Summary:
Project Geology
The western portion of the project area is dominated by the uplifted basement rocks of Angel Island which consist of metavolcanic and clastic rocks, and colluvium. The southern and eastern portions are dominated by uplifted, lacustrine sedimentary units of the Esmeralda Formation. Within the project area, the Esmeralda Formation is comprised of fine grained sedimentary and tuffaceous units, with some occasionally pronounced local undulation and minor faulting. Elevated lithium concentrations, generally greater than 600 ppm, are encountered in the local sedimentary units of the Esmeralda Formation from surface to at least 142 meters below surface grade (mbsg). The lithium-bearing sediments primarily occur as silica-rich, moderately calcareous, interbedded tuffaceous mudstone, claystone, and siltstone.
Deposit Types
The Clayton Valley deposit is reasonably well represented by the United States Geological Survey preliminary deposit model, which describes the most readily ascertainable attributes of such deposits as lightcolored, ash-rich, lacustrine rocks containing swelling clays, occurring within hydrologically closed basins with some abundance of proximal silicic volcanic rocks. The geometry of the Clayton Valley deposit is roughly tabular, with the lithium concentrated in gently dipping, locally undulating, sedimentary strata of the Esmeralda Formation. The sedimentary units are interbedded silica-rich, ash-rich mudstone and claystone, with interbeds of sandy and tuffaceous mudstone/siltstone and occasional poorly cemented silt and sandstone. The lithium concentrations are highest within the mudstone and claystone, but lithium is still also present in a siltstone unit underlying the claystone.
The deposition of the lithium-rich sediments likely occurred late in the history of the associated paleo brine lake, based largely on the stratigraphic position of the mudstone and claystone above the thick overall sandstone- and siltstone-dominated basin fill events. Such a setting would be ideal for the concentration of lithium from ash and groundwater inputs over an extensive period. As a result, the lithium-rich strata may represent several million years of lithium input and concentration within the basin. Figure 8-1 through Figure 8-3 show a conceptual sequence of depositional, erosional, and structural events which may account for the present-day nature and occurrence of the lithium deposits.
Within the lithium-bearing sediments of the deposit are oxidation and unaltered horizons that are recognizable in drill cores. Based on the drilling to date, the highest lithium concentrations occur within claystone zone 2 which has a central unaltered zone inter-layered between two oxidized layers. This distribution of mineralization may be the result of recent, oxidizing surface waters penetrating down dip within more permeable beds of the sedimentary package to create a series of oxidation-unaltered layers.
Mineralization
Elevated lithium concentrations, generally > 600 ppm, are encountered in the local sedimentary units of the Esmeralda Formation from surface to at least 142 mbsg. The lithium-bearing sediments primarily occur as silica-rich, moderately calcareous, interbedded tuffaceous mudstone, claystone, and siltstone. The overall mineralized sedimentary suite is a laterally and vertically extensive, roughly tabular zone with at least two prominent oxidation horizons (Figure 7-4). The primary area of mineralization is in a claystone unit consisting of three zones: oxidized claystone, unaltered claystone, and an oxidized claystone. The claystone unit is overlain by tuffaceous mudstone in the eastern portion of the project and underlain by a siltstone. Elevated lithium concentrations occur in all the uplifted lacustrine strata encountered; however, lithium concentrations are notably higher and more consistent in the claystone unit.