Posible Suggestions:
Gold Mine: Requires Gold in City Radius
Increases Tax Revenue or, as a wonder, produces Commerce in all squares in city already producing 1.
Coal Mine: Rquires Coal in City Radius
Increases Production in city. Perhaps needed for Coal Power Plant and/or Iron Works. Not sure of any additional benefits.
Iron Mine: Requires Iron in City Radius
Increases Production in City. Perhaps it could be needed alongside Coal Mine for Iron Works. Not sure of any additional benefits.
Diamond Mine: Requires Gems in City Radius
Increases Tax Revenue or, as a wonder, produces Commerce in all squares in city already producing 1. Also produces 1 happy face.
Or it could be used to add each of them into one game. Just a sugestion, ofcourse. Though I'm not sure if it's possible for the "Requires X to be in City Radius" but that's how it works with the Iron Works. It could also be used as an alternative graphic to the Iron Works or many other Industrial Aged Structures.