Abstract

The domestic gold mining industry has increasingly relied upon low-cost heap leach technology in the development of low-grade, bulk-minable gold deposits. Geologic and hydrologic considerations are very important in siting heap leach facilities because leach pads and solution ponds cover large areas, are constructed in total or in part of locally-derived materials, and are subject to leakage if site considerations are not taken into account during design. Site geologic and hydrologic characteristics should also be considered during designs of leach and pond liners, pregnant solution collection systems, and leak detection systems. Primary liners are usually made of a plastic; secondary liners are usually constructed with clay soils that are borrowed as close to the site as possible. A solution collection system consisting of gravel or fine crushed ore is placed on top of the synthetic liner. This layer not only protects the primary liner from damage during ore loading, but is also designed to rapidly transmit leach solution to provide for optimal solution recovery and to avoid the build up of excess hydraulic head on the liner. Leak detection systems for solution ponds are virtually universal; however, depending on the State regulatory authority and site-specific conditions, leak detection systems may be required for leach pads. Leak detection systems for solution ponds are commonly blanket type systems, consisting of geonet or permeable sand or gravel. For leach pads, leak detection systems may be blanket-type, although other types using french drains, lysimeters, or electrical resistivity grids are also used. The sand or gravel for blanket-type systems is best obtained on site, if possible.