PYRRHOTITE AND ASSOCIATED SULFIDES AND THEIR RELATIONSHIP TO ACID ROCK DRAINAGE IN THE HALIFAX FORMATION, MEGUMA GROUP, NOVA-SCOTIA

The physical disruption of sulphide-bearing metasedimentary rocks of t he Halifax Formation leads to oxidation of iron-sulphide minerals and the generation of acid rock drainage (ARD). Although pyrrhotite occurs in many places throughout the Halifax Formation, previous ARD studies have not considered in detail the mineral chemistry, texture, and dis tribution of this mineral nor how these factors may potentially influe nce the development of ARD. For this study, pyrrhotite-bearing samples of the Halifax Formation were collected in the field and from drill c ore at four locations in southwestern Nova Scotia. Samples were taken from different geological settings, such as proximal and distal to gra nitic intrusions and from different stratigraphic positions, to obtain a variety of mineral assemblages. Petrographic, microprobe and X-ray diffraction work indicate that the pyrrhotite in all samples is mainly monoclinic Fe7S8, and its composition is relatively homogeneous regar dless of geological environment. Inclusions of chalcopyrite and detect able quantities of As, Co and Ni are common. In regionally metamorphos ed, greenschist-facies areas, pyrrhotite is preferentially aligned alo ng cleavage planes and thus is easily accessible to oxidizing air and fluids. Because pyrrhotite is regionally developed, contains potential ly toxic trace elements, and occurs along cleavage planes, it is consi dered to play a, significant role in ARD development in the Halifax Fo rmation. Also, pyrrhotite oxidizes substantially faster than many othe r sulphide minerals and may be especially significant in the early sta ges of ARD.