Origin of saline, neutral-pH, reduced epithermal waters by reaction of acidic magmatic gas condensates with wall rock
- 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-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
58 GEOSCIENCES
COLORADO
MAGMA SYSTEMS
GEOTHERMAL FLUIDS
CHEMICAL COMPOSITION
SALINITY
ROCK-FLUID INTERACTIONS
NEVADA
OREGON
BRINES
CHLORITE MINERALS
GOLD ORES
HYDROTHERMAL ALTERATION
HYDROTHERMAL SYNTHESIS
MINERALIZATION
PH VALUE
PYRITE
PYROPHYLLITE
QUARTZ
SPHALERITE
WATER CHEMISTRY
ALUMINIUM COMPOUNDS
ALUMINIUM SILICATES
CHEMISTRY
DEVELOPED COUNTRIES
FLUIDS
GEOTHERMAL SYSTEMS
MINERALS
NORTH AMERICA
ORES
OXIDE MINERALS
OXYGEN COMPOUNDS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SULFIDE MINERALS
SYNTHESIS
USA
152003* - Geothermal Data & Theory- Rock-Water-Gas Interactions
580000 - Geosciences