Applicability of Na/K geothermometer to the metapelitic non-volcanic geothermal fields in the Taiwan orogenic belt

Taiwan is situated at one of the world's most active orogenic belts with arc-continent collision. This event resulted in the burial and uplift of mudstones along the continental margin, transforming them into metapelitic mountains. These mountains intercept rainfall and serve as the driving force for deep water circulation, enabling meteoric fluids to carry residual heat to the surface, thus giving rise to hot springs. The Taiwan orogenic belt is home to over 100 hot springs, and some of the hot spring reservoir temperatures such as Chingshui, Tuchang-Jentse, Lushan, Chihpon, and Chinlun exceeds 160 °C. Geothermal development in Taiwan has been carried out in such hot spring areas without any volcanic activities. In order to assess reservoir temperatures and confirm the applicability of the Na/K geothermometer to such high temperature hot spring system, the concentrations of Na⁺, K⁺, and SiO₂ were analyzed from geothermal wells. Considering the mineral composition of metapelitic host rock, a new Na/K geothermometer is proposed based on equilibrium between albite and illite: [Formula presented] This formula is only applicable in a reducing fluid and does not involve the formation of clay minerals except illite. It also requires several years to reach equilibrium. When using this formula to estimate reservoir temperatures, careful consideration of various conditions is necessary. It is advisable to incorporate both quartz geothermometer and down-hole temperature measurements for a more accurate assessment. Combining the Na/K geothermometer with the quartz geothermometer, along with other geochemical data such as Cl⁻concentrations and δD and δ¹⁸O of fluid, helps to understand whether hydrothermal fluids have undergone evaporation, dilution, and/or mixing processes.