Clemens obtained deeds and gold from many of the miners. He kept the gold locked up in the assay office. When the miners became possessed by an alien after drinking watered-down whiskey, Clemens was unaffected because he didn't drink.
When the alien gold used the stream to approach the town, searching for the rest of the gold, Clemens barricaded himself in the assay office. He met the Sixth Doctor's advice with gunfire. The stream broke apart the assay office, killing Clemens. (PROSE: Whiskey and Water)