Summary:
Silver Peak mine is located in the Clayton Valley, an arid valley historically covered with dry lake beds (playas).
The lithium resource is hosted as a solute in a predominantly sodium chloride brine. As such, the term mineralization is not wholly relevant, as the brine is mobile and can be affected by pumping of groundwater and by local hydrogeological variations. Davis et al. (1986) suggest that the current levels of lithium dissolved in brine originate from relatively recent dissolution of halite by meteoric waters that have penetrated the playa in the last 10,000 years; they suggest that the halite formed in the playa during the aforementioned climatic periods of low precipitation and that the concentrated lithium was incorporated as liquid inclusions into the halite crystals. Davis et al. (1986) are not specific about the ultimate source of the lithium.
Zampirro (2004) points to the lithium-rich rhyolitic tuff on the eastern margin of the basin as a possible source of the lithium in brine. In this regard, Zampirro (2004) agrees with previous authors (Kunasz, 1970, and Price et al., 2000); he also notes the potential role of geothermal waters, either in leaching lithium from the tuff or transporting lithium from the deep-seated magma chamber that was the source for the tuff.
In evaluating results from isotopic analysis of water and brine samples from throughout Clayton Valley, Munk et al. (2011) identified a complex array of processes affecting brine composition, depending on location. For brine from the Shallow Ash System, Munk et al. (2011) identified a process that was consistent with that suggested above by Davis et al. (1986); their results support a process whereby lithium was co-concentrated with chloride and then trapped in precipitated sodium chloride (halite) crystals.
However, in brine samples from other locations, Munk et al. (2011) found evidence that lithium did not co-concentrate with chloride and that it was introduced to the brine at levels that were already elevated. Munk et al.'s (2011) results were consistent with lithium leached from hectorite (a lithium-bearing clay mineral), and they identified two possible mechanisms for accumulation in the basin. The first process involves contact between water and hectorite to the east of the basin, with subsequent transport into the basin. The second process involves leaching of hectorite within the basin deposits, where it formed through alteration of volcanic sediments.
Previous work at the site and in Clayton Valley resulted in the definition of a six lithium-bearing aquifer system (Zampirro, 2003), as described below from depth to surface.
The aquifer systems include Lower Gravel Aquifer (LGA), Lower Aquifer System (LAS), Main Ash Aquifer (MAA), Marginal Gravel Aquifer (MGA), Tufa Aquifer System (TAS), Salt Aquifer System (SAS).
Lower Gravel Aquifer (LGA)
The LGA is the deepest aquifer and consists of gravel with a sand and silt matrix interlayered with clean gravel; it is considered alluvial material formed from the progradation of alluvial fans into the basin. Gravel clasts are limestone, dolomite, marble, pumice, siltstone, and sandstone. Zampirro (2003) reports thicknesses from 25 to over 350 ft. Ten wells drilled in 2021 and 2022 reached the base of the LGA. Thickness of the LGA in these ten wells ranged from approximately 105 to approximately 620 ft.
Lower Aquifer System (LAS)
This unit consists of air-fall and reworked ash, likely from multiple volcanic sources (Davis and Vine, 1979). The individual ash beds within the LAS are variably continuous and can occur as lenses or discontinuous beds and extensive units. Zampirro (2003) reports that this unit ranges from 350 to 1,000 ft below ground surface (bgs). The unit is interpreted to be moderately continuous north of the Cross Central Fault. An inferred origin for some of the thinner lenses may be as pluvial events carrying reworked ash possibly from surrounding highland areas into the lake environment. Permeability in the LAS is limited due to narrow lenses of ash of lesser continuity.
Main Ash Aquifer (MAA)
This unit consists of air-fall and reworked ash. Particles range in size from submicroscopic to several inches or more (ash to pumice). The Long Valley caldera eruption and ash from the Bishop Tuff (760,000 years before present) is presumed to be the source of the MAA. Zampirro (2003) reported thicknesses of 5 to 30 ft, and the depth to MAA ranges from 200 ft in the southwest to over 750 ft in the northeast. The MAA is considered a marker bed because of its continuity throughout the northeastern part of the playa.
Marginal Gravel Aquifer (MGA)
The sediments of this unit are silt, sand, and gravel. The MGA is interpreted to be alluvial fan deposits along the east-to-northeast-trending faults (Angel Fault and Paymaster Fault) where the majority of basin drop has occurred. Gravels were presumed to erode from the bedrock in the footwall of the fault (Zampirro, 2003). The faults are interpreted to act as hydraulic barriers between the brines and freshwater.
Tufa Aquifer System (TAS)
The TAS lies in the northwest sector of the playa. The unit consists of travertine deposits, likely from either subaqueous vents that discharged fluid into the ancient lake or surficial hot spring terraces composed of calcium carbonate (CaCO3). Limited drillholes indicate ring-like tufa or travertine formation (Zampirro, 2003).
Salt Aquifer System (SAS)
The SAS lies in the northeastern portion of the playa coincident with the lowest point of the valley. The SAS was formed by deposition in an arid lake and precipitation of salts (evaporites), primarily halite, from ponded water. The unit includes lenses of salts from fractions of an inch to 70 ft in thickness with interbeds of clay, some silt and sand, and minor amounts of gypsum, ash, and organic matter. Some dissolution caverns are present, which can develop into sinkholes when pumped. Salt likely precipitated in lowland standing water by concentration of minerals through evaporation. Deeper salt beds are more compact.