The characteristic of some gold deposits is the occurrence of the gold in combination with primary minerals such as tellurium, chiefly as calaverite, but partly also as the more argentiferous sylvanite, and probably to a minor extent as other gold, silver and lead tellurides. Native gold may be absent from the telluride ores, except where set free by oxidation. Pyrite is widely disseminated through the country rock and also occurs in small quantities in the fissures associated with tellurides. Galena and sphalerite are sparingly present in the majority of the veins; tetrahedrite and stibnite are of frequent occurrence; molybdenite in small quantities may be present. The tetrahedrite is usually rich in silver, and also contains gold; possibly, however, the latter is due to admixed calaverite, as the two minerals are often found in intimate intergrowth. The galena and blende rarely contain enough of the precious metals to form ore. Auriferous pyrite is often reported, but in the cases investigated the gold was found to be present as admixed tellurides. The usual minerals of the scanty gangue are quartz, fluorite, and dolomite.
In other deposits, the ore-bodies follow a dike of albite-diorite that carries a net-work of quartz and calcite veinlets. Pyrite occurs both in the veinlets and disseminated through the rock itself. Gold occurs in association with the pyrite and also native, and a large, though variable, proportion of the value of the ore may be recovered by amalgamation or gravimetric devices. Visible specks of the gold may be found. Associated minerals always present are pyrrhotite and magnetite; molybdenite is of common occurrence. Native arsenic, realgar, orpiment and arsenopyrite may be present. Stibnite may occur in small amounts with the quartz. In some areas of California, the primary ore is quartz that carries free gold in both fine and coarse particles, with from 2 to 4% of sulphides which also carry gold. Pyrite is the predominant sulphide; associated with it are galena, blende, chalcopyrite, and arsenopyrite. Subordinate accessory minerals are tetrahedrite and molybdenite. The quartz carries a little calcite. Fluid inclusions are abundant, and in many specimens are distributed in a manner dependent upon the distribution of the sulphides through the quartz.
Typical contact ores carry pyrite, chalcopyrite, bornite, pyrrhotite, specularite, and magnetite, with commonly lesser amounts of galena and blende, associated with garnet, wollastonite, epidote, amphibole, pyroxene, vesuvianite, quartz and calcite. The gold and silver content is usually low. The distinctive association of minerals in contact ores is that of primary oxides with sulphides. The primary minerals of some silver deposits are galena, tetrahedrite, chalcopyrite, sphalerite and pyrite, associated with quartz, rhodonite, rhodochrosite, and barite. The silver is contained in the galena to the extent of 22 to 30 oz/t, and in the tetrahedrite, which is probably related to freibergite, in much larger quantity. In other silver deposits the primary mineralization is composed of pyrite, arsenopyrite, tetrahedrite, and tennantite, with lesser quantities of galena and blende, in a gangue of quartz and rhodochrosite. Sparingly scattered through this ore may be found small specks of pyrargyrite, proustite, and, rarely, realgar and orpiment. This silver ore carries from 20 to 30 oz/t of silver.
Primary mineral deposits formed at the surface by hot waters are rarely of economic importance; their primary condition is commonly obscured by the action of surface agencies. The sinters characteristic of surface-formed deposits are commonly made up of silica, as opal or chalcedony, and earthy carbonates. Calcite, fluorite, celestite, barite, and many other gangue minerals may also develop in crystallized form. Stibnite, pyrite, marcasite and cinnabar are known in crystallized form, and many other sulphides have been detected chemically in such deposits. Surface waters containing atmospheric oxygen are likely to have altered these deposits greatly, and limonite, hydrous oxides, carbonates, and sulphates of the heavy metals predominate among the ore minerals, and kaolin, allophane and chloropal among the gangue minerals. The superficial formation of these deposits is commonly indicated by their structure.