NEVADA’S LI CLAY DEPOSITS, ESPECIALLY IN THE MCDERMITT CALDERA: GEOLOGIC CHARACTERISTICS AND POSSIBLE CONTRIBUTIONS FROM CLOSED-HYDROLOGIC SYSTEM DIAGENESIS (CHSD) AND HYDROTHERMAL ACTIVITY

The largest Li clay deposits (McDermitt, Rhyolite Ridge, and Tonopah Siebert Fm) are hosted by rhyolitic tuffaceous rocks deposited in closed caldera or tectonic basins that have undergone CHSD; Li smectite is the common ore mineral. Li was almost certainly sourced from rhyolitic rock or magma. All areas had partly coeval rhyolitic magmatism as a potential hydrothermal source, but hydrothermal contribution is uncertain.

Li clay deposits of the 16.4 Ma McDermitt caldera are mostly representative: hosted solely by 16.4-15.7 Ma, post-collapse, rhyolitic tuffaceous sediments deposited in a closed caldera basin. Lower grade (~4000 ppm) Li smectite deposits occur throughout the caldera. Higher grade (to 9000 ppm) Li illite deposits are only known in the southern caldera (Thacker Pass). Dissolution of rhyolitic glass in the sediments during CHSD generated high-TDS, alkaline water that, enriched by evaporation, precipitated Li smectite, zeolites, Kspar, albite, and possibly illite. Questions about McDermitt are origin of the high grade illite and the total source of Li (glass dissolution ± hydrothermal solutions). My very rough mass balance requires more than glass solution alone.

The strongest evidence for a hydrothermal component is that the McDermitt caldera has many magmatically driven hydrothermal systems: Hg, U, U-Zr, and Au. Elements enriched in Li deposits (Mg, K, Rb, Cs, F, Mo, As, Sb) partly overlap with those in hydrothermal deposits:

Hg: As, Sb, Mo, ±Zr, U, F, Tl

U: As, Tl, Sb, Mo, F, Hg

U-Zr: Y, Yb, As, Sb, F, Mo, Te, Tl ±Hg, Au, Th

Au: As, Sb, Ag, Cu ±Hg, W, Bi

Overlapping elements are those enriched by either epithermal systems or low-T redox reactions. All hydrothermal deposits, none of which are enriched in Li, formed shortly post-collapse based on adularia ⁴⁰Ar/³⁹Ar dates or restriction to McDermitt Tuff or basal sediments. Li mineralization is undated other than post-15.7 Ma, the youngest mineralized sediments, well after magmatism ceased.

A possible Li source in addition to CHSD glass solution is shortly post-collapse degassing of residual magma or high-T devitrification of McDermitt Tuff to release U, Zr, Y, Yb, etc and Li. The less mobile elements fixed in deposits, whereas mobile Li, K, As, etc were added to the intracaldera lake-groundwater system ultimately to precipitate as Li smectite and illite at low T. Absence of Li deposits in other Great Basin calderas is enigmatic.