Thacker Pass Lithium Clay Deposit, McDermitt Caldera, North-Central Nevada: Devitrification of McDermitt Tuff as the Main Lithium Source

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 midMiocene ~1000 km2 McDermitt caldera. The welded caldera-forming McDermitt Tuff
(Tuff of Long Ridge) underlies the deposit, and two basalt lava flows locally cap the
deposit. A ~15 to 30 m—thick low-grade Li-bearing zone (0.2 to 0.4 wt.% 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 wt.%), which contains the bulk
of Thacker Pass reserves, is comprised of a 15 to 30 m thick shale-rich section of Libearing 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 to 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 closedbasin 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 km3 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