AQUEOUS GEOCHEMISTRY OF THERMAL WATERS IN THE ALVORD BASIN, OREGON

The Alvord Basin is located in Harney County, Oregon and is included in the Oregon Plateau of the northern Basin and Range province of the southwestern United States. The basin is a broad, flat, graben valley bounded by several mountain ranges. Faults in the area indicate strike-slip faulting and offset resulting from East-West extension of the plateau. Rock formations in the area are of volcanic origin and sediments on the valley floor include sand, gravel, silt, and lacustrine sediments. Three main groups of hot springs are located in the valley: Borax Lake Hot Springs, Alvord Hot Springs, and Mickey Hot Springs. The geochemistry of these springs has been explored to determine processes contributing to their chemical composition. Several springs from each location were sampled on 5 separate occasions over the course of a year. Thermal waters in the area are characterized as mixed bicarbonate-chloride waters with significant levels of sulfate. These waters also have high levels of As (1-5 mg/L) and B (8-29 mg/L). There is little variation from spring to spring within each group; however, differences are evident between spring groups. The chemical compositions of the springs have not varied considerably throughout the year. Waters from Alvord Springs are the most concentrated with the highest levels of total dissolved solids, Cl^-, K⁺, Mg²⁺, and Na⁺. Waters from Borax Springs have the highest concentrations of SO₄^2- and Ca²⁺. Waters from Mickey Springs have lowest levels of Mg²⁺. Surface temperatures of the hot springs range from 30°C to over 90°C, while estimates of reservoir temperatures by chemical geothermometry vary from 120°C to over 220°C. In general, calculated reservoir temperatures are highest for Mickey Springs and lowest for Borax Springs. Thermal springs in the Alvord Basin originate from meteoric water that circulates through the crust gaining heat and chemical constituents. There is minimal mixing of deep thermal waters with shallow groundwaters. Companion isotopic studies and geochemical modeling should shed additional light on the origin of these waters.