Active and fossil travertine from the Cajón del Maipo, Andes of Central-Southern Chile: implications for geothermal fluid circulation

Abstract

The present thesis is focused on the study of active and fossil travertine deposits placed at Baños Morales and Baños Colina localities, and its implications in the understanding of the geothermal systems that originated them. This work was carried out through a morphological, mineralogical, geochemical and isotopic characterization of the carbonates that form the deposits, and the hydrochemical and isotopic characterization of the thermal waters in the area with temperatures that range between 10°C and 5|°C. To accomplish this different field campaigns where developed for deposit characterization and travertine and water sampling. Rock sampling analyzes includes X ray diffraction, optic microscopy, scanning electron microscopy, atomic absorption spectrometry, mass spectrometry with coupled plasma and isotope ratio mass spectrometry. Water sampling includes atomic absorption spectrometry, ionic chromatography and mass spectrometry with coupled plasma. Also, DNA extraction analyzes were done for the recognition of bacteria present in the thermal springs. The thermal waters in the zone correspond to sodium-chloride waters, with isotopic signatures that indicates mainly a meteoric recharge. Carbonate precipitation occurs from these thermal sources, in some cases in the presence of cyanobacteria and diatoms. Near the springs different travertine deposits morphologies were identified, including cascades, slopes, dams and fissure ridges, also fractures filled with carbonates. The mineralogy of the carbonates was mainly aragonite and calcite in different proportions. Isotopic signatures of 13C and 18O present a high correlation in its variation and have similar values to other travertine deposit originated from thermal waters registered in the literature. Based on the joint analysis of all the physical and chemical characteristics of the deposits a conceptual model for the geothermal system is proposed, where meteoric waters are transported into deep through faults and fractures, which heats the water and allow them to reach higher outcropping temperatures. Finally, travertine study can be considered as a first approach for understand geothermal systems and should be considered during surface exploration, when possible, because they sum information of the controlling factors of a geothermal system and its evolution in time, always in consideration with other methods such as geological,, hydrogeochemical and geophysical analysis.