This is a complex computation to come up with. Ideally we could set up a mining operation that didn't require additional fuel from earth. That would be costly

This is a complex computation to come up with. Ideally we could set up a mining operation that didn't require additional fuel from earth. That would be costly in CO2 if we used that as a measure. However, after being set up all that would be required for getting gold to earth from space would be de-orbiting the gold. I'm assuming that recovering a de-orbited chunk of gold would probably be cheaper than mining and transporting that gold on the surface of earth. While you are spot on to question the air pollution to get set up mining gold in space, it would require a really detailed analysis to figure out if it would be a net positive or negative for CO2 emissions. It would also need to be considered what happens when humans can start re-fueling spacecraft in space. What would that do to the shipping industry where materials are prevalent in space and can be moved to earth with minimal CO2 from earth.