Possible Mineral-Water Interactions Delimiting Li Geothermometry in Geothermal Systems

Abstract

The most robust and reliable geothermometers used in assessing active hydrothermal systems are derived from thermodynamically-based mineral solubilities (SiO2) and ion-exchange between feldspars (Na-K geothermometry) or phyllosilicates and feldspars (K-Mg geothermometry). Empirical geothermometers, such as the Na-Ca-Mg, Mg-Li and Na-Li calibrated from field measurements, are not always reliable in predicting subsurface temperatures in geothermal systems. Yet these are still widely used in the geothermal industry. The Li geothermometers appear to be site-specific as evinced by seven different Na-Li geothermometers applied to different geothermal and petroleum-bearing sedimentary systems. The Mg/Li ratios in geothermal systems of the Taupo Volcanic Zone (TVZ) are governed by Li uptake in Mg-Fe phyllosilicates smectite and chlorite and quartz. Whilst the transformation of Mg-Fe phyllosilicates to higher temperature K-bearing phases tends to decrease Mg/Li ratios in aqueous fluids, the preferential uptake of Li in quartz with increasing temperatures results to an increase in the Mg/Li aqueous fluid compositions. With several mineral alteration processes affecting Mg/Li ratios, it is not surprising that in the TVZ the estimated Mg-Li temperatures from well discharges are inconsistent with estimates from the K-Mg, Na-K and SiO2 geothermometers and can be higher by 50oC than measured well temperatures. Thus, the Mg-Li geothermometer can only be viable where the main alteration process is the alteration of Mg-Fe phyllosilicates to illitic clays and with limited quartz deposition. In the TVZ this mineral-water interaction occurs at temperatures 100oC. In the TVZ changes in the Na/Li ratios with increasing temperatures are too narrow for use as an effective geothermometer. The ineffectiveness of the Na/Li geothermometer in the TVZ can be attributed to the complex contradictory effects of a wide number of simultaneous alteration processes. These include alteration processes involving dissolution of plagioclases and pyroxenes and volcanic glass followed by deposition of secondary minerals that either incorporate (e.g., quartz, smectite, chlorite) or reject (calcite, albite) Li in their structures as temperatures increase or decrease. Furthermore, the Na/Li ratios in the fluids are also modified by alteration intensity and the composition of the original mineral being altered. In summary the Mg-Li geothermometer can only be used in geothermal systems where the well or system has low permeability, quartz is scarce, the main alteration is dominated by Mg-Fe phyllosilicates with some illitization and temperatures are