A surface and subsurface investigation of pH, oxidation–reduction potential (ORP) and spontaneous potential (SP) was carried out over sulphide-hosted gold mineralization in an area of thick and extensive glacially derived overburden. Protocols were developed to measure ORP and pH in clay slurries, peat and groundwater to test for the presence of redox and pH phenomena that were predicted to occur over mineralization. ORP protocols focused on removing memory effects caused by probe history and storage and on obtaining a reproducible measure of the ORP over mineralization relative to background areas. The results confirm the presence of distinct reduced ‘columns’ in clay and interstitial groundwater overlying sulphide mineralization that are in fact reduced ‘curtains’ that follow the trace of mineralization. These are accompanied by high pH within the reduced column, with minor lows on the flanks, and elevated H2S and CO2 in shallow groundwater over mineralization. Whereas the H2S appears to have its source in bedrock mineralization, the CO2 is likely a secondary near-surface phenomenon related to oxidation of metals in the reduced column, which produces acid and concomitant dissolution of carbonate in the clays. Wide groundwater temperature anomalies are spatially associated with mineralization. High ORP in shallow peat above mineralization may be evidence of an ‘oxidized cap’ over the reduced column. A minor SP response in surface peat was noted but is small in comparison to the redox field voltage. The presence of reduced columns and other features are consistent with a previously published redox-gradient transport model.
- © 2004 AAG/The Geological Society of London