I'd hijack LAVA's Update function (everything that's "molten" is actually LAVA with a ctype of whatever element comes after "molten") and check for neighbouring molten IRON and GOLD particles. Something like this.
Crashed how?
Edit: Nvm I got it, thanks for finding this!
Edit 2: The problem is that I set HighTemperatureTransition to 1500 instead of HighTemperature. Element #1500 doesn't exist (and in fact can't exist due to a limit set in the source code) but nothing checks HighTemperatureTransition for being valid, so you get a crash. I'll fix this later, lol.
You should totally set HighTemperature to 1500 though as that will make BGLD easier to separate from the other molten stuff. (And you shouldn't set HighTemperatureTransition at all!)