The difference is not just profit margins, but very different capex and opex. Capex on oil rigs is large (especially offshore), but opex is pretty reasonable.

The difference is not just profit margins, but very different capex and opex. Capex on oil rigs is large (especially offshore), but opex is pretty reasonable. I would imagine iron mining has much higher opex. A better approach it may be to loot at the graphs of average fixed/variable/total cost, which are very different.