Hydrochemical and age constraints of the Chapada dos Veadeiros geothermal reservoir, central Brazil

This paper reports the first conceptual model of non-magmatic system in central Brazil, constrained by structural, hydrochemical and isotopic data. The study involved analyses of carbon, oxygen and hydrogen isotopes (¹⁴C,¹⁸O, ²H and ³H) in addition to the chemical signatures of water samples from natural springs and borehole. Samples were classified into three hydrochemical groups, based on their pH, electric conductivity (EC) and temperature. Group 1 shows the lowest temperature range and lowest pH and EC values (respectively averages of 27 °C, 5.0, and 15 μS/cm); Group 3 shows higher temperature, pH, EC (42 °C, 7.1, and 215 μS/cm); and Group 2 shows intermediate values (average of 38 °C, 6.2 and 80 μS/cm). The conceptual flow model indicates that the recharge area is located in the higher plateau areas and the only recharge source is rainwater. The groundwater flows towards the southwest for 15–25 km up to the São Joaquim fault system, where the hydraulic conductivity contrast, due to different rock types, induces upward flow to the hot springs area. Buoyancy due to heating through deep circulation can also influence the upward flow, however, it is not the main process. The flow path from NE to SW is controlled by secondary brittle structures, related to fractures represented by long lineaments. Environmental radioisotopes of water (³H and ¹⁴C) were used to determine the residence time of thermal waters. ³H data indicate that samples MV2 and TOC1 (respectively from groups 2 and 3), are related entirely to the deep thermal reservoir, with no mixing influence between reservoirs. Samples SJ1 and CH3 show ³H values from 0.9 to 0.64 TU, indicating mixture of water from recent cold waters and the deep thermal reservoir. Group 3 samples yielded a raw ¹⁴C age of 9270 years, and the most coherent correction model applied was the International Atomic Energy Agency, resulting in an age of 7877 years. Group 2 yielded age of 2280 years and the most consistent correction model applied was the Pearson equation, ensuing an age of 2281 years. Group 1 samples resulted in negative age, interpreted as contamination from external carbon sources likely related to algae. Sample SJ1 (from a deep well) yielded an age of 1,470, and the correction by the Pearson model resulted in age of 1352 years. The δ²H and δ¹⁸O data are plotted above the local meteoric water line, indicating enrichment of lighter isotopes. The values measured for the total dissolved solids; electric conductivity and pH are in agreement with the isotopic data interpretations, and with the aquifer mineralogy.