Sulphide minerals in waste rock used as construction fill and their effect on groundwater chemistry at the Martha Au-Ag mine, Waihi, New Zealand

Abstract

Chemical and isotopic characterisation of groundwater, alongside geochemical testing of waste rock employed as construction fill, was used to identify sources contributing to increased levels of sulphate in monitoring wells at the Martha mine tailings storage facility, Waihi, New Zealand. Waste rock contains goethite and pyrite (about 0.3 wt%), indicating that the deposits are in an advanced stage of oxidation. Xray diffraction and shake flask tests indicate the main soluble sulphate minerals are gypsum, epsomite and iron and manganese sulphates. On average, about 0.04 wt% water-soluble sulphate is present in the deposits, but concentrations vary spatially; values up to 0.12 wt% exist in pockets within the deposit. Net acid generation tests and acid base accounting confirms that some localised regions of fill are potentially acid forming, however, most samples are classified uncertain. Analysis of isotopic signatures indicates groundwater sulphate, which currently ranges from ~3 to 100 mg/L, is predominantly the result of mixing between two primary sources: (1) pyrite released from the oxidised waste rock, (2) atmospheric sulphate derived from precipitation. In a few of the monitoring wells, some of the sulphate is derived from other sources resulting in anomalous stable isotope compositions. In wells affected by pyrite oxidation, isotope dilution calculations show between 3 and 78% of the SO4 is sourced from pyrite oxidation, with an average contribution of 25%. A conceptual model of the system includes: isotopically enriched atmospheric sulphate introduced via rainwater dissolves secondary sulphate salts that are relatively depleted in 34S, and transports them through the underlying layers towards the water table. Pyrite oxidation is limited through the armouring of grains with goethite, and the formation of preferential flow paths mediates the dissolution of secondary sulphate salts. Concentration of salts, due to evaporation, occurs at the capillary fringe, causing some un-mineralised material to return acidic paste pH results. Minor amounts of additional sulphate, with 34S values similar to that of atmospheric sulphate, yet depleted in 18O, are incorporated through the oxidation of organic sulphur in carbonaceous layers beneath the fill. Once in the shallow groundwater, bacterial sulphate reduction is a locally important process resulting in removal of sulphate and metals. In most other areas, dissolved constituents released from the fill are diluted by up to 1.5 times through additional mixing with meteoric water. The combination of stable isotopic analyses of water, dissolved sulphate and local sulphide minerals, coupled with detailed hydrogeochemical investigations, is an effective tool for identifying multiple sulphate sources, and for quantifying their relative contributions to groundwater quality.