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 High-temperature hydrothermal fluids exit from the sea floor on the East Pacific Rise at 21°N. Mixing of the hydrothermal fluids with seawater leads to the precipitation of sulfide and sulfate minerals. These precipitates may become dispersed in the water column or may aggregate to form chimney like structures and basal mounds. Our study of the distribution and textures of minerals in chimney samples collected during 1979 suggests a relatively simple model of chimney growth. Such growth starts with the rapid formation of a highly permeable wall composed of anhydrite and minor sulfides. Vertical growth rates of 30 cm/day for new chimneys were observed in 1.981 after preexisting chimneys were razed. Mixing of seawater and hydrothermal fluid through this wall causes the precipitation of additional sulfides and sulfates, which further reduces the permeability of the chimney wall. The temperature of the hydrothermal fluid inside chimneys gradually increases, and the fluid composition approaches the composition of fluid in the chimney roots. The mineralogy of chimney interiors changes gradually in response to changes in fluid temperature and composition. As chimney exteriors become increasingly isolated from the interior hydrothermal fluid, their temperature drops and new mineral assemblages are produced by reaction of ambient seawater and the mineral assemblages of the chimney walls. Weathering assemblages similar to supergene sulfide assemblages can develop during reactions of this type. The process of basal mound formation is less well understood than the process of chimney formation. Stockwork fracturing is characteristic of the footwall of many ophiolite-hosted massive sulfide deposits, which suggests that basal mound growth commences with intense localized fracturing of the sea floor. Fractures produced during such events provide conduits for rising hydrothermal fluids, and sulfide and sulfate mineral precipitates accumulate on the sea floor around the fractures and may form a basal mound. Chimney structures develop around larger openings, while smaller openings clog with precipitates to form a low-permeability crust on basal mounds; hydrothermal circulation beneath such a crust was discovered in 1981. The upper part of the mounds is a transition zone between low-temperature ambient seawater and the high-temperature fluid within the mound interior; this transition zone is probably zinc rich. The geologic preservation of sulfide-sulfate deposits on the ocean floor requires burial rates that exceed the rate of oxidation and dissolution.