Take one envious metallurgist as your raw material. Extract and refine his essence with an alien species's maturation process. Temper this lode with a deep, abiding bitterness. The result: One molten, monstrous Ore-Master.
Andy Wilson was a metallurgist. When his wife's brother, Thatcher, asked him to use his professional skills to analyze a mysterious object, he all but jumped at the chance. Ever since Thatcher was able to make Andy's gambling debts go away for a while, he had suspected his brother-in-law was in the mob, and this was his chance to rub elbows with that forbidden lifestyle.
The object was an egg-like container of some sort. Unfortunately, by breaching it, Andy exposed himself to its contents, an extraterrestrial chemical known as sorna. For the Oblati race, sorna was a key component of their physical maturation process. The typical Oblati spends time traveling and experimenting to discover their true self, then uses sorna to make that true self physical reality. The substance transforms their body to match who they are inside. When Andy Wilson came in contact, he found himself merging with the very ground beneath him, pulling its substance into and onto his own mass. He became a creature of living rock and magma.
As it happened, Wilson held a burning grudge against Gottfredson Mining, who he felt cheated him in a past dealing. Viewing his new body as a chance to avenge himself at last, he attacked the home of one of the company's executives. The superhero Cleopatra confronted him, but he was able to escape. He resurfaced to strike at one of Gottfredson's mining facilities in Canada, but this time Cleopatra defeated him.
Being composed of rock and magma, the Ore-Master can be thought of as a living, raging furnace. By firing streams of this thermal lethality from his hands and feet, he can propel himself through the air. His bulky body is super-strong.
It's not entirely clear whether his entire body is made of Earth-stuff or if his human self simply resides underneath a shell of the rock and magma. In either case, he is capable of reverting to his human form.