The mining skill affects the time between hits you can use your pickaxe. For every level, you need 0.1 seconds less time to wait between hits. This does not affect your luck of getting Rare ores. The luck skill (not in the game yet) will change that later.
The number of XP you get:
--> Get a Native Ore --> 1 XP
--> Get a Special Ore --> 2 XP
--> Get a Rare Ore --> 3 XP
--> Get a Super Rare Ore --> 4 XP