Pyrrhotite was also identified as a culprit in Quebec, where 4,000 residential foundations, as well as concrete in some public buildings, are failing.

“I saw with my very own eyes the difficult situation in which too many families live because of pyrrhotite,” Mr. Trudeau told reporters.

The copper deposits are mainly in well-marked fracture planes in serpentine; the ore is pyrrhotite, with or without chalcopyrite.