with limited venting to the surface. Boron-rich fluids were apparently present at depth in The Geysers when intrusion of silicic magma occurred because the concealed intrusion of felsite is surrounded by a halo of tourmaline-bearing hornfels. The volume of this poorly dated early to middle Quaternary intrusive body probably exceeds the 100 km 3 of erupted Clear Lake Volcanics. Similar intrusions may have occurred in the eastern part of the area at Wilbur Springs and the McLaughlin mine, where gold deposition and evidence of hydrothermal phenomena suggest more magmatic activity than is indicated by small exposed bodies of early Quaternary basaltic lava. The Clear Lake Volcanics are the present locus of volcanism in the northern Coast Ranges and other volcanic centers are progressively older to the south. Geophysical data suggest that a large silicic magma body may be centered north of The Geysers steam field providing the heat for the geothermal field. Geothermal power production has peaked at The Geysers and pressure declines indicate significant depletion of the fluid resource. The vapor-dominated field evolved from a pre-existing hydrothermal system within fractured, otherwise impermeable Franciscan metamorphic rocks. A deep water table of saline fluid has been postulated to be present under the steam field, but no chloride-rich water has been found at drillable depth. We propose that recently discovered, isotopically shifted steam in the northwest Geysers area indicates the presence not of deep connate water but rather of boiled-down, boron-rich Franciscan evolved meteoric water. This water is likely to be present in limited quantities and will not provide a significant hot water resource for geothermal power production at The Geysers or from the main Clear Lake volcanic field.

Abstract Numerous late Tertiary and Quaternary volcanic centers are scattered in the vicinity of the San Andreas Fault system across the northern California Coast Ranges (Fig. 1). These volcanic rocks are characterized by a clear age progression. The youngest volcanic rocks are found in the Clear Lake volcanic center (2.1-0.01 Ma) (Donnelly-Nolan et al., 1981). To the south, volcanic rocks become progressively older. In an area about 60-70 km north of Clear Lake, seismic tomography suggests an “unerupted magma chamber” marked by relatively low crustal velocity similar to that under Clear Lake (Benz et al., 1992). A number of causes have been suggested for the Coast Ranges volcanism. Hearn et al. (1981) proposed that the volcanism resulted from northward motion of the North American plate over a hotspot relatively fixed in the mantle. However, the trend of the Coast Ranges volcanism is difficult to reconcile with the relative motion of the North American plate derived from global inversion of plate kinematics (Minster et al., 1974). An origin of arc-volcanism is also unlikely, since volcanic rocks in the northern Coast Ranges were erupted within 50-60 km of the plate boundary, whereas the Cascades arc-volcanism typically occurs about 300 km inboard of the plate boundary (Fox et al., 1985). Most workers relate the Coast Ranges volcanism to thermal perturbations associated with the northward migration of the Mendocino triple junction (MTJ) (Dickinson and Snyder, 1979; McLaughlin et al., 1981; Furlong, 1984; Johnson and O’Neil, 1984; Fox et al., 1985; Liu and Furlong, 1992). Both

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 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 The discovery of the McLaughlin hot-spring type gold deposit in the old Knoxville mercury district sparked considerable interest and research into the origin and relation of mercury and epithermal gold mineralization in the California Coast Ranges Province. Silica-carbonate mercury mineralization occurs throughout the Coast Ranges, from Santa Barbara County in the south to Lake and Colusa counties in the north (Fig. 1). Since their discovery in the mid 1800’s California’s Coast Ranges mercury deposits have produced over 50% of the total production for the United States, and two of the mines, the New Almaden mine in Santa Clara County and the New Idria mine in San Benito County, rank as the fifth and sixth largest producing deposits in the world respectively (Bailey and others 1973). Prior to the discovery of the McLaughlin gold deposit in 1978, lode gold production from the Coast Ranges was very minor with an estimated total of about $750,000, with two thirds of this production from the Palisade and Silverado mines in the Calistoga silver-gold district (Clark, 1976). Notable quantities of lode gold have been produced from only two other areas; the Los Burros district in south western Monterey County where mesothermal gold-bearing quartz veins occur in Franciscan Complex rocks; and, from the Knoxville and Sulfur Creek mercury mining districts where gold was produced as a byproduct. The McLaughlin and Cherry Hill epithermal gold deposits (Fig. 1) were subsequently discovered in the Knoxville and Sulfur Creek mining districts respectively. Current reserves at the McLaughlin mine are

Abstract Silica carbonate alteration of serpentinite is ubiquitous in the Coast Ranges of northern California, occurring in barren, Hg-rich and auriferous hydrothermal systems. The alteration is formed by the low temperature reaction of CO 2 -rich fluids with serpentinite minerals. The alteration is considered to be an exchange of cations with little net gain or loss of oxygen. The major element flux is characterized by the addition of silica and CO2 and a depletion in all other cations. The trace element flux is different for each suite examined. Barren silica carbonate assemblages are not elevated in any of the trace elements analyzed. Mercury-rich suites are elevated in Hg and the auriferous suites are elevated in Au, As, Sb and Hg. The mineralogical changes resulting from the alteration is a halo of magnesite around a core of silicified serpentinite. The variation in alteration minerals may reflect variations in fluid temperature. Oxygen isotopes suggest that the alteration is low temperature around 20°C, and that the mineral-springs were likely active at the site of the McLaughlin deposit prior to and after the hot-spring activity, that formed the McLaughlin ore body. The sulfur isotopic composition from a variety of mercury deposits and active hydrothermal systems show fairly consistent values of about 0%o, indicating a magmatic source.

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 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).

Abstract This is a progress report on some preliminary 40 Ar/ 39 Ar incremental heating experiments on feldspar separates from four samples of the felsite unit, a complex silicic batholith that intrudes the overlying Franciscan Complex (Late Jurassic to Late Cretaceous) and underlies the Geysers Geothermal Field, northern California (Schriener and Suemnicht, 1981; Thompson, 1989,1991). The felsite unit is only found in the subsurface but it appears to be an elongate body whose axis trends northwest-southeast and whose surface is shallowest in the southeast part of the field (Figure 1). It ranges in composition from granite to granodiorite (Schriener & Suemnicht, 1981; Thompson, 1991). The apparent coincidence of the heat flow anomaly within the Geysers field (Walters and Combs, 1989) with the distribution of felsite within and below the zone of steam production suggests that the felsite unit may be the primary source of heat. Presently available K-Ar ages (0.9 Ma to 2.7 Ma) suggest, however, that the felsite unit may be too old to be the primary source of heat for the present thermal activity. Resolution of this apparent paradox should be of interest for the purposes of both exploration and field management. If the felsite unit is young (<1 Ma), for example, then it should be hot wherever it is found. If the felsite unit is old (>1 Ma), on the other hand, then it may be relatively cold outside of the region of present production. The felsite unit also may be a complex body emplaced over a significant interval of time

Abstract The Geysers “felsite” (a designation of common usage in the geothermal community and thus retained for this article) is a young, wholly concealed, hypabyssal intrusive complex of batholithic dimensions within and beneath The Geysers steam field in the Mayacmas Mountains of northwest-central California (Fig. 1). The felsite was first penetrated by geothermal wells in the 1970’s, but because these air-drilled boreholes yielded extremely minute (typically <0.1 mm in diameter) cuttings, the igneous nature of the body long eluded recognition. Bailey (1946) speculated that fumaroles at The Geysers signaled such a hidden intrusive, but the first detailed descriptions and chemical analyses of the felsite were published by Schriener and Suemnicht of Unocal Corporation (1981). The Geysers felsite shows a clear correlation with the extent and configuration of the currently exploited vapor-dominated geothermal system (Figs. 1 and 2). A portion of this system is actually hosted by the felsite (e.g. Beall and Box, 1992), and above the pluton the remainder of the steam reservoir occurs in rocks which are hydrothermally altered and mineralized in concentric zones centered on the deep intrusive (Hulen and Nielson, 1993; Hulen and others, 1992; Moore, 1992; Walters and others, 1992; Hebein, 1986; McLaughlin and others, 1983). This alteration records the prior presence of a high-temperature, liquid-dominated hydrothermal system which appears to have “dried out” to yield the modern vapor-dominant regime (e.g. Moore, 1992; McLaughlin and others, 1983). The surface projection of the steam field includes and is encircled by numerous but scattered, commonly hydrocarbon-rich mercury deposits (e.g.

Abstract This tour begins at the Lytton Springs exit from Highway 101 north of Healdsburg and proceeds through The Geysers and ends at Clearlake Highlands. The route for the trip is shown in Figures 1 and 2.

Abstract The Clear Lake Volcanics (~2.1 to 0.01 Ma) are the youngest of a series of volcanic fields in the nothern Coast Ranges of California whose ages roughly parallel the timing of northerly migration of the Mendocino triple junction (Dickinson and Snyder, 1979; Johnson and O’Neil, 1984; Fox et al., 1985). There is compelling chemical, isotopic, and textural evidence that partial melting of crustal rocks played a major role in magmatism at Clear Lake (Futa et al.,1981; Hearn et al., 1981; Johnson and O’Neil, 1984). Physical evidence includes the presence of high-grade metamorphic xenoliths in many basaltic to andesitic lavas of the area (Brice, 1953; Hearn et al., 1981). The xenolith suite is dominated by high-T, low-P granulite facies schist and gneiss, but also includes mafic igneous inclusions interpreted as cognate with the Clear Lake Volcanics (Stimac, 1991). Together, this suite is thought to represent fragments of mid- to lower-crustal mafic intrusions and their contact aureoles (Stimac et al., 1992a). Exposed basement rocks in the Clear Lake area include the Late Jurassic to Late Cretaceous Franciscan Complex (FC), the Middle Jurassic Coast Range ophiolite (CRO), and the Late Jurassic to Early Cretaceous Great Valley sequence (GVS) (McLaughlin, 1981; McLaughlin and Ohlin, 1984). The metamorphic xenoliths do not resemble exposed graywackes of the FC and GVS, although they could represent the residuum of partial melting and recrystallization of graywacke and intermediate to mafic igneous rocks of the FC or GVS. This indicates that either the exposed country rocks are metamorphosed to higher

Abstract Hot springs along Sulphur Creek in Colusa County, California, have been recognized for about 130 years. Several researchers have proposed that the hot spring fluid there is derived from mixing of “connate” or “evolved connate” water which is derived from ancient seawater deposited in the Mesozoic sedimentary rocks. This water, which is similar in composition to Complexion Spring, mixes with meteoric water to form Wilbur Springs and other hot spring waters along Sulphur Creek. A δD - δ 18 O plot shows that Complexion Spring really does not plot along this trend; it must be isotopically modified to plot along the trend. Tuscan Springs, which is located 140 km NNE of Wilbur Springs, just NE of Red Bluff, has chemical and isotopic characteristics which are similar to the Sulphur Creek hot springs. Tuscan Springs vent from the Chico Formation of the Great Valley sequence and indicate that Tuscan Springs and Wilbur Springs are both derived from waters originating in the Great Valley sequence. Also δ 11 B correlates well with Cl, δD and δ 18 O, which originate in the Great Valley sequence, suggesting a similar source for the higher d 11 B values. Chemical geothermometry of the Sulphur Creek hot springs indicates a reservoir temperature of ~ 180 °C. This temperature agrees with measured homogenization temperatures from fluid inclusion which range from 150 to 180 °C. The calculated cation geothermometer temperatures are affected by the presence of dissolved Mg, even though the concentrations appear low.

Abstract Thermal/mineral waters of the Clear Lake region, California are among the most challenging geothermal fluids in the world to study because they display enormous chemical and isotopic diversity and do not geochemically resemble fluids in typical, high-temperature (≥200°C) geothermal systems (Goff et al., 1993a, 1993b). The Clear Lake region contains no boiling hot springs, hot fumaroles, or springs actively depositing sinter, features commonly linked with high-temperature reservoirs. Regionally, the fluids display tremendous variations in chemical and isotopic composition, caused more by variations in bedrock composition than by subjacent magmatic heat sources (Goff et al., 1977; Thompson et al., 1981a; 1992; Donnelly-Nolan et al., 1993). The distribution of fluids is roughly coincident with the late Tertiary and Quaternary Clear Lake volcanic field (2.1 Ma to 10 ka; Donnelly-Nolan et al., 1981; Hearn et al., 1981). The region lies northeast of The Geysers steam field, the largest geothermal field in the world, yet drilling of approximately 25 exploration wells has not found a commercially exploitable geothermal system. Because conditions in most of these wells are very hot (≥200°C at 2000 m) but relatively impermeable, the Clear Lake region is rated as one of the best hot dry rock geothermal prospects in the United States (Goff and Decker, 1983). Gas geochemistry is becoming more widely used for geothermal prospecting, especially in areas where spring chemistry is ambiguous or where spring waters are not directly derived from deep reservoirs (Goff et al., 1985; 1991; Janik et al, 1991; 1992). Geothermal gases commonly originate from

Abstract Within the central part of the Sulphur Creek Mining District six epithermal gold-mercury deposits have been mined, the West End, Central, Cherry Hill, Empire, Manzanita, and Wide Awake (Fig. 1). The mining district also includes the Abbott, Elgin and Rathburn mercury deposits and several hot springs that are depositing gold and cinnabar. The association of gold and mercury in the district was recognized early and gold was mined at the Cherry Hill and the Manzanita Mercury Mine from 1865 to 1891 with total production being about 3,000 oz of Au (Whitney, 1865, and Bradley, 1916). Whitney (1865) describes cobbles of cinnabar from the Sulphur Creek Mining District containing plumes of gold distributed through out the cinnabar. Fine placer gold is also present in drainages within the Sulphur Creek district, but has not been mined. Total mercury production from the district has been about 33,000 flasks, primarily from the Abbott mine. The Wide Awake mine was reported to have reserves of 24,000 flasks in 1899 and substantial unreported production may have come from this mine. In 1977, Homestake Mining Co. delineated a small gold deposit in the area of the Cherry Hill, West End, and Wide Awake Mines. The mineralization extends under the Sulphur Creek valley which separates these mines. Although numerous high-grade gold veins are present, typically greater than 0.3 oz of Au with multiple oz assays common, the veins are widely spaced and the deposit is presently uneconomic.

Abstract This tour begins and returns to Clearlake Highlands. The route for the trip is shown in Figure 1. Note that several stops on mis trip are described in detail in the field guide that is part of the Paper by Goff and Janik (this volume).

Abstract The Geysers geothermal area and associated Clear Lake Volcanics have aroused much scientific interest and have been subjected to many sorts of geophysical investigations. Summaries of these studies have been published by Isherwood (1981) and Majer and others (1992). In this Paper we provide a broad tectonic interpretation of the Clear Lake region and the relationship to the local gold mineralization, by interpreting magnetic and gravity data together with other available geologic and geophysical information. These results stem from work in progress and should be considered preliminary.

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