PGE-Cu-Ni Mineralization of Mafic-Ultramafic Massifs of the Khangai Upland, Western Mongolia

Abstract

The mafic-ultramafic massifs with the PGE-Cu-Ni mineralization located in North-Central Mongolia: Oortsog, Dulaan, Mankhan, Yamat, and Nomgon were investigated. For the first time we consider these massifs as a single magmatic association and as fragments of Khangai batholith caused by the action of the plume responsible for the formation Permian Khangai LIP. The massifs fractionated from peridotite to gabbro have a similar typomorphic ore mineralogical and geochemical features, which change depending on the degrees of fractionation of magma and evolution of the sulfide melt. The least fractionated Oortsog massif originated from Ni-rich high-Mg basaltic magma. It is characterized by predominance of pyrrhotite mineralization due to exsolution of monosulfide solid solution (MSS). The most fractionated is the Nomgon massif originated from Cu-rich basaltic magma with bornite-chalcopyrite mineralization, formed as an exsolution of intermediate solid solution (ISS). The rest of the massifs have a medium characteristics between these two. The compositions of sulfides in the studied massifs change in accordance with the increase in sulfur fugacity from peridotite to gabbro: enrichment of pentlandite in Ni and pyrrhotite in S. The composition of PGM changes from Pt minerals in Oortsog massif to Pd minerals in Nomgon massif in the same direction. These massifs can be considered as potential for the PGE.

Petrology and Age of the Yamaat Uul Mafic Complex, Khangai Mountains, Western Mongolia

“…Mantle heterogeneity in the Permian may have influenced the development of metallogeny in the Khangai region. There are mafic-ultramafic intrusions with PGE-Cu-Ni mineralization in the Khangai Mountains, Oortsog Uul and Nomgon [32,63], which, together with the studied intrusion, belong to a single metallogenic province formed above the Khangai plume [24,32].…”

“…The Central part of the complex is characterized by sulphide disseminations only, while in other parts, both sulphide dissemination and sulphide schlieren are found. The sulphide mineralization represented by the chalcopyrite-pentlandite-pyrrhotite composition has been previously described [32].…”

“…Thus, the data obtained allow us to date the formation of the Yamaat Uul complex to the Permian (255-262 Ma) rather than to the Early Palaeozoic as previously thought [62,63]. The following mafic-ultramafic intrusions of the Khangai Mountains formed during the same time interval: the Nomgon troctolite-anorthosite-gabbro intrusion (256 ± 2.1 Ma) [30], the Oortsog Uul peridotite-troctolite-gabbro intrusion (257-278 Ma) [32], the Dzaraa Uul monzogabbro intrusion (269 Ma) [15] and the Nariintolburingol intrusion (249-252 Ma) [63]. The rocks of some of them are enriched in PGE [16,32,64].…”

“…The following mafic-ultramafic intrusions of the Khangai Mountains formed during the same time interval: the Nomgon troctolite-anorthosite-gabbro intrusion (256 ± 2.1 Ma) [30], the Oortsog Uul peridotite-troctolite-gabbro intrusion (257-278 Ma) [32], the Dzaraa Uul monzogabbro intrusion (269 Ma) [15] and the Nariintolburingol intrusion (249-252 Ma) [63]. The rocks of some of them are enriched in PGE [16,32,64]. There is every reason to suggest that the Yamaat Uul complex together with the forenamed intrusions of Mongolia, localized in the single CAOB structure, are the products of Permian magmatism belonging to a single igneous province.…”

“…In addition, we will consider the sulphide Figure 1. A geologic map of the mafic-ultramafic intrusions location within Khangai batholith (modified after [31,32]). 1-Central Asian orogenic belt (CAOB); 2-volcano-plutonic belt (SVVPB-Selenga-Vitim volcano-plutonic belt, GAVPB-Gobi-Altay volcano-plutonic belt); 3-Mesozoic-Cenozoic troughs; 4-Late Palaeozoic granitoids of the Khangai batholith; 5-Permian gabbros of the Khangai Mountains; 6-9-orogens: 6-Middle-Late Palaeozoic (Hercynides), 7-Early-Middle Palaeozoic (late Caledonides), 8-Vend-Early Palaeozoic (early Caledonides), 9-Neoproterozoic; 10-tectonic blocks with the Early Precambrian basement; 11-tectonic blocks with the Pre-Vend orogenic basement; 12-main tectonic boundaries.…”