Hydrochemical characteristics of carbonated waters and non-carbonated thermal waters of the Heyuan Deep Fault Zone in Guangdong, Southern China

Abstract

This paper studied both carbonated geothermal waters and generic geothermal waters, to better understand deep groundwater circulations in deep fault-basin system in the Heyuan Deep Fault Zone. Uniquely occurring carbonated geothermal waters are not well known in their origin-evolution process, geologic settings of deep ground water systems, and their flow behaviors in response to the unique physicochemical properties. Through deep-borehole data, and field samplings of various isotope and aqueous geochemistry data, we characterized these carbonated and generic geothermal waters, and analyzed water-rock reactions in relevant silicate rocks. Carbonated waters have high contents of HCO₃⁻ and dissolved CO₂, and enhance their leaching effect in such a way consequently leading to much higher major ion concentrations than non-carbonated thermal waters. According to δD and δ¹⁸O data, both carbonated waters and non-carbonated waters are of local meteoric origin and recharge from the hilly areas scattering throughout the study area at elevations ranging from 492 to 602 m (masl). The δ¹³C results confirm that the high DIC contents in carbonated waters have their deep sources of both mantle and carbonate rock thermal metamorphism genesis. Thermal reservoir temperatures are estimated for water samples, yielding a temperature range of 48.2 to 134.8 °C, based on chalcedony and quartz geothermometers. A 3500-m-deep borehole for carbonated water has revealed the relatively low geothermal gradient (2.35 °C/100 m) in the carbonated water convergence zone, which is related to the localized depression of the granitic-igneous basement within the study area. Relatively low-temperature environments have formed in localized basement depressions with thick cover of sedimentary rocks, and thus have provided excellent storage condition for deep-derived CO₂ rising along deep faults, forming carbonated waters when combined with groundwater systems. The deep crustal groundwater flow was further examined for the crustal thermo-structure, showing the heat flow and temperature distribution in each layer of the crust. The findings and insights could provide further understanding of deep ground water flow systems and deep groundwater circulations. The implications involve effective exploitations and utilizations for carbonated water and non-carbonated thermal water. This study is the first of the kind to study the deep ground water systems including carbonated waters and non-carbonated thermal waters in deep fault-basin systems, in terms of the deep sourcing, ground-water circulation, and thermal structure of the deep crust, and deep groundwater circulation in deep fault settings.