Petrogenesis of Holocene siliceous sinters from the Los Geysers geothermal field, northern Trans-Mexican Volcanic Belt

Abstract

Sinters are siliceous rocks precipitated from geothermal waters typically formed in continental volcanic provinces, where heat flux is significant and deep source fluids become silica-rich due to water-rock interaction processes. Research on this type of rock is useful to elucidate key aspects on the reconstruction of hydrothermal processes operating during their deposition. This study aimed to understand the petrogenesis of sinters exposed within the Los Geysers geothermal field, Trans-Mexican Volcanic Belt (TMVB) north-central sector. Petrographic observations, 14C ages, whole-rock geochemistry, and Sr isotopic analyses were undertaken to address the primary goal of this research. The siliceous sinters precipitated in Los Geysers were characterized by a lack of diagenetic maturation, being composed of non-crystalline opal-A, classified as “dry opal” (4.53–8.08 wt% of the H2O+). Petrographic observations revealed a high abundance of diatom frustules. Middle and distal apron lithofacies were associated with a relatively calm depositional pool environment (warm to mesothermal conditions). Sinter growth occurs mainly by evaporation of the geothermal water mobilized by capillary creep through the porous matrix. The earliest hydrothermal activity in the Los Geysers area is dated at ~6.7 kyr. B.P. The difference in deposition ages between two sinter mounds (~2.0 kyr) suggests a relationship between palaeo-seismic events (>6.9 Mw) and the hydrothermal activity. Chemically, the sinters were composed mainly of SiO2 (91.63–97.60 wt%) with minor amounts of CaO (0.17–0.98 wt%), Na2O (0.08–0.31 wt%), and MgO (0.06–0.11 wt%). Trace elements such as Cs, Co, Ga, B, Be, Sr, and Li are enriched because of their hydrothermal nature and presented similar contents relative to the Upper Continental Crust (UCC). Sr-isotopic values (0.706038–0.706955) and rare earth elements (REE) patterns normalized with the Post-Archean Australian Shale (PAAS) standard (LaN/SmN = 0.36–0.80; YbN/SmN = 0.26–0.67; Eu/Eu⁎ = 1.33–1.67) reflect the bulk composition of the precursor liquid.