Possible Clues:

• Mineral that's covering gold man comes upon
• Nero or I (anag)
• Smooth alternative to European source of heavy metal
• Taconite
• Mineral yielding common metal
• Limonite, for one
• Siderite, for one
• Refined find
• Mineral from which a hard magnetic metal may be profitably extracted
• Press old engineers to show where metal comes from