Lithium at the Thacker Pass deposit, Humboldt County, Nevada, U.S.A.
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Authors
Ingraffia, James T.
Issue Date
2020
Type
Thesis
Language
Keywords
Economics , Hectorite , Illite , Lithium , McDermitt , Thacker
Alternative Title
Abstract
Lithium (Li) is a primary component of modern rechargeable batteries manufactured at exponentially increasing volumes to satisfy the global demand and shift to electrification of the automotive transportation sector. The Thacker Pass deposit, located in Humboldt County, north-central Nevada, is the world’s largest Li clay reserve with ~1.56 Mt of Li supply. Ore-grade lithium occurs in a ~100-m-thick sequence of interbedded lacustrine dark greenish brown to dark greenish gray carbonaceous shale and volcaniclastic siltstone, and thin, light gray rhyolitic ash beds. The strata were deposited inside the south end of the mid-Miocene ~1000 km2 McDermitt caldera. Pyritic lacustrine strata rest unconformably on an intensely oxidized surface of welded McDermitt Tuff, an intracaldera peralkaline, high-silica rhyolite dated between ~ 16.33 - 16.39 Ma that occupies 1000 km3 of space within the caldera and beneath the intracaldera sedimentary basin (Benson et al., 2017a,b; Henry et al., 2017). Li-bearing lacustrine strata and the McDermitt Tuff are tilted 5-15˚SE and the lacustrine rocks thicken to the south and east; tilting is consistent with caldera resurgence. The Thacker Pass deposit is faulted on its north by E-striking, S-dipping normal faults. Tholeiitic basalts locally surround and cap the lithium clay deposit, and give an 40Ar/39Ar age of 14.9± 0.74 Ma (Henry et al., 2017).The Thacker Pass stratigraphy has no outcrop, and this study has been conducted entirely from drill core. The lithologies have been divided into 5 units, from youngest to oldest, of shale, tephra, basalt, and a basal lithic tuff. The uppermost unit of the examined lithium deposit area is capped by two layers of basalts intercalated by silty shales and tephra. The low grade Li zone (LGZ; 0.2 �" 0.4 wt. %) occurs in the second and third unit from the top, is ~50-100 ft. thick, occurs in light brown laminated shales spatially associated with >5 layers of altered tephra, and is smectitic (hectorite; Castor, 2010). The LGZ is underlain by the high-grade Li zone (HGZ; 0.4 �" 0.8 wt. %), which is comprised by Li-bearing illite clays (tainiolite?; Morisette, 2012) that constitute a 50 - 100-thick section of replaced shaley beds in the third and fourth units from the top. The LGZ:HGZ boundary is clearly apparent in stratigraphy by bleached and replaced intervals of shale surrounding fractures and slickensides associated with sharp chemical increases of Li (+ 0.15 wt. %), As (> 200 ppm above background concentrations; Turekian and Wedepohl, 1961) and S (>2.0 wt. %). The LGZ:HGZ horizon is also marked by a redox boundary, visible by the appearance of ubiquitous pyrite in the HGZ. Li grades generally continue to increase with depth, with variable rises in associated elements (e.g. As, S, Mo, F) and with a consistent background concentration of 0.1-0.2 wt. % Ti. The HGZ ends 25-30 ft. above the oxidized and lithic-rich McDermitt tuff. Within this final 25-30 ft of lacustrine sediments are silicified zones that carry up to 940 ppm Zr. The deposit LGZ and HGZ are never seen without Ca-bearing minerals, either Fe-rich dolomitic calcite in the LGZ, pure calcite in an LGZ-HGZ transition zone, or calcite pseudomorphically replaced by purple or light grey fluorite in the HGZ. Lithium ore minerals are also consistently associated with Fe in the form of chlorite, hematite, secondary pyrite, and marcasite; the presence of sulfides is dependent in part upon the localized oxidation state of the ore rock. Other minerals observed in the deposit are analcime in the LGZ, K-feldspar in the HGZ, and secondary or detrital quartz throughout. HGZ euhedral pyrite was found to contain As and Mo and occurred as disseminations, veinlets, and bedding-parallel bands within individual lacustrine? units. Analyses of ore Li-illites via SHRIMP microprobe indicate a mean of 2.11 wt. % Li, a substitute for Mg within the octahedral layer of these sheet silicates. This is higher than the concentration predicted by the published formula [KLiMg2(Si4O10)F2] of the Li-mica tainiolite, a mineral that shares the same stoichiometric chemistry as the Thacker Pass Li-illite. This economic implication is a direct contribution to the deposit bottom line profitability. Investigation of the Thacker Pass lithium deposit origin has been renewed since its initial descriptions in the late 1970s (e.g. Glanzman and others, 1978; Glanzman and Rytuba, 1979). High Li contents (~1482-1646 ppm) in pre-degassed quartz melt inclusions within the McDermitt Tuff indicate high initial Li contents in magmas, suggest an ultimate volcanic source, and substantiate a mass balance of 29 Mt of lithium that was available for geologic intracaldera mobilization (Benson et al., 2017a). Castor and Henry (2020) investigated the potential of intracaldera sediment lithium enrichment as an exclusive result of leaching and redistribution from Li-rich rhyolitic tephra glass, released by low temperature diagenetic processes. They calculated an available total of 22.4 �" 40.0 Mt of Li within the sediments from that tephritic input, but found those totals to be insufficient and that other geologic processes/sources must be necessary to account for the observed Li enrichment throughout the greater McDermitt caldera. This study postulates that degassing and devitrification of the underlying McDermitt Tuff was responsible for the enrichment of Li in the caldera and Thacker Pass deposit. Observation of vapor phase altered, degassed and devitrified McDermitt Tuff with 30-40 ppm Li beneath the Thacker Pass Li-clay deposit. This supported the hypothesis that Li, and other elements of high concentrations within the deposit, were released on eruption of the McDermitt Tuff which functioned as the principal Li source. Multiple counts of textural and geochemical evidence, indicative of hydrothermal activity within the ore zones, makes clear that hydrothermal input was significant in secondarily concentrating the lithium at Thacker Pass. These findings of the McDermitt Tuff as the principal Li source and Li’s subsequent redistribution through hydrothermal activity related to caldera magmatism, were compared to those suggestive of a solely diagenetic origin for Li ore genesis in a post-caldera arid lake setting. The resultant model exhibits Li (likely complexed as a fluoride) transported via high-temperature gases and brought to the intracaldera surface by fumarolic activity as the ~1 km-thick intracaldera McDermitt Tuff degassed. A layer of volcanogenic sublimates enriched in lithium and other elements (less mobile at relatively low temperatures) formed on the ancient intracaldera surface. The Li-rich and relatively soluble sublimate layer would have provided an enormous quantity of surface Li, available for remobilization and concentration by a combination of hydrothermal and diagenetic processes. Li-associated elements fixed in caldera sublimates, and therefore also available for further concentration by secondary processes, included Na, K, Cs, Rb, Be, Mg, As, Sb, Mo, S, and F.
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Creative Commons Attribution 4.0 United States