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The Late Devonian Tsagaan Suvarga deposit (255 Mt at 0.55% Cu, 0.02% Mo) is located on the NW margin of the Tsagaan Suvarga Complex (TSC), which extends ENE over 15 × 10 km and comprises mainly medium-grained equigranular hornblende-biotite quartz monzonite and monzodiorite. Distinct mineralized intrusions are inferred from distribution of Cu–Mo mineralization but are not clearly discernible. The Tsagaan Suvarga Complex is a window within Carboniferous volcanic and sedimentary rocks, and wall rocks to the TSC are not known or exposed in the nearby district. Whole-rock analyses and Sr–Nd isotopes, 87Sr/86Sr0 = 0.7027 to 0.7038 (n = 12) and eNd0 = + 4.26 to + 2.77 (n = 12), show that the granitoids are subduction-related I-type, high K-calc-alkaline to shoshonitic series and derived from a mantle source. They exhibit fractionated light rare earth elements, without depleted Eu and depleted middle heavy rare earth elements and Y, typical of oxidized, fertile porphyry magmatic suites. Early porphyry-style quartz veins include A- and B-type. Molybdenite occurs in monomineralic veins (1–5 mm) or A veins. Copper mineralization occurs mainly as chalcopyrite and subordinate bornite, disseminated and associated with quartz–muscovite veins. Pyrite (vol%) content is less than chalcopyrite and bornite combined. Deep oxidation to about 50 m depth has formed zones of malachite and covellite in late fractures. The most important alteration is actinolite–biotite–chlorite–magnetite replacing hornblende and primary biotite. Quartz–K-feldspar alteration is minor. Late albite replaces primary K-feldspar and enhances sodic rims on plagioclase crystals. Quartz–muscovite (or sericitic alteration) overprints actinolite–biotite and porphyry-type quartz veins. Field observations and petrographic studies suggest that the bulk of the chalcopyrite–bornite mineralization at the Tsagaan Suvarga formed together with coarse muscovite alteration.

To further constrain the tectonic evolution of the convergent margin of the West Mongolian segment in the Paleo–Asian Ocean, we have investigated coarse-grained kyanite metapelites associated with quartz veins in the Gobi Altai Mountains, Mongolia, where the metamorphic belt is in a thrust contact with a Paleozoic accretionary complex. The metapelites contain coarse-grained granoblastic kyanite grains that are surrounded by sillimanite (Sil-II) with moat-like andalusite (And-II). The garnet grains in the garnet-bearing kyanite metapelites contain inclusions of andalusite (And-I) in their cores and sillimanite (Sil-I) in their mantles. Detailed textural characterization of the aluminosilicate phases indicates five distinct generations in the sequence andalusite (And-I), sillimanite (Sil-I), kyanite (Ky), sillimanite (Sil-II), and andalusite (And-II). Thermobarometric calculations and thermodynamic modeling suggest a hairpin-shaped P–T evolution with prograde conditions starting from ~530 °C at 3.5 kbar, reaching a peak of 600 °C at 6 kbar, and then a retrogression to ~550 °C at 4 kbar. U-Pb zircon and U–Th–Pb monazite ages indicate that the multistage aluminosilicate formation has occurred during c. 260–245 Ma. The detrital zircon age distribution is essentially similar to those in the accretionary sedimentary rocks from the entire Altai Range (a sharp peak at 550–490 Ma and a broad peak at 900–700 Ma), but is characterized by more abundant Proterozoic zircon grains and an older maximum deposition age (Early Ordovician) compared with Devonian pelitic gneisses in the Mongolian Altai. These petrological and geochronological results indicate the following development of the orogen: (1) Subduction of an oceanic plate beneath a microcontinental block as early as 550 Ma, evidenced by the youngest peak of detrital zircon ages. (2) Development of the accretionary wedge and volcanic front due to maturation of the arc until c. 360 Ma. The development of the volcanic front due to the magmatism has provided abundant Paleozoic detritus to the accretionary wedge and has stemmed supply of Proterozoic detritus from the inland at the same time. (3) Burial of the youngest accretionary wedge at c. 360–350 Ma, which is not observed in the current samples. Ridge subduction has also been suggested to have occurred in this period. (4) Arc magmatism in the upper (older) part of the accretionary wedge during the late Permian that resulted in prograde high-T and low-P metamorphism (formation of And-I). (5) Burial of the heated accretionary wedge to a depth of ~20 km due to subsequent collision (formation of Sil-I and Ky) at c. 260 Ma. (6) Rapid exhumation of the buried accretionary wedge to a shallow depth by thrusting (formation of Sil-II and And-II) at c. 245 Ma. In summary, the metapelites in the Altai Range clearly preserve the evolution of both the accretion and collision events as well as their transformation processes, which are likely to be a good natural example of the Wilson cycle for a period of ~300 Myr.

Four amphibolite facies pelitic gneisses from the western Mongolian Altai Range exhibit multistage aluminosilicate formation and various chemical-zoning patterns in garnet. Two of them contain kyanite in the matrix and sillimanite inclusions in garnet, and the others have kyanite inclusions in garnet with sillimanite or kyanite in the matrix. The Ca-zoning patterns of the garnet are different in each rock type. U–Th–Pb monazite geochronology revealed that all rock units experienced a c. 360 Ma event, and three of them were also affected by a c. 260 Ma event. The variations in the microstructures and garnet-zoning profiles are caused by the differences in the (i) whole-rock chemistry, (ii) pressure conditions during garnet growth at c. 360 Ma and (iii) equilibrium temperatures at c. 260 Ma. The garnet with sillimanite inclusions records an increase in pressure at low-P (~5.2–7.2 kbar) and moderate temperature conditions (~620–660 °C) at c. 360 Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (~7.0–8.9 kbar at ~600–640 °C). The detrital zircon provenance of all the rock types is similar and is consistent with that from the sedimentary rocks in the Altai Range, suggesting that the provenance of all the rock types was a surrounding accretionary wedge. One possible scenario for the different thermal gradient is Devonian ridge subduction beneath the Altai Range, as proposed by several researchers. The subducting ridge could have supplied heat to the accretionary wedge and elevated the geotherm at c. 360 Ma. The differences in the thermal gradients that resulted in varying prograde P–T paths might be due to variations in the thermal regimes in the upper plate that were generated by the subducting ridge. The c. 260 Ma event is characterized by a relatively high-T/P gradient (~25 °C km−1) and may be due to collision-related granitic activity and re-equilibrium at middle crustal depths, which caused the variations in the aluminosilicates in the matrix between the rock units.

High-Mg, high-Al metasedimentary gneisses from the Altai Mountains, Mongolia, belonging to a subduction-accretion complex within the Central Asian Orogenic Belt can be divided into five rock types on the basis of mineral assemblages. Most rock types have high MgO and Al2O3 content and low CaO, Na2O, Rb, and Sr content. All rock types experienced a similar medium-pressure metamorphism characterized by a “hairpin”-shaped counterclockwise pressure-temperature path. U-Pb zircon and U-Th-Pb monazite ages indicated metamorphism at ca. 356 Ma and 277 Ma and inherited ages of 510–379 Ma, suggesting possible provenance to granitoids comparable to those in the Altai Mountains, China. The zircons that newly nucleated at ca. 356 Ma are characterized by high concentrations of light rare earth elements without a Ce anomaly—features common in zircons from hydrothermally altered rocks and a reducing environment. Petrological and geochronological results in this study suggest the following tectonic evolution: (1) ...

Geochemical, mineralogical and geochronological investigations have been carried out on the lamprophyre dike in the Tsagaan Tsahir Uul gold deposit area, Bayankhongor, Central Mongolia. The lamprophyre is composed of olivine, amphibole, and clinopyroxene as phenocrysts with plagioclase, potassium feldspar, biotite, and amphibole as the groundmass. The amphibole is mostly pargasite; some grains are rich in titanium and constitute kaersutite. Pyroxene is slightly enriched in titanium. Although the olivine ranges from Fo92.9 to Fo84.6, a distinct compositional variation between the core and rim in a single grain is lacking. Biotite and phlogopite are dominant. Plagioclase is mainly andesine (An45Ab49Or6) and oligoclase (An12Ab74Or14), and potassium feldspar is orthoclase. The major oxides content of the lamprophyre has a good correlation with SiO2. TiO2, FeOt, MgO, CaO, and MnO decrease, and Al2O3 and K2O increase with increased SiO2. Trace element composition is characterized by high values of Large Ion Lithophile Elements (LILE) (Ba, Rb, Sr) and also some compatible elements (Cr and Ni). The MORB normalized spider diagram shows slight depletion of Nb in contrast to Ba and Ce. The REE pattern shows LREE enrichment, and rapid decrease in HREE practically without an Eu anomaly. The Rb-Sr internal isochron yields an age of 248.61 ± 18.5 Ma (Zechstein, Late Permian) with 87Sr/86Sr initial ratio of 0.706096 ± 0.00002. Mineral assemblage and chemical characteristics are close to those of calc-alkaline lamprophyres (CALs). An Al2O3 versus TiO2 tectonic discrimination diagram of the shoshonitic and potassic igneous rock shows that the Tsagaan Tsahir Uul lamprophyre might have originated within an intra-plate tectonic setting. The high-Ti content of the lamprophyre also suggests that the magma could have originated in an intra-continental tectonic setting. Small but similar intrusions emplaced during the Permian period are spatially and temporally associated with the main gold and copper mineralization in the Bayankhongor metallogenic belt. The obtained age of the lamprophyre dike (248.61 ± 18.5 Ma) demonstrates that its intrusion was a contemporary event. This was the youngest plutonic body in the Tsagaan Tsahir Uul area, which is close in age to the gold-bearing quartz veins. Although there is no direct genetic evidence associating the auriferous veins and the lamprophyre in the field, they occur together and possibly share a similar tectonic setting.

Geochemical and geochronological investigations have been carried out on the Baidrag batholith, Tsagaan Tsahir Uul, Bayanhongor, central Mongolia to characterize the batholith, and to investigate its origin. The batholith mainly consists of biotite granite and muscovite-biotite granite. Major mineral constituents are plagioclase, potassium feldspar, quartz, biotite±muscovite. The batholith has experienced deuteric alteration and chloritization. Mineral compositions were changed to some extent during those episodes, but bulk rock chemical change seems insignificant except for highly chloritized samples. Field occurrence, mineral assemblage and chemical and isotopic characteristics of the granite, including the high initial 87Sr/86Sr ratio, demonstrate that the rock has the S-type characteristics. They show Nb-depletion in MORB normalized diagram, and abundances of other trace element suggest that the parental material might be mixture of subduction-related igneous rocks similar to TTD and old continental materials. The determined age of the Baidrag batholith is 647±10 Ma by the Rb-Sr whole rock isochron method.