Funny you should ask...
I had a customer that had a lump of what I would say looked like moon rock. It consisted of many different metals including but not limited to gold, silver, copper, etc. At some time in their families history, a house had burnt down. This metal mass was the result of a fire that had been hot enough to melt the coins into a mass of fused metals, and partially alloyed some of them.
Needless to say I was able to refine the metals, and to my surprise there was actually a large amount of gold and silver. I would never have guessed it just by looking at it, perhaps by weight but I still was surprised after everything was said and done.
There is a great story about a couple Nobel Peace Price winners during WWII who saved the gold out of which their medals were made, by dissolving them in Aqua Regia. Check this out for an interesting reading:
How aqua regia saved Nobel Prize medals from the Nazis | A Schooner of Science
Scott