HR: 1340h
AN: T23B-0558    [Abstracts]
TI: Exploration of a Basin and Range-Type Geothermal System Using Soil pH Analysis
AU: * Owens, L
EM: lara@nmt.edu
AF: New Mexico Institute of Mining and Technology Dept. of Earth and Environmental Sciences, 801 Leroy Place, Socorro, NM 87801 United States
AU: Hill, G
EM: hill@actlabs.com
AF: Activation Laboratories, 1775 West Sahuaro Drive, Tucson, AZ 85745 United States
AU: Norman, D I
EM: dnorman@nmt.edu
AF: New Mexico Institute of Mining and Technology Dept. of Earth and Environmental Sciences, 801 Leroy Place, Socorro, NM 87801 United States
AB: The Socorro Peak, NM Known Geothermal Reservoir Area (KGRA) is a Basin and Range-type extentional-fault geothermal system boasting thermal gradients upwards of 420 mW/m3 in an uplift Precambrian fault block. Structural and geophysical evidence suggests that a low-to-mid temperature (60-100C) geothermal aquifer may reside within the fault-bounded alluvial basin, capped and insulated by over 1000meters of Tertiary mudstone aquitard strata and Quaternary flanglomerates. Select Ion Leach Analysis Geochemistry (SILG) and pH analysis of soils were employed to investigate the location and extent of the Socorro KGRA. The SILG geothermal exploration method is commonly used for mineral and oil exploration; the soil pH is method is a new method being developed. Soil samples for SILG were collected at 100m intervals over the alluvial basin and range bounding fault. Oxide Suite elements (As, V, I, Hg) and alkali elements (Rb, Sr) were observed in a series of nested halos of anomalously high concentrations surrounding a central core of anomalously low concentrations. The center of this 3 km-wide anomaly is located just in to the east of the range bounding fault. This pattern is interpreted as an oxidizing environment surrounding a reduction chimney created by the geothermal waters. Field pH analyses were also performed on a 25 meter interval grid over the same exploration area. A 25 g soil sample screened to 18 mesh is mixed with 10 cc water, stirred and pH measured. Two dimensional plots indicate a central region of 7.0 to 7.5 pH values surrounded by a 1.5 km radius semi-halo of 5.0 to 6.5 pH values . This pattern corresponds with those observed by SILG. If a geothermal reservoir is responsible for the oxidizing/reduction environment causing the volatilization of select ions through the substrate, then we would also expect to see a decrease in pH caused by the release of free hydrogen ions. Spikes of acidic pH values (5.0-6.0) were also observed along sub alluvium faults indicated by SILG and soil gas-flux analyses some of which are blind; others are inferred from fault scarps. Soil pH variations due to vegetation, pollution and wet-precipitation were observed and must be adjusted for. Despite these variables and restrictions, the soil pH method proved to be a cheap, fast and reproducable substitute for SILG that costs more than 20 dollars/sample and takes several weeks to perform. Our data suggest that soil pH analyses can be used for exploration of buried geothermal reservoirs lacking surficial manifestations.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1065 Major and trace element geochemistry
DE: 1194 Instruments and techniques
DE: 1894 Instruments and techniques: modeling
SC: Tectonophysics [T]
MN: Fall Meeting 2005