The ocean is full of sulfate, so if you get the water there, I'd think it would be just a matter of diluting it, as long as sulfate is the only contaminant of concern. I assume that the problem is with diluting the sulfate out into the freshwater system around these mines. I don't have specific information on sulfate-accumulating plants. Generally plants uptake sulfate for incorporation into the amino acids cysteine and methionine, but that is a minor demand. The downside there is that the pH of the treated water is going to be outrageously high, which could theoretically be brought down with carbon dioxide, I guess.
Calcium sulfate is reasonably inert.
You could make wallboard with it, I guess. So you have already been there, done that, it doesn't work.