Abstract

An investigation of surface and subsurface redox and electrical field conditions was carried out over a volcanogenic massive sulphide (VMS) deposit in thick glacial clay and sand terrain of the Abitibi clay belt of northeastern Ontario. Thirty-seven boreholes were drilled to 9 m depth on two lines crossing the strike of mineralization and were completed in either sand or clay. An additional three were drilled 30 to 50 m and completed in bedrock. Monitoring wells were installed in all holes and groundwater was later sampled. A variety of oxidation–reduction potential (ORP) and spontaneous potential (SP) techniques were used, the methodologies for which were introduced earlier and are further developed here. The best definition of redox conditions was obtained by ORP measurement of clay and sand slurries from split-spoon samples collected and analysed during the drilling programme. Related techniques included groundwater ORP and down-hole platinum-SP (PtSP) testing, the latter measuring total potential field, which is the sum of the electrical and electrochemical components of voltage. Although the performance of the methods differed, the results consistently show chemically reduced areas (reduced ‘columns’) in the phreatic zone above mineralization with overlying acidic ‘caps’ in the vadose zone. Other features noted over mineralization include carbonate remobilization, partial extraction metal anomalies in shallow soils, CO2 enrichment in groundwater, and elevated groundwater temperature. These varied features are related and provide evidence for upward transport of reduced species from mineralization. They agree well with phenomena predicted in a previously published redox-gradient transport model.

SP was measured, both down-hole and on surface, and shows a weakly negative electrical field response over mineralization. This response is small in comparison to the redox response and is interpreted to occur because, in addition to being oxidizable, the sulphide is also conductive and SP currents within it produce a small electrical field. It is postulated that oxidizable but non-conductive features such as sphalerite, would also produce a reduced column but without an electrical field, whereas, conductive, non-oxidizable features such as graphite would produce an electrical field but with little or no redox response.

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