Action Comics #219

Superman suddenly uses his powers to smelt raw gold into ingots. crush coal into diamonds, and collect giant pearls. All these he takes to a cave, and when Lois follows him there she is astounded to find that he has become a super miser. With the location revealed, Superman realizes that crooks will attempt to steal his treasure, so he fashions a steel door to hang across the entrance to the cave. The whole scheme is designed to trap the crook who stole an experimental atomic cutting torch.