Before I suggest changing it, is there a reason why civilian mining colonies only spawn on bodies with Duranium or Sorium? Why not let them spawn on any planet with a collective mineral wealth of 15,000 tons? I don't see why civilians would only be interested in deposits with some of the most common substances in the galaxy.
It's actually duranium and gallicite, originally it was duranium only as it is the most-used mineral in the game thus considered by Steve to be the highest priority to mine. One big benefit of CMCs is that they give you mines that you don't have to build, so it's valuable to have those "free" mines (which you actually do pay for but only with wealth, not BPs) on the most-used resource in the game to help the player generate a good inflow early on while we are still building up our planetside industry especially because moving mines off of Earth isn't economically viable in most games until some time has passed just because Earth's minerals are usually so much better than anything except 1-2 comets or asteroids usually.
Gallicite was added as an optional requirement instead of duranium based on playtesting during Steve's Crusade campaign as the need to produce MSPs with gallicite makes that mineral even more critical than it was in VB6 when engines already demanded a lot of the stuff.
I think this way works well for being a very simple implementation, as duranium and gallicite are nearly always critical resources so this really helps build a strong economy early in the game or in new major systems later in the game. The only others I would even think of as being close to the same broad importance are corundium and neutronium, both of which are not as heavily used but a crunch of those minerals can be very crippling.
