Groundwater was sampled with a straddle-packer system at the Stratmat Main Zone deposit, New Brunswick, Canada, to better understand the origin of self-potential (SP) anomalies associated with massive sulphide mineralization and to investigate processes of sulphide oxidation below the water table. Groundwater displays a marked increase in total dissolved solids (TDS; 200 to 1600 mg/l) through the sulphide zone (84–89 m in depth). Groundwater ranges from low TDS CaHCO3-type water at shallow depth to higher TDS CaSO4-type water within the massive sulphides. The massive sulphides are coincident with a large SP anomaly (−600 mV) compared to the host rocks with an associated positive temperature anomaly of 0.05°C. With increasing depth through the massive sulphide zone, base metal (Cu, Zn, Mo, Pb, Cd) and Fe concentrations increase (e.g. Fe increases from 40 to 2300 mg/l). Stable isotopic compositions in groundwater also vary with depth in the sulphide zone with δ18OVSMOW increasing from −12.32 to −12.00‰ and δ13CVPDB from −13.55 to −10.31‰. The stable isotopes, TDS and straddle-packer recovery time data indicate that groundwater flow is greatest in the upper parts of the massive sulphides.

The large SP anomaly is interpreted to result from decoupled sulphide oxidation occurring between oxygenated waters impacted by the borehole and non-pyritic sulphide phases within the sulphide mass. Electrons pass from sulphide phases (e.g. pyrrhotite) through intervening pyrite to dissolved oxygen in the borehole, which renders a cathodic character to the wall of the borehole and an anodic character to the interior of the sulphide. The higher hydraulic conductivity of the upper massive sulphide has allowed penetration of dissolved oxygen, which produces a similarly decoupled oxidation reaction between the upper and lower sulphides, at least proximal to the borehole. The increasingly anodic nature of the sulphide mass with increasing depth is demonstrated by the chemical evolution of groundwater with depth. Increasing SO42−, H+, Fe2+ and Zn2+ concentrations are evidence of the oxidation reactions occurring in deeper sulphides, whereas the higher pH in the upper massive sulphide is evidence of O2 reduction. Increased δ13CVPDB with depth probably results from enhanced carbonate mineral dissolution as acid is produced. The unusual positive temperature anomaly associated with the borehole may be partly a result of exothermic oxidation of the sulphide but is more likely due to the higher thermal conductivity of massive sulphide relative to country rock. The sulphide appears to be acting as a conduit for heat from deeper areas, which suggests that a large sulphide mass extends beneath the intersected section.

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