This might not be revolutionary, but I haven't seen anyone else post it here, at least in a long while. So I decided to share my findings with you all, to hear some opinions.
I think you can see what minerals a certain level was made of, by the minerals you find in that level. It seems the quantities you find in a level relate linearly to the tray that the level was created from.
So instead of creating gates all of one/two colors to see what comes out, you could just look at the minerals in any level and determine what relative quantities of minerals caused the level to be created.
Anyone?
I believe this idea has been mentioned and discussed before, however we are still just as unsure about it as any other hypothesis. I have noticed trends in certain minerals appearing in certain types of levels. The obvious example being the abundance of Crimsonite in Battle Arena levels (I think we can all justify this relationship by now). Perhaps this hypothesis is a rough draft of what happens; a more general way of thinking about it.
When you think about the logic behind it, though, why would the Devs do this? Once a mineral gets rolling, it starts popping up everywhere: you create a certain level-type based on a certain mineral and suddenly this mineral is a bit too abundant. This results in an abundance of the same level type (CT anyone?). However, I would not doubt if this is how it's roughly shaped. Perhaps the problem I just suggested is the reason they have put random colour minerals in terminals (and created Treasure Vaults). This is to not ridiculously overpopulate one specific mineral, and in turn, one specific level.
My best guess would be that the majority of a mineral that appears in a certain level is based on the composition of that level (what was used to make it). Although, it seems there are some exceptions... so who knows. As it has been brought up before, I believe we need some more official information from the Devs. (Not to mention those still-missing Materials...)