I can tell you from my experience building my Blacksmith and Ores mod here that the mechanic currently in the game is the “random chance based on the material type” sort of method. Meaning, that for a given ground material that is being mined (grass, dirt, rock, etc.) there are a set of weighted values that determines the random chance of a given result. Whether this is temporary or not I’m not sure but I’d wager that if ore-like ground types are added to the world generation then the random chances are still there but just much higher for the particular material that is the primary consistency.
I personally like this method over the other ways as this method is much more suitable to both the gameplay and visual style of Stonehearth. Plus it’s super easy to change the materials that the world produces simply by altering the mining loot table instead of having to regenerate all of the world terrain. Therefore the special rock that is an “ore vein” is simply called “Ore Rich Rock” (or something similar) which would yield higher percentages of metal ores and lower percent of plain stone when compared to normal rock.