I. Clare Robinson

B. Sc. Honours Thesis

Pyrrhotite composition and its relationship to acid drainage potential in the Halifax Formation, Meguma Group, Nova Scotia

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The physical disruption of sulphidic slates of the Halifax Formation of the Meguma Group in Nova Scotia, leads to oxidation of sulphides and the generation of acid rock drainage (ARD). Acid runoff has been a serious problem at the Halifax International Airport (HIA) since the early 1960's, resulting in fish kills, as well as severely degraded water quality and aquatic habitat in receiving streams. The seriousness of these accidents and the high cost of abatement and remediation procedures have lead to revised guidelines by the Nova Scotia Department of Environment (1995) which require strict predictive test procedures before allowing physical disruption of Halifax Formation slates.

Previous ARD studies in Nova Scotia have not considered in detail the mineralogy, texture, distribution and mode of occurrence of sulphides nor how these factors may influence the potential for ARD. Recent research at the Department of Earth Sciences, Dalhousie University, Halifax, N.S., has shown that the most abundant sulphide from the problem area of the HIA is monoclinic pyrrhotite (Fe7S8) which is the most iron-deficient and magnetic form of pyrrhotite. It also oxidizes substantially faster than pyrite, and thus generates damaging pulses of ARD.

Due to the widespread distribution of the Halifax Formation throughout southwestern Nova Scotia, it is important to determine the type and composition of sulphides in other areas. Representative samples of sulphidic slates from the Halifax Formation were collected in the field and from drillcore at four locations throughout southwestern Nova Scotia, in areas with different geological settings, such as proximal to distal from granitic intrusions and contrasting trace element lithochemistry, to obtain a variety of mineralogical assemblages.

Petrographic, microprobe and X-ray diffraction work confirm that pyrrhotite composition is consistent and relatively homogeneous regardless of geological environment and is of the monoclinic type, thus posing the worst risk in terms of ARD pulses when the sulphidic rocks are exposed to air and water. Two widely accepted acid drainage static tests, the BC Research Initial Test and the US Environmental Protection Agency's Acid-Base-Accounting procedure, were performed on samples from the four field areas. The two tests yield slightly different results for the samples, but confirm that these rocks are potential net acid producers, despite some containing carbonates.

Keywords: acid rock drainage (ARD), Halifax Formation, sulphides, slates, monoclinic pyrrhotite, magnetic, acid drainage static tests, BC Research Initial Test, US Environmental Protection Agency's Acid-Base-Accounting, net acid producer
Pages: 112
Supervisors: Marcos Zentilli