Comment by munk-a

It's dependent on the minerals being prospected for and the mining techniques as well as general awareness and regulations that were in place during operation. Yellowknife has a huge ecological issue due to Arsenic being actively used in Giant Mine[1] but that was introduced to ease mining (mercury is also commonly used for gold extraction and purification, though occasionally offsite). So it really depends on the mine. Mines are never good for the environment but how toxic they are will vary.

1. https://en.wikipedia.org/wiki/Giant_Mine