Most gold mines, at least in the western U.S., are in sulfide ores. They have variable, but sometimes significant, amounts of silver, lead, and other metals - but as noted above if you were looking for the gold the other stuff often just got dumped. That's why the tailing piles can sometimes be re-mined for other things, and even for gold using modern recovery methods (mostly using cyanide). Also, mines usually were abandoned because they no longer made money enough to cover expenses and/or make suitable profits for the investors and/or justify more investment - in which case In Those Days everybody just picked up their tools and left. As also noted above, there's no real prospect of cleaning up the remaining mess with money from the miners.

Another issue with sulfide ores, which probably contributed in this case, is that when exposed to air and water they produce acid. The drainage water from sulfide mines can have a pH below 0. And of course that kind of stuff rots the rocks further, which releases more metals, and makes more acid, which makes a royal mess. That's the kind of stuff that dumped into the river; nobody will be fishing it for years, and the towns that use the water may have to re-think their treatment processes. The farmers that irrigate directly from the river are probably SOL - nobody will buy crops from them knowingly even after things are running fairly clear again.

I haven't seen any "official" comment on the mess other than a big "oops" from EPA and the warnings about staying away from the rivers. Probably too soon. Unless somebody has more references, for instance about the concrete plugs.