Geothermal systems ancient and modern: a geochemical review

Epithermal deposits are important sources of gold and silver that form at 20 wt percent NaCl equiv for Ag-Pb-Zn deposits. Stable isotope data indicate that hydrothermal solutions were composed mostly of deeply circulated meteoric water, with a nil to small and variable component of magmatic water. Epithermal deposits associated with quartz + alunite ± pyrophyllite ± dickite ± kaolinite assemblages contain Au ± Ag ± Cu ores. Native gold and electrum are the main ore-bearing minerals, with variable amounts of pyrite, Cu-bearing sulfides and sulfosalts such as enargite, luzonite, covellite, tetrahedrite, and tennantite, plus sphalerite and telluride minerals; enargite dominates the Cu sulfides and indicates a high-sulfidation state. Quartz (both massive and vuggy) and alunite are the main gangue minerals with kandite minerals (dickite and/or kaolinite) and/or pyrophyllite. Concentric patterns of hydrothermal alteration envelop the zone of vuggy and massive quartz alteration, which hosts ore. Outward, these comprise zones of quartz and alunite, dickite ± kaolinite or pyrophyllite, and illite or smectite alteration, surrounded by regional propylitic alteration. Zones of illite or pyrophyllite alteration occur in the roots beneath some deposits. Fluid inclusion data indicate that salinities are typically 30 wt percent NaCl equiv. Stable isotope data indicate that the altering fluids are composed mostly of magmatic fluids with a minor to moderate component of meteoric water. Critical genetic factors include: (1) at several-kilometers depth, the development of oxidized and acidic versus reduced and near-neutral pH solutions, controlled by the proportions of magmatic and meteoric components in solution, and the amount of subsequent water-rock interaction during ascent to the epithermal envi

Geological characteristics of epithermal precious and base metal deposits

Long Valley caldera, a 17- by 32-km elliptical depression on the east front of the Sierra Nevada, was formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68-0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende-biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende-biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self-sealing of near-surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated northwest-trending Sierra Nevada frontal faults that tap hot water at depth.

Volcanism, structure, and geochronology of Long Valley Caldera, Mono County, California

Epithermal environments and styles of mineralization: Variations and their causes, and guidelines for exploration

Geothermal mineral equilibria

The effect of salinity on the temperature-depth relations of a brine of constant composition, enclosed in a vein system, but freely connected to the surface, and everywhere at the boiling point for the hydrostatic head, was calculated by using a mathematical model. The Na-Ca-K-Cl brines which are found in thermal springs and in fluid inclusions in ore minerals were approximated by the available data for vapor-saturated NaCl-H 2 O solutions. In general, the results are similar to those calculated by D. E. White in 1968 for H 2 O, except that the gradients are steeper because of the increase in density and the decrease in vapor pressure caused by the dissolved salt. As a practical rule, the depth to an isotherm in a 5, 10, 15, 20, and 25 wt percent NaCl brine system is, respectively, 92, 84, 77, 70, and 63 percent (+ or -2 percent) of the depth to the same isotherm in an H 2 O system. From the data presented, the minimum depth to the growth site of crystals containing fluid inclusions which indicate boiling of the brine can be estimated. Among other applications, these results are useful toward the understanding of the behavior of brines in geothermal areas which may or may not contain compositional stratification.

John L. Haas

Papers

The effect of salinity on the maximum thermal gradient of a hydrothermal system at hydrostatic pressure