The major phases found in the vicinity of the 1866 m flow zone are K-feldspar, epidote, pyrite, hematite, and chlorite. These are found to exist in primarily four, two-phase assemblages, the dominant being K-feldspar+pyrite, epidote+pyrite, epidote+hematite, and chlorite+pyrite. Complete reaction relations among these isobaric isothermal divariant phase assemblages oriented in log fO2-pH space are compatible with stable growth at 300 ¿C. Sensitivity of reaction relations to total sulfur, temperature, pressure, and thermodynamic values is investigated. The aqueous fluid sampled at 1866 m is out of equilibrium with the more reducing phase assemblages lying stratigraphically above and below the flow zone as well as pyrite-bearing assemblages in the flow zone and shows aspects of mixing with other, hotter, flow zones. High concentrations of CO2 may indicate contribution to the fluid of CO2 from hotter portions of the system. Low total sulfur and high total iron are due to possible gas loss during down hole sampling and reaction of the fluid with well casing and lost circulation materials. ¿ American Geophysical Union 1988