| Title | Distribution, Isotopic Composition and Origin of Lithium in the Salton Sea Geothermal Field |
|---|---|
| Authors | J. HUMPHREYS, M. BROUNCE, M. A. MCKIBBEN, P. DOBSON, N. PLANAVSKY, H.R. HOOVER, B. WENZEL |
| Year | 2023 |
| Conference | Stanford Geothermal Workshop |
| Keywords | lithium, salton sea geothermal field, isotopes, mineralogy |
| Abstract | Hypersaline brines from the Salton Sea Geothermal Field (SSGF) may contain up to 32 million metric tons of lithium carbonate equivalent, making it one of the largest brine deposits of lithium in the world. With rising global and domestic demand for lithium needed to manufacture electric vehicle batteries, there is increased interest in extracting lithium directly from geothermal brines such as those found in the SSGF. This type of lithium resource is also of interest because it has a smaller footprint and thus the environmental impacts of extracting lithium from deep brines that are already being produced and reinjected are substantially lower compared to more traditional forms of lithium extraction: open pit hard rock mines and evaporative ponds (salars). This study aims to understand the sources of lithium as well as the long-term viability of lithium extraction from the SSGF by quantifying the lithium concentrations in individual minerals within the reservoir rocks and their unaltered equivalents. Understanding the mineralogical sources and sinks of lithium in this system will inform long term viability models, because lithium contained in mineral phases within the geothermal reservoir could potentially recharge the reinjected brine as it becomes depleted over time. Preliminary analyses of polished sections of host rock minerals using laser ablation ICP-MS yield Li concentrations as high as 91 ppm in rhyolite matrix glass and 315 ppm in authigenic chlorite. Lithium abundances are lower in rocks where the major mineral constituents are epidote, alkali feldspars, and anhydrite, with groundmass concentrations of Li in these rocks being as low as 15 ppm. When chlorite is present in addition to these minerals, groundmass Li concentrations reach as high as 173 ppm. Chlorite encasing pyrite and epidote within anhydrite bearing rocks records the highest Li values for a single mineral grain at 315 ppm Li. Unaltered surface rhyolite glasses record Li concentrations up to 91 ppm while their buried counterparts record Li concentrations of up to 68 ppm. Li isotopic compositions of the SSGF reservoir brines are relatively light and narrowly confined (δ7Li 3.7 to 4.7 ‰) compared with global evaporative salar deposits (3 to 41 ‰) and are more akin to global spodumene values (-3 to 11 ‰), likely reflecting the hydrothermal/metamorphic origins of Li in the SSGF. Fresh surface rhyolites show a wider δ7Li variation from 3.5 to 10.3 o/oo, while older buried, altered intrusive and extrusive rhyolites exhibit a tight δ7Li clustering from 6.4 to 7.6 ‰. Metasediment δ7Li values range from 1.8 to 7.8 ‰. Metamorphic phyllosilicates, their sedimentary precursors, and rhyolitic glasses therefore may play key roles as both sources and sinks of Li to the brines. |