Lithium clays at McDermitt caldera, Nevada: M.S. thesis defense (Apr. 2020)

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2020Apr 29

0:00 Intro, 6:23 Presentation Thesis defense by me, J. Ingraffia, and advised by Mike Ressel, Ph.D. April 21st, 2020. Contacts below. Reach Mike Ressel, Ph.D. YouTube: / @mikeressel3812 LinkedIn: / mike-ressel-20a60034 Reach James Ingraffia, M.S., M.B.A. (IP) LinkedIn: / jamesingraffia A peer-reviewed paper of this content is being published by the Geological Society of Nevada (https://gsnv.org) in the June 2020 proceedings. The complete thesis of this content will also be published in June 2020. If you would like to know more, be involved with Li work and research, feel free to reach out to myself or Dr. Ressel at our LinkedIn pages above. Enjoy! Abstract: Thacker Pass is the world’s largest Li clay reserve. Ore-grade Li occurs in a ~100 m thick sequence of interbedded lacustrine shale and siltstone, and thin, light gray rhyolitic ash beds. The strata were deposited inside the south end of the mid-Miocene ~1000 sq. km McDermitt caldera. The welded caldera-forming McDermitt Tuff (Tuff of Long Ridge) underlies the deposit, and two basalt lava flows cap the deposit. A ~15-30 m—thick low-grade Li-bearing zone (0.2 to 0.4% Li) occurs above the ore zone in smectite-altered shale alternating with numerous rhyolite tephras. The underlying high-grade Li zone (0.4 to 0.8 %), which contains the bulk of Thacker Pass reserves, is comprised of a 15-30 m thick shale-rich section of Li-bearing illite clays (Morissette, 2012) also enriched in Mg, Rb, Cs, and F. Other minerals occurring with high-grade Li include abundant Mg-calcite, fluorite, pyrite and marcasite. Silicification in the form of fine-grained quartz, including drusy quartz in open-space cavities, are present, particularly at the deposit’s base. Paragenetic relationships indicate the illite and pyrite likely formed contemporaneously at early stages; calcite nodules formed later and were replaced by fluorite. The origin of the Thacker Pass Li deposit is controversial. High Li contents (~1154-1646 ppm) in quartz melt inclusions from alkaline rhyolites at McDermitt are interpreted to indicate high initial Li contents in magmas prior to degassing and therefore, an ultimate volcanic source (Benson and others, 2017a). Because of closed-basin hydrologic conditions of the McDermitt intracaldera lake, prevailing models for Li concentration postulate leaching and redistribution of Li from Li-rich rhyolitic glass by meteoric water and precipitation through cool, shallow-burial diagenesis within an ephemeral alkali lake setting without hydrothermal input. However, simple Li mass balance determinations do not explain all of the Li within the McDermitt caldera moat sediments (Castor and Henry, 2020). Other models invoke a combination of diagenetic and hydrothermal processes to account for Li concentration at Thacker Pass (Benson and others, 2017a). We suggest that the degassing ~1000 cu. km largely intracaldera McDermitt Tuff ultimately sourced most Li at Thacker Pass and other areas containing Li-bearing sediments at McDermitt caldera. High initial temperatures indicated by the tuff’s rheomorphic textures and anhydrous mafic mineralogy would have made the degassing of magmatic volatiles entrained within the ash-flow a more efficient process. A caldera lake developed immediately after caldera collapse and was sustained during and after resurgence. We suggest that degassing of the McDermitt Tuff during compaction and devitrification released large quantities of Li and other large-ion lithophile elements, part of which were eventually sequestered in groundwaters and in lake sediments after Li was deposited in fumarolic sublimates at and near the caldera floor. Hydrothermal circulation associated first with the initial high thermal gradient associated with caldera volcanism (e.g. fumarolic activity) and with later resurgence-related intrusion, solubilized and mobilized lithium, associated alkalis, alkaline earths, and common ore-related elements in the near-surface lacustrine setting. The interface between degassing tuff and overlying sediments, in part in a subaqueous setting, served as the locus for Li concentration, whereas closed-basin diagenesis may have further enhanced alkali enrichment, particularly under more arid conditions. Key Words: lithium clays, Thacker Pass, McDermitt caldera, caldera lake, degassing, hydrothermal alteration, devitrification