resource chance discovery is (1+earthmana)/50000ish (not exact number, but close enough). This means that getting 1 earth mana doubles your base chances, 2 triples, etc. But it also means that each earth mana makes less and less of a change (subjectively - first one doubles, 2nd increases by 3/2, third by 4/3, fourth by 5/4 etc). Another thing to note is that resource chance discovery is random chance per turn - so every turn a mine will have a 1/50000 chance of discovering a material. Therefore, if you had 50000 mines, you would probably discover a new resource every turn.
-Colin