It's a 50/50 explanation. Some of it is explained, some isnt. The Ghost People were explained as having been workers exposed to the corrosive material in the cloud by running into a pipe of it, but the cloud corroded the locks on the people. The same dust is also explained as what has prisinely preserved the Sierra Madre as a whole. However, we're never really given a full explanation of it's origins. This is likely done to add to the horror mystique of a gone wrong utopia gone wrong feeling it posesses.
I remember a God comment: "Is that smell copper or Old World gold?"
I think the OP mightbe right about the gold in some way. Maybe a few bars or so somehow helped create the cloud, with the help of the poorly constructed villa made of cheap material and such.