Abstract Steamboat Springs geothermal area, Nevada is at the boundary between the Sierra Nevada and the Great Basin, and 10 mi (16 km) south of downtown Reno (Fig. 1). Access to the thermal area is convenient by bus or passenger car, from Nevada 431 at the northern edge of the area and U.S. 395 at the eastern edge.

Abstract In a critical period, when all energy resources must be evaluated, geothermal energy is an attractive alternative to fossil fuels. Geothermal resources can be categorized by three main mechanisms of heat transfer: (1) hydrothermal convection systems, (2) hot igneous systems where magma generated in the mantle or deep crust moves upward, and (3) conduction-dominated regimes. Most geothermal exploration has been directed toward the hydrothermal convection systems in areas of young volcanic or seismic activity. Temperatures above 200° are most actively sought. Compared with other sources, geothermal generation of electricity is small. However, the growth rate—a doubling time of 10 years—is significant. Geothermal energy is especially important to certain countries, including El Salvador, Mexico, Japan, New Zealand, and in California. It is rapidly increasing in importance in the Philippines, and probably will in Chile and Nicaragua. Although the emphasis at present is on the easily exploited vapor-dominated systems, within the next 5 years the more abundant hot-water systems will become more important. With changes in price and advancement of technology, low-temperature hot-water convection systems and geopressured pore fluids may eventually become more extensively utilized.

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.

Abstract a. All fluid inclusion studies to date suggest that Na-Ca-Cl brines are the universal ore fluids for Zn, Pb, and some other metals. b. Those natural waters highest in heavy metal content are also Na-Ca-Cl brines. The Salton Sea geothermal brine is a particularly outstanding example; oilfield waters and the recently discovered Red Sea geothermal brines also support this view. c Known examples of Na-Ca-Cl brines are stoichiometrically richer in sulfophile metals than in sulfide; the metals evidently are present as chloride complexes, stable at high temperature even in the presence of some sulfide. Mechanisms for locally supplying sulfide in quantities sufficient to form ore deposits have been proposed by Lovering, White, Barton, and others.

Abstract Most saline waters of marine sedimentary rocks wereprobably similar initially to present-day ocean water. Many early diagenetic changes in sediments and waters are related to organic content and bacterial activity;ion exchanges and perhaps some other early changes areinorganic. Diagenetic and later changes in sedimentaryrocks cannot be understood without considering the associated fluids, which are mobile and leave little direct and easily interpretable evidence of their changingcompositions with time. Compaction of sediments and escape of interstitial water start at the time of deposition and probably continue for millions of years. The evidence is now convincing that fine-grained sediments behave as semipermeable membranes, permitting selective escape of water and concentrating dissolved components in remaining porefluids. The initial driving force is lithostatic pressure; after maximum compaction has been attained, salt-filtering may continue under certain circumstances of topography, structure, and lithology, with meteoric water providing the driving energy.

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.