RELATIVE RATES OF SULPHIDE OXIDATION BY CHEMICAL AND MICROBIAL MEANS: THE ROLE OF MINERALOGY AND TEXTURE IN ACID ROCK DRAINAGE (ARD) FROM THE MEGUMA SUPERGROUP, NOV A SCOTIA

Abstract

The Meguma Supergroup in Nova Scotia contains abundant sulphide minerals that potentially can cause acid rock drainage (ARD), which is environmentally damaging and expensive to ameliorate. The problem is most intense in the basal Halifax Group where a well-defined suite of sulphide minerals is dominated by monoclinic pyrrhotite and lesser pyrite. Meguma rocks also contain hexagonal pyrrhotite, arsenopyrite, chalcopyrite, galena, and sphalerite. These occur in different textures, sizes, orientations, or mineral associations, depending on such factors as rock type, stratigraphy, composition, structure, and metamorphism. This project investigated the relative rate of chemical and microbially assisted oxidation of a selected suite of sulphide minerals. Six pairs of polished thin sections containing a wide selection of sulphide minerals and textures were oxidized in a controlled laboratory experiment with two treatments. One thin section of each pair was placed in a natural uncultured sample of ARD collected from a quarry near the Halifax International Airport (pH between 3.62 and 3.76). The matching thin section of the pair was placed in ARD (pH = 3 .16) from the same source but double filtered at 0.2 microns to remove bacteria. Surface changes were monitored regularly and documented with photomicrography, and final surface characteristics were documented in detail with the SEM. Tarnish (colour changes} and etching of pits, cracks and polishing scratches were interpreted as oxidation. Photomicrographic evidence indicates a significant difference between the treatments. In general, sulphide minerals exposed to unfiltered, biologically active ARD oxidize faster than in filtered ARD. In the microbial treatment, the relative degree of oxidation among sulphides was galena > hexagonal pyrrhotite > monoclinic pyrrhotite > anhedral pyrite >> arsenopyrite, sphalerite > euhedral pyrite > chalcopyrite. In the sterile treatment, the relative degree of oxidation was galena > anhedral pyrite > monoclinic pyrrhotite>> hexagonal pyrrhotite, sphalerite, arsenopyrite> euhedral pyrite> chalcopyrite. This study confirms the essential role played by biological agents in ARD, but also shows that sulphide mineralogy, texture, grain orientation and geology must be considered in the evaluation of ARD potential in Meguma rocks.