 The surface outcrop of a gold-bearing vein may be oxidized, so that the gold occurs as native gold, and the ore needs only to be crushed and washed (free milling ore).The first miners may at first build a simple arrastra to crush their ore; later, they may build stamp mills to crush ore at greater speed.As the miners venture downwards, they may find that the deeper part of vein contains gold locked in sulfide or telluride minerals, which will require smelting.