REDOX REACTIONS ACCOMPANYING HYDROTHERMAL WALL ROCK ALTERATION

Wall rock alteration processes often include redox reactions involving iron and sulfur. The effect of these reactions on the composition of hydrothermal solutions can be represented conveniently on 1ogfs2-1ogfo2 diagrams. Contours of total dissolved sulfur concentration at 250° C and 350° C, calculated on the basis of the available thermochemical data and partially checked by direct experiment, were superimposed on the mineralogical boundaries of the Fe-S-O system in logf s2 - logfo 2 diagrams. During the replacement of iron oxides or silicates by pyrite, solutions containing a higher concentration of reduced than oxidized sulfur species move toward lower for values, whereas solutions containing a higher concentration of oxidized than reduced sulfur species move toward higher for values. The sequence of iron sulfide-iron oxide minerals in wall rock alteration zones depends both on the trend of these reaction paths and on the initial total dissolved sulfur concentration in the hydrothermal solutions. The observed sequences of minerals in alteration zones at Butte, in the northern part of the Boulder Batholith, and at Ely, Nevada, can be interpreted readily in terms of the proposed scheme. The concentration of total dissolved sulfur in the solutions in these three areas was probably less than 10 -2 moles/kg of solution. In the northern part of the Boulder Batholith the chemistry of the hydrothermal solutions was affected strongly both by cation metasomatism and by redox reactions. © 1969 Society of Economic Geologists, Inc.