INFLUENCE OF SIDERITE ON THE PORE-WATER CHEMISTRY OF INACTIVE MINE-TAILINGS IMPOUNDMENTS

Siderite (FeCO3) precipitation and dissolution can influence strongly the geochemistry of pore water associated with the alteration of sulfide minerals in mine tailings. Laboratory determinations of siderite solubility were made over a pH range of 4.1 - 6.5 and a FeSO4 concentration range of 0.0 - 1.8 m. These conditions are representative of the geochemical zones where siderite has been observed to dominate the chemistry of tailings pore water. The laboratory data were analyzed with the Fitter ion-interaction model, which is applicable over the large range in FeSO, concentration studied. The Fitter model and new constants derived from this solubility study were used to calculate saturation indices for pore-water chemistry data obtained from a tailings impoundment at the Heath Steele mine, New Brunswick, Canada. Sulfide minerals in this impoundment had been oxidizing for approximately 20 years at the time of sampling. The model calculations suggest that the tailings pore water, which contains up to 1 m FeSO4, is at or near saturation with respect to siderite for the majority of the water samples collected. Mineralogical study indicates siderite is present in the tailings solids at approximately the same locations where the pore water approaches saturation with respect to siderite.