Geological note and historical review of the Pathé geothermal zone, Hidalgo

Abstract

The Pathé geothermal zone, in the central portion of the Trans-Mexican Volcanic Belt, attracted a variety of scientists and explorers since late 18th century. Joseph Burkart (1798-1870), a German mining engineer and a contemporary of Alexander von Humboldt (1769-1859), provided in 1836 a geological description of the zone because of which Pathé was the first geothermal system to be studied in Mexico under modern science principles. At the beginning of the 20th century, this zone was investigated by the Mexican geologist José Guadalupe Aguilera Serrano (1857-1941), who described in 1907 the local volcanic rocks and their alteration. His main interest were the kaolin deposits of Yexthó, located one km to the west of the geothermal manifestations. By the middle of the century, the area was already explored for geothermal energy, which resulted in the first geothermal power plant in all America, operating between 1959 and 1972 with a nominal capacity of 3.5 MW. The thermal manifestations of Pathé are located at the junction of two regional normal fault systems, striking N-S and E-W, and are hosted by a volcanic unit (tuff and lava) of basic to intermediate composition and Upper Miocene age. Surficial hydrothermal alteration is conspicuous and largely controlled by faults of these two systems. Associated with the N-S faults is a calcite-gypsum alteration assemblage, indicating oxidized, alkaline sulfate-rich hydrothermal fluids. On the other hand, linked to the later E-W faults are the kaolin deposits; they are the product of an advanced argillic alteration that is characteristic of a steam-heated environment and indicates acidic conditions (pH: 2 to 3) and temperatures around 100 °C. Because of its unambiguous relation to faults and the absence of recent (Quaternary) volcanism, Pathé is likely to be a fault-controlled geothermal system (non-magmatic, extensional domain type). The successive alteration events suggest that the hydrothermal activity is long-lived and dates back to before the Pliocene.