Free Milling Gold Ores
Free milling ores are defined as those from which Cyanidation can extract approximately 95% of the gold when the ore is ground to size 80% < 75 µm, as commonly applied in industrial practice, without incurring prohibitively high reagent consumptions. Frequently some of the gold is recovered by gravity concentration and amalgamation and gangue minerals composition does not significantly affect the processing requirements. The two main classes of free milling ores are palaeo-placers and quartz vein gold ores. Some epithermal deposits may be free milling but more commonly contain significant concentrations of sulphides and are considered in other classes.
Palaeo-placers are literally fossilized placers, the most famous being the Witwatersrand lake bed reefs in South Africa. Other includes Jacobina (Brazil), Blind River Elliot Lake (Canada) and Tarkwa (Ghana). This kind of deposits consists of lithified (the formation of massive rock from loose sediment) conglomerates which contain small rounded pebbles of quartz in a matrix of pyrite, fine quartz, micaceous material and small quantities of heavy resistant materials such as magnetite, uraninite, platinum group metals, titanium minerals and gold.
From mineral processing point of view palaeo-placers differ from young alluvial placers as the gold is unliberated and the ore is consolidated. Crushing and grinding is required to liberate the gold to an extent which allows efficient gold extraction. Palaeo-placer gold deposits have been mined at depths of up to 3 km and therefore both mining and mineral processing costs are generally more than an order of magnitude greater than those for young placer deposits.
Various non-placer gold ore types can be classified as free milling. These are usually formed as a result of deposition from hydrothermal solutions. Epithermal deposits may fall in this category, but quite often have some refractory components. Quartz-gold veins or lode comprise a variety of deposits which are essentially hydrothermal veins of quartz and gold that either replace wall rock or fill open spaces along fracture ones. Most are pre-Cambrian or tertiary in age and can occur to depth in excess of 1 km. the main categories are as follows:
- · Auriferous vein, lodes, sheeted zones and saddle reefs in faulted or folded sedimentary rocks.
- · Gold/silver veins, lodes or stock works and irregular silicified bodies in fractures, faults, shear, breccia or sheeted zones in volcanic rocks.
- · Gold/ silver occurrence as above, though in a complex geological environment comprising sedimentary, volcanic and various igneous intrusive and granitized rocks.