Gold mining is mostly strip mining tons of ore, spraying it with cyanide, collecting the runoff, and refining that. Then you pack up and leave before someone makes you clean up.

A very rough estimate: World wide Bitcoin mining operation spends per year around 60% as much electricity as all of USA's bank offices: https://news.ycombinator.com/item?id=15615601

With Bitcoin mining there is a high incentive to actually use renewable energy sources and could be physically outsourced to places where building green power plants is otherwise not practical due to the limits of creating a power grid to make such a power plant profitable.