Significance For Mineral Exploration of Sulphate Concentrations In Groundwaters

CIM Bulletin, Vol. 69, No. 776, 1976

Roland B. Hoag, Jr., Research Associate, and G. Roger Webber, Associate Professor, Department of Geological Sciences, McGill University, Montreal, Quebec, and Mineral Exploration Research Institute, Montreal, Quebec

Sulphate concentration in groundwater is a good measure of the extent of oxidation of sulphides, provided that sulphide minerals are the primary source of the sulphate. A study was conducted near the Eustis Mine in Quebec to determine where and how the sulphate in groundwaters originated. Atmospheric precipitation is the only source of sulphate other than oxidation of sulphides, and in this study area it is equal to 0.04 millimole per litre. The oxidation of sulphides can be divided into subsurface oxidation (mainly inorganic processes) and surface oxidation (including both organic and inorganic processes). The_ maximum sulphate concentration due to subsurface oxidation of sulphides depends on the solubility of 02 in water and is 0.25 millimole per litre. Surface oxidation is, therefore, responsible for any amount greater than atmospheric plus subsurface contributions (0.29 millimole per litre). Surface inorganic oxidation occurs in waters with a pH greater than 4.5 and is controlled by the kinetics of the oxidation processes. Organic (bacterial) surface oxidation is dominant only in acid (pH < 4.5) waters. The bacterial action accelerates the oxidation of sulphides, and large quantities of sulphate may be introduced into solution.

Keywords: Geochemical exploration, Sulphate concentrations, Groundwaters, Eustis mine, Oxidation, Zinc