i've been branch mining at y-8, y-12 and y-18 (as far as 200-300m in any direction) for quite some time and found quite a bit of zinc, usually in groups of 2-4 blocks. right now im looking at 7 stacks of zinc dust (run through a pulverizer for the x2 dusts). im right underneath a swamp biome, next to an ocean and an extreme hills biome if that helps any. as for adamantine...ya thats gonna be a rare find. definitely spawns at higher y levels tho. you need to branch mine a lot to find some of this stuff, or start cave diving and look at the walls, thats where ive found most of the adamantine so far. atlarus too..found enough to make me an uber tico hammer and pickaxe with all the bells n whistles.