Geochemical dispersion in groundwater from a weathered Cu-Zn deposit in glaciated terrain

Weathering of mineral deposits under glacial cover may slowly release elements to the groundwater that disperse forming secondary geochemical halos. The Tillex Cu-Zn deposit subcrops beneath a sand aquifer and glacial clay/silt aquitard that extends to the surface. A conceptual model was developed based on hydrologic, geochemical and mineralogical data to describe dispersion in the aquifer and aquitard overlying the mineralization. In the recharge zone, dissolved oxygen oxidizes detrital sulphides in the aquifer and aquitard, which promotes the dissolution of carbonate minerals, increasing the concentrations of Ca and Mg. Iron released by sulphide oxidation precipitates as Fe-(III) oxyhydroxides. Trace metals released during sulphide oxidation may be subsequently adsorbed onto, or co-precipitated with, clays and Fe oxyhydroxides. Sulphate-reducing conditions occur deep in the aquifer over the mineralization and there is no evidence of metal transport upward through the overlying sand and clay/silt overburden. The sulphate-reducing conditions are not favourable for the development of a plume of weathering products that could represent a target for mineral exploration. The one-dimensional reactive transport model PHREEQC was used to test the conceptual model by simulating die hydrogeochemical conditions in the aquitard. The observed data correspond well to the model results, suggesting that the conceptual model is reasonable.