Traditionally gold has been associated to iron, arsenic and copper sulphide minerals. There appears to be a difference of opinion as to whether the gold is in combination with the sulphur or iron contained in pyrite, or in a state of fine mechanical comminuted condition in the mineral. According to some geologists and metallurgists, on very finely divided auriferous pyrite, by the action of cyanide and microscopic observation, it is generally considered that gold is not totally combined with sulphur, but merely locked up in the crystals in the metallic state. The presence of gold in chalcopyrite varies considerable from few grains to several ounces per ton, a good average copper ore contains 5 g/t of gold and the copper concentrate reports 1-2 ounces per ton of gold.
The invariable presence of gold in pyrite contained in lodes has naturally suggested the idea that the iron sulphide is an essential factor as to the presence of gold in its association with silica, and when it is considered that pyrites have been found to be produced from a solution of iron sulphate by reducing agents, and consequently if during such reduction gold was present in solution, it would doubtless be precipitated with the crystals.
It has been established that gold deposits with high sulphization are produced by the action of magmatically fluids with host rocks. Essentially, a volatile fluid transporting hydrogen sulphide and hydrochloric acid leaves the magmatic source and is depressurized during its migration to epithermal crustal levels, promoting the liberation of volatile compounds and the formation of acid fluids. Gold deposits with high content of sulphides usually develop without repeated activation of dilational structures that are found in low sulphidization deposits take hydrothermal zones from meteoric waters to promote the formation of banded quartz veins. Basically, the volatile fluid is moved quickly than the fluid portion and reacts with the host rocks to produce the typical alteration. High sulphidization fluid flow is influenced by permeability control systems such as structural, lithological and breccia.
Copper and polymetallic sulphides deposits contain gold in several forms, free or as fine particles hosted in iron sulphides. In general, gold associated with a particular base metal is recovered by flotation with the mineral, but sometimes when gold is hosted by pyrite appear some problems to the process. Basically, coarse free gold can be recovered by gravimetric devices ahead of flotation circuit. Fine gold is recovered into the copper concentrate. It is important to indicate that porphyry gold-copper deposits were developed as a result of focusing of the mineralization fluids at depths of 1-13 km in the cooler outgrowths to magmatic sources at greater depths and extended from intrusion host rocks to wall rocks.
Gold prospectors must not deny the importance of copper-gold deposits. Probably, the best examples are OK Tedi and Frieda River in Papua New Guinea. These deposits were identified by follow up of skarn float well downstream and the presence of skarn helped to maintain the interest in the alteration system of the zone.