Mines are hit with decay at a rate of, say, palisades. Require block of wood to be repaired.
On the final stages, you cannot enter the mine unless it is repaired.
When a final decay hits, mine collapses and is available for prospecting again.
I am seeing a number of built and never used mines, especially in new grids. Also some of unused mines are near dead villages. When so many people are trying to find a mine for themselves and fail because the mines were discovered and built by older players, it seems a good way to allow newer players to prospect these mines.