The Thacker Pass Deposit sits sub-horizontally beneath a thin alluvial cover at Thacker Pass and is partially exposed at the surface. The Thacker Pass Deposit contains the targeted multi-phase mining development of the Thacker Pass Project. It lies at relatively low elevations (between 1,500 m and 1,300 m) in moat caldera lake sediments that have been separated from the topographically higher deposits to the north. Exposures of the sedimentary rocks at Thacker Pass are limited to a few drainages and isolated road cuts. Therefore, the stratigraphic sequence in the deposit is primarily derived from core drilling.

The sedimentary section, which has a maximum drilled thickness of about 160 m, consists of alternating layers of thick claystone and thin volcanic ash. The claystone comprises 40% to 90% of the section. In many intervals, the claystone and ash are intimately intermixed. The claystones are variably brown, tan, gray, bluish-gray and black, whereas the ash is generally white or very light gray. Individual claystone-rich units may laterally reach distances of more than 152 m; though unit thickness can vary by as much as 20%. Ash-rich layers are more variable and appear to have some textures that suggest reworking. All units exhibit finely graded bedding and laminar textures that imply a shallow lacustrine (lake) depositional environment.

Surficial oxidation persists to depths of 15 m to 30 m in the moat sedimentary rock. Oxidized claystone is brown, tan, or light greenish-tan and contains iron oxide, whereas ash is white with some orange-brown iron oxide. The transition from oxidized to unoxidized rock occurs over intervals as much as 4.5 m thick.

The moat sedimentary section at Thacker Pass overlies the intra-caldera Tuff of Long Ridge. A zone of weakly to strongly silicified sedimentary rock, the Hot Pond Zone (HPZ), occurs at the base of the sedimentary section above the Tuff of Long Ridge in most of the cores retrieved from the Thacker Pass Deposit. Both the HPZ and the underlying Tuff of Long Ridge are generally oxidized.

Lithium enrichment in the Thacker Pass Deposit and deposits of the Montana Mountains occur in the lowest portions of the caldera lake sedimentary sequence, just above the intra-caldera Tuff of Long Ridge. The uplift of the Montana Mountains during both caldera resurgence and Basin and Range faulting led to increased rates of weathering and erosion of a large volume of caldera lake sediments. As a result, the deposits of the Montana Mountains have minimal overburden and the Li-enriched interval occurs close to the surface. Along the southern and eastern margins of the Montana Mountains, caldera lake sediments dip slightly away from the center of resurgence.

Dips on the sediments in the vicinity of the Thacker Pass Deposit were slightly restored during the collapse event associated with the Tuff of Thacker Creek; most of the sediments within this deposit are sub-horizontal. Because of the lower elevations in Thacker Pass, a smaller volume of the overburden eroded south of the Montana Mountains. As a result, the amount of overburden increases with distance from the Montana Mountains. The proposed pit mining activity is concentrated along the southern margin of the Montana Mountains in Thacker Pass where lithium enrichment is close to the surface with minimal overburden.

The shallow and massive nature of the deposit makes it amenable to open pit mining methods. The mining method chosen is a modified panel mining method which employs excavators and surface miners. In this method, a section along the length of the pit is mined to the entire width and depth before moving to the next section of the pit. The ore body is perfectly set up for this as it is massive and the floor is fairly consistent.

Waste removal will be done by means of an excavator and haul truck operation. Once the ore has been exposed and a running surface prepared to a relatively consistent profile, the excavator will move to the next panel section. Following the waste removal, the surface miner will mine the exposed ore and load the haul trucks directly.

The ore will be hauled to the head of an overland ore conveyor or to nearby short-term stockpiles. A front-end-loader will be used for any rehandling of ore and for managing the short-term stockpiles.

During the first year of pre-production, mine waste will be hauled to the plant site to be used for construction fill material and will also be used to construct the tailings embankment. During the second year of preproduction, mine waste continues to be used for construction with any excess mine waste hauled directly to the waste dump. The waste dump has been designed to accommodate sufficient material such that when it is complete the remaining waste mined for the life of the mine can be backfilled directly into the mined-out pits, less any that is used for subsequent tailing embankment construction.

Due to the sequence of mining, the majority of in- pit ramps will be temporary, and some will be built on backfill. Exposure to final pit walls will also be temporary.

A bench height of 5 m was chosen to limit dilution. Double benching was included to increase the bench widths while still maintaining the inter-ramp slope requirements.

Three (3) geotechnical zones were included in the pit design. A delineation between soil and bedrock occurs around 30 m depth. The inter-ramp angle for the soil is 25 degrees for all areas of the pit. For total pit wall depths less than 90 m, the bedrock slope is 47 degrees. Areas of the pit with wall depths between 90 m and 120 m have a bedrock inter-ramp angle of 39 degrees.

The recovery process consists of the following major components:
i) Ore preparation and leaching and
ii) lithium processing.

The Ore Preparation will prime the ore for lithium extraction in a leaching circuit. Ore will be delivered to the run of mine (“ROM”) stockpile from the mining operation. The ore in the ROM stockpile will be sized using toothed roll crusher (sizer) to optimize the efficiency of leaching lithium.

The ore will be transferred from ROM by reclaim feeders to a toothed roll crusher for size reduction prior to being mixed with filter wash solution in attrition scrubbers. The clay will readily disengage from coarse gangue size fraction during scrubbing. The gangue (defined as screen oversize material) will be transferred to the clay tailings facility for storage and reclamation.

After Ore Preparation, the ore will be transferred as a slurry to the Leaching circuit. Sulfuric acid will be mixed in with the slurry to liberate the lithium from the clay. The leaching process will take place in stirred reactors designed to (a) maximum lithium dissolution from the ore and (b optimize sulfuric acid consumption. The lithium-bearing solution, i.e. “lithium brine”, will be separated from the leach residue by filtration. The filtered residue will be washed to recover any remaining free lithium, and then conveyed to the clay tailings facility. The wash solution will be recycled to the slurry ore in the attrition scrubbers. Crushed limestone and residue from the neutralization filters will be added to the leach clay residue to produce a geotechnically stable clay tailings.

To prepare the lithium brine for subsequent processing, pH-neutralization is required. Neutralization will predominantly use high-pH hydroxide-based filter residue from the liming process. Limestone may be intermittently added for nominal pH control. Waste solid compounds will precipitate from the neutralization step and will be filtered from the lithium brine. The filter residue will be washed with process water to recover any residual lithium, resulting in an enriched wash solution. The wash solution and lithium brine will be combined and processed in the lithium processing plant. The resultant lithium brine is a sulfate solution dominated by lithium, magnesium, potassium, and sodium cations.

The Lithium Processing Plant will take the lithium brine from the Ore Preparation and Leaching circuit and separate out lithium from the remaining salts in the brine, i.e. magnesium, potassium and sodium. The Plant will produce lithium chemicals that meet or exceed market requirements, including use in making lithium batteries.

The first step in lithium separation involves purifying the lithium brine through crystallization of magnesium sulfate, followed by removal of residual magnesium with the addition of quicklime. Soda ash will then be added to the brine to precipitate out lithium as a carbonate solid. The remaining solution, containing potassium and sodium sulfates, will be put through another crystallization step to remove the remaining salts as solid salt tailings. The left-over water will be recovered and returned to the process.

The Magnesium Sulfate evaporator and crystallizer will be used to remove magnesium sulfate from the lithium brine. The evaporated water produced from this step will be reused in upstream processes. The magnesium sulfate salt will be conveyed to a dedicated storage facility. In the future, the magnesium sulfate salt, otherwise known as Epsom salt, could be sold to a variety of industrial and agricultural end users.

The purpose of the liming step is to remove residual magnesium and calcium impurities. The lithium brine from the magnesium sulfate crystallizer will be reacted with quicklime to increase the pH, resulting in precipitation of magnesium hydroxide and calcium sulfate crystals. The alkaline solids will be filtered and transferred to a tank in the Neutralization area. Transferring the alkaline solids to the Neutralization tank beneficially raises the pH of the acidic slurry from the Leaching circuit and further recovers lithium contained in the alkaline residue.

Following liming, the purified lithium brine will be transferred to a Lithium Carbonate Precipitation circuit. The precipitation system will react the lithium in the lithium brine with saturated soda ash solution to create lithium carbonate solids. The lithium carbonate solids will be filtered from the barren brine and washed with water. The precipitation and filtration system will be engineered to produce consistent, high quality product that will meet or exceed a variety of industry standards, namely specifications corresponding to lithium ion batteries. The purified lithium carbonate will be dried and packaged in bulk bags for shipment to customers.

The barren brine drawn from the lithium carbonate process will be reacted with sulfuric acid to remove any remaining carbonates. A zero-liquid discharge crystallizer will recover the water and precipitate a combination of sodium and potassium sulfate salts. The sodium and potassium sulfate salt mixture will be transferred to a dedicated storage facility. In the future, the salt mixture could be processed to yield potassium sulfate and sodium sulfate for sale into the fertilizer and industrial markets.

The zero-liquid discharge crystallizer will purge a small amount of barren lithium brine back to the liming system to maintain a predetermined lithium concentration in the crystallizer that will optimize lithium recovery.