An important class of gold deposits is that in which the gold bearing minerals are disseminated as minute particles, or narrow seamlets, throughout a large mass of enclosing country rock. The number of such mineralization whose primary ore is high gold grade is probably small, but these deposits, especially those that contain copper, are of the greatest importance where enriched by secondary processes. Disseminated mineralization are probably due in great part to metasomatic replacement, but the occurrence of their minerals as sparse disseminations, or impregnations, which are structural terms, is sufficient to warrant their description as a separate type.
It is important to mention that disseminated mineralization are most frequent in schists and in intrusives; these rocks appear to favor disseminated mineralization in much the same way that limestone appears to induce segregation and localization of introduced minerals. A characteristic feature of disseminated gold deposits is the occurrence of the most thorough mineralization in the areas most fissured or shattered. In many cases the mineralization accompanies reticulated quartz veinlets through the shattered rock, and often, especially where the dissemination occurs in the mineralizing intrusive, the introduced minerals are abundant along joint planes as well as fracture planes, and occur in much less quantity in the interior of the masses bounded by such surfaces. Disseminated mineralization is commonly closely associated with intrusives, and where important, extends through very large rock masses.
For instance, a great mass of intrusive monzonite carries throughout an irregular but persistent mineralization of finely disseminated pyrite and chalcopyrite that carry high values in gold. In the fresh monzonite these minerals may occur as minute grains scattered through the rock, and along joint planes, and also embedded in irregular quartz veinlets. A correlation of gold assays with structure indicates that the values are highest where the fissuring and veining are most pronounced. Through secondary enrichment this deposit has yielded copper-deposits. Intrusive monzonite-porphyry may have a disseminated mineralization of pyrite and chalcopyrite, with subordinate blende, associated with quartz and a sericitization of the containing rock. This mineralization may be intense in and along certain veins through the monzonite-porphyry, but extends as impregnations through the rock, along joint planes, and associated with quartz veinlets, for long distances either side of the mineralizing veins. The containing rock may be the mineralizing intrusive. A large area of the intrusive carry sulphides, but the mineralization is apparently most intense in the vicinity of the centers of intrusion.