Sometimes gold deposits present base metal sulphide minerals, which carry gold and silver. Probably, the most typical sulphide is pyrite, which is common in most quartz veins in granite and in eruptive rocks may yield both gold and silver, but usually the former. There are some deposits more characterized by pyrite than others and these are generally gold producing districts. Some mines have strong pyritiferous veins in eruptive dykes. Also, it is possible to find large argentiferous veins with pyrite. In limestone, pyrite is rare and its place is filled by some iron oxide. In the deeper deposits, this iron oxide is the beginning to pass down into the iron sulphide or pyrite from which it was derived. Iron pyrites may be distinguished from chalcopyrite by its paler, more brassy color, by its superior hardness and by it crystallizing in cubes. A vein may glitter with showy pyrites and yet be quite valueless. It is usually yields more gold in its decomposed, oxidized condition than in its non-altered state.
A typical silver mineral is tetrahedrite. Besides the ordinary pyrite common in some deposits, this is generally a rich silver bearing ore running from 50 ounces to some thousands per ton. It usually occurs massive, rarely showing its pyramidal crystals. In appearance it is not unlike a freshly broken piece of bronze. It is more common in fissure veins and eruptive rocks. Argentite or silver sulphide is a blackish, lead gray color. Other silver minerals are stephanite, which is closely allied to argentite; polybasite, which is flaky, scaly and graphitic appearance. These silver minerals sometimes line little cavities in limestone with a dark sooty substance, which under the microscope proves to be crystals of one of the silver sulphosalts. Sometimes, a rock is stained all through a blackish gray by these silver minerals. Iron and manganese may produce the same effect, but an assay will reveal the difference. It has been noted that associated with such a rocks, flakes or wires of native silver emerge from the sulphide state.
Pyrargyrite and proustite (ruby silver) are seen in spots or crystals on a mass of ore of a deep red or blackish tint. When scratched with a knife it shows a bright or deep red color. In some mines this very rich ore occur as specimen, but in others it is present in sufficient quantity to largely influence the value of the ore. The main disadvantage of these minerals is the presence of antimony and arsenic, which may yield some penalties to the concentrate or make complex the treatment process.
Galena, Sphalerite, Chrysocolla and Quartz. 0.7 g/tAu, 53 g/tAg 1.1%Pb, 5.2%Zn, 0.43%Cu