DISTRIBUTION AND ISOTOPIC COMPOSITION OF Li IN THE SALTON SEA GEOTHERMAL FIELD

The behavior of lithium during interactions between geothermal brine and their host rocks in the Salton Sea Geothermal Field (SSGF) is under-constrained. Here, we present new measurements of bulk rock and brine Li abundance and δ⁷Li, and in situ Li abundances of minerals from the California State 2-14 scientific drill core as well as commercial wells in the SSGF to identify the mineral hosts of Li and constrain Li behavior during brine-rock interactions.

In rocks recovered from the California State 2-14 drill core, Li concentrations are highest in authigenic chlorite and decrease with increasing depth (270-580 ppm at ~2358 m and 70-100 ppm at ~2882 m); groundmass containing chlorite has up to 250 ppm Li at ~2358 m. Chlorite from ~2358 m depth is observed to encase pyrite in anhydrite-bearing rocks, indicating that mineral precipitation upon reaction between the hot brine and the metasedimentary rocks is important to fixing Li from the brine into the rocks at these depths. Metasediments from State 2-14 have δ⁷Li (relative to LSVEC) = +1.8 - +7.9‰. Below 1.5 km depth, there is a positive correlation between depth (and temperature) and δ⁷Li: rocks from the chlorite-calcite metamorphic zone have δ⁷Li = +2.0 to +4.3‰ and rocks from the deeper, hotter biotite metamorphic zone have δ⁷Li = +4.3 to +7.9‰. Conversely, whole rock Li concentration decreases from ~80 ppm at ~2358m to ~10 ppm at ~2882 m.

The geothermal brines sampled at commercial wells in a 8 km radius of State 2-14 have δ⁷Li = +3.7 to +4.7‰. The values of the rocks and brines overlap with the known δ⁷Li of many Li-bearing fluids and solid Earth reservoirs of Li, obscuring the origin of Li in the Salton Sea Geothermal Field. The difference in Li concentration and δ⁷Li between chlorite-bearing rocks at depth suggests that the partitioning of Li in chlorite between the brine and rock is temperature dependent – lithium becomes more incompatible in chlorite in the biotite metamorphic zone, indicating an important control of temperatures above and below 325ᵒC on the behavior of Li upon brine-rock interaction. Simple closed-system batch modeling using reasonable K and alpha values does not replicate the geothermal system, suggesting open-system behavior when it comes to the behavior of Li within the Salton Sea Geothermal Field.