Silica melts at ridiculously high temperatures. Being careless with molten glass can cost a person an eye, more so with silica. Like most other people, I have no idea what you're talking about "melting" pyrite -- FeS will decompose. I guess you mean melting the sulfides to iron? Then yes, the gold will definitely alloy with the iron. But there's too much else going on. Like others said, the traces of gold (and you're only going to find traces, its gold after all) will be lost, even if they theoretically separate.