This ore group is also called free-milling ores and is formed by deposits with gold contents higher than 5 g/t and the process extraction recovery is as high as 90%. Normally, the ore treatment process follows crushing, comminution, gravimetry and cyanidation. This kind of ores are found in placers, quartz vein gold ores, oxidized ores, silver-rich ores, copper sulphide ores, and some iron sulphide and arsenic sulphide ores.
An aspect very important is to perform mineralogical studies of gold behavior in various ores and metallurgical products in order to classify the ore and address issues and problems related to gold ore processing. It is widely used as a predictive and trouble-shooting tool in gold ore processing, and provides useful information on process selection, flowsheet development, recovery improvement, and reagent consumption optimization.
The gold behavior of the several ore types can be summarized as follows: gold in placers, quartz vein gold ores, and oxidized ores are easily liberated, and can be recovered by gravity, flotation, and cyanidation; gold in copper sulphide ores is often coarse-grained and associated with copper minerals, and can be recovered into a copper concentrate by flotation; gold in silver-rich ores is often present as electrum or associated with silver minerals, which can be recovered by gravity, flotation, and/or direct cyanide leaching. The problem in recovering gold from silver-rich ores is that the greater reactivity of silver can influence the metallurgical performance of gold in flotation, leaching, and recovery processes by the formation of silver sulphide or silver sulphate coatings.
One good example of leachable ore is the Witwatersrand deposit in South Africa. This deposit is the bigger auriferous concentration in the world. Since was discovered in 1886, the estimated gold production is near of 45,000 tonnes, which represents approximately 40% of total production of gold in the world. In 1983 some mined zones had a gold content of 10 g/t. Nowadays the gold content is lower. This type of deposits consists of lithified conglomerates which contain rounded pebbles of quartz in a matrix of pyrite, fine quartz, uraninite, and gold. There are have been some hypothesis that tried to establish an association between gold and uranium oriented to determine if uraninite is a chemical precipitated or not.
Figure shows a Witwatersrand fan formation. Gold has a higher concentration in the middle zone.