For further tests, if what was said about ore spawning is true, one should test with mystcraft world, flat.
Flat worlds are very high ones too, so it maximizes the ore spawning, and if no caves are chosen as symbols, it should definitely be accurate.
But yeah, it is definitely a low chance to stumble upon one of such ores, and the only "reliable" way to produce iridium is through mass/matter fabricator, depending on the pack. UU matter has a secret recipe for iridium, so go over the IC2 wiki to learn more about that, maybe?