Mining gains work two ways, unlike most skills.
1. You can gain from mining any metal, up to 100. It is difficulty-based in relation to the chance to successfully mine, which becomes 100 at 100.0 skill.
2. You can gain from smelting. This is based on the difficulty to smelt the metal, which is as follows -
a. Iron can be mined at 0 skill, but cannot be smelted successfully until 25 skill (0.2% chance at 25.1), and stops giving gains at 75.0
b. Dull Copper starts being available at 65 skill, and each additional metal up through verite in 5 point increments (with Valorite at 99 skill). Each colored metal starts at 50% chance to smelt, and becomes 100% chance to smelt 25 skill points higher.
c. As long as you are not 100% to smelt a specific ore, you can get a gain from smelting that ore.