If surface mines tend to cause greater environmental impacts, why are they used?

Surface mines, in metallic mining called open pits are used because they are much more economical for large low grade deposits in which the mineralization as in copper is widely disseminated and needs a bulk mining method such as an open pit. The size is usually numbered in say approximately 50 million to say well over a billion tonnes (metric tons) and underground mining methods mostly could not move this amount of material. Electric shovels and trucks up to or greater than 200 tonnes per truck are used to move this material and it is not uncommon to move 300,000 tonnes per day.

Underground is much more labour intensive and much more expensive so needs a high grade orebody. An open pit can do a lot to preserve the environment by reclaiming the waste dumps ie re-sloping and planting grass for the deer. Also, open pits are no hunting areas so allow a safe haven for large mammals like the moose in Canada.