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 The Modoc prospect is a Plio-Pleistocene epithermal gold-bearing hot springs system located in the northwestern Salton Trough. To date, Modoc offers the best preserved and developed fossil analogue of the currently active geothermal systems in the continental rift. A surficial hot springs origin for the system is evidenced by interbedded, nearly flat-lying chalcedonic sinter beds exhibiting a strong layered morphology, hummocky pool structures and well preserved fossil reed casts. The primary structural control on the hot springs system is a major ENE-striking, southerly dipping, oblique-slip fault system separating Jurassic granodiorite on the north from Plio-Pleistocene continental sediments on the south. This “Truckhaven” fault system appears to be coincident with the edge of a large basin related to crustal thinning and progradation of the Colorado River Delta into the proto Gulf of California. Gold mineralization occurs as electrum and native gold (8 to 1800 microns) and is associated with sheeted and banded chalcedony veins, silicified and adularized sandstone and conglomerate, and siliceous sinter beds. Gold and chalcedony precipitated simultaneously in numerous episodes and preliminary fluid inclusion data indicate temperatures ranging from 90 to 135°C. Gold mineralization is locally high grade (up to 1.4 ounces of gold per ton over a 20′ (6.1 m) width) and is found in highly anomalous concentrations over an area measuring 8,000′ (2,438 m) long by up to 2,000′ (610 m) wide. Correlation coefficient data indicate a strong gold, silver, copper affinity. The typical hot springs pathfinder elements As, Sb and Hg are present in unusually low concentrations. The Modoc hot springs system is more akin to the active geothermal fields situated on the margins of the rift than those in the center of the Trough. It is strikingly similar to the high-silica Dunes geothermal system, which has no surface expression largely due to the development of hydrothermally induced caprocks. The rediscovery of the Modoc hot springs gold prospect leaves exploration geologists wondering how many other such occurrences underlay the seemingly barren sediments of the Imperial and Mexicali Valleys.

Abstract The characteristics of five explored hot spring systems are reviewed, focusing attention on features that may be preserved after the activity has ceased. Four systems, including the Upper Basin and Norris Basin of Yellowstone Park, Steamboat Springs, Nevada, and Wairakei, New Zealand have relatively high temperatures at the surface and at depth. They are volcanic in origin, deriving heat, much mineral matter, and part of their water from a volcanic source. Surficial acid leaching related to oxidation of H 2 S occurs above the water table in three of the four areas. Kaolinite and alunite are characteristic of mild acid alteration immediately below the water table, if the environment is favorable. Where acid alteration is absent, fresh rock may occur near the surface. Alteration minerals formed at depth include adularia, montmorillonite, illite, sericite, kaolinite, chlorite, pyrite, calcite, zeolites, and sodic plagioclase. All have been found at Steamboat Springs, most were described from the Upper Basin and Wairakei, but only a few occur at Norris. Tridymite and “orthoclase” (sandine), previously described in welded tuffs at Norris by Fenner, are related to pneumatolitic alteration when the tuffs were deposited, and are not related to present activity. Chalcedony, quartz, and calcite are typical vein minerals. Opal is dominant near the surface in sinter and in veins, but it seldom occurs below 100 feet. Where the temperature is much above 100° C., opal may form if deposition is sufficiently rapid, but it is generally converted to chalcedony. Cinnabar and stibnite occur at Steamboat and orpiment and realgar in Norris Basin, restricted to shallow depths. Sinter at Steamboat and Wairakei contains gold and silver. In many respects, the four high-temperature spring systems are similar to epithermal gold-silver deposits. The fifth explored system, Sulphur Bank, California, is the only one that has been mined extensively for its metal content, having produced 126,000 flasks of quicksilver. Rock alteration and present temperatures differ notably from the other systems. Most alteration is of the superficial acid type but some replacement calcite, pyrite, and argillic minerals occur in deeper workings. Cinnabar and opal are intimately associated and persist to depths of at least 260 feet, probably because temperatures of the past and present are relatively low. The water as well as the quicksilver may be derived from sedimentary rather than volcanic rocks. The near-surface activity of the spring systems is dependent on many factors, including structure, topography, ground-water relations, compositions of the waters, temperature relations, and rates of discharge. The waters of all five hot spring systems are believed to be near-neutral at depth, but those of the Upper Basin are somewhat more alkaline than the others. The possible existence of deep acid water in Norris Basin is also recognized. As the near-neutral waters approach the surface they may become more alkaline because of boiling and relative loss of CO 2 , or under favorable conditions, more acid because of surficial oxidation of H 2 S. Thermal springs may form siliceous sinter, travertine, other special types of deposits, or no significant deposit at all. Four of the five well-explored systems are of the high temperature, sinter-depositing type. Most other types, unfortunately, have not been explored at depth, or the associated vein and alteration minerals have not been studied. Nearly all epithermal ore deposits that show a relatively close relationship to thermal springs fall into four major groups: Quicksilver (and antimony), manganese (and tungsten), gold-silver, and fluorite. A significant number of quicksilver deposits are associated with thermal springs, and some others are characterized by abnormally high temperatures and gases (principally CO 2 and CH 4 ). Some quicksilver has probably been derived from a volcanic source but much may have come from sedimentary rocks, with connate water as the transporting agent. Some manganese deposits are clearly of hot-spring origin, and a few contain notable tungsten. Most bedded deposits in marine and non-marine sediments overlie basic and intermediate volcanic rocks that have been altered or albitized. The volcanic rocks appear to be the source of the manganese, which was probably transported by circulating ocean water or saline meteoric water of the non-marine sedimentary basins. Silver and gold deposits of the epithermal type appear to be closely related to the high-temperature volcanic hot springs; mineralogy of the veins and altered rocks are strikingly similar except for the scarcity of precious metals in Quaternary hot springs systems. Fluorite deposits are not well known but they may be related to travertine-depositing spring waters that contain volcanic components, and have also been influenced by contact with carbonate rocks. In summary, some ore epithermal deposits were formed by solutions closely related to volcanism, but others were probably deposited from circulating meteoric and connate waters. Much additional study is needed, particularly concerning criteria that distinguish waters of different origin.

Abstract The beauty and lure of Yellowstone National Park (YNP) to visitors from all around the world, in large part, comes from one of the very things that gives it its own name; The bright and colorful clay-sized mineral assemblage that make up its grand mountains, alluring valleys and brilliant hot springs. Proximity of The Clay Minerals Society (CMS) 2009 annual meeting to one of the largest terrestrial geothermal area in the world was ample reason to run the first “inquiry-based” CMS Workshop Lectures series (WLS), where both lectures and field excursions were emphasized. Inquiry teaching is investigating with questions, emphasizing evidence, forming explanations, making connections to the larger scientific community, and communicating conclusions. This accompanying volume brings another first to the CMS-WLS, which is electronic formatting and the benefit of color graphics, where a picture is worth a 1000 words. This volume includes displays and maps designed to bring the reader closer to YNP and perhaps let them better virtually sense the heat of hot springs, the smell of fumaroles, the sounds of gushing geysers, and the panorama of colors. This CMS-WLS also joins the mainstream realization that most Earth surface mineralization processes are mediated in the presence of biological communities. In fact, it may be harder to find an exception than a rule to this understanding. Microbiological metabolic pathways that exchange protons, electrons, and essential ions in nature are inextricably linked to clay sized mineral reaction pathways that do the same. For example, the reduction of iron in a smectite-bearing hot spring may be linked to the oxidation of N in a microbe (and visa versa). To understand the patterns that shape the clays and clay minerals we see today in YNP, one must embrace both the caldera-scale range of igneous, glacial, and hydrological processes, as well as the atomic-scale processes that occur along steep redox, pH, anions, cations, and temperature gradients. To this end, this CMS-WLS volume is holistically designed to cover (1) broad scale features of YNP as seen by remote sensing technology, (2) the evidence and processes by which biomineralization of clay sized materials occurs within terrestrial hot springs, and (3) the importance of spring chemistry, microbial communities, and other coexisting clay-sized, but non-clay mineral phases that are present. These perspectives lead to better understanding of the occurrences and variations of clay mineral assemblages existing within the environs of YNP and perhaps their analogs on early Earth and other planets.