Summary:
Deposit Types
Lithium enrichment (greater than 1,000 ppm) in the Thacker Pass deposit and deposits of the Montana Mountains occur throughout the caldera lake sedimentary sequence above the intra-caldera Tuff of Long Ridge. The exact cause for the lithium enrichment in the caldera lake sediments is still up for debate. The presence of sedimentary carbonate minerals and magnesium-smectite (hectorite) throughout the lake indicates that the clays formed in a basic, alkaline, closed hydrologic system.
It is likely that two primary mechanisms play a role in the genesis of the Thacker Pass deposit: (1) neoformation of smectite in a closed lake, rich in lithium due to the leaching of nearby and underlying volcanic glass (Benson et al., 2017b); and (2) alteration of a portion of the smectite-bearing clays to illite during intracaldera hydrothermal alteration associated with the uplift of the Montana Mountains.
Caldera lake sediments of the McDermitt Caldera contain elevated lithium concentrations compared to other sedimentary basins. Exploration results support the proposed model and have advanced the understanding of the geology of the Thacker Pass deposit.
Geology
The Project is located within an extinct 40x30 km supervolcano named McDermitt Caldera, which was formed approximately 16.3 million years ago (Ma) as part of a hotspot currently underneath the Yellowstone Plateau. Following an initial eruption and concurrent collapse of the McDermitt Caldera, a large lake formed in the caldera basin. This lake water was extremely enriched in lithium and resulted in the accumulation of lithium-rich clays.
Late volcanic activity uplifted the caldera, draining the lake and bringing the lithium-rich moat sediments to the surface resulting in the near-surface lithium deposit which is the subject of the Project.
The Thacker Pass deposit sits sub-horizontally beneath a thin alluvial cover and is partially exposed at the surface. The sedimentary section consists of alternating layers of claystone and volcanic ash. Basaltic lavas occur intermittently within the sedimentary sequence. The moat sedimentary section at the Project site overlies the indurated intra-caldera Tuff of Long Ridge. A zone of silicified sedimentary rock, the Hot Pond Zone (HPZ), occurs at the base of the sedimentary section above the Tuff of Long Ridge.
Clay in the Thacker Pass deposit includes two distinct types of clay mineral, smectite and illite. Smectite clay occurs at relatively shallow depths in the deposit and contain roughly 2,000 – 4,000 parts per million (ppm) lithium. Higher lithium contents (commonly 4,000 ppm lithium or greater) are typical for illite clay which occurs at relatively moderate to deep depths and contain values approaching 9,000 ppm lithium in terms of whole-rock assay.
Mineralization
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 is the target of a multi-phase mining development as the Thacker Pass Project. It lies at relatively low elevations (between 1,500 m and 1,300 m) in caldera lake sediments that have been separated from the topographically higher deposits to the north due to post-caldera resurgence and Basin and Range normal faulting. Exposures of the sedimentary rocks at Thacker Pass are limited to a few drainages and isolated road cuts. Therefore, the stratigraphic sequence in the Thacker Pass 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 claystone and volcanic ash. Basaltic lavas occur intermittently within the sedimentary sequence. 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 hard, dense, indurated 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.
Most of the moat sedimentary rocks drilled in the Thacker Pass basin contain high levels of lithium (>1,000 ppm). Intervals that consist mostly of ash or volcanic rock have lithium contents of less than 800 ppm whereas intervals dominated by claystone contain more lithium (>1,000 ppm). Many intervals have very high lithium contents (>4,000 ppm). Intervals with extreme lithium contents (>8,000 ppm) occur sporadically in the Thacker Pass deposit.
There is no obvious change in lithium content across the boundary between oxidized and unoxidized rock. The highest lithium grades generally occur in the middle and lower parts of the sedimentary rock section, or in sections where these rocks have been uplifted to surface.
The lithium content of the Thacker Pass deposit claystone can generally be correlated to the color and texture of the rock, as well as the amount of mixed-in ash. Intervals with the highest lithium grades (>4,000 ppm) generally contain gray to dark-gray or black claystone with less than 10% ash. Intervals of bluish-gray claystone with low ash content have moderate lithium content (generally 2,500 ppm to 3,000 ppm). Intervals of light-colored claystone (e.g., tan, light gray, greenish-tan) have lower lithium grades (generally 1,500 ppm to 2,500 ppm). Intervals of mixed claystone and ash are common and have variable lithium contents (generally 1,500 ppm to 3,000 ppm) depending on the type of claystone and proportion of ash present.
Mineralogy
Clay in the Thacker Pass deposit includes two distinctly different mineral types, smectite and illite, based on chemistry and X-ray diffraction (XRD) spectra. Smectite drill intervals contain roughly 2,000 – 4,000 ppm Li. The chemistry and structure of the smectite at McDermitt is most similar to hectorite, a subtype of smectite (Na0,3(Mg,Li)3Si4O10(OH)2), though chemically the clay is intermediate between hectorite and two other smectites, stevensite and saponite (Morissette, 2012). Supported hectorite clay occurs elsewhere in the McDermitt Caldera and has been documented by several authors (e.g., Odom, 1992; Rytuba and Glanzman, 1978; Morissette, 2012; Castor and Henry, 2020).
Drill intervals with higher lithium contents (commonly 4,000 ppm Li or greater) contain clay typical for illite (Morissette, 2012; Castor and Henry, 2020). This illite clay occurs at relative moderate to deep depths in the moat sedimentary section and sporadically occurs in intervals that contain values approaching 9,000 ppm lithium in terms of a whole-rock assay, higher than what a hectorite crystal can accommodate. The Li-rich illite is similar in character to tainiolite, a subtype of illite (K2[Mg4Li2]Si8O20(OH,F)4) (Morissette, 2012; Castor and Henry, 2020).