Kerry Knee

B. Sc. Honours Thesis

Magnetic Susceptibility of Halifax Formation Slates at the Halifax International Airport: Correlation with Potential for Acid Drainage

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The disruption by excavation or quarrying of sulphide-rich slate units of the Halifax Formation of the Meguma Group of Nova Scotia leads to oxidation of the sulphides and generation of acid drainage. Construction within the area of the Halifax International Airport (HIA) since 1955 has caused the most serious problems. Expensive measures were required to avoid fish kills in rivers draining from the HIA. Previous studies assumed that the mineral responsible for the acid drainage was pyrite (FeS2) but recent work at Dalhousie University has shown, and this study has confirmed, that the most abundant sulphide in the slates of the HIA is monoclinic pyrrhotite (Fe7S8). Monoclinic pyrrhotite is a magnetic mineral. This thesis is a pilot study which measured the magnetic susceptibility of representative rock powders and drill cores as a means of predicting the percentage amount of pyrrhotite in slates from the HIA as an indirect measure of their acid drainage potential. To this purpose, the laboratory conducted magnetic susceptibility measurements of representative rock powders and drill core samples. Reflected light microscopy and microprobe analyses of the same samples was used to determine the mineralogy. As a first approximation, pyrrhotite has a positive correlation with magnetic susceptibility in solid drillcore samples, although the inhomogeneous distribution of sulphides in the rock leads to substantial uncertainty at the scale of the experiment. The correlation is poor in powdered samples stored for several years; however, these results may reflect sampling error or recent oxidation of the sulphides in the vials.

Keywords: magnetic susceptibility, sulphide-mineralogy, pyrrhotite, acid drainage, Halifax
Formation, Halifax International Airport.
Pages: 95
Supervisor: Marcos Zentilli