GEOCHEMICAL CHARACTERIZATION OF LACUSTRINE LITHOLOGY IN MCDERMITT CALDERA, OR/NV

McDermitt Caldera is an early Yellowstone eruptive center exposed in southeastern Oregon and northern Nevada. Historically home to productive mercury, uranium, and opalite mines, McDermitt Caldera has recently made global headlines as a promising economic Li resource contained in crater lake clay sediments. The intracaldera lacustrine sedimentary stratigraphy at McDermitt consists mostly of well-bedded tuffaceous siltstones and claystones with some locally mineralized and/or silicified zones. These lacustrine sediments are commonly exposed between a lower and upper nonwelded portion of the McDermitt Tuff, and preliminary single crystal 40Ar/39Ar ages confine sedimentation to between 16.46 +/- 0.29 Ma to 16.26 +/- 0.04 Ma. Representative samples of the sedimentary stratigraphy were taken over 1 cm intervals from 1-3m outcropping sections at 8 sites around the caldera, proximal to historic or prospective mineralization at depth. Observations of the presence of hectorite (Htr) and montmorillonite (Mmr) clay were made in key samples using X-ray Diffraction (XRD) along with fine grained calcite, quartz, and sanidine/orthoclase. The presence of Htr and Mmr are also supported by observations of characteristic short-wave infrared (SWIR) absorption peaks in the clay samples at ~1410nm (with doublet at ~1390nm in Htr), ~1900nm and either ~2220nm (Mmr) or ~2310nm (Htr). Preliminary laser-induced breakdown spectroscopy (LIBS) measurements were also used to obtain initial estimates of Li concentration in 64 samples from the 8 key locations. Li-poor units (tuffs, silicified tuffs, and sand/carbonate) ranged from below 75 ± 44 ppm to 691 ± 180 ppm Li whereas samples with Htr contained between 1017 ± 201 ppm and 4395 ± 636 ppm Li, suggesting a strong association between spectroscopic detection of that clay type and enrichment across the caldera. Inductively coupled plasma mass spectrometry (ICP-MS) and emission spectroscopy (ICP-OES) analysis are planned to validate and expand upon this data to generate a comprehensive geochemical assessment of the stratigraphic column and inform models of Li enrichment zones in the caldera.