Prediction of acid rock drainage (ARD) from sulphidic slates using GIS analysis of mineralogical, geochemical, magnetic and geological parameters: A test case in southern Nova Scotia.

摘要

The physical disruption of sulphide-bearing rocks in humid environments leads to the oxidation of the two most common iron-sulphide minerals, pyrite (FeS2) and pyrrhotite (Fe1-xS), and the generation of acid rock drainage (ARD). ARD, also called acid mine drainage (AMD), is typically associated with mining operations that create waste rock piles and tailings impoundments. However, it also occurs in any area that causes physical disruption of the bedrock, such as highway construction, quarry operations, and urban development or expansion. The resulting drainage from these areas generally has acidic pH values in the range of 2 to 4, and high contents of potentially harmful elements that are toxic to local ecosystems.;ARD chemistry, and the overall intensity and duration of the drainage, very much depends on local conditions and the mineralogical components of the bedrock. Acidic drainage from bedrock dominated by pyrite may be very different from bedrock dominated by pyrrhotite, since pyrrhotite reacts much more quickly than pyrite. In pyrrhotite-rich areas, this difference in reactivity rate could lead to toxic "pulses" of low pH waters released into surrounding waterways.;The area selected to test these hypotheses is southern Nova Scotia, which includes the site of one of the most serious cases of ARD in Eastern Canada, the Halifax International Airport. In this study, sulphide mineral textures, compositions, and associations were analysed in detail throughout an area of several hundred square kilometres. Monoclinic pyrrhotite, with varying proportions of pyrite, are the predominant sulphide minerals. The location of pyrrhotite can be detected by magnetic susceptibility measurements made with a hand-held meter, field-scale magnetometer surveys, and regional-scale, airborne magnetic surveys.;Regional-scale stratigraphic, structural, and geophysical data that are presently available in digital form, were incorporated into a geographical information system (GIS), and used as evidence to predict areas that have a high potential of generating ARD. The potential or "favourability" maps generated through expert-driven Boolean logic and fuzzy logic, as well as data-driven, weights of evidence modelling proved very useful for outlining areas that may produce ARD in the future, if the bedrock is disrupted and exposed to surface oxidizing conditions. Due to the high cost of ARD treatment, and the limited success of presently available treatment technology, prediction and avoidance is the best option. In areas where avoidance is impossible, detailed mineralogical studies are necessary in order to plan for, and establish, the best approach to treatment and amelioration.;The conclusions of this study should be applicable in other areas of the world underlain by sulphidic-rich black slate, including the carbonaceous and sulphidic slate of the Anakeesta Formation in North Carolina and Tennessee (southern Appalachians), and the black shale formations of the Karelia Supergroup in eastern Finland.