James Ingraffia—Tuesday, April 21 at 3:30 PM
Gilberto Martinez—Monday, April 27 at 3:00 PM

M.S. Thesis Defense: James T. Ingraffia
Stratigraphy, Geochemistry, and a Possible Origin for the Thacker Pass Lithium Clay Deposit Humboldt County, Nevada, USA
April 21 at 3:30 PM via Zoom; Advisor: Mike Ressel

(Contact Mike Ressel mressel@unr.edu for meeting access.)

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 deposited inside the south end of the mid Miocene ~1000 km2 McDermitt caldera. High‐grade Li zone (0.4 to 0.8 %) occurs in Li‐bearing illite clays also enriched in Mg, Rb, Cs, and F. Other minerals occurring with high‐grade Li include abundant Mg‐calcite, fluorite, pyrite, marcasite, and quartz. Silicification in the form of fine‐grained quartz, including drusy quartz in open‐space cavities, is present 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. Li ores contain high concentrations of As, Sb, Hg, Tl, Mo, and S, mainly in Fe sulfides.

The origin of the Thacker Pass Li deposit is controversial. High Li contents (>1600 ppm) in quartz melt inclusions from rhyolites indicate high initial Li contents in magmas prior to degassing. 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 and later precipitation through cool, shallow‐burial diagenesis within an ephemeral alkali lake setting. However, simple Li mass balance determinations do not explain all of the Li within the McDermitt caldera moat sediments. Other models invoke a combination of diagenetic and hydrothermal processes to account for Li concentration at Thacker Pass. 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.

Gilberto Martinez
Monday, April 27 at 3:00 PM.
Tommy Thompson and Mike Ressel, advisors;
Ph.D. thesis title: Productive Versus Non-productive Porphyry Systems Surrounding the Peñasquito Diatreme-Porphyry System, Zacatecas, Mexico.

Zoom defense, contact: Mike Ressel, mressel@unr.edu for meeting ID and password.

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