Ore textures that affect gold recovery by heap leaching

Heap leaching is a developing technology for recovering native gold and electrum from low grade deposits and from tailings. Recovery depends on permeability of the rock and ore minerals, size of particles in the heap, and ore textures that influence exposure of gold grains to cyanide solutions. These ore textures include gold-bearing veinlets and microveinlets along fractures and microfractures, gold grains in porous minerals and rocks, and gold grains occurring along grain boundaries of other minerals. On the other hand, small encapsulated gold grains are not likely to be exposed even with fine grinding. Most gold grains that occur along grain boundaries of other minerals can be exposed with a moderate to fine grind, but the resulting material will be too fine-grained to be permeable in a heap. Such finely ground ore, as well as tailings from flotation operations, can however be prepared for heal? leaching by agglomerating the material into porous briquettes. Oxidized and weathered zones above orebodies are frequently amenable to heap leaching because the gold-bearing pyrite and arsenopyrite have become permeable by conversion to porous iron oxides and ferric oxyhydroxides. A review of ore textures in several types of gold deposits suggests that: (1) the gold in vein deposits would generally be amenable to heap leaching of both oxidized ores and fresh non-refractory ores, (2) the gold in Carlin-type deposits and in volcanogenic massive sulfide deposits is not likely to be amenable to heap leaching fresh ore, but oxidized zones above the orebodies are generally amenable to cyanidation, and (3) the gold in porphyry copper-gold deposits and in Witwatersrand-type deposits would likely be amenable to heap leaching of both fresh and oxidized ores but with high cyanide consumption.