Le système hydrothermal actuel de Los Humeros (Mexique): Etat du système SO −− 4 —SH2 à 300°C,origine du soufre et phénomènes d'oxydation associés à l'ébullition du fluide ascendant

In the geothermal system of Los Humeros, hydrothermal fluids are low in sulfur (7.7 mM/kg) and are slightly alkaline (pH of ca. 8.5) at depth. Nevertheless, the aqueous sulfate-sulfide species are in isotopic and chemical equilibrium. The “freezing” of these equilibria at 290 °C is tentatively related to boiling of water at this temperature. In the LH1 borehole δ³⁴S values for pyrite (ca. 0‰) and present aqueous SH₂ (ca. ?12‰) are very different. Measured δ³⁴S value of the bulk aqueous sulfur is close to ?11‰ These data suggest a possible magmatic origin of pyrite (ca. 0‰) and, in contrast, a sedimentary source for the present hydrothermal sulfur. The CO₂/CH₄ molar ratio of 58 determined in the fluid from this well is much lower than the ratio calculated for 300° and 350 °C. This divergence is eliminated when the equilibrium constant is reestimated (log K_G=?7.9 at 300 °C). Based on fluid inclusion study the boiling of hydrothermal fluid before the geothermal area was opened by drilling is proven in neoformed minerals sampled at different levels of the volcanic sequence. In this way, the pervasive oxidation of the primary reduced paragenesis (pyrite + pyrrhotite + magnetite) can be explained. The boiling of the ascending water does induce a H₂ loss from the liquid and creates more oxidizing conditions in the ascending liquid. Moreover, the continuous vaporization of liquid water induces its gradual cooling and increases the ascension speed. Furthermore, a persistence of chemical equilibrium between primary paragenesis and the hydrothermal fluid at different levels would require a very fast and unrealistic adjustment of the fO₂-fH₂ parameters to the local equilibrium conditions. This expected disequilibrium induces the afore mentioned oxidation processes under steady-state conditions.