Tectonics,magmatism, and mineralization of Circum–Balkash–Junggar area in the Central Asian Orogenic Belt

ABSTRACT

The Circum–Balkhash–Junggar area, including mostly Kazakhstan, NW China, Russia, Kyrgyzstan, Tajikistan, Uzbekistan, and Mongolia, occupies an important tectonic position of the Central Asian Orogenic Belt (CAOB) (Figure 1). Tectonically, this vast area records the complicated geodynamic processes, among which the most prominent stages are the formation of the U-shaped Kazakhstan Orocline and its interactions with adjacent Altai (Altay), Junggar (West Junggar, Junggar Basin, and East Junggar), and Tianshan orogenic collages in the Palaeozoic, bearing large-scale mineral deposits. The formation of the Late Palaeozoic mineral deposits is related to the tectonic evolution of the Devonian and Carboniferous–Permian volcano-magmatic arcs in the region. However, the link between the metallogeny and the evolution of the volcano-magmatic arcs is not well understood and existing geodynamic models have not explained satisfactorily the mechanism of the huge metallogenic belt. Therefore, this special issue focuses on the formation of the Late Palaeozoic porphyry Cu deposits and their link to the tectonic evolution of the Devonian and Carboniferous–Permian volcano-magmatic arcs with emphasis on comparative studies across the international borders.     

Origin of anomalously Ni-rich parental magmas and genesis of the Huangshannan Ni–Cu sulfide deposit,Central Asian Orogenic Belt,Northwestern China

The Huangshannan Ni–Cu sulfide deposit at the southern margin of the Central Asian Orogenic Belt (CAOB) is an important recent discovery in the Eastern Tianshan Region, Northwestern China. The Huangshannan Intrusion is composed of mafic and ultramafic rocks, and its websterite and lherzolite sequences host the sulfide orebodies. Olivine is the dominant mineral in the Huangshannan Intrusion, occurring as olivine inclusions hosted by pyroxene oikocrysts, as olivine crystals in magmatic sulfides, and as poikilitic crystals in the lherzolite. Small olivine inclusions always coexist with large poikilitic olivine crystals in the same sample, resulting in a heterogeneous texture on the scale of the oikocrysts. The Ni abundance ranges from 1540 to 3772 ppm in poikilitic olivine grains, from 2114 to 3740 ppm in olivine grains hosted by sulfide minerals, and from 2043 to 4023 ppm in olivine inclusions hosted by pyroxene oikocrysts. For the three types of olivine, the ranges in forsterite (Fo) content are 78.97–84.92 mol.%, 81.57–84.79 mol.%, and 80.33–84.68 mol.%, respectively. The Ni content of olivine in the lherzolite is anomalously high relative to the range found in most within plate olivine-bearing mafic-ultramafic rocks. The composition of olivine is controlled mainly by that of the parental magma, fractional crystallization and reactions with interstitial silicate and sulfide melts. Both fractional crystallization and reaction with interstitial silicate may cause a decrease in the Ni content of olivine. The possibility that Ni–Fe exchange causes the anomalously high Ni contents in olivine can be excluded because the olivine grains contained in sulfide have similar or lower Ni content than the olivine grains hosted in the silicate rock. Most of the olivine grains are unzoned, and they have anomalously high Ni contents throughout the crystal. Assuming a partition coefficient of Ni between olivine and silicate magma to be 7, the measured Ni content of olivine in the lherzolite (1540–4023 ppm with a mean of 2907 ppm) indicates that the parental magma contains 220–575 ppm (average of 415 ppm) Ni. This value is higher than that found in basaltic magmas that crystallized olivine with similar Fo contents compared to the Huangshannan Intrusion. As mentioned above, the symmetric and reproducible variations in both Fo and Ni contents from core to margin in most of the olivine grains cannot be explained by fractional crystallization and reactions with interstitial silicate or sulfide melts but may reflect the equilibration of the olivine with new fluxes of magma as the chamber was replenished. The anomalously Ni-rich composition of the parental magmas of the Huangshannan Intrusion, relative to those of many other mineralized olivine-bearing mafic-ultramafic intrusions, may be produced by upgrading and scavenging of metals from a previously formed sulfide melts by a moderately Ni-rich magma. The mass-balance calculations of PGE data indicate that the parental magma that formed lherzolite contains 0.04 ppb Os, 0.02 ppb Ir and 0.4 ppb Pd, whereas the parental magma that formed websterite has 0.02 ppb Os, 0.009 ppb Ir and 0.75 ppb Pd. Rayleigh modeling using PGE tenors indicates that the massive sulfides may be produced by monosulfide solid solution (MSS)-sulfide liquid fractionation from the magma that formed the websterite. Rayleigh modeling of Fo and Ni contents of olivine shows that the parental magma that formed the lherzolite has experienced previous sulfide segregation and olivine crystallization.     

Geological characteristics and genesis of the Jurassic No. I porphyry Cu–Au deposit in the Xiongcun district,Gangdese porphyry copper belt,Tibet

The Xiongcun district, located in the western segment of the Gangdese porphyry copper belt (GPCB), hosts the only known Jurassic mineralization in the GPCB, Tibet, PRC. The No. I deposit in the Xiongcun district is related to the Middle Jurassic quartz diorite porphyry (167–161 Ma) and the mineralization was formed at ca. 161.5 ± 2.7 Ma. Ore-bearing Middle Jurassic quartz diorite porphyry emplaced into the Early Jurassic volcano-sedimentary rock sequences of the Xiongcun Formation. Veinlets and disseminated mineralization developed within the Middle Jurassic quartz diorite porphyry and the surrounding metamorphosed tuff, hosting a measured and indicated resource of 1.04 Mt copper, 143.31 t gold and 900.43 t silver with an average grade of 0.48% copper, 0.66 g/t gold, and 4.19 g/t silver. The mineralization can be assigned to four stages, including three main stages of hypogene mineralization and one epigenetic stage. The main alteration associated with mineralization is potassic. Seven mineralization-related hydrothermal veins have been recognized, including quartz–sulfide, biotite–sulfide, magnetite–sulfide, quartz–molybdenite–sulfide, chalcopyrite–pyrite–pyrrhotite, pyrite and polymetallic veins. The S and Pb isotopic compositions of the ore sulfides and the Re contents of the molybdenite suggest a mantle source for the ore-forming materials with minor contamination from the subducted sediments. Hydrogen and oxygen isotope compositions of quartz in the ores suggest that both magmatic and meteoric waters were involved in the ore-forming process. The ore-bearing porphyry (167–161 Ma) and ore-forming (161.5 ± 2.7 Ma) ages of the No. I deposit correspond to the time of northward subduction of Neo-Tethys oceanic slab. The geochemical data of the ore-bearing porphyry indicate that the No. I deposit formed in an intra-oceanic island arc setting and the ore-bearing porphyry originated from the partial melting of mantle with limited contribution of subducted sediments. The genesis of the ore-bearing porphyry and No. I deposit is interpreted as being related to northward intra-oceanic subduction of Neo-Tethys oceanic slab in the Middle Jurassic time (167–161 Ma).     

Geological,geochronological, and H–O isotopic constraints on the genesis of the Tongjing Cu–Au deposit in the Ningwu basin,east China

The Tongjing Cu–Au deposit is a medium-sized deposit within the Ningwu volcanic basin, east China, and is hosted by Cretaceous volcanic rocks of the Dawangshan and Niangniangshan Formations. The veined and lenticular Cu–Au orebodies are spatially and temporally related to the volcanic and subvolcanic rocks of the Niangniangshan Formation in the ore district. The wall-rock alteration is dominated by silicification, siderite alteration, carbonation, sericitization, chloritization, and kaolinization. On the basis of field evidence and petrographic observations, two stages of mineralization are recognized: (1) a siderite–quartz–sulfide stage (Stage 1) associated with the formation of chalcopyrite and pyrite in a quartz and siderite gangue; and (2) a quartz–bornite stage (Stage 2) cutting the Stage 1 phases. Stage 1 is the main mineralization stage. Quartz that formed in Stage 1 has δ¹⁸O_H2O values of − 4.3‰ to 3.5‰ with δD values of fluid inclusion waters of − 97.1‰ to − 49.9‰, indicating that the ore-forming fluids were derived from early magmatic fluids and may have experienced oxygen isotopic exchange with meteoric water during Stage 1 mineralization.LA–MC–ICP–MS zircon U–Pb dating of the mineralization-related nosean-bearing phonolite and nosean-bearing phonolitic brecciated tuff at Tongjing yields ages of 129.8 ± 0.5 Ma and 128.9 ± 1.1 Ma, respectively. These results are interpreted as the crystallization age of the volcanic rocks of the Niangniangshan Formation. A hydrothermal sericite sample associated with Cu–Au mineralization at Tongjing yields a plateau ⁴⁰Ar–³⁹Ar age of 131.3 ± 1.3 Ma. These results confirm a genetic link between the volcanism and associated Cu–Au mineralization. The Tongjing Cu–Au deposit in the Ningwu basin is genetically and possibly tectonically similar to alkaline intrusion-related gold deposits elsewhere in the world.     

Geodynamics of the origin of granulites in the Sangilen block of the Tuva–Mongolian terrane,Central Asian Orogenic Belt

The Tuva–Mongolian terrane of the Central Asian Orogenic Belt is a composite structure with a Vendian–Cambrian terrigenous–carbonate cover. The Sangilen block in the southern part of the belt is a smaller composite structure, in which tectono–stratigraphic complexes of different age that were produced under various conditions were amalgamated in the course of Early Paleozoic tectonic cycle. The P–T parameters of the Early Paleozoic metamorphism in the western part of the Sangilen block corresponded to the amphibolite facies. The gneisses of the Erzin Complex contain relict granulite-facies mineral assemblages. The granulites are dominated by metasediments typical of deep-water basins on passive continental margins. The only exception is granulites of the Lower Erzin tectonic nappe of the Chinchlig thrust system: these rocks are metatholeiites, tonalites, and trondhjemites, whose REE patterns are similar to those of MORB. The composition of these granulites and their high Sm/Nd ratios indicate that the rocks were derived from juvenile crust that had been formed in an environment of a mature island arc or backarc basin. It is reasonable to believe that these rocks are fragments of the Late Riphean basement of the Sangilen block. The average ²⁰⁶Pb/²³⁸U zircon age of the garnet–hypersthene granulites is 494 ± 11 Ma. With regard for the zircon age of the postmetamorphic granitoids, the granulite-facies metamorphism occurred within the age range of 505–495 Ma. The peak metamorphic temperature reached 910–950°C, and the pressure was 3–4 kbar, which corresponds to ultrahigh-temperature/low-pressure (UHT–LP) metamorphism. The garnet–hypersthene orthogranulites were formed at a temperature that decreased to ~850°C and pressure that increased to ~5.5?7 kbar. It can be hypothesized that the earlier UHT–LP granulites were produced at an elevated heat flux and were later (in the course of continuing collision) overlain by a relatively cold tectonic slab, and this leads to a certain temperature decrease and pressure increase. This relatively cold slab could consist of fragments of the Vendian elevated-pressure metamorphic belt whose development terminated at the Vendian–Cambrian boundary before the onset of the Early Paleozoic regional metamorphism.     

Geology and ore genesis of the late Paleozoic Heijianshan Fe oxide–Cu (–Au) deposit in the Eastern Tianshan,NW China

The Heijianshan Fe–Cu (–Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic–intermediate volcanic and mafic–felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic–hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (–Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote–calcite–tourmaline–sericite alteration assemblage indicates a pre-mineralization Ca–Mg alteration event. Stage II Fe and Stage IV Cu (–Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore-forming fluids.Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189–370 °C), II (301–536 °C), III (119–262 °C) and V (46–198 °C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic–hydrothermal-derived Fe mineralization fluids were characterized by high temperature (>300 °C), medium–high salinity (21.2–56.0 wt% NaCl equiv.) and being Na–Ca–Mg–Fe-dominated. These fluids were overprinted by the external low temperature (<300 °C), medium–high salinity (19.0–34.7 wt% NaCl equiv.) and Ca–Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (–Au) mineralization, as supported by quartz CL images and H–O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic–hydrothermal), III (basinal brine-related) and IV (magmatic–hydrothermal). Stage II disseminated pyrite has δ³⁴S_fluid values of 1.7–4.3‰, comparable with sulfur from magmatic reservoirs. δ³⁴S_fluid values (24.3–29.3‰) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite–chalcopyrite–pyrrhotite assemblage (with low δ³⁴S_fluid values of 4.6–10.0‰) may have formed at low fO₂ and temperature (119–262 °C). The continuous basinal brine–volcanic/volcaniclastic rock interactions during the basin inversion (∼325–300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like δ³⁴S_fluid values (1.5–4.1‰). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (∼160 °C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins.The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.     

The Late Carboniferous–Early Permian Ocean-Continent Transition in the West Junggar, Central Asian Orogenic Belt: Constraints from Columnar Jointed Rhyolite

The West Junggar of the western Central Asian Orogenic Belt is one of the typical regions in the term of ocean subduction, contraction and continental growth in the Late Paleozoic. However, it is still controversial on the exact time of ocean-continent transition so far. This study investigates rhyolites with columnar joint in the West Junggar for the first time.Based on zircon U-Pb dating, we determined that the ages of the newly-discovered rhyolites are between 303.6 and 294.5 Ma, belonging to Late Carboniferous–Early Permian, which is the oldest rhyolite with columnar joint preserved in the world at present. Geochemical results show that the characteristics of the major element compositions include a high content of SiO_2(75.78–79.20 wt%) and a moderate content of Al_2O_3(12.21–13.19 wt%). The total alkali content(K_2O +Na_2O) is 6.14–8.05 wt%, among which K_2O is 2.09–4.72 wt% and the rate of K_2O/Na_2O is 0.38–3.05. Over-based minerals such as Ne, Lc, and Ac do not appear. The contents of TiO_2(0.09–0.24 wt%), CaO(0.15–0.99 wt%) and MgO(0.06–0.18 wt%) are low. A/CNK=0.91–1.68, A/NK=1.06–1.76, and as such, these are associated with the quasi-aluminum-weak peraluminous high potassium calc-alkaline and some calc-alkaline magma series. These rhyolites show a significant negative Eu anomaly with relative enrichment of LREE and LILE(Rb, Ba, Th, U, K) and depletion of Sr, HREE and HFSE(Nb, Ta, Ti, P). These rhyolites also have the characteristics of an A2-type granite, similar to the Miaoergou batholith,which indicates they both were affected by post-orogenic extension. Combining petrological, zircon U-Pb dating and geochemical characteristics of the rhyolites, we conclude that the specific time of ocean-continent transition of the West Junggar is the Late Carboniferous–Early Permian.     

Yanshanian–Himalayan geodynamic transformation of the northwestern Junggar Basin,southwestern Central Asian Orogenic Belt (CAOB), and its significance for petroleum accumulation

The northwestern Junggar Basin in the southwestern Central Asian Orogenic Belt is a typical petroliferous basin. The widely distributed reservoirs in Jurassic–Cretaceous strata indicate that the region records Yanshanian–Himalayan tectonic activity, which affected the accumulation and distribution of petroleum. The mechanism of this effect, however, has not been fully explored. To fill the knowledge gap, we studied the structural geology and geochemistry of the well-exposed Wuerhe bitumen deposit. Our results indicate that deformation and hydrocarbon accumulation in the northwestern Junggar Basin during the Yanshanian–Himalayan geodynamic transformation involved two main stages. During the Yanshanian orogeny, a high-angle extensional fault system formed in Jurassic–Cretaceous strata at intermediate to shallow depths owing to dextral shear deformation in the orogenic belt. This fault system connected at depth with the Permian–Triassic oil–gas system, resulting in oil ascending to form fault-controlled reservoirs (e.g., a veined bitumen deposit). During the Himalayan orogeny, this fault system was deactivated owing to sinistral shear caused by far-field stress related to uplift of the Tibetan Plateau. This and the reservoir densification caused by cementation formed favorable hydrocarbon preservation and accumulation conditions. Therefore, the secondary oil reservoirs that formed during the Yanshanian–Himalayan tectonic transformation and the primary oil reservoirs that formed during Hercynian–Indosinian orogenies form a total and complex petroleum system comprising conventional and unconventional petroleum reservoirs. This might be a common feature of oil–gas accumulation in the Central Asian Orogenic Belt and highlights the potential for petroleum exploration at intermediate–shallow depths.     

The Kalatongke magmatic Ni–Cu deposits in the Central Asian Orogenic Belt, NW China: product of slab window magmatism?

The Permian Kalatongke Ni–Cu deposits in the Central Asian Orogenic Belt are among the most important Ni–Cu deposits in northern Xinjiang, western China. The deposits are hosted by three small mafic intrusions comprising mainly norite and diorite. Its tectonic context, petrogenesis, and ore genesis have been highly contested. In this paper, we present a new model involving slab window magmatism for the Kalatongke intrusions. The origin of the associated sulfide ores is explained in the context of this new model. Minor amounts of olivine in the intrusions have Fo contents varying between 71 and 81.5?mol%, which are similar to the predicted values for olivine crystallizing from coeval basalts in the region. Analytic modeling based on major element concentrations suggests that the parental magma of the Kalatongke intrusions and the coeval basalts represent fractionated liquids produced by ～15% of olivine crystallization from a primary magma, itself produced by 7–8% partial melting of depleted mantle peridotite. Positive ε _Nd values (+4 to +10) and significant negative Nb anomalies for both intrusive and extrusive rocks can be explained by the mixing of magma derived from depleted mantle with 6–18% of a partial melt derived from the lower part of a juvenile arc crust with a composition similar to coeval A-type granites in the region, plus up to 10% contamination with the upper continental crust. Our model suggests that a slab window was created due to slab break-off during a transition from oceanic subduction to arc–arc or arc–continent collision in the region in the Early Permian. Decompression melting in the upwelling oceanic asthenosphere produced the primary magma. When this magma ascended to pond in the lower parts of a juvenile arc crust, it underwent olivine crystallization and at the same time triggered partial melting of the arc crust. Mixing between these two magmas followed by contamination with the upper crust after the magma ascended to higher crustal levels formed the parental magma of the Kalatongke intrusions. The parental magma of the Kalatongke intrusions was saturated with sulfide upon arrival primarily due to olivine fractional crystallization and selective assimilation of crustal sulfur. Sulfide mineralization in the Kalatongke intrusions can be explained by accumulation of immiscible sulfide droplets by flow differentiation, gravitational settling, and downward percolation which operated in different parts of the intrusions. Platinum-group element (PGE) depletion in the bulk sulfide ores of the Kalatongke deposits was due to depletion in the parental magma which in turn was likely due to depletion in the primary magma. PGE depletion in the primary magma can be explained by a relatively low degree of partial melting of the mantle and retention of coexisting sulfide liquid in the mantle.     

Geochemistry and U–Pb detrital zircon dating of Paleozoic graywackes in East Junggar,NW China: Insights into subduction–accretion processes in the southern Central Asian Orogenic Belt

The southern Central Asian Orogenic Belt (CAOB) is characterized by multiple and linear accretionary orogenic collages, including Paleozoic arcs, ophiolites, and accretionay wedges. A complex history of subduction–accretion processes makes it difficult to distinguish the origin of these various terranes and reconstruct the tectonic evolution of the southern CAOB. In order to provide constraints on the accretionary history, we analyzed major and trace element compositions of Paleozoic graywackes from the Huangcaopo Group (HG) and Kubusu Group (KG) in East Junggar. The HG graywackes have relatively low Chemical Index of Alteration (CIA) values (50 to 66), suggesting a source that underwent relatively weak chemical weathering. The identical average Index of Compositional Variability (ICV) values (~ 1.1) for both the KG and HG samples point to an immature source for the Paleozoic graywackes in East Junggar, which is consistent with an andesitic–felsic igneous source characterized by low La/Th ratios and relatively high Hf contents. These graywackes are geochemically similar to continental island arc sediments and therefore were probably deposited at an active continental margin. U–Pb dating of detrital zircons from the lower subgroup of the HG yielded a young age peak at ~ 440 Ma, indicating a post-Early Silurian depositional age. However, the youngest populations of detrital zircons from the KG graywackes and the upper subgroup of the HG yielded ²⁰⁶Pb/²³⁸U ages of ~ 346 Ma and ~ 355 Ma, respectively, which suggest a post-Early Carboniferous depositional age. Because of similarities of rock assemblages, these two units should be incorporated into the Early Carboniferous Nanmingshui Formation. The detrital zircon age spectrum of the Early Paleozoic HG graywackes resembles that of the Habahe sediments in the Chinese Altai, which suggests that the ocean between East Junggar and the Chinese Altai was closed before the deposition of the sediments and that the Armantai ophiolite was emplaced prior to the Early Devonian. The differences in age spectra for detrital zircons from the post-Early Carboniferous graywackes in East Junggar and the Harlik arc indicate that the emplacement of the Kalamaili ophiolite postdates the Early Carboniferous. Therefore, a long-lasting northward subduction–accretion process is suggested for the formation of East Junggar and the reconstruction of the Early Paleozoic evolution of the southern CAOB.     

Geochemistry,mineralogy and genesis of the Ayazmant Fe–Cu skarn deposit in Ayvalik, (Balikesir), Turkey

The Ayazmant Fe–Cu skarn deposit is located approximately 20 km SE of Ayval?k or 140 km N of Izmir in western Turkey. The skarn occurs at the contact between metapelites and the metabasites of the Early Triassic K?n?k Formation and the porphyritic hypabyssal intrusive rocks of the Late Oligocene Kozak Intrusive Complex. The major, trace, and rare earth-element geochemical analysis of the igneous rocks indicate that they are I-type, subalkaline, calc-alkaline, metaluminous, I-type products of a high-level magma chamber, generated in a continental arc setting. The ⁴⁰Ar–³⁹Ar isochron age obtained from biotite of hornfels is 20.3 ± 0.1 Ma, probably reflecting the age of metamorphic–bimetasomatic alteration which commenced shortly after intrusion into impure carbonates. Three stages of skarn formation and ore development are recognized: (1) Early skarn stage (Stage I) consisting mainly of garnet with grossular-rich (Gr_75–79) cores and andradite-rich (Gr_36–38) rims, diopside (Di_94–97), scapolite and magnetite; (2) sulfide-rich skarn (Stage II), dominated by chalcopyrite with magnetite, andraditic garnet (Ad₈₄–₈₉), diopside (Di₆₅–₇₅) and actinolite; and (3) retrograde alteration (Stage III) dominated by actinolite, epidote, orthoclase, phlogopite and chlorite in which sulfides are the main ore phases. ⁴⁰Ar–³⁹Ar age data indicate that potassic alteration, synchronous or postdating magnetite–pyroxene–amphibole skarn, occurred at 20.0 ± 0.1 Ma. The high pyroxene/garnet ratio, plus the presence of scapolite in calc-silicate and associated ore paragenesis characterized by magnetite (± hematite), chalcopyrite and bornite, suggests that the bulk of the Ayazmant skarns were formed under oxidized conditions. Oxygen isotope compositions of pyroxene, magnetite and garnet of prograde skarn alteration indicate a magmatic fluid with δ¹⁸O values between 5.4 and 9.5‰. On the basis of oxygen isotope data from mineral pairs, the early stage of prograde skarn formation is characterized by pyroxene (Di_94–97)-magnetite assemblage formed at an upper temperature limit of 576 °C. The lower temperature limit for magnetite precipitation is estimated below 300 °C, on the basis of magnetite–calcite pairs either as fracture-fillings or massive ore in recrystallized limestone-marble. The sulfide assemblage is dominated by chalcopyrite with subordinate molybdenite, pyrite, cubanite, bornite, pyrrhotite, galena, sphalerite and idaite. Gold–copper mineralization formed adjacent to andradite-dominated skarn which occurs in close proximity to the intrusion contacts. Native gold and electrum are most abundant in sulfides, as fine-grained inclusions; grain size with varying from 5 to 20 µm. Sulfur isotope compositions obtained from pyrrhotite, pyrite, chalcopyrite, sphalerite and galena form a narrow range between ? 4.8 and 1.6‰, suggesting the sulfur was probably mantle-derived or leached from magmatic rocks. Geochemical data from Ayazmant shows that Cu is strongly associated with Au, Bi, Te, Se, Cd, Zn, Pb, Ni and Co. The Ayazmant mineralizing system possesses all the ingredients of a skarn system either cogenetic with, or formed prior to a porphyry Cu(Au–Mo) system. The results of this study indicate that the Aegean Region of Turkey has considerable exploration potential for both porphyry-related skarns and porphyry Cu and Au mineralization.     

Provenance of the Permo–Carboniferous sediments in the northern Alxa and its tectonic implications for the southernmost Central Asian Orogenic Belt

As the southernmost segment of the Central Asian Orogenic Belt (CAOB), the northern Alxa orogenic belt (NAOB) connects the southeastern and southwestern segments of the CAOB. The NAOB amalgamated with the closure of the Paleo-Asian Ocean; however, the closure time of the Paleo-Asian Ocean is still on great debate. In this study, we reported new detrital zircon U–Pb geochronology and Hf–O isotopes for the Permo–Carboniferous sediments in the northern Alxa to constrain the provenance and its tectonic implications. The Permo–Carbonifereous Amushan Formation is composed of volcanic-carbonite-clastic rocks and was deposited in a shallow marine environment. Based on the zircon U–Pb geochronology, the Amushan Formation was deposited in the late Carboniferous to early Permian, but some outcrops of volcanic and clastic rocks in the Quaganqulu area were likely formed in the middle to late Permian. The integrated zircon age spectrum for the clastic rocks shows a wide range from late Archean to Paleoproterozoic, Mesoproterozoic (with a peak age at 1458 ​Ma), early Neoproterozoic (with peak ages of 988 ​Ma and 929 ​Ma), early Paleozoic (with a peak age at 447 ​Ma) and late Paleozoic (with a peak age at 294 ​Ma). Combined with the zircon Hf–O isotopes, the provenance was considered to be the Alxa Block, the Shalazhashan terrane and the Zhusileng–Hangwula block (and the southern Beishan orogenic belt). The multiple source regions to the south and north of the Paleo-Asian Ocean indicate the closure of this ocean before the late Carboniferous. The absence or small proportion of depositional age-approximated zircons in most samples makes their age spectra similar to extensional basins. Combined with the intra-plate volcanism, the deposits were considered to be formed in extensional settings. Accordingly, after the closure of the Paleo-Asian Ocean, the NAOB stepped into an extensional stage.     

Magma mixing origin for the Aolunhua porphyry related to Mo–Cu mineralization,eastern Central Asian Orogenic Belt

Many Cu–Mo–Au deposits are considered to be related to adakitic porphyries formed in non-arc settings, e.g., in collisional orogenic zones and intra-plate environments, but their genesis is still under discussion. The Aolunhua porphyry complex and its related Mo–Cu deposit from the eastern Central Asian Orogenic Belt (CAOB) provide important insights into this issue. The porphyries are characterized by high Sr (496–705 ppm) and Sr/Y and La/Yb ratios similar to those of typical adakitic rocks, and low I_Sr ratios (0.7049–0.7052) and positive ε_Nd(t) (+ 0.5 to + 1.4) and ε_Hf(t) (+ 3.5 to + 9.8) values. These features, along with the occurrence of mafic microgranular enclaves (MMEs), compositional and textural disequilibrium of plagioclase phenocrysts and relatively high Mg# values (45–52), indicate that they were derived from mixing of felsic magma from partial melting of a juvenile arc-type lower crust and mafic magma from a lithospheric mantle previously metasomatized by subduction zone fluids/melts. High Sr/Y and La/Yb ratios are indicative of contribution from enriched mantle-derived materials (with high LILEs; e.g., Sr, La), which were strengthened by subsequent fractionation of ferromagnesian phases such as pyroxene and hornblende. MMEs hosted by the ore-bearing porphyry have zircon U–Pb ages of ca. 132 Ma, similar to those of the host rocks. The enclaves have elevated Mg^# (56–63), LILEs (e.g., Sr = 660–891 ppm), LREE (La_N = 68–150, (La/Sm)_N = 3.0–4.0, (La/Yb)_N = 12.0–19.6) and ratios of radiogenic isotopes of Nd- and Hf (ε_Nd = + 0.7 to + 1.6; ε_Hf = + 3.3 to + 10.9), suggesting that their parental magmas were derived from the metasomatized mantle source. The Mo–Cu mineralization was probably related to the high water content, high oxygen and sulfur fugacity of hybrid magma. Formation of the adakitic porphyries and related Mo–Cu deposits of the eastern CAOB could be related to the Early Cretaceous lithospheric extension, caused by the subduction of the Paleo-Pacific plate and its induced reactivation of juvenile arc-type lower crust.     

Paleozoic convergence processes in the southwestern Central Asian Orogenic Belt:Insights from U–Pb dating of detrital zircons from West Junggar,northwestern China

The West Junggar orogen,located in the southwestern Central Asian Orogenic Belt(CAOB),preserves an abundant record of tectonic processes associated with the evolution of the Junggar Ocean.In this study,we use detrital zircon U–Pb age data from Ordovician to Carboniferous sandstones in the southern and central West Junggar domains,complemented by literature data,to better constrain the tectonic evolution of the southwestern CAOB.The Kekeshayi,Qiargaye,and Laba formations in the southern West Junggar domain were deposited during the Darriwilian-Sandbian,Katian-Aeronian,and Homerian-Emsian,respectively.Detrital zircon provenances of these formations display a marked shift from the southern West Junggar domain to the Paleo-Kazakhstan Continent(PKC).This suggests that the southern West Junggar intra-oceanic arc might have gradually accreted to the northern margin of the PKC prior to the Emsian,which has significantly contributed to the lateral growth of the PKC.The Carboniferous strata,Xibeikulasi,Baogutu,and Tailegula formations,in the central West Junggar domain represent a coherent sequence of volcaniclastic turbidites and were deposited in a progressively shrinking remnant oceanic basin during the Visean to Moscovian.They contain unimodal detrital zircon distributions and are derived from the local and coeval magmatic rocks in the central West Junggar domain.We propose that the final closure of the Junggar Ocean likely occurred in the end of the Late Carboniferous in response to regional amalgamation events in the southwestern CAOB,which marks the final assembly of the Kazakhstan Orocline.The central and southern West Junggar domains underwent individual evolution in the Paleozoic,and were recombined by the significant intra-continental reworking along the large-scale strike-slip faults.     

Geological,fluid inclusion and isotopic studies of the Yinshan Cu–Au–Pb–Zn–Ag deposit,South China: Implications for ore genesis and exploration

The Yinshan Cu–Au–Pb–Zn–Ag deposit is located in Dexing, South China. Ore bodies are primarily hosted in low-grade phyllite of the Neoproterozoic Shuangqiaoshan Group along EW- and NNW-striking fault zones. Pb–Zn–Ag mineralization is dictated by Jurassic rhyolitic quartz porphyries (ca. 172 Ma), whereas Cu–Au mineralization is associated with Jurassic dacite porphyries (ca. 170 Ma). The main ore minerals are pyrite, chalcopyrite, galena, sphalerite, tetrahedrite–tennatite, gold, silver, and silver sulphosalt, and the principal gangue minerals are quartz, sericite, calcite, and chlorite. Two-phase liquid-rich (type I), two-phase vapor-rich (type II), and halite-bearing (type III) fluid inclusions can be observed in the hydrothermal quartz-sulfides veins. Type I inclusions are widespread and have homogenization temperatures of 187–303 °C and salinities of 4.2–9.5 wt.% NaCl equivalent in the Pb–Zn–Ag mineralization, and homogenization temperatures of 196–362 °C and salinities of 3.5–9.9 wt.% NaCl equivalent in the Cu–Au mineralization. The pervasive occurrence of type I fluid inclusions with low-moderate temperatures and salinities implies that the mineralizing fluids formed in epithermal environments. The type II and coexisting type III inclusions, from deeper levels below the Cu–Au ore bodies, share similar homogenization temperatures of 317–448 °C and contrasting salinities of 0.2–4.2 and 30.9–36.8 wt.% NaCl equivalent, respectively, which indicates that boiling processes occurred. The sulfur isotopic compositions of sulfides (δ³⁴S = −1.7‰ to +3.2‰) suggest a homogeneous magmatic sulfur source. The lead isotopes of sulfides (²⁰⁶Pb/²⁰⁴Pb = 18.01–18.07; ²⁰⁷Pb/²⁰⁴Pb = 15.55–15.57; and ²⁰⁸Pb/²⁰⁴Pb = 38.03–38.12) are consistent with those of volcanic–subvolcanic rocks (²⁰⁶Pb/²⁰⁴Pb = 18.03–18.10; ²⁰⁷Pb/²⁰⁴Pb = 15.56–15.57; and ²⁰⁸Pb/²⁰⁴Pb = 38.02–38.21), indicating a magmatic origin for lead in the ore. The oxygen and hydrogen isotope compositions (δ¹⁸O = +7.8‰ to +10.5‰, δD = −66‰ to −42‰) of inclusion water in quartz imply that ore-forming fluids were mainly derived from magmatic sources. The local boiling process beneath the epithermal Cu–Au ore-forming system indicates the possibility that porphyry-style ore bodies may exist at even deeper zones.     

Provenance and origins of a Late Paleozoic accretionary complex within the Khangai–Khentei belt in the Central Asian Orogenic Belt,central Mongolia

We have investigated the petrography, geochemistry, and detrital zircon U–Pb LA-ICPMS dating of sandstone from the Gorkhi Formation of the Khangai–Khentei belt in the Ulaanbaatar area, central Mongolia. These data are used to constrain the provenance and source rock composition of the accretionary complex, which is linked to subduction of the Paleo-Asian Ocean within the Central Asian Orogenic Belt during the Middle Devonian to Early Carboniferous. Field and microscopic observations of the modal composition of sandstone and constituent mineral chemistry indicate that the sandstone of the Gorkhi Formation is feldspathic arenite, enriched in saussuritized plagioclase. Geochemical data show that most of the sandstone and shale were derived from a continental margin to continental island arc setting, with plutonic rocks being the source rocks. Detrital zircon ²⁰⁶Pb/²³⁸U ages of two sandstones yields age peaks of 322 ± 3 and 346 ± 3 Ma. The zircon ²⁰⁶Pb/²³⁸U age of a quartz–pumpellyite vein that cuts sandstone has a weighted mean age of 339 ± 3 Ma. Based on these zircon ages, we infer that the depositional age of sandstone within the Gorkhi Formation ranges from 320 to 340 Ma (i.e., Early Carboniferous). The provenance and depositional age of the Gorkhi Formation suggest that the evolution of the accretionary complex was influenced by the intrusion and erosion of plutonic rocks during the Early Carboniferous. We also suggest that spatial and temporal changes in the provenance of the accretionary complex in the Khangai–Khentei belt, which developed aound the southern continental margin of the Siberian Craton in relation to island arc activity, were influenced by northward subduction of the Paleo-Asian Ocean plate.     

Discovery and Genetic Mechanism of Basic Granulite in the Altay Orogenic Belt, Xinjiang, NW China

Altay granulite (AG), which represents the product of high-grade metamorphism in the lower crust, was newly found in the Wuqiagou area, Fuyun County in the Altay orogenic belt, Northwest China. It is composed mainly of hypersthene, augite, basic plagioclase, amphibole and brown biotite. Its mineral compositions of amphibole and biotite are rich in Mg/(Mg+Fe2+) and Ti. Geochemically, the AG is enriched in Mg/(Mg+Fe2+) and A12O3, and poor in CaO, with depletion of U, Th, K and Rb contents. Furthermore, geochemical data reflect that the protolith of the AG is igneous-genetic calc-alkaline basalt formed under an island arc environment. The AG has ZREE of 92.38-96.58 ppm and enriched LREE model with weak positive Eu anomaly of 1.09-1.15. In the MORB normalized spider diagram, the AG shows tri-doming pattern with a strong negative Nb anomaly and medium negative P and Ti anomalies, reflecting that the AG has tectonic relation with subduction or subduction-related materials. The P-T conditions of peak metamorphi     

Nature and significance of the late Mesozoic granitoids in the southern Great Xing’an range,eastern Central Asian Orogenic Belt

ABSTRACT

Abundant late Mesozoic granitic rocks are widespread in the southern Great Xing’an Range (GXAR), which have attracted much attention due to its significance for the Mesozoic tectonic evolution in the eastern Central Asian Orogenic Belt. However, controversy has still surrounded the late Mesozoic geodynamic switching in the continental margin of east China, especially the spatial and temporal extent of the influence of the Mongol-Okhotsk and Palaeo-Pacific tectonic regimes. In order to better understand the Late Mesozoic evolutionary history of the southern GXAR, a number of geochemical, geochronological, and isotopic data of the granitoids in this region are collected. Magmatism in the southern GXAR can be divided into six phases: Late Carboniferous (325–303 Ma), Early-Middle Permian (287–260 Ma), Triassic (252–220 Ma), Early Jurassic (182–176 Ma), Late Jurassic (154–146 Ma), and Early Cretaceous (145–111 Ma). Mesozoic magmatic activities in the southern GXAR peaked during the Late Jurassic to Early Cretaceous, accompanied by large-scale mineralization. Sr–Nd–Hf isotopic evidence of these granitic rocks suggested they were likely originated from a mixed source composed of lower crust and newly underplated basaltic crust. Assimilation-fractional crystallization (AFC) or crustal contamination possibly occurred in the magma evolution, and a much more addition of juvenile component to the source of the Early Cretaceous granitoids than that of Late Jurassic. The closure of Mongol-Okhotsk ocean and the break-off of the Mongol-Okhotsk oceanic slab at depth in the Jurassic triggered extensive magmatism and related mineralization in this region. The Jurassic intrusive activities was affected by both the subduction of the Palaeo-Pacific plate and the closure of Mongol-Okhotsk ocean. Less influence of the Mongol-Okhotsk tectonic regime on the Early Cretaceous magmatism, whereas, in contrast the Palaeo-Pacific tectonic regime possibly continued into the Cenozoic.     

Magnetite geochemistry of the Longqiao and Tieshan Fe–(Cu) deposits in the Middle-Lower Yangtze River Belt: Implications for deposit type and ore genesis

Magnetite is a common mineral in many ore deposits and their host rocks, and contains a wide range of trace elements (e.g., Ti, V, Mg, Cr, Mn, Ca, Al, Ni, Ga, Sn) that can be used for deposit type fingerprinting. In this study, we present new magnetite geochemical data for the Longqiao Fe deposit (Luzong ore district) and Tieshan Fe–(Cu) deposit (Edong ore district), which are important magmatic-hydrothermal deposits in eastern China.Textural features, mineral assemblages and paragenesis of the Longqiao and Tieshan ore samples have suggested the presence of two main mineralization periods (sedimentary and hydrothermal) at Longqiao, among which the hydrothermal period comprises four stages (skarn, magnetite, sulfide and carbonate); whilst the Tieshan Fe–(Cu) deposit comprises four mineralization stages (skarn, magnetite, quartz-sulfide and carbonate).Magnetite from the Longqiao and Tieshan deposits has different geochemistry, and can be clearly discriminated by the Sn vs. Ga, Ni vs. Cr, Ga vs. Al, Ni vs. Al, V vs. Ti, and Al vs. Mg diagrams. Such difference may be applied to distinguish other typical skarn (Tieshan) and multi-origin hydrothermal (Longqiao) deposits in the MLYRB. The fluid–rock interactions, influence of the co-crystallizing minerals and other physicochemical parameters, such as temperature and fO₂, may have altogether controlled the magnetite trace element contents of both deposits. The Tieshan deposit may have had higher degree of fO₂, but lower fluid–rock interactions and ore-forming temperature than the Longqiao deposit. The TiO₂–Al₂O₃–(MgO + MnO) and (Ca + Al + Mn) vs. (Ti + V) magnetite discrimination diagrams show that the Longqiao Fe deposit has both sedimentary and hydrothermal features, whereas the Tieshan Fe–(Cu) deposit is skarn-type and was likely formed via hydrothermal metasomatism, consistent with the ore characteristics observed.     

Late Devonian–early Permian accretionary orogenesis along the North Tianshan in the southern Central Asian Orogenic Belt

The Palaeozoic orogenic process in the North Tianshan of the southern Central Asian Orogenic Belt is controversial. Systematic field study indicates that the ophiolitic fragments of the North Tianshan are mainly thrust slices and blocks of a late Palaeozoic accretionary complex, which was intruded by granitoids. U-Pb zircon dating of plagiogranites from the North Tianshan ophiolite yielded a mean age of 343.1 ± 2.7 Ma. These are typical oceanic plagiogranites but with a supra-subduction zone (SSZ) signature. Ophiolitic basalts display N-MORB, E-MORB, and OIB compositions. One gabbro with an age of 301.9 ± 2.2 Ma shows E-MORB geochemistry mixed with N-MORB and OIB. Some andesites show clear island arc characters indicated by enrichment of LILEs relative to HFSEs. Mean ages of 344.9 ± 4.2 and 298.7 ± 2.4 Ma were obtained for a granite porphyry and a mylonitic granite, respectively. The two granitoids display an island arc geochemical signature evidenced by enrichment of LILEs and depletion of HFSEs. Combined with an eastward migration of Late Devonian to Carboniferous arc magmatism and related Cu-Au-Mo deposits, we propose that trench retreat and slab roll-back took place during subduction of the Junggar Ocean spreading ridge beneath the North Tianshan arc, and that the accretion may have lasted into early Permian time, an important late stage of the long-lived accretionary orogenesis in the southern Central Asian Orogenic Belt.