| Abstract |
Calcium carbonate tufa columns have formed around many subaqueous springs in terminal lakes in the Great Basin, western United States. These subaqueous springs can be either thermal or non-thermal, as long as the spring waters have sufficient calcium to react with atmospherically derived CO2 dissolved in lake water to precipitate calcium carbonate. In contrast to large calcium carbonate travertine deposits that form around subaerial springs, which are usually not associated with electricity grade geothermal systems, large tufa deposits in the Great Basin have formed around thermal springs whose fluid geothermometry suggests reservoir temperatures as high as 150-170°C (e.g. Needle Rocks and Pyramid Rock, Nevada), which is well within the temperature range necessary for electricity production. The reason that large subaerial calcium carbonate travertine deposits are usually not associated with electricity grade geothermal systems is that high temperature geothermal waters usually do not contain sufficient levels of dissolved CO2 to precipitate significant amounts of calcium carbonate around subaerial spring orifices. But if these same thermal springs form in lake waters saturated with atmospheric CO2, precipitation of calcium carbonate can still occur as long as the spring waters contain sufficient amounts of calcium. Many springs that formed tufa columns in the Great Basin are currently dry because the associated lakes have dried up and groundwater tables have dropped significantly, making it unlikely for springs to form. Consequently, tufa columns can provide evidence of possible underlying geothermal activity where no hot springs are present. For this reason it becomes important to discriminate tufa columns that formed around thermal springs from tufa columns associated with non-thermal springs. Trace element geochemistry is being evaluated for its ability to distinguish thermal from non-thermal tufa deposits. We report on the results of systematic sampling of spring-related tufa and chemical analysis using inductively coupled plasma mass spectroscopy (ICP-MS). Higher temperature geothermal waters tend to have different concentrations of some trace elements as compared to shallower cooler groundwaters. In a few cases, coprecipitated silica or possible sulfate minerals may also serve to distinguish thermal from non-thermal tufa. Carbon and oxygen isotopes are less likely to be useful, because the CO2 in tufa deposits is ultimately derived from the atmosphere. |