Distribution of hypogene alteration and fluid evolution in the Los Humeros Geothermal Field (Puebla, Mexico): Multiple sourced fluids, interrelations, and processes in a superhot system

Abstract

The Los Humeros Geothermal Field (LHGF) is one of the highest producing geothermal areas in Mexico. This paper aims essentially to determine high-resolution patterns of distribution of hypogene alteration assemblages and the distribution of temperatures in space and time of geothermal fluids in the LHGF. The zoning of hypogene alteration assemblages in the LHGF consists of (1) shallow advanced argillic alteration assemblages that grade into (2) argillic and (3) zeolitic assemblages with depth, followed by (4) low-temperature and (5) high-temperature propylitic, and (6) deep high-temperature high-silica advanced argillic alteration assemblages. The distribution of deep advanced argillic alteration assemblages is associated with the occurrence of acidic supercritical fluids prior to their deep boiling. Capping these, high-temperature propylitic assemblages are associated with the occurrence of subcritic brines (barely exceeding 13 wt% NaCl equiv.) that stemmed from magmatic supercritic fluids (upon degassing and/or conductive cooling) and that were diluted by deeply circulated and thermally equilibrated meteoric water. Conductively cooled subcritic fluids, which eventually kept being diluted on their way to the surface, would be likely associated with the generation of shallow low-temperature propylitic assemblages. Relatively fresh meteoric water might have been increasingly relevant in such dilution as upwelling fluids neared the surface. Shallow boiling occurred mostly between high- and medium-temperature propylitic zones, and generated acidic vapors that eventually condensated in the phreatic water table and, in part, in the argillic alteration zone. Shallow hypogene advanced argillic (and argillic) alteration consisting of kaolinite, platy alunite, native sulfur and the newly found cuprocopiapite occurred in steam-heated grounds. Fluid inclusion and present-day geothermal fluid data allow to introduce relative time as a variable in the characterization of hydrothermal fluids in the LHGF. These sets of temperature data allow to independently determine patterns of temperature distribution as isotherms. Concave shapes in isotherms are interpreted to spearhead preferential passageways for hydrothermal fluids. Some areas (for instance, in the southern part of the LHGF) record relatively stable passageways through time, but these may significantly shift their positions in other areas. Stable passageways, apparently undisturbed by geothermal exploitation, may identify robust portions of the system that ensure a long-lasting exploitation. They also correspond to sound inputs of high-salinity and hot magmatic fluids, which may be sandwiched between cooler low-salinity fluids, dominantly meteoric in origin, and collectively make a case for convection (and partial mixing). Also, we determined that “older” inclusion fluids are ∼50 °C higher than “younger” fluids from geothermal wells, but such decrease can be compatible with the cooling down of the system due to geothermal exploitation. Therefore, such differences in temperature do not necessarily reflect an actual cooling in the system.