| Abstract: |
Lithium/magnesium, lithium/sodium, and to a lesser extent, potassium/magnesium ratios in calcium carbonate tufa columns provide a fingerprint for distinguishing tufa columns formed from thermal spring waters versus those formed from non-thermal spring waters. These ratios form the basis of the Mg/Li, Na/Li, and K/Mg fluid geothermometers commonly used in geothermal exploration, which are based on the fact that at elevated temperatures, due to mineral-fluid equilibria, lithium preferentially partitions into thermal waters relative to magnesium. Cooling of thermal waters during their ascent to the surface can lead to an imbalance in Li/Mg, Li/Na, and K/Mg ratios between fluids and host rocks which in turn can lead to preferential precipitation of lithium and potassium relative to magnesium and sodium in host minerals, thus increasing the Li/Mg and K/Mg ratios of mineral precipitates to levels higher than that normally seen in the surface environment. These lithogeochemical trace element ratios have significance for geothermal exploration, because tufa columns potentially overlie electricity grade geothermal resources at depth. As defined here, tufa columns form in lacustrine lakes. Lakes were abundant in the Great Basin in wetter periods of the Pleistocene to Recent Epochs, but as the lakes dried up, the springs also dried up, leaving the tufa towers behind as the only physical evidence in many cases that thermal waters might still be present in the subsurface. In addition to providing a potentially diagnostic lithogeochemical tool for geothermal exploration, the analysis of lithium and other elements in tufa deposits could serve as exploration guides for hot spring lithium deposits. Even though lithium clays such as those at McDermitt, Nevada occur at shallow depths, they are easily eroded and/or covered by thin layers of post-mineral sediments or colluvium. Tufa columns, because of their vertical height and greater degree of lithification, offer more resistance to erosion and are more likely to remain exposed, where they can be mapped and sampled. |