Especies sulfurosas del sistema hidrotermal actual del campo geotérmico de Los Azufres, Michoacán: distribución y comportamiento isotópico

Abstract

Los Azufres show a mineralogy of copper sulphides represented by idaite, digenite, covelite, bornite (which have a local distribution), and chalcopyrite.Pyrite (the most abundant species), pyrrhotite and marcasite are present in well-defined zones.

The paragenetic succession of neoforming opaque minerals is greatly controlled by a reductive medium found at great depth; boiling seems to control the massive precipitation of mainly calcite and hematite. Pyrite is found in an oxidant medium, associated to zones of low permeability and acid pH.

The boiling (amply testified in the beds) propitiates a massive separation of hydrogen and other volatile elements initially dissolved in the liquid phase. The ascending water is more oxidant than the one present in the deep reservoir and than the volcanic rock itself, with which it comes in contact, a phenomenon which is reflected in the evolution of the mineral paragenesis of pyrrhotite to pyrite and to marcasite and pyrite+ hematite. The thermodynamic adjustments imply an increase in the fugacity of oxigen (fO₂) and a simultaneous lowering of the fugacity of hydrogen (fH₂. The hydrothermal system of Los Azufres belongs to “low sulphidation” type. The isotopic composition of pyrite presents two groups: the first (from deep areas of the field, without boiling ) has values that go from FeS₂ d ³⁴S = - 1.57 %o to - 2. 97⁰/00, and the second (from the more shallow, steam dominated zones) has values of FeS₂ d ³⁴S = - 4 %0 to - 4. 83⁰/00. The mean value of pyrite is FeS₂ d ³⁴S = - 2.5⁰/00, which is very similar to that of H₂S in H₂S d ³⁴S = - 2.0⁰/00. The isotopic composition of the dissolved sulphates presents a composition of SO₄d ³⁴S=+ 21.47⁰/00, which was found to be in isotopic equilibrium with sulphides. Boiling seems to control the precipitation of sulphides as well as its isotopic composition.