That's the kind of scientific reasoning (radioactivity causes mutations, uranium is radioactive, pitchblende contains uranium: therefore animals near pitchblende get macromutations) that I think we'd better take care to avoid. It's a valid way to structure a universe, and I'd love to see a game that consistently applies that theme. But this is mythical fantasy, not Victorian or fifties horror SF.
A fitting way to get that kind of effect in the game would be to form an association between a god of growth and pitchblende: things around pitchblende could then grow (slighty) faster, and (slightly) larger than their ordinary counterparts (much like Pippin and Merry ended up larger than the other halflings, because of their stay with Treebeard and his Ent-style drinks. If anything is associated with growing large and sturdy, it's trees.) Mind you, that probably should be randomized from game to game, and it might as well be orthoclase, stibnite, the colour yellow or dancing in the moonlight that makes creatures grow. At all times, it should be congruent with the symbolism that is used in that world (but not necessarily our world). To balance stuff,
AFA gigantic turtles etc. are concerned, they are mobile in principle, so they're not really limited to exist near a certain mineral. We could assume that they sleep for very long times, so they don't need to sleep nearby where a food source is right now - they might have been sleeping for a few thousand years.. That way, the only limit on their occurrence might be game balance, or the player's sanity [PLACEGIGTURTLES=6513].