I'll have to correct my first post in the interests of historical accuracy. It turns out that tin and lead were the first metals that were smelted from ore; copper ore needs about 200° more than a wood fire can attain. It seems that copper smelting had to wait until pottery kilns were invented. So that suggests that we already had the idea that metals can be extracted from 'special rocks', rather than a lucky find. Why it took as long as it did for alloys to appear is opaque, it seems. They already had tin when copper was first smelted. Now there was a growing metallurgic science, mining was important during the Chalcolithic. But, there's evidence of early bronze alloys, perhaps they were too difficult to work at first, or, working bronze had to wait for more technology to be invented; necessity and all that . . .