There are some gold ores appropriate to heap leaching and other group present some problems. These ores are fractured volcanics and schists; gossans; porous rocks with sulphides; disseminated gold in limestone; and silver ores. Fractures volcanics and schists located near high gold grade veins present coarse gold, which is detected easily. This material needs long leach times in order to get acceptable gold recovery and to develop a mine plan and estimated reserves is not a big deal since old mine operations have exposed low grade gold ores. In the western of United States was common to develop heap leaching operation on fractured material and the best example is Round Mountain in Nevada whose operation was designed to process 7,100 t/d.
Some gold ores are associated with pyrite and arsenopyrite in metamorphic rocks. If the gold ore is not refractory, the material needs to be crushed as fine as 10 mm. In this case is applicable for small deposits with high grade. It’s important to mention that the cost of the crushing circuit is very important for the economy of the project. Gossans used to present gold that has been enriched by leaching activity and they are typical on massive sulphide deposits. Iron oxide minerals are present and they are not a problem unless a high lateritic zone contains gold and iron oxides. The problem is that oxides tend to cover free gold particles or gold bearing minerals.
Other type of gold ore is porous rocks with sulphide and soluble base metals. In this material, gold is deposited on sulphide iron minerals (pyrite, marcasite) located near the oxidation zone. Gold recovery is acceptable and the consumption of cyanide and lime is relatively high. Sulphide veins tend to be oxidized in form of rocks and clayey material. The latter could be a problem due to the percolation rate and permeability of heap is affected. In some deposits are present oxide copper minerals, which increase the cyanide consumption and the operation needs a mineralogical control in order to detect high levels of oxidized copper.
Gold is found disseminated in hydrothermal limestone and the gold dissolution rate is high if the rock is porous and permeable, but when carbon is present, the dissolution of gold is reduced at low levels. The best examples of this material are Carlin, Cortez and Winfall in Nevada. Gold grains are very, very tiny and it is necessary to perform assays to detect gold. This type of gold ore is problematic and the design could include only heap leaching or more than two recovery processes to achieve acceptable gold recovery. It believed that some silver deposits could be treated successfully by heap leaching and the gold recovery must be done by Merrill-Crowe process and not by Activated carbon due to the presence of high levels of silver in the pregnant solution.
Unfractured and metamorphic rocks may contain coarse gold and the rate dissolution of gold is very slow due to the rock is impermeable and the primary rock is difficult to leach well. Probably, weathering is responsible for gold deposition as fine and disseminated particles. The presence of clay is an extra problem and the solution may be not simple. Basically, the production of fine material must be reduced as much as possible. The blasting method is important because fracture the rock and this is the first point of fine particles production.