We actually did this same thing. Adjacent to a gravel pit operation. Now, there were significant numbers of commercial pit operations and USGS soils studies to show that the minerals were the same across the properties. The interesting thing was that the "market value" for land was less than half of what the mining company was offering, and I was given the offers for review. My SOW was what was it worth only as land, not considering the operation. So we had dirt, discounted for the operation. Conversely, if it was sold to the mining operation, big increase in value as the mining operation was beginning to mine out the land it had currently owned.
As to sales, if you can find sales of land adjacent, then great. I would talk to the operators of the mining operations in the area and see if you can pry any info out, maybe enough to put together the necessary data.
USGS, Federal soil bank information, all can give you data as to whether the minerals will extend under the subject site. If they don't, then you're looking at a locational obsolescence with no premium to market to the mining company.
Don't let the gripers bug you. If you don't ask, you'll never learn. You'll always have someone get out the "license" level and the "competency" mantra. That's what you're trying to determine, if you license will allow you to do this, given the complexity of the appraisal. As to competency, you are taking steps to cure the competency issue.
Good luck.