SOURCE CHARACTERIZATION OF THE BORAX LAKE GEOTHERMAL SYSTEM, ALVORD BASIN, OREGON

The Borax Lake geothermal system comprises approximately 175 springs linearly aligned along a northeast-southwest trending normal fault in the Alvord Basin of southeast Oregon. The reservoir is located on the edge of the Great Basin, where geothermal systems arising from extensional tectonics and high crustal heat flow are common. As a result, previous investigations of the Borax Lake springs have concluded the thermal discharge is of non-magmatic origin; however, the level of boron in the geothermal discharge is more commonly associated with magmatic geothermal systems. The fluids discharging from the Borax Lake system are sodium-bicarbonate-chloride-type waters with high sulfate content. The average pH of the springs is 7.3, and pH ranges from 5.8 to 8.6. Geothermometry studies have suggested deep reservoir temperatures between 200 – 250^oC, although spring temperatures at the land surface average 62.1^oC, and range between 23.9^oC and 95.5^oC.

To assess the origin of the Borax Lake geothermal fluids, we analyzed filtered water samples from 175 geothermal springs and wells for major and trace elements. We found that major cation chemistries trend from juvenile waters in a sample obtained from an artesian geothermal well towards more equilibrated water/rock systematics in the lower-temperature springs along the trace of the Borax Lake fault. In addition to the unusually high boron content (13.5 ppm) of the geothermal discharge, the high SO4^2-/Cl^- and low Mg²⁺/Na⁺ ratios are also characteristic of a magmatic influence on the water chemistry. There are two likely explanations for these chemical trends in the Borax Lake geothermal system: the fluid chemistry may be influenced by a magma chamber at depth, or the fluids may be non-magmatic, but have had significant interaction with silicic volcanics at depth. Because the basin is underlain by mainly basaltic and andesitic rocks, with minor dacitic and tuffaceous extrusives, extensive interaction with silicic rocks appears unlikely. Although not conclusive, our preliminary results are therefore consistent with a magmatic fluid signature in the geothermal discharge.