Board Thread:General Mod Discussion/@comment-27752412-20160221173323/@comment-26236093-20160221174958

"Mithril ore can only be found in the Meneltarma biome or in the Misty Mountains (Yes, it is also in the Misty Mountain Foothills) at layer 16 or below. Every chunk in the Misty Mountains has a 1/4 chance of spawning a mithril vein. Each vein can be up to six blocks in size, which leads to an average of 2.2 mithril ore per 10,000 blocks.

In Meneltarma, every chunk will spawn a mithril vein (barring other factors such as caves), but the max size is only four, which leads to an average of 6.1 mithril ore per 10,000 blocks. For comparison: In the vanilla world it's about 10 diamond ore in 10,000 blocks..."

- Taken from the Mithril Ore page