Abstract Sulphur Bank is the most productive mineral deposit in the world that is clearly related to hot springs. The ore is late Quaternary and is localized in rocks immediately below the water table that existed prior to mining. The hydrothermal alteration and the mineralogy of the veins have been controlled largely by the water table. The upper part of an andesite flow has been above the water table and is extensively altered by sulfuric acid formed by oxidation of H2S. Characteristic alteration minerals are opal, cristobalite, and anatase where leachi ng has been intense, and kaolinite, halloysite, alunite, soluble sulfates, and perhaps jarosite and montmorillonite where acid attack has been less intense. Native sulfur without cinnabar was abundant near the surface, but, as the water table was approached, sulfur decreased, and cinnabar became abundant. The principal ore bodies were at and below the water table and consisted of cinnabar, marcasite, pyrite, dolomite, calcite, quartz, a zeolite mineral, and all the minerals of the original rocks. Metacinnabar and stibnite were locally common. The waters deep in the spring system appear to be nearly neutral, but near the water table they become slightly acid because of mixing with downward percolating waters containing H 2 SO 4 resulting from oxidation of H 2 S. Films of condensate in the areas of most intense acid leaching may have pH values of 1 or less. The present thermal waters are very highin total CO 2 , boron, ammonia, sodium, and iodine and are low in silica and potassium as compared to many thermal and mineral waters. Chemically and isotopically they are unlike most thermal waters associated with recent volcanism. The present rate of discharge of water of deep origin is calculated to be about 50 gpm. T h e average concentration of quicksilver in the ore solutions was probably 0.05-8 ppm, assuming an interval of deposition between 10,000 and 100,000 years and a rate of discharge of water of 50–1000 gpm. The most reasonable estimate is believed to be 0.1–1 ppm. Present temperatures are relatively low compared to other hot-spring systems of clearly volcanic origin . The present heat flow is on the order of 200,000 cal per sec, or about 12 times “normal” for the area; total heat flow inthe past may have been as much as 20 times as much. The heat is almost certainly volcanic in origin, but, despite association with Quaternary volcanic rocks and volcanic heat, the chemical and isotopic compositions of the water and gases now being discharged indicate that these fluids are nonvolcanic i n origin.

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 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 During the past 25 years our understanding of hydrothermal ore deposits has progressed remarkably because of combined approaches through detailed study of actual deposits, laboratory experimental study of ore and gangue minerals and fluid inclusions, and study of active geothermal systems. My review emphasizes the active systems, which have recently become a focus of interest in a worldwide search for alternative energy sources. Sulphur Bank, California, and Ngawha, New Zealand, have provided several keys for understanding the generation of many mercury deposits. Major requirements probably are: deep source regions of fluids and Hg at temperatures >200°C; metamorphic environments above subduction zones on continental margins; a through-going (rather than local) vapor phase enriched in CO 2 or other gases, migrating along with liquid water; and instability of HgS at high temperatures, decomposing to Hg 0 and S 0 , with the migrating vapor required for major transport of Hg at temperatures <200°C; a coexisting liquid phase is generally required to transport SiO 2 and other nonvolatile constituents. This two-phase mechanism best explains the general absence of other significant ore metals. Vapor-phase transport of the Hg associated with other metals at higher temperatures is probably not essential. Epithermal precious metal ore deposits are probably the fossil equivalents of high-temperature geothermal systems like Broadlands, New Zealand, and Steamboat Springs, Nevada. The evidence suggests that the fossil and active systems are similar in their rare chemical elements, ranges in temperature, pressure, compositions of fluids, isotope relationships, and mineralogy of ore, gangue, and alteration minerals. Broadlands and Steamboat Springs show a depth zoning of the “epithermal” chemical elements, Au, As, Sb, Hg, Tl, B, and some Ag, that selectively concentrate near the surface. Much Ag, base metals, and probably Se, Te, and Bi precipitate at somewhat greater depths and higher temperatures. Nolan (1933) divided the epithermal precious metal deposits into a gold-rich group (Au > Ag by weight) and a silver-rich group. The concepts of depth zoning in active geothermal systems, if applied to epithermal deposits, suggest that some gold-rich deposits (including the recently recognized Carlin-type) form at relatively shallow depths and low temperatures. These may grade down into deposits enriched in Ag and base metals, perhaps in places separated by a relatively barren zone resulting from changes in the dominant complexing agent, Cl vs. S. This possibility, even if remote, justifies close examination. Active systems that might form base metal ore deposits were virtually unknown 25 years ago. Discovery of the Salton Sea, Red Sea, and Cheleken thermal chloride brines in the early 1960s focused on Cl as the probable critical agent, permitting transport of base metals as metal-chloride complexes. Also, some oil field waters were found to have Pb and Zn contents in the range of a few parts per million (ppm) to many tens of ppm. The low-temperature brines have no sulfide within detection limits; only at temperatures >200°C can small quantities of sulfide coexist with the base metals in solution. All of these metal-bearing brines are deficient in sulfide; most of their metals can precipitate as ore deposits only where supplemental sulfide can be provided by any one of several proposed mechanisms. Comparable brines in the past probably formed low-temperature epigenetic deposits like those of the Mississippi Valley, as well as many marine sediment-hosted syngenetic and early diagenetic ore deposits. Ore fluids rich in both base metals and reduced sulfide species probably require very high salinity, high temperature, and rock-water reactions buffered at low pH (thus, with little free S −2 immediately available). Hostile environments of extreme temperature and salinity, such as those indicated in generating porphyry copper deposits, cannot be drilled by present methods, even if we knew where to drill. Visual observation of comparable environments seemed unlikely until early in 1979, when Cu and Zn sulfides were found to be precipitating from spring vents on the spreading axis of the East Pacific Rise at temperatures exceeding 350°C. Low-temperature discharges on the sea floor had been known for a few years, but the activity in this hydrothermal area, known as Twenty-One North, is the first that bears directly on the origin of volcanogenic massive sulfide deposits (and indirectly on other deposits formed at extreme temperatures and salinities). Brines of many origins can form base metal deposits; origin of the water may be less important than the physical and chemical environments of the brines and source rocks. Ocean water alone, ocean and fresh waters plus evaporites, evolved connate waters of marine sedimentary rocks, and magmatic waters are all effective solvents of base metals in suitable environments. Precipitation of metals from these brines can occur by decreasing temperature, mixing with low-salinity water, access of supplemental sulfide, and neutralizing reactions with wall rocks, as well as various combinations of these. Suggestions for exploration for concealed deposits of the major groups considered here are offered, resulting from improved understanding of various genetic models.