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Liu J.-J.,China University of Geosciences | Yang L.-B.,McGill University | Dai H.-Z.,China University of Geosciences | Yuan F.,China University of Geosciences | Zhu B.-Y.,National 305 Project Office
Geological Journal | Year: 2014

A group of Cu-Zn intermetallic compounds has been discovered in breccias at the Jingxi gold deposit, NW China. The host rocks, silicified tuff and andesite, are cut by pyrite-bearing quartz veins and are intensely silicified which leads to a high SiO2 content (over 85%). The main component of breccia is siliceous rock, with the cement mainly siliceous and tuffaceous. Under the reflected light microscope, the Cu-Zn intermetallic compounds have quadrilateral shapes which range in size from 6 to 28μm. Scratches and pitting can be seen on the surfaces of them. The reflection colour of these intermetallic compounds is golden yellow, similar to gold but slightly more white, with a reflectivity that is lower than that of Au and is more close to that of pyrite. They are weakly bireflectant, ranging from bright yellow to golden yellow. Under polarized reflected light, they show a weak anisotropism and no internal reflections. The EMPA (WDS) results of the intermetallic compounds reveal that the ranges of concentration (wt%) are Cu: 81.35~85.36 (on average 83.75), Zn: 6.08~10.10 (on average 7.39), Sn: 4.13~6.75 (on average 5.23) and Ni: 0.88~1.38 (on average 1.05), with minor S: b.d.~5.05. Compared with other Zn-Cu intermetallic compounds previously described, these Cu-Zn intermetallic compounds can be classified as α portion phase which has an over 80% Cu content. In the Jingxi gold deposit, the Cu-Zn intermetallic compounds occur as xenomorphic grains and are paragenetically closely linked with the formation of planar silicification. The characteristics of ore type, ore mineral assemblages and ore textural relations demonstrate that these Cu-Zn intermetallic compounds were formed during the early stage of mineralization (I stage) under conditions of both low fO2 and fS2. Generally speaking, the native elements and their intermetallic compounds formed at the high melting temperatures under strongly reducing conditions, with O and S absent. Because of the sudden drop of the confining pressure, we believe that fast upwelling and cooling of magmatic hydrothermal fluid make reactions between S and Cu/Zn difficult when S is either absent or present. As a result, Cu-Zn intermetallic minerals will deposit. © 2014 John Wiley & Sons, Ltd. Source


Yang Z.,Chinese Academy of Geological Sciences | Ma H.,National 305 Project Office | Wang Z.,Chinese Academy of Geological Sciences | Xiao W.,Chinese Academy of Geological Sciences
Acta Petrologica Sinica | Year: 2012

Binggou ophiolite melange, the eastern part of Hongliugou ophiolite, Altyn Tagh, is composed of serpentinites, harzburgites, pyroxenites and gabbros etc. Geochemistry analysis shows that the serpentinites are characterized by high Mg contents, with Mg/Fe ratio greater than 9, and low Al, Ca, Na, K contents. Determined by rare earth elements and trace elements compositions, the gabbro is very similar to MORB, and ultrabasic rocks are close to PRIMA. The SHRIMP U-Pb age of zircon from the gabbro is 449. 5 ± 10. 9 Ma which is belongs to Middle to Late Ordovician. The country rocks of Binggou ophiolite mélange are mainly composed of clastic rocks, pyroclastics, volcanics, and part of carbonates which combined to be a huge thick sequence. These rocks contain basalt which has characteristics of MORB and the pillow structure, and radiolarian cherts which formed in Middle to Late Ordovician that is consistent with the gabbro SHRIMP age. These evidences further convinced that the Early Paleozoic ocean basin has been existed in the Hongliugou area. Source


Han Q.,Xinjiang University | Han Q.,Geological Research Academy of Xinjiang | Gong X.-P.,Xinjiang University | Ma H.-D.,National 305 Project Office | And 6 more authors.
Geology in China | Year: 2015

There exist widely-developed volcanic rocks of Dahalajunshan Formation along the Awulale metallogenic belt in West Tianshan Mountains. They are volcano-sedimentary rocks composed mainly of a set of rhyolite, trachyte, trachyandesite and intermediate-acidic tuff as well as a small amount of basalts. In this paper, the authors used LA-ICPMS zircon U-Pb dating method to determine the rock-forming age of volcanic rocks from Songhu, Shikebutai and Beizhan iron ore deposits. In addition, the authors sorted out the high-precision geochronological data previously obtained from the Awulale metallogenic belt. There exists the gradual change regularity from west to east: Chronologically, the age tends to change from old to young eastward, the Early Carboniferous volcanic rocks tend to pinch eastward, and the lithologic character varies in order of andesite lithology - basaltic andesite - rough endoplasmic lava - pyroclastic rocks. On the basis of data obtained and remote sensing image features, the authors determined the temporal and spatial distribution of the volcanic rocks in Dahalajunshan Formation, extracted the characteristics of the caldera and judged the spatial relationship between typical ore deposits and the caldera, thus investigating the genetic relationship between typical deposits and volcanic apparatuses. In combination with tectonic evolution characteristics, the geological significance is also discussed. Source


Jian P.,Chinese Academy of Geological Sciences | Kroner A.,Chinese Academy of Geological Sciences | Kroner A.,University Mainz | Jahn B.-M.,National Taiwan University | And 4 more authors.
Lithos | Year: 2013

We dated and geochemically characterized peridotite-bearing mélanges in the Chinese South Tianshan and within the Main Tianshan Shear Zone. The Yushugou-Tonghuashan mélange in the Chinese South Tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite (predominantly lherzolite) massif with a high-grade metamorphic terrane (ca. 10km long; protolith age≥ca. 445-466Ma). Metamorphic zircons of a mafic granulite (εNd(t)=5.0) yielded a weighted mean 206Pb/238U age of 341±8Ma that we interpret as the time of granulite-facies metamorphism. The youngest zircon rims of an intermediate granulite (εNd(t)=-4.3) have a mean age of 332±13Ma that records a retrogressive metamorphic event. These ages determine the timing (ca. 341-332Ma) of mantle diapirism through continental crust. A dolerite dike (εNd(t)=2.3) emplaced into metaperidote has a crystallization age of 335±5Ma, that, on the basis of geochemistry, we interpret as representing E-MORB-OIB magmatism that accompanied mantle diapirism. An undeformed pink granite (εNd(t)=-3.6) intrudes the mélange matrix and has an emplacement age of 324±5Ma, thus providing an upper time limit for a tectonic movement that led to mélange formation. The older time limit of deformation (ca. 362-352Ma) is constrained by the youngest ages of thermo-tectonically modified zircons in a mylonitized metagabbro and a foliated meta-andesite. Magmatic zircons in the meta-andesite (εNd(t)=-3.3) and a dacite (εNd(t)=-5.6) have eruption ages of 433±4Ma and 435±3Ma that date formation of the mélange matrix. A microgabbro dike (εNd(t)=10.0) cutting metasediments (i.e. mélange matrix) has an emplacement age of 279±3Ma and contains abundant zircon xenocrysts ranging in age from Archean to late Paleozoic. Similarly, a dolerite dike, a meta-andesite, an intermediate granulite, and a metagabbro all contain inherited Precambrian to Paleozoic zircons. We accordingly conclude that the mélange was formed by mantle diapirism in the early Carboniferous and was subsequently affected by an early Permian volumetrically minor mafic episode. Two foliated garnet granites (εNd(t)=-7.0 and -6.9), in direct contact with metaperidotite within the Main Tianshan Shear Zone have emplacement ages of 341±2Ma and 329±4Ma that reflect an early Carboniferous episode of crustal anatexis. They also contain Neoproterozoic zircon xenocrysts. Also within the Main Tianshan Shear Zone, a gabbro (εNd(t)=8.7) has an emplacement age of 297±3Ma and contains abundant Neoproterozoic to late Paleozoic zircon xenocrysts. Based on the regional geological relationships, we associate early Carboniferous mantle diapirism and crustal anatexis with lower crustal delamination-induced asthenospheric upwelling. The numerous Precambrian zircon xenocrysts support a Chinese Tianshan-Tarim connection. The early Permian mafic episode coincided in time with the development of an early Permian unconformity in the Chinese North Tianshan. © 2013 Elsevier B.V. Source


Shi Y.,Chinese Academy of Geological Sciences | Jian P.,Chinese Academy of Geological Sciences | Kroner A.,Chinese Academy of Geological Sciences | Kroner A.,University Mainz | And 4 more authors.
International Geology Review | Year: 2014

We present new zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the crustal evolution of the Chinese Central Tianshan (Xinjiang, northwest China) in the early to mid-Palaeozoic. The Early Ordovician (ca. 475-473 Ma) granitoid rocks have zircon εHf(t) values either positive (+0.3 to +9.5) or negative (-6.0 to -12.9). This suggests significant addition of juvenile material to, and coeval crustal reworking of, the pre-existing continental crust that is fingerprinted by numerous Precambrian zircon xenocrysts. The Late Ordovician-Silurian (ca. 458-425 Ma) rocks can be assigned to two sub-episodes of magmatism: zircon from rocks of an earlier event (ca. 458-442 Ma) has negative zircon εHf(t) values (-6.3 to -13.1), indicating a predominantly crustal source; zircon from later events (ca. 434-425 Ma) has positive zircon εHf(t) values (+2.6 to +8.9) that reveal a predominantly juvenile magma source. The Early Devonian (ca. 410-404 Ma) rocks have near-zero zircon εHf(t) values, either slightly negative or positive (-1.4 to +3.5), whereas the Mid-Devonian rocks (ca. 393 Ma) have negative values (-11.2 to -14.8). The Late Devonian (ca. 368-361 Ma) granites are undeformed and are chemically similar to adakite but have relatively low negative whole-rock εNd(t)values (-2.4 to -5.3). We interpret the Early Ordovician to Mid-Devonian magmatic event to reflect combined juvenile crustal growth and crustal reworking processes via episodic mafic underplating and mantle-crust interaction. The Late Devonian episode may signify delamination of the over-thickened Chinese Central Tianshan crust. © 2014 © 2014 Taylor & Francis. Source

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