Unraveling the origin of geothermal heat in absence of recent volcanism: The Santiago Papasquiaro hydrothermal area, Central-Eastern Sierra Madre Occidental, México

Abstract

In western Mexico, the central part of the Sierra Madre Occidental (SMO) large igneous province hosts several thermal springs with groundwater temperatures up to 74 ºC within late Eocene to Oligocene extensional structures. Given that in this region the last pulse of silicic volcanism ended >30 Ma ago this hydrothermal activity cannot be associated with cooling magma chambers and/or ongoing crustal thinning as in conventional geothermal sites. A possible explanation for the anomalous water temperature includes two non-mutual excluding processes: 1) heat production by radioactive decay of abundant U, Th, K radioisotopes in the silicic rocks forming the upper crust of the SMO and accumulated below the low conductive sediments filling the tectonic depressions, and 2) water heating by convection through deeply rooted faults. To prove this hypothesis, we studied the Santiago Papasquiaro hydrothermal area, located at the intersection between the NNW trending Santiago Papasquiaro half-graben and the NW trending San Luis-Tepehuanes fault system. We provide a detailed stratigraphy of the area supported by new U-Pb ages and we estimated heat production with in-situ gamma ray measurements of radioelements (U, Th, K), and groundwater He isotopes analyses.The stratigraphy of the region consists of 1) a Late Cretaceous-Paleocene continental volcanosedimentary succession (Garame Group), 2) Early Eocene rhyolitic lavas and ignimbrites, 3) Late Eocene volcanosedimentary succession, 4) Late Eocene-Early Oligocene pyroclastic succession capped by rhyolitic lavas and domes, and 5) a siliciclastic succession (Papasquiaro Formation) filling the graben interbedded with minor basaltic lavas (∼12–11 Ma) in its upper part. We also mapped two intermediate composition intrusive bodies, a Late Cretaceous (66 Ma) granodiorite and a Late Eocene (∼39 Ma) diorite.Heat production values are in the range of 1 µWm−3 to 8 µWm−3, with extreme values up to 310 µWm−3 for carbonaceous beds within the Garame Group. Late Cretaceous to Eocene volcanosedimentary and volcanic successions have lower heat production values than the Oligocene volcanic succession and the Oligocene-Quaternary siliciclastic succession. As a whole, heat production of Santiago Papasquiaro rocks exceeds the average for the upper crust as well as that of continental granite/rhyolite, being in the range of the high heat production granites worldwide, and above plutonic and volcanic rocks from other areas of Mexico. Noble gasses isotopes show a major crustal radiogenic input (79.4–96.1%), consistent with the radiogenic heat source hypothesis for the thermal springs, and a secondary mantle input (1.9 – 9.4%), which can be related to the ascent of mantle fluids through a deep crustal zone at the intersection of the regional structures. The isotopic He composition differs from the volcanic geothermal systems of Mexico such as Los Azufres, Los Humeros and from the unconventional geothermal area of the Juchipila Graben, located 300 km southeast in the same tectonic setting. The first two nearly represent a binary mixture between mantle and air saturated water end-members, whereas the latter also includes a minor crustal signal. Modeling of He isotopes data confirm that the 3He signature is associated to the ascent of mantle fluids through a deep crustal zone of highly fractured rocks at the intersection of regional structures. The signal of mantle fluids is diluted over the time by 4He sourced from the Oligocene high-heat production rocks.