USE OF AN ILLITE/SMECTITE GEOTHERMOMETER TO TEST THE DEPOSIT MODEL FOR HECTORITE AND LI-RICH CLAY FROM LACUSTRINE BASINS OF THE WESTERN US

Lithium-rich clay is found in Neogene lacustrine sediments in several western U.S. basins. Lithium-clay minerals from these locations have been identified as smectite, mixed layer smectite-illite, and illite. It is the illite structure that appears to contain the highest concentration of lithium with up to 12,400 ppm Li observed in illite and up to 4190 ppm Li observed in smectite, in clay separates from Thacker Pass in the McDermitt Caldera in Nevada.

The USGS deposit model for hectorite and Li-rich clay invokes a depositional environment that includes a “hydrologically closed basin containing evaporative lakes and receiving discharge of geothermal waters”. This work tests the hypothesis that the Li clay sediments were associated with geothermal waters.

Chemical analyses of Li clay separates (<2um ) from sediments in 7 basins show a positive correlation between Li and F (0.2 – 6.5 wt % F). Previous work suggests that the presence of F is an indicator of hydrothermal influence in the depositional history. Additionally, trace elements commonly attributed to hydrothermal fluids, i.e., As, Cs, Mo, Rb, Sb, Sr, and Ti, are found enriched in clays with high Li and F concentrations.

Maximum temperature in the clay thermal histories was estimated with an illite/smectite geothermometer employing the illite/smectite ratio and Reichweite (i.e., ordering of the illite/smectite stratification). Clay modeling was performed with Jade Pro^TM software.

Correlations among the temperature estimate, Li and F concentrations, and the trace element suite are discussed in the presentation along with assumptions and considerations for use of the illite/smectite geothermometer.