On a more limited scale, in-place leaching has been applied to fill in old mine stopes, caved areas over block caving operations underground, and in peripheral portions of conventional open pits where the grade is too low to permit mining the material. This method is not well enough understood, nor has enough experience been gained to apply it to a virgin, high-grade ore deposit with assurance of control and predictable recovery of values being leached. The method holds great promise, because capital costs are low and there are fewer environmental problems compared to the movement of vast tonnages of rock in conventional mining. Open pit gold mine operators have begun to follow this practice, particularly where the pregnant liquor can be pumped to the precipitation section of an existing metallurgical plant. Usually no special consideration is given to the preparation of the mine dump for leaching, and in fact the decision to leach often comes after the dump was laid down.
All vegetation is removed over the dump area.
The surface of the dump area is compacted and overlain by impervious material such as clay. Fine material should be separated. A long, narrow dump may be desired to promote natural aeration. The surface of the dump is ripped, or otherwise uncompacted. The dump material may be moistened as it is laid down, inducing oxidation while the material is still in direct contact with atmospheric air.