Fluid-inclusion Gas Chemistry of the Dixie Valley (nv) Geothermal System

Hydrothermally altered samples from outcrops along the eastern Stillwater Range (NV), and scale and vein samples from Dixie Valley geothermal wells were collected for fluid-inclusion analyses. Fluidinclusion microthermometry and gas analysis by quadropole mass spectrometry were applied to establish the chemistry of the fluids trapped during alteration. Relationships between CO2/CH4, N2/Ar, and H2S were used to evaluate the origins of the inclusion fluids. Most geothermal vein samples from the wells are interpreted as mixtures of shallow meteoric and evolved meteoric (“crustal”) fluids. Fluid-inclusion gases from epidote-bearing fault gouge appear to have a strong crustal signature (with low CO2/CH4 ratios). Hematite-bearing vein assemblages exhibit gas compositions that are oxidized (with high CO2/CH4 ratios), and meteoric (N2/Ar = 50-100) in origin. Analyses with high N2/Ar ratios (up to 300) indicate a magmatic origin for some fluid-inclusion gases. Actinolite-bearing veins associated with Miocene-age basaltic dikes contain mixtures of magmatic and meteoric gases. There also appears to be a small magmatic component to the gases in quartz-calcite veins from production wells. This result was unexpected because the Dixie Valley system is thought to be a deep-circulation, nonmagmatic geothermal system. However, vapor-rich fluid inclusions in production well scales and production fluids also contain some magmatic helium, and have slightly higher N2/Ar ratios than gases with a purely meteoric origin. INTRODUCTION AND GEOLOGIC SETTING The Dixie Valley geothermal system in west-central Nevada is a typical deep-circulation, fault-related geothermal system. Wells a few kms south of the geothermal field produce the hottest (265275oC) fluids in the entire Basin and Range Province (Benoit, 1994; Blackwell et al., 2000). Permeable zones in production wells in the field are associated with the Dixie Valley fault, a large-displacement normal fault with a long history of activity. In the southern Stillwater Range, up to 6 km of vertical displacement has been recorded along this fault system since the mid-Miocene (Parry et al., 1991). The Dixie Valley fault continues to be one of the most active fault systems in the Basin and Range Province (Bell and Katzer, 1987; Caskey et al., 1996; 2000a; Caskey and Wesnousky, 2000b). The fluids in the Dixie Valley geothermal system are dominantly meteoric in origin, and the source of the heat for the geothermal system is thought to be the deep circulation of fluids in an area of above average geothermal gradients (Williams et al, 1997; Blackwell et al., 2000). Isotopic studies of Dixie Valley production fluids have demonstrated that the fluids are essentially unexchanged meteoric water of late Pleistocene age (Nimz et al., 1999). However, gases collected at the wellheads appear to contain a small component of magmatic fluid. Elevated helium contents and helium isotopic signatures suggest that as much as 7.5 % of the helium may have a magmatic source. Therefore, at least some of the heat in the Dixie Valley geothermal system may be magmatic in origin (Kennedy et al., 1996). In this paper, we examine the sources and chemistry of fluids trapped in alteration minerals and wellbore scales associated with the geothermal system and recorded by the fluid-inclusion gas compositions. The samples collected for analysis represent a variety of depth intervals, mineralogies, and alteration types. Outcrop samples collected from the Stillwater range front include silicified gouge from the Dixie Valley fault, corrensite-rich clay from range front fault splays, geothermal veins with variable amounts of quartz, calcite, dolomite, barite and hematite, fumarolic encrustations (gypsum), and fossil hot spring sinters and quartz breccias from the Dixie Comstock Mine (Vikre, 1994; Fig. 1). For comparison, actinolite-bearing veins associated with Miocene-age basaltic dikes were also sampled. In all, thirteen outcrop samples were analyzed, resulting in 133 fluid-inclusion gas analyses. Subsurface samples consist of vein fragments hand-picked from the cuttings of three geothermal wells and carbonate scales from four production wells. These eleven samples yielded 93 gas analyses. The objectives of this study were to determine the importance and distribution of magmatic, crustal or meteoric derived volatiles during the evolution of the geothermal system.