LITHIUM AT THACKER PASS DEPOSIT, MCDERMITT CALDERA, NORTH-CENTRAL NEVADA: HYDROTHERMAL OR FOSSIL BRINE ORIGIN IN AN INTRACALDERA LACUSTRINE SETTING?

Thacker Pass (TP), the world’s largest Li clay reserve, occurs in a 100 m thick sequence of interbedded lacustrine shale, volcaniclastic siltstone, and thin rhyolitic tephras deposited at the south end of the >1000 km² McDermitt caldera. Tephras are good markers because they have relatively homogeneous, silicic compositions. Tephras at the top of the section locally preserve glass shards, whereas tephras in the underlying TP lithium deposit are mostly or entirely altered to clays. Clays are vertically zoned from smectite and I-S above the high Li zone, to illite within the productive zone, to smectite at the base (Ehsani et al., 2018, Lithium Americas Corp. published prefeasibility study). The strata are locally capped by thin olivine basalt flows and floored by the ~16.4 Ma intracaldera McDermitt tuff, a high-Si rhyolite exposed in the central resurgent dome (Henry et al., 2017) and intersected by drilling. High Li (>50 ppm) occurs in all lacustrine rocks but appears most concentrated (0.3-0.9 wt. %) in medium gray to black shale crosscut by abundant veinlets and breccia infill containing illite and variable amounts of alkali feldspar, drusy quartz, pyrite, fluorite and calcite. Li strongly correlates with K, Rb, Cs, Be, Mg, and F (Group 1), constituents of clays at TP , and negatively correlates with Al. Rocks with high Li contain correlative S, As, Sb, Hg, Tl, and Mo (Group 2), which likely occur in sulfides. Elements of Group 1 and 2 share weak to moderate positive correlations. Vein and breccia textures indicate the involvement of a fluid(s) that precipitated a mineral assemblage containing both Group 1 and 2 elements.

The origin of Li at TP is controversial. Likely, Li was ultimately sourced from peralkaline to weakly peraluminous igneous rocks (Benson et al., 2017). Possibilities of how the Li deposit formed at TP include a primary diagenetic origin involving a low-T brine, a hydrothermal origin related to post-caldera magmatism, or a combination of these processes. Our preliminary studies test origins of Li at TP through: 1) detailed core logging to examine the stratigraphy as it relates to the spatial distribution of Li and other alkali elements, 2) petrographic and SEM characterization of the alteration and vein assemblages, and 3) geochemical assessment of Li-bearing rocks to identify and interpret elemental associations.