MTGNexus

The mechanic is "Spend only mana produced by basic lands to cast this spell." (yeah it's Imperiosaur)
What does sound cooler to an english speaker? Untainted or Uncontaminated?

Those are both valid words. Untainted gets a bonus as a mechanic name because it's three syllables versus six. For keywords, you should have shorter and pithier names if you can.

Neither; nonbasic lands aren't contaminated. Adamant has already been taken, but something closer to that, I suppose.

Of the two options, I would go with 'untainted' due to it being fewer syllables. That said, I will agree that it feels weird for nonbasic lands to be tainted / contaminated. I'd probably go with something more positive instead - maybe 'primal'? I could see 'pristine'. 'pure', or 'simple' working instead, depending on the flavor.

I second "primal", but I'd also like to suggest "basal", to tie it in with "basic".

Basal would be neat.

From the already proposed words I like "pure" and "primal". Let me throw in "radical" and "native". Everything depends on the flavor.