I believe you are correct. These forces are so powerful, however, I am not sure an ancient man could have thought, "if I can reproduce that, I can melt rocks". IMO, it started in campfires with simple metals like lead which melts at low temperature. And probably progressed as pottery making progressed. Pottery furnaces enabled early man to produce temperatures necessary to see controllable cause and effect. So much for my opinion.
About veins of pure metal? I suppose they have and do exist but they are very rare. Most metals are very reactive and when liquid easily combine with other chemicals.
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