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

The McDermitt caldera, an early Yellowstone hotspot igneous center, has major Li clay deposits hosted exclusively in intracaldera, rhyolitic tuffaceous sediments. The sediments accumulated 16.4-15.7 Ma in an irregular lake around the resurgent dome after 16.4 Ma eruption of the McDermitt Tuff and caldera collapse. Li smectite deposits with maximum grades of ~4000 ppm occur throughout the caldera. Li illite deposits with maximum grades of ~9000 ppm are only known in the southern caldera (Thacker Pass). The tuffaceous sediments underwent CHSD, in which rhyolitic glass dissolved to generate high-TDS, alkaline water that, enriched by closed-basin evaporation, precipitated smectite, zeolites, authigenic Kspar and albite, and possibly illite. CHSD generated at least the Li smectite deposits. Questions about McDermitt deposits are 1. origin of the high grade, illite part, 2. the total source of Li (dissolution of glass ± contributions of hydrothermal solutions). My very rough mass balance indicates glass solution alone is insufficient.

The strongest evidence for a hydrothermal component is that the caldera, unlike other hotspot centers, has many hydrothermal systems related to caldera magmatism, including Hg, U-Zr, U non-Zr, and Au. Elements enriched in Li deposits (Mg, K, Rb, F, Mo, As, Sb) partly overlap with those in hydrothermal deposits:

Hg: As, Sb, Mo, ±Zr, U, F, Tl

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

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

Au: As, Sb, Ag, Cu ±Hg, W, Bi

Overlapping elements are those enriched by either hydrothermal systems or low-T redox reactions. No known hydrothermal deposit is enriched in Li. All hydrothermal deposits formed shortly post-collapse based on adularia ⁴⁰Ar/³⁹Ar dates (Moonlight U-Zr; McDermitt Hg) or restriction to McDermitt Tuff or basal sediments. Li mineralization is undated other than post-15.7 Ma, the youngest mineralized sediments, and 14.87±0.05 Ma on authigenic Kspar (illite zone, Thacker Pass).

One 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 highly mobile Li, K, As, etc were added to the intracaldera lake-groundwater system ultimately to precipitate as Li smectite and illite at relatively low T.