Origin of saline, neutral-pH, reduced epithermal waters by reaction of acidic magmatic gas condensates with wall rock

Conference · Thu Apr 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5067375

Reed, M H ^[1]

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1. Univ. of Oregon, Eugene, OR (United States). Dept. of Geological Sciences

Fluid inclusions in quartz and sphalerite of epithermal veins containing galena, sphalerite and chalcopyrite with silver sulfides and electrum commonly have salinities of 2 to 10 weight percent NaCl equivalent. Examples include Bohemia, OR, Comstock, NV, and Creede, CO. Salinities in such base metal-rich systems are apparently greater than those in gold-adularia, base metal-poor systems such as Sleeper, NV, Republic, WA, and Hishikare, Kyushu. Saline epithermal fluids are commonly assumed to have been derived from saline magmatic brines, from local host formations, as has been suggested for Creede, or from evaporative concentration (boiling) of more dilute meteoric ground water. Another possibility, which may be the most common origin, is reaction of wall rocks with magmatic gas condensates rich in HCl and sulfuric acid. A mixture of one part Augustine Volcanic gas condensate in 10 parts cold ground water has a pH of 0.7 and the dominant cation is H[sup +] by a factor of 10[sup 4]. Calculated reaction of this condensate mixture with andesite at 300 C to a water/rock ratio (w/r) of 4.6 yields an NaCl-dominated fluid with a total salinity of 2.1 wt %. and pH 3.7. Further reaction, to w/r 0.14 yields a fluid salinity of 2.6 wt % and pH of 5.7; this fluid is in equilibrium with a propylitic alteration assemblage. Aqueous sulfide accumulates during the rock reaction as sulfate is reduced to sulfide when ferrous iron is oxidized to ferric iron. Sulfide concentration in the latter fluid is 32 ppm, far exceeding sulfate concentration. In the overall reaction, hydrogen ion is exchanged for base cations (including base metals) and sulfate is reduced to sulfide.

• OSTI ID: 5067375
• Report Number(s): CONF-9305259--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:5
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
152003* -- Geothermal Data & Theory-- Rock-Water-Gas Interactions
58 GEOSCIENCES
580000 -- Geosciences
ALUMINIUM COMPOUNDS
ALUMINIUM SILICATES
BRINES
CHEMICAL COMPOSITION
CHEMISTRY
CHLORITE MINERALS
COLORADO
DEVELOPED COUNTRIES
FLUIDS
GEOTHERMAL FLUIDS
GEOTHERMAL SYSTEMS
GOLD ORES
HYDROTHERMAL ALTERATION
HYDROTHERMAL SYNTHESIS
MAGMA SYSTEMS
MINERALIZATION
MINERALS
NEVADA
NORTH AMERICA
OREGON
ORES
OXIDE MINERALS
OXYGEN COMPOUNDS
PH VALUE
PYRITE
PYROPHYLLITE
QUARTZ
ROCK-FLUID INTERACTIONS
SALINITY
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SPHALERITE
SULFIDE MINERALS
SYNTHESIS
USA
WATER CHEMISTRY