SuperSoldierRCP,
My understanding is that each resource represents a different 'deposit'. Each mine is set up for a specific resource deposit. A Corp can only take from the deposit it's built for, in the same way that an Oil well in Texas, can't extract from Oil deposits in California.
The deposit is selected when a new Corporation is built, from the top (or bottom, can't remember which) of the country's resources list. There is no feature to specifically select which deposit you want to extract. You simply have to use each one in the order it was spawned into the country by the GMs.
Signed President of DanNation on LU