Like I said, difference between book and screen. A goodly bit of time passed in the book. Virtually none on TV. The "melted" gold was drippy and "running" not liquified in the book. It was also far less volume than as depicted in the show.
It was a flat out crappily made scene in the show.
And yes, camp fires typically burn between 700 and 1200 degrees, but depending on what you make your fire with, how you stoke and tend it, and etc., you can get one up to a temperature hot enough to smelt steel,which is more than adequate to melt gold. Again, from the book, the fires in question were described as being 10 feet tall, not the little cook fires we saw in the show.