In my experience masterwork is pretty much then only time I find iron, gold, or fuel on my embarks and I think I normally end up with all three at once. I normally set mineral scarcity to low (which I've checked and it's actually 500) if using standard world generation or 2,000 on advanced so that could be why, but unless in standard df you're setting your mineral scarcity to 50 or something then there shouldn't be much of a decrease. There is something else that might effect it, but I expect Meph has already taken that into account. In the vanilla game the ores of iron are hematite, limonite, and magnetite. In Masterwork though there's only hematite bearing rock. However I'm guessing Meph made it so hematite occurs in the same places the other iron ores occur so it's not rarer.