So it's 2036 and Mars has about 15mil population, so I decide to drop 10 factories on Mars and slowly build a commercial spaceport. Since Mars does not produce any of the needed minerals in any quantity, I import them from Earth. As soon as they hit the ground, the mass driver fires part of them back to Earth.
So I turn off the mass driver, import more minerals, and start the spaceport construction, hoping once it starts, the minerals will be deleted from the stockpile. Not the case, apparently. The minerals seem to be used up gradually throughout the course of the construction.
So, do I just have to leave the mass driver off for the seven or so years it takes to finish construction? Not a big deal in this particular case, but I could see where this would be a problem building up facilities on a planet that is the prime source of a critical element like duranium or sorium.