Search

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 HCO 3 − dominated waters, although dissolution of underlying Mesozoic evaporitic rocks occasionally causes SO 4 2 − and Cl − enrichments. In this area, few Na + –HCO 3 − and Na + –SO 4 2 − waters occurred, being likely produced by a Ca 2 + –Na + exchange during water–rock interactions. Differently, the chemical features of Ca 2 + –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 (~ 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 3 He/ 4 He ratios (up to 3.9 Ra) measured in the fumaroles on this area evidenced a significant crustal contamination by radiogenic 4 He. The latter was likely due to (i) degassing from 4 He-rich magma batches residing in the crust, and/or (ii) addition of fluids interacting with sedimentary rocks. This interpretation is consistent with the measured δ 13 C-CO 2 values (from − 13.2 to − 5.72‰ vs. V-PDB) and the CO 2 / 3 He ratios (up to 14.6 × 10 10 ), which suggest that CO 2 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 + , Mg 2 + and Ca 2 + 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 ~ 300 °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 δ 13 C-CO 2 , Rc/Ra, CO 2 / 3 He 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 Planchón–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. [ABSTRACT FROM AUTHOR]

Lastarria volcano (25°10′ S, 68°31′ W; 5,697 m above sea level), located in the Central Andes Volcanic Zone (northern Chile), is characterized by four distinct fumarolic fields with outlet temperatures ranging between 80°C and 408°C as measured between May 2006-March 2008 and April-June 2009. Fumarolic gasses contain significant concentrations of high temperature gas compounds (i.e., SO, HCl, HF, H, and CO), and isotopic ratios (He/He, δC-CO, δO-HO, and δD-HO) diagnostic of magmatic gas sources. Gas equilibria systematics, in both the HO-H-CO-CO-CH and alkane-alkene C system, suggest that Lastarria fumarolic gasses emanate from a superheated vapor that is later cooled and condensed at relatively shallow depths. This two-stage process inhibits the formation of a continuous aquifer (e.g., horizontal liquid layer) at relatively shallow depth. Recent developments in the magmatic gas system may have enhanced the transfer and release of heat causing shallow aquifer vaporization. The consequent pressure increase and aquifer vaporization likely triggered the inflation events beginning in 2003 at the Lastarria volcano. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the chemical and isotopic composition of water and gas thermal discharges from six hydrothermal systems in the Tarapacà and Antofagasta regions (northern Chile): Surire, Puchuldiza-Tuja, Pampa Lirima, Pampa Apacheta, El Tatio and Torta de Tocorpuri, to determine the chemical–physical conditions at the fluid source. The chemical facies of the thermal discharges vary from Na+–Cl− (El Tatio, Puchuldiza-Tuja and part of Surire where SO4 2 −-rich waters also occur) to Na+(Ca2+)–Cl−(SO4 2 −) (Pampa Lirima), Ca2+–SO4 2 −(HCO3 −) (Torta de Tocorpuri) and Ca2+–SO4 2 − (Pampa Apacheta). The gas seeps are characterized by the dominance of CO2, H2S and CH4 with N2/Ar ratios that occasionally are up to 450. Significant amounts of SO2 and HCl are present at Pampa Apacheta. Water and gas geothermometry suggests that El Tatio and Puchuldiza-Tuja fluid reservoirs have relatively high reservoir equilibrium temperature (up to 270°C). Gases from Pampa Apacheta apparently equilibrated at unusually high temperature for hydrothermal fluids (up to 350°C), being likely related to an active magmatic system, as testified by the presence of highly acidic gas compounds such as SO2 and HCl. On the contrary, low equilibrium temperatures were calculated for the Surire fluids (<200°C), suggesting interactions of deep-originated fluids with meteoric water at shallow depth. Shallow, low temperature aquifers mask any signal of deep fluids in the Pampa Lirima and Torta de Tocorcupi thermal discharges. Despite the variations in temperatures, He isotopic compositions and redox conditions of the fluid reservoirs consistently indicate a significant contribution from magmatic degassing. Although geophysical studies are necessary to better constrain the geothermal potential of northern Chile, the geothermometric estimations carried out in the present study indicate that El Tatio, Pampa Apacheta, Surire and Puchuldiza-Tuja can be regarded as the most promising areas and deserve more detailed geological investigations. [Copyright &y& Elsevier]

Abstract: Tacora volcano is a nearly unknown and understudied 5980m high volcano located on the northernmost border between Chile and Peru. Tacora is characterized by intense fumarolic activity with extensive, white-colored, hydrothermal areas along the NW and W flanks of the volcanic structure. The chemical and isotopic features of gas emissions indicate that the hydrothermal reservoir is significantly affected by contributions from magmatic degassing, despite relatively low outlet temperatures (82–93°C). Water and gas isotopic signatures stem from degassing of andesitic magmas produced from the sediment-poor subducting slab. Isotopic data suggests CH4–CO2 isotopic equilibration occurs at ~400°C, likely in the deepest portion of the hydrothermal-magmatic fluid reservoir. The H2–H2O, CO–CO2, H2–Ar, CH4–CO2 and C2–C3 alkenes/alkanes pairs tend to approach a chemical equilibrium at temperatures between 200 and 320°C when redox conditions are more oxidizing than those determined by the typical FeO/FeO1.5 rock buffer system. Boiling occurs at shallow depth at 85°C within a discontinuous, thin aquifer, enough as thermal buffer but not able to scrub the uprising magmatic-related acidic gases. [Copyright &y& Elsevier]