Granitoids in Mongolia occupy about 30 percent of all territory and are forming elongated belts, comprised mainly by calc-alkaline granitoid series. Porphyry systems with Cu-Mo, Cu-Au, Mo-W and W-Sn greisen and vein mineralization are associated with calc-alkaline granitoids.
Porphyry systems in Mongolia are the main type of mineralization associated with granitoids occurred in island arc and continental arc environment. Porphyry mineralization is located mainly in porphyritic intrusions, dikes, or small bodies postdate the large intrusions of calc-alkaline series. Mineralized intrusion age is very close to age of intrusive series.
The major type of granitoids, which contains Cu-Mo mineralization, is represented by calc-alkaline series high or medium K, I type, oxidized, represented by gabbrodiorite, diorite, granodiorite and granite. Orebodies are associated with pervasive alteration: quartz-sericite and silica in the granitoid host rocks. Mineralized granodiorite porphyry is depleted in Nb, Sr, Ti and HREE, and shows signature of continental arc environment. The quartz-sericite alteration grades outwards into intermediate argillic and to propylitic on the peripheries. Altered granites are depleted in LREE and enriched in Al and Mo. Fluid inclusion data indicate the involvement of an aqueous, moderate to high salinity fluids during ore deposition.
The Cu-Au porphyry systems are associated with monzonitic granitoids: monzodiorite to quartz syenite. Quartz mozodiorites are enriched in K, Rb, depleted in Nb, Sr, Ti and HREE, and show adakite-like characteristics. Mineralization and alteration is characteristic with high sulfidation systems which occur above and partially telescoped onto underlying copper-gld porphyry systems.
Mo porphyry and Mo-W systems are associated with leucocratic biotite granites, which occur more inland toward the subduction zone. Ore bearing intrusions are late phases of composite bodies or small plutons, represented by biotite leucocratic granites. Wein type Mo-W, W and W-Sn mineralization is associated with leucocratic intrusions and occurs in intrusion or host rocks. Granites are enriched in K, Al, Rb, and show post-orogenic signatures, close to A2 type. Special type is highly evolved granites enriched in Li, F, Rb and known as Li-F granites after Kovalenko et al (1977) with Sn and Ta-Nb mineralization. These granites show within-plate (post-orogenic) signatures.
Examples of largest mineral deposits related to porphyry Cu-Mo, Cu-Au, and vein Mo-W, W-Sn will be discussed in detail.