title: A Study of Fluid Inclusions and Geochemical Mechanisms for Gold Deposition, Cannon Mine, Wenatchee, Washington
Creators: name: Klisch, Michael P.
Publisher: name: Western Washington University
Year: 1989

description: <p>The Carmen Mine is located 1 km south of the city of Wenatchee, Washington. The mine is hosted by lower to middle Eocene fluvial arkoses of unknown correlation. An unaltered rhyodacite dome, dated at 43.2 ± 8 Ma, bordered by perlite, intrudes the sediments and is separated from the orebody by a zone of sheared and faulted sediments. The western border of the orebody is a brecciated and weakly altered and mineralized andesite of probable early Eocene age. The structural geology of the mine area is quite complex and related to the formation of the Chiwaukum graben during the middle Eocene.</p>
<p>Hydrothermal alteration consists dominantly of pervasive silicification of the sediments. Argillically altered (chlorite+kaolinite+smectite) arkoses cap the silicified zones. Gold occurs in quartz-adularia-calcite veins which occupy 2 generations of fractures in the silicified zones. Ore minerals are pyrite, electrum, pyrargyrite, tetrahedrite, chalcopyrite, and acanthite. Gangue minerals are quartz,calcite, adularia, and clays. Post mineralization shearing has disrupted the silicified zones.</p>
<p>Fluid inclusions were studied in quartz and calcite veins from the silicified zones. Fluid inclusions and textures of veins and wall rocks indicate that the fluid was boiling. Three types of fluid inclusions were identified in quartz: 2 phase, liquid-vapor, liquid rich, with variable degree of filling; 2 phase L-V, vapor rich; and 3 phase, liquid-vapor-solid. The most common are L-V, liquid rich, with homogenization temperatures ranging from 58° C to over 400° C. The mean homogenization temperature is 285° C, except in late-stage, low temperature quartz. Salinities are quite variable, ranging from 1.6 weight % to 23 wt. % NaCl equivalent, and averaging 8 wt. % NaCl equivalent. No microthermometry was performed on the vapor-rich inclusions due to their opaque nature, but their occurrence together with liquid-rich inclusions suggests the system was boiling. Rare L-V-S inclusions contain an unidentified solid phase. Ice melting temperatures as low as -33° C reveal a more complex system than NaCl-H<sub>2</sub>0, possibly NaCl-(Ca,Mg)Cl<sub>2-</sub>H<sub>2</sub>0. Calcite contains 2 phase, L-V inclusions with consistent degrees of filling. Homogenization temperatures range from 99° C to 220° C, averaging 185° C. Salinities range from 0.09 wt. % to 1.13 wt. % NaCl equiv., averaging 0.87 wt. %.</p>
<p>Inclusions were decrepitated by stepwise heating. Following each step, leachates were analysed by ion chromatography, and the results applied to chemical geothermometers, widely applied to the investigation of active geothermal fields. The most common cations are Na+ and K+, with minor amounts of Ca<sup>2+</sup>, Mg<sup>2+</sup>, Li+, and NH<sub>4 </sub>+. The most common anion is S0<sub>4</sub><sup>2-</sup>, with lesser amounts of Cl-. The sulfate probably represents original H<sub>2</sub>S, which was oxidized during sample preparation.</p>
<p>The Na-Ca-K geothermometer gave results consistent with fluid inclusion geothermometry, while the Na-K geothermometers gave high but generally consistent results. The Na-Li geothermometer gave variable,unusable results.</p>
<p>Thermochemical calculations and fluid inclusion results indicate that gold was transported as a gold-bisulfide (thio) complex rather than a gold-chloride complex, in dilute, high ΣS/Cl- fluids. The hydrothermal fluids were probably undersaturated with respect to gold, and the gold content of the source rocks was the limiting factor in the amount of gold transported. Gold was probably precipitated as the ascending hydrothermal fluids encountered open space in the host rocks. The sudden drop in pressure allowed the fluids to devolatilize and boil, precipitating gold. Tectonic activity related to the formation of the Chiwaukum graben produced the fractures for the fluid to flow through. Magmatic activity coeval with graben formation provided a heat source for the circulation of the hydrothermal fluids.</p>
<p>Hydrothermal activity and gold deposition took place in a near-surface environment similar to the boiling geothermal systems at The Geysers, California, and Broadlands, N.Z. Fluid inclusion evidence for boiling fluids with a high ΣS/Cl- ratio may be an important, inexpensive exploration tool for similar types of gold deposits.</p>