Zinc isn't a fantasy metal, it was just broken in the configs.
Adamantite is a fantasy metal and rare to find (though I lessened the Rarity of fantasy metals in 1.0.10, or maybe it was 1.0.9)
I guess I should format my replies better.
You're aware of the super-unstable nature of the metallurgy configs, right? There are bugs on metal spawning near air-gaps which cause too much around caves, but when turned low an error in the gen makes them almost never appear EXCEPT around caves. We discovered it was quite possible for it to NEVER spawn at much lower than half the default.
Metallurgy's spawn code is just... we're seriously considering doing all spawning with CoFH world very soon. You may want to consider the same.