Kezhang Qin

Paleozoic multiple subduction-accretion processes of the southern Altaids

The formation and development of the southern Altaids is controversial with regard to its accretionary orogenesis and continental growth. The Altay-East Junggar orogenic collage of North Xinjiang, China, offers a special natural laboratory to resolve this puzzle. Three tectonic units were juxtaposed, roughly from North to South, in the study area. The northern part (Chinese Altay), composed of variably deformed and metamorphosed Paleozoic sedimentary, volcanic, and granitic rocks, is interpreted as a Japan-type island arc of Paleozoic to Carboniferous-Permian age. The central part (Erqis), which consists of ophiolitic mélanges and coherent assemblages, is a Paleozoic accretionary complex. The southern part (East Junggar), characterized by imbricated ophiolitic mélanges, Nb-enriched basalts, adakitic rocks and volcanic rocks, is regarded as a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites, superimposed by Permian arc volcanism. A plagiogranite from an imbricated ophiolitic mélange (Armantai) in the East Junggar yields a new SHRIMP zircon age of 503 ± 7 Ma. Using published age constraints, we propose the presence of multiple subduction systems in this part of the Paloasian Ocean in the Paleozoic. The intraoceanic arcs became accreted to the southern active margin of the Siberian craton in the middle Carboniferous-Permian. During the long accretionary processes, in addition to large-scale southward-directed thrusting, large-scale, orogen-parallel, strike-slip movements (for example, Erqis fault) in the Permian translated fragments of these intraoceanic arcs and associated accretionary wedges. This new tectonic model has broad implications for the architecture and crustal growth of Central Asia and for other ancient orogens.

SIMS zircon U-Pb geochronology and Sr-Nd isotopes of Ni-Cu-Bearing Mafic-Ultramafic Intrusions in Eastern Tianshan and Beishan in correlation with flood basalts in Tarim Basin (NW China): Constraints on a ca. 280 Ma mantle PLUME

Zircon SIMS U-Pb dating of the Poshi, Hongshishan, Bijiashan, and Huangshan Ni-Cu-bearing and Xiangshan Ni-Cu-Ti-Fe-bearing mafic-ultramafic intrusions in the Eastern Tianshan and Beishan Rift yields a relatively restricted range of 278.6 Ma to 284.0 Ma. The histogram of compiled age data of basalts in the Tarim Basin and mafic-ultramafic intrusions in the Eastern Tianshan and Beishan Rift has a peak of 280 Ma, which probably represents the time of mantle plume activity. The basalts have lower εNd(t) values in the range of −9.2 ∼ −1.7 and Mg# of <50, and higher TiO2 contents (>2 wt.%), indicating that they were generated directly from a peripheral zone of the mantle plume by low degree of melting. The mafic-ultramafic intrusions have higher εNd(t) of −1.3 ∼ 11.2 and Mg# of 33 ∼ 90, and lower TiO2 < 1.8 weight percent, suggesting that their parental magmas were produced from lithospheric mantle source by high degree of melting due to higher temperature of the mantle plume head. A possible mantle plume model beneath lithospheric mantle of the Tarim Basin, Tianshan and Beishan and its spatial framework is suggested.

Types, characteristics, and time–space distribution of molybdenum deposits in China

Molybdenum exploration activity in China has accelerated tremendously during the past decade owing to the continuous, increasing demand for Earth resources. China possesses the largest Mo reserves in the world (exceeding 19.6 Mt). The major ore deposits are of porphyry, porphyry–skarn, skarn, vein, and sedimentary types. Porphyry molybdenum deposits contain 77.5% of the Chinese Mo reserves, with lesser amounts in porphyry–skarns (13%), skarns (5.1%), and veins (4.4%). Exploitation of sedimentary-type molybdenum deposits thus far has been uneconomical. The six Mo provinces are in the Northeast China, Yanliao, Qinling–Dabie, middle–lower Yangtze River Valley, South China, and Sanjiang areas. We recognize six ore-forming periods: (1) Precambrian (>541 Ma), (2) Palaeozoic (541–250 Ma), (3) Triassic (250–200 Ma), (4) Jurassic–Early Cretaceous (190–135 Ma), (5) Cretaceous (135–90 Ma), and (6) Cenozoic (55–12 Ma). The abundance of Mo ore deposits in China reflects the occurrence of multiple periods of tectonism, involving interactions between the Siberian, North China, Yangtze, India, and Palaeo-Pacific plates. Precambrian molybdenum deposits are related to Mesoproterozoic volcanism in an extensional setting. Palaeozoic Cu–Mo deposits are related to calc-alkaline granitic plutons in an island arc or a continental margin setting. Triassic Mo deposits formed in the syn-collision–postcollision tectonic setting between the Siberian and North China plates and between the North China and Yangzi plates. Jurassic–Early Cretaceous molybdenum deposits formed along the eastern margin of Asia and are associated with the palaeo-Pacific plate-subduction tectonic setting. Cretaceous Mo deposits are related to high-K calc-alkaline granitic rocks and formed in a lithospheric thinning setting. Cenozoic molybdenum deposits formed in a collision setting between the Indian and Eurasian continents and the subsequent extensional setting.

Occurrence of an Alaskan-type complex in the Middle Tianshan Massif, Central Asian Orogenic Belt: inferences from petrological and mineralogical studies

The Xiadong mafic–ultramafic complex lies in the central part of the Middle Tianshan Massif (MTM), along the southern margin of the Central Asian Orogenic Belt (CAOB). This complex is composed of dunite, hornblende (Hbl) clinopyroxenite, hornblendite, and Hbl gabbro. These rocks are characterized by adcumulated textures and variable alteration. Orthopyroxene is an extremely rare mineral in all rock units and plagioclase is absent in dunite and Hbl clinopyroxenite. Hbl, Fe-chromite, and Cr-magnetite are common phases. Olivines have forsterite (Fo) contents ranging from 92.3 to 96.6. Clinopyroxenes are Ca-rich, Ti-poor diopsides, and mostly altered to tremolites or actinolites. Chromites display low TiO2 and Al2O3 contents and high Cr# and Fe2+/(Fe2+ + Mg) values. Primary and secondary Hbls show wide compositional variations. These petrological and mineralogical features as well as mineral chemistry are comparable to typical Alaskan-type complexes worldwide, which are widely considered to have formed above subduction zones. The chemistry of clinopyroxene and chromite supports an arc plate-tectonic origin for the Xiadong complex. Its confirmation as an Alaskan-type complex implies that the MTM, with Precambrian basement, was probably a continental arc during oceanic plate underflow and further supports the hypothesis of southward subduction of the Palaeozoic Junggar Ocean.

Geochronologic-petrochemical studies of the Hongshishan mafic-ultramafic intrusion, Beishan area, Xinjiang (NW China): Petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting. Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ϵNd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.

Scheelite elemental and isotopic signatures: Implications for the genesis of skarn-type W-Mo deposits in the Chizhou Area, Anhui Province, Eastern China

Abstract Scheelite is well developed in hydrothermal deposits, providing a window into genetic processes and facilitating comparative studies, however, few studies have focused on characterizing scheelite in skarn-type W-Mo deposits. The primary ore mineral in the Jitoushan and Baizhangyan skarn-type W-Mo deposits (Anhui Province, Eastern China), scheelite was analyzed for major, trace, and rare earth element (REE) abundance and for Sr-Nd isotopes. The analysis revealed two unique geochemical characteristics that distinguish the scheelite from skarn-type W-Mo deposits to that from vein-type Au-W and porphyry-type W-Mo deposits: higher Mo content with a negative correlation between MoO3 and WO3 and a strong HREE depletion. Skarn-type scheelite mainly inherited REE signatures from ore-forming fluids, and the early precipitation of skarn minerals (e.g., garnet, diopside, and amphibole) has most likely resulted in the observed strong HREE depletion in scheelite and the decoupling of LREEs and HREEs. Of the numerous substitution mechanisms suggested by previous workers, 3Ca2+ = 2REE3+ + □Ca (where □Ca is a Ca-site vacancy) is preferred for the substitution of REE3+ for Ca2+ and in this study, particularly given the low salinity of ore fluids. As the scheelite Eu anomalies were inherited from ore-forming fluids with variable redox conditions and pH, the complex δEu/Mo correlation indicates that Mo increasingly entered the scheelite under oxidizing conditions and reached a maxim at δEu values of 0.8 to 1. In contrast, under reducing conditions, Mo contents in scheelite decrease gradually and Mo is precipitated as molybdenite as a result of the change in dominant valence state. Unlike the Sr-Nd isotope compositions of scheelite from vein-type Au-(W) and W-(Sb-Au) deposits, the scheelite from skarn-type W-Mo deposits has low (143Nd/144Nd)(t) (most <0.5125) and intermediate (87Sr/86Sr)(t) values (most between 0.708 and 0.715). The εNd(t) values of the scheelite varied from -16 to -12.3 in the Baizhangyan deposit and from -9.5 to -9.1 in the Jitoushan deposit, indicating that the ore-forming materials in the two W-Mo deposits were mainly derived from crustal sources.

On the Possibility of Porphyry Copper Mineralization in Japan

Why no porphyry copper deposit has been discovered thus far in the islands of Japan remains a perplexing question. A new attempt is made to address this question in this paper, with eight prerequisites for the formation of porphyry copper deposits being presented. The authors then review certain unfavorable conditions for formation of porphyry coppers, discussing especially the spacetime relationship between Kuroko-type and porphyry copper-type deposits, and suggest that there should be some inherent linkage between them. The possible existence of porphyry copper in the islands of Japan cannot be ruled out. Indeed, in some uplifted areas in the subaerial volcanic belt, porphyry coppers probably were formed and will be found in the deeper parts of the alteration zone of some high-sulfidation-type epithermal Au deposits or vein Pb-Zn-Cu deposits, or even at the periphery of some skarn Pb-Zn-Cu deposits.

Geochronologic and isotope geochemical constraints on magmatism and associated W-Mo mineralization of the Jitoushan W-Mo deposit, middle-lower Yangtze Valley

The Jitoushan W–Mo ore body is a typical skarn-type deposit with the potential for porphyry Mo mineralization at depth. As it is newly discovered, only a few studies have been conducted on the geochronology and ore genesis of this deposit. The ore district consists of Cambrian to Silurian sedimentary and low-grade metasedimentary strata, intruded by granodiorite, diorite porphyry, granite porphyry, and quartz porphyry. Skarn W–Mo ore bodies are hosted in the contact zone between the granodiorite and Cambrian limestone strata. Within the granodiorite near the contact zone, quartz vein type and disseminated sulphide mineralization are well developed. The Mo-bearing granite porphyry has been traced at depth by drilling. Our results reveal two discrete magmatic events at ca. 138 and ca. 127 Ma in the study area. The molybdenite Re–Os isochronal age of 136.6 ± 1.5 million years is consistent with the first magmatic event. The zircon Hf isotope (ϵHf(t) = −12.55−3.91), sulphide isotopes (δ34S = 3.32–5.59‰), and Re content of molybdenite (Recontent = 6.424–19.07 μg) indicate that the ore-forming materials were mainly derived from the deep crust. The regional tectonic system switched from a Late Jurassic transpressive regime to an earliest Cretaceous extensional regime at ca. 145 Ma, and at ca. 138 Ma, the Jitoushan W–Mo deposit formed in an extensional setting.

Assessing the magmatic affinity and petrogenesis of granitoids at the giant Aktogai porphyry Cu deposit, Central Kazakhstan

Most mineralized porphyries associated with large to giant oxidized porphyry Cu deposits show an affinity with high Sr/Y rocks, while barren or weakly mineralized granitoids show typical low Sr/Y features. The Aktogai giant porphyry Cu deposit occurs in the Koldar pluton and provides a good natural laboratory in which to investigate this relationship, while determining the petrogenesis of the pluton and its mineralization. Zircon U-Pb dating, mineral chemistry, whole rock geochemistry and Sr-Nd-Pb and zircon Hf-O isotopic analyses were carried out on the pre-ore granodiorite (the major component of the Koldar pluton) and on the mineralized granodiorite porphyry. Zircon U-Pb ages indicate that the pre-ore granodiorite and mineralized granodiorite porphyries were emplaced at 345 and 328 to 331 Ma, respectively. Distinctly higher apatite SO3 contents in the granodiorite porphyry relative to the granodiorite suggest an increase in fO2 during the petrogenesis of the mineralized porphyries (>NNO+1). Although all rocks share similar geochemical characteristics (calc-alkaline, strong depletion in Nb, Ta and Ti, and enrichment in LREE and LILE), the pre-ore Koldar pluton has normal arc related magmatic features [low Sr/Y and (La/Yb)N, high Y and YbN], while the granodiorite porphyries and diorite (trace component of Koldar pluton) exhibit high Sr/Y and (La/Yb)N, low Y and YbN features. All samples show similar Sr-Nd-Pb-Hf-O isotopic compositions [(87Sr/86Sr)i = 0.70369 to 0.70413, εNd (t) = + 3.6 to + 5.6, (206Pb/204Pb)i = 18.16 to 19.32, zircon εHf (t) = + 11.8 to + 15.9, and δ18O = + 3.8 to + 5.9 ‰], and very young whole rock T2DM (Nd) (640 – 680 Ma) and zircon TDMC (Hf) (320 – 590 Ma) values, suggesting that they were probably derived from partial melting of juvenile lower crust. Geochemical patterns and partial melt modeling indicate that the high Sr/Y rocks were probably formed by partial melting of eclogitized, thickened lower crust, while the Koldar pluton formed by partial melting of normal thick lower crust. We propose that pre-ore low Sr/Y rocks were probably generated earlier via subduction of Junggar-Balkhash oceanic crust, and that the high Sr/Y rocks were formed later by partial melting of sulfide-enriched, thickened juvenile lower crust. High oxygen fugacity and the high melting temperature of the high Sr/Y rocks ensured that all sulfide was dissolved in the magma, which intruded the previously emplaced low Sr/Y pluton and resulted in significant mineralization.

Olivine Compositional Mapping of Mafic-Ultramafic Complexes in Eastern Xinjiang (NW China): Implications for Cu-Ni Mineralization and Tectonic Dynamics

Early Permian mafic-ultramafic complexes in eastern Xinjiang (新疆) are mainly distributed in the Beishan (北山) area, Mid-Tianshan (天山) massif and Jueluotage (觉罗塔格) belt. Systematic compositional mapping of olivines from these Early Permian mafic-ultramafic complexes demonstrates that an apparently spatial distribution and heterogeneous partial melting in the mantle source exists from the Beishan area, across the Mid-Tianshan massif, to the Jueluotage belt from the south to the north. This is probably consistent with the spatial evolutional differences and tectonic features of these three belts. The decreasing degree of partial melting, as revealed by decreasing Fo contents of olivines, from south to north and from east to west reflects the southward subduction of the Paleo-Asian Ocean and the south location of the indistinct mantle plume in the Permian. Simultaneously, NiO and Fo-mapping in olivine also indicates that sulfide segregation before olivine crystallization played an important role in Ni-Cu mineralization in the mafic-ultramafic complexes. Olivines with the compositional range of Fo (77–86) and NiO (less than 0.22 wt.%) are more favorable for Ni-Cu sulfide mineralization.