It’s very similar to what SimCity does. If we take its idea to those things (called “data layers”), we could end up with something like this:
(I like the style of it, using different colors for different ores and different shades instead of generic yellow/orange/red. Of course the curves would probably not fit very well into SH’s design, but even with voxelized borders it could look cute)
The concentration of ore is displayed in different shades of… auburn. Each mine is drawing from this layer, within a radius, slowly but steadily, therefore slowly decreasing the concentration.
If we utilised something like these data layers for Stonehearth, we could have many (I suggest two to begin with) layers: The first would be the normal ore, any other would be the level n-th layer, which requires a level-n-deep mine. Every level would have access to more ores, i.e.
ores_at_level(n) = ores_at_level(n-1) + some_amount, where
some_amount is potentially increasing the deeper we go.
Speaking of data maps, I think something like that could be really cool in this game, although it might not fit entirely (Stonehearth is, so far, no game where an omnipotent player exists). You could have the visible maps blank at first and have geologist search for ores by surveying the area, again similar to Settlers 3. The result could be viewed and switched-through as you please.
If we ever settle for such a one-point-of-entry type of mine, we could define multiple mines that have advantages: For example, you could start a mine on a cliff wall (that is sufficiently deep). This would yield you a cave mine, which would be most efficient and very quickly built. On the plains, an elevator mine would be built, with slightly reduced efficiency and longer building time (as we’ve got to get through all that dirt first). Just two examples of how mining could be diversified again, to have a little knack for those who want to go the extra mile.