GEOSPATIAL MODELLING OF PYRRHOTITE IN BEDROCK AND RISK ACROSS CANADA

Pyrrhotite is a chemically reactive iron sulfide mineral (Fe_(1-x)S) that causes substantial structural damage to infrastructure when present in the coarse aggregate fraction of a concrete mix. In Trois-Rivières, Québec, for example, expansion of concrete due to oxidation of pyrrhotite-bearing aggregate in foundations of homes has led to several million dollars of property damage. Accordingly, constraining the distribution and risk of pyrrhotite in bedrock is critical for informing decisions regarding concrete aggregate extraction.

This research investigates the distribution of pyrrhotite occurrences and pyrrhotite-bearing geology across Canada. We generated a series of national-scale maps, based on publically available bedrock geology and provincial/territorial mineral occurrence datasets, to illustrate pyrrhotite occurrences with respect to bedrock geology units. A goal is to assess the effectiveness of several different map styles at modelling the relative risk of pyrrhotite for defined rock units. The most effective strategies involve normalizing the number of pyrrhotite occurrences with respect to rock unit area or total number of mineral occurrences per rock unit, and defining different risk classes based on pyrrhotite occurrence density. Such geospatial products overcome sampling biases in the mineral occurrence datasets; therefore, may be most appropriate for assessing which rock units pose the greatest risk for pyrrhotite. National-scale maps of pyrrhotite risk across Canada can be compared with related datasets from the United States to assess the distribution and risk of pyrrhotite across North America.

Geospatial modelling can help inform the selection of potential aggregate extraction sites, as well as guide investigations of possible pyrrhotite occurrences in coarse aggregate used for concrete. Such results may also be valuable for targeting key areas to pursue detailed geotechnical tests of coarse aggregate used to develop concrete standards.