The minerals present in gold deposits frequently show a basis upon which to judge its origin. Some minerals are distinctively primary, some distinctively secondary, others, and among them are some of the most important ore minerals, are in some instances primary and in others secondary. The presence of a mineral of secondary origin proves the action of surface agencies; a mineral that is known to be sometimes of secondary origin indicates that surface agencies may have enriched the ore under consideration, and casts doubt upon the primary character of the ore containing it. Genetic classification of minerals is useful to study the mineralogy of the deposit.
Distinctively primary minerals are arsenopyrite, bismuthinite, cobaltite, stibnite, pyrrhotite, and tellurides. The gangue minerals are albite, biotite, diopside, fluorite, garnet, graphite, hornblende, ilmenite, muscovite, orthoclase, rhodonite, rutile, scapolite, specularite, spinel, topaz and tourmaline. Minerals of both primary and secondary origin are argentite, bornite (usually secondary), chalcopyrite (usually primary), enargite, galena, gold, pyrargyrite, proustite, pyrite, polybasite, sphalerite, stephanite, tetrahedrite and tennantite. Distinctively secondary minerals are chalcedony, pyrolusite, cuprite chlorides, chalcocite, sulphates of the heavy metals, covellite, carbonates of the heavy metals, kaolin, phosphates of the heavy metals, limonite, silicates of the heavy metals, opal and arsenates of the heavy metals
In primary ores some metals exhibit a tendency to associate themselves with certain minerals. Among the more prominent of these primary associations are gold with quartz, gold with pyrite, gold with chalcopyrite, silver with galena, silver with copper, silver with manganese, copper with pyrite and lead with barium. Of these associations that of gold with chalcopyrite is probably stronger than its association with either quartz or pyrite, and the association of silver with lead is most marked. Tetrahedrite is a guide to high silver and gold values in most deposits in which it occurs. In quartz veins, the presence of finely disseminated galena or chalcopyrite, or the presence of fluorite, are often indicative of high gold or silver values. In quartz veins that carry gold and silver it is frequent that quartz of a certain texture carries high values, while associated quartz of other textures is low grade It is generally supposed that galena having curved crystal faces carries higher silver values than galena of cubical cleavage; it is not unusual that where the crystal faces of galena are curved the associated minerals have a similar structure; the supposed relation, therefore, is not always a reliable guide.
The fact is well known that a fine grained or granular galena is likely to carry more silver than the coarsely or well-crystallized mineral. Well crystallized pyrite is usually quite lean in copper. It seems probable that the presence of silver in galena and of copper in pyrite tend to interrupt crystallization, and it is not unusual that information as to the content of these associated or contained metals may be gained by an inspection of the casts left in the outcrop after the solution and removal of the principal sulphide. The relations between segregations of values and accessory minerals, or varying textures are important in the study of gold deposits, and often form valuable guides in exploration.