They recycled old power plant. That is a huge CO2 offset, building new plant would had huge emmisions.
It's similarly not a "CO2 offset": the offset is latent in the plant remaining dormant, or possibly being scrapped. By turning the plant back on, these miners have converted a CO2 offset into an ongoing source of CO2 emissions.
Clearly the couldn't sell energy before if they were about to shutdown, so there must be something different about the miners compared with their usual market, can't just be cheaper.
In the article they seem to be suggesting they couldn't buy renewable power in 2020 which seems odd, but no follow up detail on it, apparently they're phasing coal out in a year so it may just have been a short term thing.
And knowing Montana, it is difficult to get electricity connection there.
Yes, obviously. And what's the downside of that?
And even more obviously—-Not mining/creating a desired product that would allow you to make a profit on your investment.