Computer simulation in hydrothermal systems with allowance for nonideality of sphalerite and pyrrhotite

Abstract

To enhance the computer simulation of hydrothermal processes using the HCh program package, an external ZnS_FeS module has been created on the basis of a nonideal asymmetric model of sphalerite solid solution. FeS and ZnS activity coefficients computed in line with this model within a temperature range 200–350°C lead to the decrease in FeS mole fraction (X _FeS) in sphalerite by 3.0–1.5 times as compared with the ideal model. The calculated data on composition of sphalerite at the pyrite-pyrrhotite buffer with allowance for pyrrhotite nonideality are consistent with experimental results within the limits of 2% X _FeS of its value (0.215). A nonlinear relationship logX _FeS versus $\left( {\log f_{S_2 } } \right)$. has been established, involving additional calculated data on equilibria of sphalerite with pyrite and magnetite, as well as pyrite and barite. With transition from pyrrhotite to magnetite and barite, a FeS mole fraction in sphalerite decreases to 0.1 and 0.006, respectively, because of increase in sulfur fugacity. The feasibility of using the calculation results based on the nonideal model of sphalerite for interpretation of natural data is exemplified in the Rainbow ore occurrence at the Mid-Atlantic Ridge (MAR). The computed pyrite-pyrrhotite and pyrite-cubanite-chalcopyrite buffer equilibria (X _FeS = 0.215 and 0.10–0.12, respectively) are consistent with compositions of sphalerite in the pyrrhotite-cubanite-sphalerite and sphalerite ores (X _FeS = 0.20–0.33 and 0.05–0.14, respectively).