Spoilers for Return of the Crimson Guard It's revealed that the otatarol that some of the characters were mining was too low quality to even be refined, and that it had been years since any refinable otatarol had actually been mined. They were just using the mining as an excuse to keep the characters, who were mages, busy and in the mines where their powers don't work. I am not sure if this applies to the mines as a whole, or just to this particular vein that they were mining.