A RECONSTRUCTION OF WATER-ROCK INTERACTIONS, BASED ON CHEMICAL ANALYSES OF GEOTHERMAL WATERS IN THE ALVORD BASIN, OREGON, USA

We attempted to reconstruct the water-rock interaction history of hydrothermal fluids discharging the Borax Lake fault, in the Alvord Basin of southeast Oregon, USA. The reconstructions used a mass-balance approach, and assumed interaction with the major rock units (basalts, andesites, and tuffs) that comprise the ranges bounding the Alvord Basin. Water discharging from the fault-controlled springs is of the sodium-bicarbonate type, with significant amounts of chloride and sulfate, and is near neutral in pH, ranging in temperature from about 25°C to as high as 95°C. Although we identified inverse alteration reactions that could have resulted in chemistries similar to those observed in the springs, important differences were noted for several major constituents, primarily of Na⁺, HCO^3-, SO4^2-, and Cl^-. Dissolution of large quantities of gypsum and pyrite or other sulfide minerals, or a magmatic input of volatiles, would be required to account for the presence of these species, but such reactions are not consistent with a conceptual model where the hydrothermal system receives recharge from the topographically-high Steens Mountain/Pueblo Mountains, interacting primarily with the Steens Basalt and the andesites and tuffs of the Steens Mountain Volcanics. As an alternative, we hypothesize the hydrothermal fluids discharging from the Borax Lake springs circulate principally within the thick alluvial and lacustrine basin-fill, and have relatively little direct interaction with the surrounding igneous units. This alternative conceptual model provides a mechanism for developing elevated levels of species such as SO4^2-, which would otherwise be present in relatively small quantities, through a process of accumulation and concentration in the basin-fill sediments. The hypothesized reservoir geometry is similar in many respects to models proposed by investigators in the 1970s to explain geophysical observations pointing to relatively shallow lateral flow in hydrothermal reservoirs and, in the case of the Borax Lake system, has additional support from strontium and sulfur isotope data, thermal conduction/convection modeling, and unpublished logs from geothermal exploration wells in the basin.