Reply to Norini and Groppelli's comment on “Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)” by Urbani et al. (2020)

Abstract

Abstract. Structural studies in active caldera systems are widely used in geothermal exploration to reconstruct volcanological conceptual models. Active calderas are difficult settings to perform such studies mostly because of the highly dynamic environment, dominated by fast accumulation of primary and secondary volcanic deposits, the variable and transient rheology of the shallow volcanic pile, and the continuous feedbacks between faulting, secondary porosity creation, and geothermal fluid circulation, alteration and cementation that tend to obliterate the tectonic deformation structures. In addition, deformation structures can be also caused by near- and far-field stress regimes, which include magmatic intrusions at various depths, the evolving topography and regional tectonics. A lack of consideration of all these factors may severely underpin the reliability of structural studies. By rebutting and providing a detailed discussion of all the points raised by the comment of Norini and Groppelli (2020) to the Urbani et al. (2020) paper, we take the opportunity to specify the scientific rationale of our structural fieldwork and strengthen its relevance for geothermal exploration and exploitation in active caldera geothermal systems in general and, particularly, for the Holocene history of deformation and geothermal circulation in the Los Humeros caldera. At the same time, we identify several major flaws in the approach and results presented in Norini and Groppelli (2020), such as (1) the lack of an appropriate ranking of the deformation structures considering an inventory method for structural analysis; (2) the misinterpretation and misquoting of Urbani et al. (2020) and other relevant scientific literature; and (3) irrelevant and contradictory statements within their comment.

The impact of hydrothermal alteration on the physiochemical characteristics of reservoir rocks: the case of the Los Humeros geothermal field (Mexico)

“…However, high-temperature alteration minerals, such as garnet, diopside, and wollastonite, were also observed at shallower depth levels as shown in fracture fillings of sample H38-4 (1950 m) in the central to northern zones of the geothermal field (also referred to as the central collapse area). This observation is well in line with findings from (i) Martıńez-Serrano (2002), who defined the beginning of a skarn zone at about 1800 m depth in the central collapse area, (ii) Jentsch et al (2020), who reported the highest temperature anomalies at Loma Blanca within the central collapse area, as well as (iii) Urbani et al (2021), who discussed existing borehole-temperature profiles in the same area. However, our findings show that high-temperature secondary mineral assemblages are not limited to the central collapse area as previously described in Prol-Ledesma and Browne (1989).…”

“…We interpret silicification at shallower depth levels within the propylitic alteration zone as well as the local occurrence of high-temperature minerals at different depth levels as possible evidence of shallow intrusions, in agreement with the shallowest geometry of the Los Humeros post-caldera stage magmatic plumbing system (Lucci et al 2020) and with the results from field-constrained analog modeling proposed by Urbani et al (2020Urbani et al ( , 2021. Shallow cryptodomes (e.g., Urbani et al 2020Urbani et al , 2021 and/or magma pockets (Lucci et al 2020) could be also responsible for thermal metamorphism in limestones and lavas that locally underwent transformation to marble, hornfels, and skarn, respectively. Furthermore, the occurrence of low and high-temperature alteration minerals at the same depth or even within one sample (Martıńez-Serrano 2002;Prol-Ledesma 1998), plus the high variability of alteration facies and intensity in the upper section of the pre-caldera group, points to the possibility of several heterogeneous heat sources.…”

“…Further sources of acidic fluids might be shallow intrusions (degassing of supercritical fluids), temperature/pressure changes, boiling, or reactivation of fractures (González-Partida et al 2022). We interpret silicification at shallower depth levels within the propylitic alteration zone as well as the local occurrence of high-temperature minerals at different depth levels as possible evidence of shallow intrusions, in agreement with the shallowest geometry of the Los Humeros post-caldera stage magmatic plumbing system (Lucci et al 2020) and with the results from field-constrained analog modeling proposed by Urbani et al (2020Urbani et al ( , 2021. Shallow cryptodomes (e.g., Urbani et al 2020Urbani et al , 2021 and/or magma pockets (Lucci et al 2020) could be also responsible for thermal metamorphism in limestones and lavas that locally underwent transformation to marble, hornfels, and skarn, respectively.…”

“…In particular, the Holocene LHVC post-caldera volcanic activity is associated with a complex magma plumbing system vertically distributed within the whole crust (Lucci et al 2020) and shallow intrusions in the Los Potreros caldera within the Los Humeros geothermal field (< 1 km depth; Urbani et al 2020Urbani et al , 2021Deb et al 2019), which causes several very localized temperature anomalies mainly along the NNW-SSE trending Maxtaloya and Los Humeros lineament (Jentsch et al 2020;Deb et al 2019). Except for the water-producing well H1, the produced fluids are predominantly steam with an enthalpy of more than 2000 kJ kg −1 (Romo-Jones et al 2020).…”

“…Various geological (Ferriz and Mahood 1984;Carrasco-Núñez et al 2017a, b), geochemical (Prol-Ledesma and Browne 1989;Martıńez-Serrano 1993Izquíerdo et al 2011Izquíerdo et al , 2015, geophysical (Lermo et al 2008;Arzate et al 2018), and hydrological studies (Tello 2005) have been performed in the past and most recently within the framework of the GEMex project (EU-H2020, GA Nr. 727,550;Jolie et al 2018;Weydt et al 2018Weydt et al , 2021bLucci et al 2020;Urbani et al 2020Urbani et al , 2021 to improve reservoir understanding and to create conceptual (Cedillo 2000;) and 3D geological models (Calcagno et al 2020;Deb et al 2019). However, even after more than 40 years of exploration, information on rock properties of the different geological units in the study area were scarce or not available.…”