Hong-Rui Fan

Mineral Inclusions in Zircon from Gneisses in the Ultrahigh‐Pressure Zone of the Dabie Mountains, China

Mineral inclusions in zircon from gneisses in the ultrahigh‐pressure (UHP) zone of the Dabie Mountains, China, are studied by using laser Raman spectrophotometry. We identified numerous mineral inclusions including coesite, quartz, clinopyroxene, phengite, barite, anhydrite, and feldspar. Coesite is found to occur as zircon‐hosted inclusions in all types of gneisses, indicating that these gneisses had shared the ultrahigh‐pressure metamorphic history with the UHP eclogite blocks that reside in them. The occurrence of barite and anhydrite probably implies the existence of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

A Large-Scale Cluster of Gold Deposits and Metallogenesis in the Eastern North China Craton

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Dual origins of Xiaoqinling gold-bearing quartz veins: Fluid inclusion evidence

\end{document} ‐bearing fluids during the UHP metamorphic stage. We also found no simple ways to correlate growth zones of zircon as shown by cathodoluminescence images with metamorphic conditions determined from inclusion in any specific zircon grains. Of particular interest is the discovery of coesite included in zircons from gneiss of the “cold” eclogite zone that was previously considered to have not experienced UHP conditions.

REE Daughter Minerals Trapped in Fluid Inclusions in the Giant Bayan Obo REE-Nb-Fe Deposit, Inner Mongolia, 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.

Geochemistry and zircon U–Pb chronology of charnockites in the Yinshan Block, North China Craton: tectonic evolution involving Neoarchaean ridge subduction

The eastern North China craton contains the largest association of gold deposits in China. Hundreds of gold deposits of different scales, commonly in groups or belts, constitute seven subclusters. In contrast, there are no industrial gold deposits in the neighboring areas of the UHP Sulu region and the northern Yangtze craton. The host rocks of the gold deposits are chiefly Precambrian high-grade metamorphic rocks and granites (anatectic melts derived from Precambrian metamorphic rocks). Traditionally the gold deposits were considered to be of the Archean greenstone type. However, recent geochronology of granitoid bodies and veins related to gold mineralization and gold-bearing minerals in ore lodes reveals that the main metallogenic episode was 110-130 Ma in all seven subclusters. Geochemical data also indicate that the metallogenic materials mainly came from the Precambrian basement of the North China craton and its underlying mantle. Sr-Nb-Pb and S-O-H isotopic data of the ore lodes show crust-mantle mixing characteristics, corresponding to the metamorphic host rocks and basic-alkaline dikes. The main structures controlling the mineralization are Mesozoic faults that trend roughly NE-ENE and NW. Therefore, Precambrian rocks and Mesozoic magma-tectonic events provide fundamental controls on the mineralization. Problematic relationships between the Mesozoic tectonic events (Yanshanian movements) and large-scale metallogenic activity have attracted substantial attention from Chinese and foreign geologists. Mesozoic tectonic inversion in eastern China mainly involved a thick lithosphere that thinned at depth and a reconstructed basin-and-range structure at the surface, which resulted from a catastrophic lithosphere-asthenosphere event. This tectonic event led to a large-scale mantle upwelling, which induced disturbance and adjustment of thermal, density, and chemical phenomena, resulting in large-scale crustal remelting (especially the lower crust), material exchange and mixing between mantle and crust, movement and circulation of fluids, and finally a new magma-fluid-mineralization system.

Coesite inclusions in zircon from gneisses identified by laser Raman microspectrometer in ultrahigh pressure zone of Dabie Mountains, China

The North China craton (NCC) hosts numerous gold deposits and is known as the most gold-productive region of China. The gold deposits were mostly formed within a few million years in the Early Cretaceous (130-120 Ma), coeval with widespread occurrences of bimodal magmatism, rift basins and metamorphic core complexes that marked the peak of lithospheric thinning and destruction of the NCC. Stable isotope data and geological evidence indicate that ore-forming fluids and other components were largely exsolved from cooling magma and/or derived from mantle degassing during the period of lithospheric extension. Gold mineralization in the NCC contrasts strikingly with that of other cratons where gold ore-forming fluids were sourced mostly from metamorphic devolatization in compressional or transpressional regimes. In this paper, we present a summary and discussion on time-space distribution and ore genesis of gold deposits in the NCC in the context of the timing, spatial variation, and decratonic processes. Compared with orogenic gold deposits in other cratonic blocks, the Early Cretaceous gold deposits in the NCC are quite distinct in that they were deposited from magma-derived fluids under extensional settings and associated closely with destruction of cratonic lithosphere. We argue that Early Cretaceous gold deposits in the NCC cannot be classified as orogenic gold deposits as previously suggested, rather, they are a new type of gold deposits, termed as “decratonic gold deposits” in this study. The westward subduction of the paleo-West Pacific plate (the Izanagi plate) beneath the eastern China continent gave rise to an optimal tectonic setting for large-scale gold mineralization in the Early Cretaceous. Dehydration of the subducted and stagnant slab in the mantle transition zone led to continuous hydration and considerable metasomatism of the mantle wedge beneath the NCC. As a consequence, the refractory mantle became oxidized and highly enriched in large ion lithophile elements and chalcophile elements (e.g., Cu, Au, Ag and Te). Partial melting of such a mantle would have produced voluminous hydrous, Au- and S-bearing basaltic magma, which, together with crust-derived melts induced by underplating of basaltic magma, served as an important source for ore-forming fluids. It is suggested that the Eocene Carlin-type gold deposits in Nevada, occurring geologically in the deformed western margin of the North America craton, are comparable with the Early Cretaceous gold deposits of the NCC because they share similar tectonic settings and auriferous fluids. The NCC gold deposits are characterized by gold-bearing quartz veins in the Archean amphibolite facies rocks, whereas the Nevada gold deposits are featured by fine-grained sulfide dissemination in Paleozoic marine sedimentary rocks. Their main differences in gold mineralization are the different host rocks, ore-controlling structures, and ore-forming depth. The similar tectonic setting and ore-forming fluid source, however, indicate that the Carlin-type gold deposits in Nevada are actually analogous to decratonic gold deposits in the NCC. Gold deposits in both the NCC and Nevada were formed in a relatively short time interval (<10 Myr) and become progressively younger toward the subduction zone. Younging of gold mineralization toward subduction zone might have been attributed to retreat of subduction zone and rollback of subducted slab. According to the ages of gold deposits on inland and marginal zones, the retreat rates of the Izanagi plate in the western Pacific in the Early Cretaceous and the Farallon plate of the eastern Pacific in the Eocene are estimated at 8.8 cm/yr and 3.3 cm/yr, respectively.

Oxygen isotope compositions of eclogites in Rongcheng, Eastern China

Fluid inclusion research in Archean metamorphic rocks, Yanshanian granite and gold-bearing quartz veins shows that regional metamorphic fluids are high temperature and high saline, Yanshanian post-granitic fluids are CO2-bearing low saline, and ore-forming fluids are also CO2-rich low saline. In gold-barren/free parts of quartz veins in gold deposits, daughter mineral-bearing high saline inclusions retated with metamorphic fluids remained. This proclaims quartz veins hosting gold ores might have been formed in early metamorphism, and overlapped or mineralized by late Yanshanian gold-bearing fluids.

Ore-forming fluids in the dongping gold deposit, northwestern Hebei Province

The Bayan Obo REE-Nb-Fe deposit hosts the worlds largest known REE resource. The deposit consists of replacement bodies hosted in dolomite marble and of magnetite, REE fluorocarbonates, fluorite aegirine, amphibole, calcite, and barite. Three types of fluid inclusions have been recognized: two-phase aqueous liquid-vapor (L-V), two- to three-phase CO2 (C), and three-phase liquid-vapor-solid (L-V-S) inclusions. Microthermometry measurements indicate that the carbonic phase in C inclusions is nearly pure CO2. During heating experiments, hexagonal or irregular-shaped daughter minerals in L-V-S inclusions complete dissolution at temperatures of 420-480°C and recrystallize again at about 400-320°C. These show that daughter minerals in multiphase inclusions in mineralizing veins were crystallized from trapped fluids, and are real daughter minerals. REE-carbonates, halite, sylvite, barite, calcite, and pyroxene (?) have been identified on the basis of crystal habit (microscopic and SEM) and EDX analysis. By comparison with Raman spectra of reference REE-carbonate mineral crystals, hexagonal or irregular-shaped daughter minerals in L-V-S inclusions might be cebaite and bastnaesite. The presence of REE-carbonates as an abundant solid in the ore-forming veins shows that the original oreforming fluids were very rich in REE, and therefore had the potential to produce economic REE ores at Bayan Obo.