Open Tunnel spells... the main explanation for cheap gold and dungeons.
ST 12 dwarf with proper tools opens 5 hexes per 10 hour work shift.
Open Tunnel spells opens 2.5 hexes/day. (25/fatigue a day, all rolls success.)
Open Tunnel is a better option for vertical movement though and Pathfinder your way to the ore. (Or hand the dwarf a tool with that's Bound Spirit a Stone elemental.)