Classicly

The five men were “salting” the worthless mine; that is, they had loaded the shotgun-shells with fine gold, and were blowing the gold into the breast of the level. When the intended victim came to take his samples of the vein, he would chip off pieces of the doctored rock, and when the rock was assayed, it would show the mine to be a heavy “gold-producer.” On this showing, unless the intended victim was warned, a hundred thousand dollars would change hands, and Captain Lawless, of the Forty Thieves, whoever he was, would be that much richer.