Sulphide oxidation and groundwater transport of base metals at the Halfmile Lake and Restigouche Zn–Pb massive sulphide deposits, Bathurst Mining Camp, New Brunswick

Abstract

A detailed hydrogeochemical study at the undisturbed Halfmile Lake and Restigouche Zn–Pb deposits, Bathurst Mining Camp, New Brunswick, was initiated to elucidate processes controlling the oxidation of sulphide minerals, factors influencing base-metal dispersion in ground and surface waters, and to improve hydrogeochemical exploration methods. Groundwaters were collected using flow-through bailer and straddle-packer technologies; the latter of which proved to be the most effective under the fracture-flow dominated conditions. Groundwaters from the two deposits are compositionally distinct. At the Halfmile Lake deposit, groundwaters are dominated by low total dissolved solids (TDS < 250 mg l −1 ) and Ca–HCO 3 -type compositions. Groundwaters at the Restigouche deposit vary in composition from shallow Ca–HCO 3 - and Ca–SO 4 -type waters to high TDS Na–Cl-type waters. Elevated sulphate and base metal contents (up to 4200 μg l −1 Zn and 1400 μg l −1 Pb) of shallow Ca–SO 4 -type groundwaters proximal to massive sulphides indicate that the sulphides are undergoing natural oxidation. The degree to which the massive sulphides are oxidized depends on the depth of penetration of dissolved oxygen and flux rates, which are controlled by the permeability of the rocks and local hydrological gradients. Transport of metals away from massive sulphides depends on physical factors that include hydrology, groundwater flow, and the degree of connectivity between groundwater and surface water environments, as well as groundwater pH, metal adsorption and colloid mobility.