GEOCHEMICAL CONSTRAINTS ON LI-RICH AUTHIGENIC MINERALIZATION OF LACUSTRINE DEPOSITS IN THE MCDERMITT CALDERA

The McDermitt Caldera (Nevada-Oregon) contains lacustrine sediments with some of the richest lithium (Li) mineralization known (up to 9,000 ppm Li). Stratiform sequences of Li-enriched illite or smectite (also called hectorite) are found in the intracaldera sedimentary deposits throughout the 40 x 30 km caldera. The sedimentary section near Thacker Pass is approximately 190 m thick and is composed of tuffaceous sandstones, siltstones, mudstones, tephra, minor conglomerates and carbonates overlying silicified welded intracaldera ignimbrite. Carbonates are found as thin primary beds, in veins, and as ‘snowballs’ that crosscut the sedimentary laminations. The snowball carbonates within the illitic portion of the sequence at Thacker Pass have elevated uranium concentrations (50-75 ppm) and high ²³⁸U/²⁰⁶Pb, yielding a LA-ICP-MS isochron age of 14.29±1Ma 2sigma. This age is similar to authigenic feldspar ages reported by Henry et al. (2017, Geosphere) and suggests that the snowball carbonates may have precipitated syn-feldspar mineralization. K/Ar ages from clays within the deposit will further clarify the formation of the deposit. Clumped isotopes on these carbonates are expected to yield relatively low temperatures of mineralization and element mapping of the carbonates, in the context of the Li bearing laminated strata, will provide details on the fluid responsible for this authigenic mineralization. These cross-cutting carbonates can place a minimum age on Li enrichment of Thacker Pass strata.