Chloride Boride Relationship in Pannonian Basin Geothermal Systems

Abstract

The Pannonian Basin is a Neogene extensional basin that lies between the Carpathian, Alpine and Dinaride thrust belts. The Pannonian Basin was filled by large deltaic systems that originated from the surrounding mountain ranges. Several sub-basins were formed, separated by metamorphic core-complexes (Tari et al., 1992). These sub-basins were filled with thick sediments of Miocene to Quaternary age. During the sedimentation, marine to brackish, syn-rift sediments were deposited. The pre-Tertiary basement rocks, overlain by the Miocene and Quaternary sediments, are mainly composed from crystalline Precambrian rocks or Mesozoic sediments. The basement rocks have undergone multiple and complicated tectonic events. Due to the crustal extension, crustal thinning occurred, resulting in relatively high geothermal gradient. For instance the average geothermal gradient in Hungarian geothermal systems is about 50 °C/km (Tulinius et al., 2010), compared to about 30 °C for the surrounding countries. Consequently hidden geothermal systems are well known from the area. Due to the seawater origin of the Miocene and Quaternary sediments, the geothermal water is relatively saline. The purpose of this work is to study the chloride boride relationship in geothermal water from the Pannonian Basin. The Cl and B content of geothermal water have been widely studied for various purposes. Arnórsson and Andrésdóttir (1995) studied processes which controlled the distribution of chloride and boron in natural waters in Iceland. One of the main conclusions was that both Cl and B act essentially as incompatible elements at all temperatures in the natural water-volcanic rock environment in Iceland. The absence of clay minerals that tend to absorb B in low temperature geothermal systems in Iceland and high water/rock ratios was considered the main reason for the incompatible of B at low temperatures. The chloride and boron relationship in geothermal fluid from various geothermal systems within the Pannonian Basin was studied in this work. The geothermal systems which were studied are either low enthalpy geothermal systems, with temperatures less than 100 °C or medium high enthalpy systems, with temperatures between 100 and about 170 °C. The chloride boron relationship geothermal fluid from Pannonian Basin geothermal systems is such that the Cl/B ratio increases with increasing chloride content. The chloride and boron content evolvement broadly follows a mixture of geothermal fluid with initial chloride content of 6 ppm and Cl/B ratio of 2 and seawater. The seawater component originates from the time when the area was immersed in seawater during Neogene. Geothermal fluid which has evolved at temperatures above 100 °C has generally much higher boron content than fluids from low enthalpy systems. This suggests that boron in Pannonian Basin geothermal systems acts as a reactive element in low enthalpy geothermal systems. However, in medium enthalpy geothermal systems Boron behaves as an incompatible element.