Quote from Mineral Vein

Veins are vast, rare areas, that are rich on a particular resource. These veins are multiple chunks wide and long, their respective resources don't exist outside them. Veins aren't made completely of given material, it's just very common there, and often multiple veins of different materials overlap. This ore distribution changes mining for resources completely - you have to scout wide areas for a vein, and when you finally find one, it can supply you for quite a long time. In general, the average density of resources/chunk is simillar, but the distribution is changed completely.

Could be added with a Vein resource:

Vein(Block[.BlockData],OreSize,OreFrequency,VeinSize,VeinFrequency,VeinRarity,MinAltitude,MaxAltitude)

Suggested by @Burckhart.