Here's how markets work: the further they get stretched from the long-term norm in one direction, the more violent the move tends to be when they eventually swing in the other direction. It's like a pendulum. So the bigger the bull market, the bigger the bear market that follows, and vice versa.
Mining stocks have already experienced one of the biggest, most destructive bear markets in history. The Gold / XAU ratio has been bounced around through most of history from between 6 on the high side, which would be undervalued to 3 or 4, on the downside which would be overvalued to the price of gold. In the 2008 bear market the ratio broke out of this long term channel and miners became quite undervalued versus gold.
Then we had the bull market, but as the price of gold moved up and completed the second phase of what I consider its secular bull market, the miners never got to a point where we would consider them to be overvalued compared to the price of gold. Then, by 2015 we had the gold bear market, unlike anything we'd seen so far.
This is where the pendulum comes into play. After one of the most destructive bear markets in history, I think it's going to generate one of, if not the, largest bull markets in history and valuations have started moving back to a more normal level.