Chemical and isotopic features of cold and thermal fluids discharged in the Southern Volcanic Zone between 32.5 degrees S and 36 degrees S: Insights into the physical and chemical processes controlling fluid geochemistry in geothermal systems of Central Chile

Abstract

The Principal Cordillera of Central Chile is characterized by two belts of different ages and lithologies: (i) an eastern Mesozoic belt, consisting of limestone- and gypsum-rich sedimentary rocks at the border between Central Chile and Argentina, where the active volcanic arc occurs; and (ii) a western belt of Cenozoic age containing basaltic to andesitic volcanic and volcanoclastic sequences. This distinctive geological setting controls water chemistry of cold and thermal springs in the region, which are fed by meteoric water that circulates through deep regional structures. In the western sector of Principal Cordillera, water-rock interaction processes produce low TDS, slightly alkaline HCO3- dominated waters, although dissolution of underlying Mesozoic evaporitic rocks occasionally causes SO42- and Cl- enrichments. In this area, few Na+-HCO3- and Na+-SO42- waters occurred, being likely produced by a Ca2+-Na+ exchange during water-rock interactions. Differently, the chemical features of Ca2+-Cl- waters was likely related to an albitization-chloritization process affecting basaltic to andesitic rocks outcropping in this area. Addition of Na+-Cl- brines uprising from the eastern sector through the west-verging thrust faults cannot be excluded, as suggested by the occurrence of mantle He (similar to 19%) in dissolved gases. In contrast, in the eastern sector of the study region, mainly characterized by the occurrence of evaporitic sequences and relatively high heat flow, mature Na+-Cl- waters were recognized, the latter being likely related to promising geothermal reservoirs, as supported by the chemical composition of the associated bubbling and fumarolic gases. Their relatively low He-3/He-4 ratios (up to 3.9 Ra) measured in the fumaroles on this area evidenced a significant crustal contamination by radiogenic He-4. The latter was likely due to (i) degassing from He-4-rich magma batches residing in the crust, and/or (ii) addition of fluids interacting with sedimentary rocks. This interpretation is consistent with the measured delta C-13-CO2 values (from -13.2 to -5.72% vs. V-PDB) and the CO2/He-3 ratios (up to 14.6 x 10(10)), which suggest that CO2 mostly originates from the limestone-rich basement and recycling of subducted sediments, with an important addition of sedimentary (organic-derived) carbon, whereas mantle degassing contributes at a minor extent. According to geothermometric estimations based on the Na+, K+, Mg2+ and Ca2+ contents, the mature Na+-Cl- rich waters approached a chemical equilibrium with calcite, dolomite, anhydrite, fluorite, albite, K-feldspar and Ca- and Mg-saponites at a broad range of temperatures (up to similar to 300 degrees C) In the associated gas phase, equilibria of chemical reactions characterized by slow kinetics (e.g. sabatier reaction) suggested significant contributions from hot and oxidizing magmatic gases. This hypothesis is consistent with the delta C-13-CO2, Rc/Ra, CO2/He-3 values of the fumarolic gases.

Accordingly, the isotopic signatures of the fumarolic steam is similar to that of fluids discharged from the summit craters of the two active volcanoes in the study area (Tupungatito and Planchon-Peteroa). These results encourage the development of further geochemical and geophysical surveys aimed to provide an exhaustive evaluation of the geothermal potential of these volcanic-hydrothermal systems.