The mineralization of conglomerated beds is characterized by the presence of groups of minerals, and is part of the most important gold mining districts in the world. The origin of these deposits is not clear, and while to a certain extent similar, they posses features that render difficult their classification with the more common and better understood gold deposits. For example, in the Michigan copper deposits and the gold deposits of the Witwatersrand the ore consists of native metals in the cementing material of conglomerate beds, and in both districts the mineralization is remarkably persistent over great areas, and to great depths.
In the Witwatersrand, South Africa, beds or reefs of conglomerate are intercalated with quartzitic sandstones and, more rarely, slates; there are eight groups of these reefs, certain of which have been exploited over a length of 75 meters. In thickness the reefs vary between a maximum of several meters to a complete wedging out. The thinner reefs are commonly the richer. The intervening strata are practically barren, although exceptions to this rule are known. The reefs are composed of pebbles, commonly ranging from a hazel-nut to a hen's egg in size, of quartz and quartzite, more rarely of siliceous schist, and occasional rounded pyrite granules; the pebbles are often deformed, being flattened or splintered, and they rarely carry any mineralization. The cementing material is composed of small quartz granules and pyrite with associated particles of gold. The pyrite occurs in rather irregular distribution, and frequently forms crusts around the pebbles of the conglomerate, and occasionally is concentrated in delicate films parallel to the stratification. The mineralization, while exhibiting local irregularity, is relatively uniform and persistent over long distances both in strike and in depth.
Where mineralization has proceeded contemporaneously with the deposition of the enclosing bed the resulting deposit is known as a seam, or bedded deposit; in this class are also included those replacements of similar occurrence the origin of whose mineralization is not apparent. The criteria for distinguishing between bedded gold deposits and intercalated gold veins is not difficult. Bedded deposits are occasionally recognizable by contained fossils, which may have become mineralized. These deposits, of which a majority contain iron or manganese, are commonly of large horizontal dimensions as compared with thickness, and where the strata are folded, they follow all the sinuosity of the containing bed; in occurrence they are comparable with coal seams. A study of the stratigraphy of the bed containing the gold ore and of the associated strata is imperative in the investigation of such gold deposits.
Certain types of bedded deposits terminate through wedging out around their peripheries, others toward their edges become gradually poorer through the occurrence of barren partings, which increase in proportion to the gold ore until the mass has no an important value. Bedded gold deposits whose mineralization was contemporaneous with the deposition of the containing stratum are commonly persistent over large areas. Bedded gold deposits whose mineralization is later than the deposition of the containing bed are less likely to be persistent over large areas, and their contained mineralization is likely to have been controlled by some constituent of the bed, such as carbonaceous material, and to fail over such parts of the bed as did not contain such precipitants. Where two or more bedded deposits occur in the same series they are persistently parallel through all the sinuosity of the associated strata.