Then why haven't they made a single drill hole to attempt to fill the gap?

Drilling is very expensive and the number of holes required to fill the gap would be astronomical. Why not extend three drifts out into the gap that can be used to haul ore back to the A shaft. Once you are out there, exploration drifts can be built off of the main drifts and shorten the length of the drill cores by 100s of meters ($250.00 per meter). Sounds like a plan to me.