But was the Canadian Shield exposed before the last glaciation? If it was buried under a large layer of other stuff before, the fact that it wasn't mined may not tell us much.
Again, not a geologist (or glaciologist), but glaciations happen frequently, in geological terms - every 40k? years or so. And, they don't dig very deep. Mines go >1km deep, so are below what I think any glaciation would have an effect on.