Editorial for Special Issue “Metallogenesis of the Central Asian Orogenic Belt”

1. Introduction

The Central Asian Orogenic Belt (CAOB), or Altaid orogenic collage, is one of the largest Neoproterozoic to Early Mesozoic accretionary orogenic belts on Earth. The belt was formed through a process of prolonged subduction and the closure of the Paleo-Asian Ocean. Extending from the Ural Mountains in the west to the Pacific Ocean in the east, it is delimited by the Siberian Craton to the north and by the Tarim–North China Craton to the south. The CAOB is a record of a long and intricate history of oceanic subduction, arc–continent and continent–continent accretion, and post-collisional processes, leading to the development of diverse tectonic terranes and large-scale metallogenic systems. These processes have produced multi-stage, multi-type mineralization across the belt, making the CAOB one of the world’s three principal metallogenic domains. Hosting diverse, world-class mineral deposits, its complex arc terranes, ophiolites, metamorphic belts, and granitoids record multi-stage tectonics, crucial for understanding Eurasian assembly and mineralization in accretionary orogens.

This Special Issue focuses on the tectono-magmatic processes of the CAOB and their genetic links to ore formation, with particular focus on the connection to igneous-related mineral systems.

2. Contents of This Special Issue

This Special Issue comprises six articles published in 2024, with an emphasis on “Metallogenesis of the Central Asian Orogenic Belt”. The first contribution, “The Evolution of Permian Mafic–Ultramafic Magma of the Yunhai Intrusion in the Northern Tianshan, Northwest China, and Its Implications for Cu–Ni Mineralization”, by Pei et al. (2024) [1], presents new petrological, geochronological, and geochemical data from the Yunhai mafic–ultramafic intrusion. The study constrains the timing of Cu–Ni mineralization, identifies the magmatic source, unravels the evolutionary history of the magma, and provides new insights into the Cu–Ni metallogenic processes in the western segment of the Junggar–Tianshan–East Kazakhstan Orogenic Belt, Northern Tianshan. Ultimately, the conclusion is drawn that the primitive magma of the Yunhai intrusion may have been derived from 2% to 10% partial melting of very limited (ca. 5%) metasomatized subcontinental lithospheric mantle (with 2% spinel lherzolite and 1% garnet peridotite) in a post-collisional extensional tectonic setting and is characterized by a high Mg basaltic and a PGE-undepleted composition. In contrast, the Cu–Ni mineralization at Yunhai is presented as primarily a product of low-degree partial melting of the metasomatized mantle, early sulfide segregation at depth, and olivine crystal–liquid differentiation at shallower levels.

The second article, “Timing of Ore Mineralisation in Deposits of the Baikal–Muya Belt and the Barguzin–Vitim Super-Terrain (Transbaikalie)”, by Vanin et al. (2024) [2], constrains the ages in Transbaikalia, Russia, using robust geochronology on Au ores from the Yubileinoe, Irokinda, and Uryakh deposits (Baikal–Muya fold belt) and Pb–Zn ores from the Ozernoe deposit (Barguzin–Vitim super-terrain). Ages of 265 ± 33 Ma for Yubileinoe, mesothermal low-sulfide gold–quartz and gold–(pyrite)–galenite–sphalerite types); 276 ± 13 Ma for Irokinda, quartz veins; and 287 ± 7 Ma for Uryakh, quartz–carbonate veins, define two mineralization stages: 330–320 Ma disseminated and 290–270 Ma vein types. These correspond to early and late Angara–Vitim batholith emplacement, clarifying the metallogenic evolution and tectonic framework of the Central Asian Orogenic Belt.

In the third article, “Petrogenesis and Geochronology of Late Devonian Intrusive Rocks in Eastern Tianshan, Xinjiang, China: Subduction Constraints of the North Tianshan Ocean”, by Meng et al. (2024) [3] investigate the petrology, geochemistry, and zircon U–Pb geochronology of Late Devonian intrusive rocks in the Tulargen area of the Eastern Tianshan Orogenic Belt, Xinjiang. Through comprehensive geochemical analyses, they reveal that these intrusive rocks were emplaced in an island-arc tectonic setting, reflecting a subduction-related origin linked to the northward subduction of the North Tianshan Ocean at about 382 ± 5 Ma (gabbro) to 362.8 ± 4.4 Ma (biotite monzogranite), confirming that the source magma of the biotite monzogranite was the product of partial metabasalt melting at a medium crustal depth combined with the melting of lower crustal rocks. The study concludes that the Late Devonian intrusive rocks in this area formed within the island-arc tectonic setting are associated with the subduction of the North Tianshan Ocean.

The fourth article, “The Genesis of Ultramafic Rock Mass on the Northern Slope of Lüliang Mountain in North Qaidam, China”, by Guo et al. (2024) [4], focuses on a large ultramafic rock mass exposed on the northern slope of Lüliang Mountain. LA-ICP-MS zircon U-Pb dating of serpentinite in the North Qaidam orogenic belt provides an age of 480.6 ± 2.4 Ma, placing formation during the Ordovician northward subduction of the North Qaidam Ocean. Interestingly, chromite is shown to have a zonal texture; i.e., FeCr₂O₄, with minor MgAl₂O₄, FeFe₂O₄, high Cr₂O₃ and Cr#, and elevated Fe³+# but low Al₂O₃ and TiO₂, representing primary chromite. Meanwhile, its rim is formed of Cr-rich magnetite with high Fe³+# but depleted Cr₂O₃, Al₂O₃, TiO₂, and Cr#, indicating hydrothermal alteration. Here, ultramafic rock, an ophiolitic cumulate derived from a depleted fore-arc mantle in a supra-subduction zone, is shown to crystallize at 1303–1308 °C and 3.37–3.46 GPa (avg. 1306 °C, 3.41 GPa). In summary, the Fe-rich hydrothermal alteration found in this study marks a later uplift–denudation stage.

In the fifth article, entitled “Petrogenesis of Carboniferous–Permian Granitoids in the Kumishi Area of Tianshan, China: Insights into the Geodynamic Evolution Triggered by Subduction and Closure of the South Tianshan Ocean”, by Kang et al. (2024) [5] report the zircon U–Pb, Hf, and Pb data for monzonitic and granitic intrusions in the Kumishi area, yielding ages of 284.5 ± 2.4 Ma and 283.4 ± 3.9 Ma. The quartz-monzonites and syenogranites are shown to be high-K calc-alkaline to shoshonitic I-type granites, with εHf(t) of +14.9–+15.5 and +6.6–+14.9. They are LREE-enriched and HFSE-depleted (Nb, Ta, Ti), showing flat HREE patterns and negative Eu anomalies, features that collectively indicate partial melting of the lower crust with mantle input, driven by asthenospheric upwelling in a post-collisional extensional setting. Combined with regional magmatism, the data show a tectonic transition from subduction to post-collision in the Late Carboniferous–Early Permian, marking the final closure of the South Tianshan segment of the Paleo-Asian Ocean and illuminating Late Paleozoic geodynamics of the southern CAOB.

The concluding article, “Rb–Sr Pyrite Dating and S–Pb Isotopes in the Fang’an Gold Deposit, Wuhe Area, Eastern Anhui Province”, by Wang et al. (2024) [6], reports the in situ sulfur isotopic compositions of pyrite and lead isotopic compositions of sulfides within the ores as having ²⁰⁶Pb/²⁰⁴Pb ratios ranging from 16.759 to 16.93,²⁰⁷Pb/²⁰⁴Pb ratios ranging from 15.311 to 15.402, and ²⁰⁸Pb/²⁰⁴Pb ratios ranging from 37.158 to 37.548,which are close to the Xigudui Formation, relatively distant from the Mesozoic granites, indicating that the Xigudui Formation was the source of lead for the Late Mesozoic ores of the deposit. It is therefore possible that the degassing of mantle-derived magma in the shallow parts of the crust was the source of ore-forming sulfur in the Wuhe area, with Rb–Sr dating of pyrite from the Fang’an gold deposit revealing that the mineralization occurred at 126.89 ± 0.58 Ma.

3. Conclusions

This Special Issue on Metallogenesis of the Central Asian Orogenic Belt presents a series of cutting-edge studies that shed new light on the tectono-magmatic evolution and metallogenic processes within one of the world’s most prolific accretionary orogens, highlighting the genetic links between subduction–accretion, arc magmatism, and post-collisional tectonic processes and the formation of diverse mineral deposits. In light of these findings, it is our hope that this Special Issue will encourage new comparative research into the metallogenesis of other accretionary orogens.