| Title | Constraints of the Las Derrumbadas Geothermal Conceptual Model by Gas Geochemistry Puebla, Mexico |
|---|---|
| Authors | Typhaine TUAL, Laurent ESCARE, Snorri GUDBRANDSSON, Elodie MULLER, Marie-Noëlle GUILBAUD, Carole CORDIER, Manuel MOREIRA, Lorraine TUAL |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Volcanic gas geochemistry, noble gas, stable isotopes, geothermal exploration, geothermal reservoir |
| Abstract | The Las Derrumbadas volcanoes are a geothermal prospect located in the Mexican state of Puebla. The geothermal target is a rhyolitic complex made of twin domes in the eastern part of the Trans-Mexican Volcanic Belt. They break through the sedimentary basin of Serdán-Oriental, mostly composed of Mesozoic limestones. Recent geochronological data suggest that Las Derrumbadas is merely 2000 years old. Fumaroles as well as intensive hydrothermal alteration at the summit indicate the presence of a high enthalpy geothermal system. These fumaroles were sampled in order to constrain the fluids origin and characterize the geothermal resources. Las Derrumbadas gases present high concentrations of CO2 and CH4. No H2S was detected in the fluids sampled in the area. The isotopic compositions of gases show a magmatic signature suggesting the presence of a deep, still active heat source. Fumaroles are highly enriched in He (up to 38 ppm) with R/Ra ratios of respectively 4.37 and 4.56 measured on two samples, indicating the influence of a high mantellic contribution. The δ13CCO2 (-9.26 ‰, and -9.56 ‰), δ13CCH4 (-28.24‰ and -28.43‰) and δDCH4 (-144.27 ‰ and -143.06 ‰) compositions also suggest a hydrothermal origin without any Mesozoic sedimentary contribution. However, isotopic values of condensates, δDH2O (from -126.53‰ to ¬-143,1‰) and δ18OH2O (from -18.87 ‰ to -19.97‰), show that water was freshly infiltered. The enigmatic absence of sulfur in fumaroles associated with rhyolitic volcanoes could result from neutralization by scrubbing in the geothermal aquifer. This is also in line with the CH4 content, which isotopy confirms the geothermal origin, generally linked to the reduction of CO2 in the geothermal aquifer. Gases geothermometers suggest temperatures higher than 300 °C. The geothermal aquifer is expected to be located in the igneous basement, below the upper limestone formations and sealed by a strong alteration layer acting as a barrier for liquids. This alteration cap allows sulfur and carbon dioxide reduction, but remains permeable to gases. Interaction between rainwater and hot gases at the surface causes their vaporization. Estimated temperatures are comparable to well measurements at the Los Humeros geothermal powerplant located 45 km to the North. This study provides key constraints on the conceptual geological and geothermal models, and confirms the strong potential of the Las Derrumbadas prospect. |