R. W. Charles

Geochemistry and isotopes of fluids from sulphur springs, Valles Caldera, New Mexico

Abstract Detailed geochemistry supported by geologic mapping has been used to investigate Sulphur Springs, an acid-sulfate hot spring system that issues from the western flank of the resurgent dome inside Valles Caldera. The most intense activity occurs at the intersection of faults offsetting caldera-fill deposits and post-caldera rhyolites. Three geothermal wells in the area have encountered pressures 4 ⩽8000 mg/l. These conditions cause argillic alterations throughout a large area. Non-condensible gases consist of roughly 99% CO 2 with minor amounts of H 2 S, H 2 , and CH 4 . Empirical gas geothermometry suggests a deep reservoir temperature of 215 to 280°C. Comparison of 13 C and 18 O between CaCO 3 from well cuttings and CO 2 from fumarole steam indicates a fractionation temperature between 200 and 300°C by decarbonation of hydrothermally altered Paleozoic limestone and vein calcite in the reservoir rocks. Tritium concentrations obtained from steam condensed in a mudpot and deep reservoir fluids (Baca #13, ∼278°C) are 2.1 and 1.0 T.U. respectively, suggesting the steam originates from a reservoir whose water is mostly >50 yrs old. Deuterium contents of fumarole steam, deep reservoir fluid, and local meteoric water are practically identical even though 18 O contents range through 4‰, thus, precipitation on the resurgent dome of the caldera could recharge the hydrothermal system by slow percolation. From analysis of D and 18 O values between fumarol steam and deep reservoir fluid, steam reaches the surface either (1) by vaporizing relatively shallow groundwater at 200°C or (2) by means of a two-stage boiling process through an intermediate level reservoir at roughly 200°C. Although many characteristics of known vapor-dominated geothermal systems are found at Sulphur Springs, fundamental differences exist in temperature and pressure of our postulated vapor-zone. We propose that the reservoir beneath Sulphur Springs is too small or too poorly confined to sustain a “true” vapor-dominated system and that the Sulphur Springs system may be a “dying” vapor-dominated system that has practically boiled itself dry.

Molybdenum mineralization in an active geothermal system, Valles caldera, New Mexico

Shallow, sub-ore-grade molybdenite mineralization has been discovered in the active, high-temperature geothermal system penetrated by Continental Scientific Drilling Program corehole VC-2A at Sulfur Springs, in the western ring-fracture zone of the Valles caldera, New Mexico. This mineralization is hosted by fractured, quartz-sericitized, intracaldera ash-flow tuffs younger than 1.12 Ma. The molybdenite is an unusual, poorly crystalline variety that occurs in vuggy veinlets and breccia cements also containing quartz, sericite (illite), pyrite, and fluorite, as well as local sphalerite, rhodochrosite, and chalcopyrite. Fluid-inclusion data suggest that this assemblage was deposited from very dilute solutions at temperatures near 200/sup 0/C. Geochemical modeling indicates that under restricted pH and fO/sub 2/ conditions at 200/sup 0/C, the molybdenite and associated phases would be in equilibrium with hydrothermal fluids now circulating in the deep subsurface. The shallow molybdenite zone intersected in VC-2A may be the near-surface expression of deep, Climax-type stockwork molybdenum mineralization.

Rock-fluid interactions in a temperature gradient: biotite granodiorite + H2O

Abstract A biotite granodiorite was reacted in a controlled temperature gradient with distilled water for 60 days at 1/3 kbar P Tot = P H 2 o . Polished rock prisms were placed in the gradient at 72, 119, 161, 209, 270, and 310°C. Scanning electron microscope and microprobe analyses show the appearance of several secondary phases: Ca montmorillonite at 72°C and 119°C; zeolite, either stillbite or heulandite, at 161°C; and another zeolite, thomsonite, at higher temperatures. Solution analyses show a constant composition after about 2 weeks of reaction. The phase assemblages observed in this experiment are connected by reaction relationships. The reactions show the transition from clay to zeolite I (stilbite or heulandite) to zeolite II (thomsonite) with increasing temperature and decreasing chemical potential of silica. Additional relations can be used to predict mineral assemblages not directly observed in this experiment but which may occur at different pressures and temperatures as well as at different chemical potentials of water and silica.

Thermodynamic parameters and experimental data for the NaKCa geothermometer

Abstract The standard literature values for the thermodynamic parameters of a quartz—two-feldspar reaction are not consistent with the constants of Fournier and Truesdell. The constants in the geothermometer expression have been re-evaluated by the method of leastsquares using both the data compiled by Fournier and Truesdell and new experimental data. The resultant thermodynamic parameters also eliminate feldspar-quartz as the controlling phase assemblage and, in conjunction with experimental observations, suggest that many different alteration phase reactions involving thermodynamically similar clays and zeolites may buffer the solution to the observed compositions. A single formulation of the NaKCa geothermometer may, therefore, be appropriate over a given temperature range for dilute aqueous systems in a number of different altered rocks containing framework silicates. The new constants using the least-squares fit of the Fournier and Truesdell data yield the following revised NaKCa geothermometer equations: log ( Na + K + ) − 6.3 log ( Ca 2+ Na + ) = −22200/T+64.2 for T log ( Na + K + ) + 0.055 log ( Ca 2+ Na + ) = 1416/T+−1.69 for T

Status of the Los Alamos experiment to extract geothermal energy from hot dry rock

For the past four years, the Los Alamos Scientific Laboratory (LASL) has been investigating a method for extracting geothermal energy from hot, but essentially impermeable, rocks at moderate depths. The concept is to drill a hole into the hot, relatively impermeable rock and create a large hydraulic fracture that serves as a downhole heat exchanger. A second hole is drilled to intersect the fracture, creating a circulation loop for the injected water. Two deep holes (GT-2 and EE-1) have been drilled at the Fenton Hill site in northern New Mexico. The first hole, GT-2, has a depth of 2,928 m and a bottom-hole temperature of 197 °C. The second hole, EE-1, has been temporarily halted at at depth of 3,062 m, where the temperature is 205.5 °C. Hydraulic fractures have been created in both holes, and fluid communication between the holes was established in October 1975. Circualtion experiments are now being conducted between the two holes.