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  • 40Ar/39Ar dating, fluid inclusions and S–Pb isotope systematics of the Shabaosi gold deposit, Heilongjiang Province, China

    JUN LIU1, GUANG WU1*, HUANING QIU2 and YUAN LI3 1MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of

    Geological Sciences, Beijing, China 2State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,

    Guangzhou, China 3Department of Earth Science, Rice University, Houston, TX, USA

    The Shabaosi deposit is the only large lode gold deposit in the northern Great Xing’an Range. The gold ore bodies are hosted by sandstone and siltstone of the Middle Jurassic Ershi’erzhan Formation, and are controlled by three N–S-trending altered fracture zones. The gold ore bodies are composed of auriferous quartz veinlets and altered rocks. Fluid inclusion studies indicate that the ore-forming fluids belong to a H2O–NaCl–CO2–CH4 system, with salinities between 0.83 and 8.28wt% NaCl eq., and homogenization temperatures ranging from 180 to 320 °C. The δ34S values of sulphides show a large variation from �16.9‰ to 8.5‰. The Pb isotope compositions of sulphides are character- ized by a narrow range of ratios: 18.289 to 18.517 for 206Pb/204Pb, 15.548 to 15.625 for 207Pb/204Pb, and 38.149 to 38.509 for 208Pb/204Pb. The μ values range from 9.36 to 9.51. These results suggest that the ore-forming fluids/materials were mainly of magmatic hydrothermal origin, derived from magmas produced by partial melting of the lower crust. The 40Ar/39Ar age of auriferous quartz veinlets from the Shabaosi gold deposit is about 130Ma. The Shabaosi gold deposit has counterparts in similar orogenic gold deposits, and was formed during the post-collisional setting of the Mongolia–Okhotsk Orogen. Copyright © 2014 John Wiley & Sons, Ltd.

    Received 19 February 2014; accepted 3 May 2014

    KEY WORDS 40Ar/39Ar dating; S and Pb isotopes; fluid inclusions; Shabaosi gold deposit; Orogenic gold deposit; Mongolia–Okhotsk Orogen; NE China

    1. INTRODUCTION

    Orogenic lode gold deposits occur in metamorphic belts throughout the world and have accounted for over a quarter of the total historic global production of gold (Groves et al., 1998; Goldfarb et al., 2005). A considerable proportion of the world’s placer deposits also originated from erosion of primary orogenic lode gold deposits (Groves et al., 1998). The northern Great Xing’an Range is a famous accumula- tion area of placer gold deposits in NE China (Lv et al., 1992), but lode gold deposits are rarely discovered, which has long confused geologists. The geology and mineraliza- tion of the northern Great Xing’an Range are poorly under- stood because of the dense forest coverage. The Shabaosi gold deposit, the only large lode gold deposit in the northern Great Xing’an Range, is located 45 km northwest of Mohe County, Heilongjiang Province in NE China. The geological

    characteristics of the Shabaosi gold deposit are different from those of gold deposits hosted in the Mesozoic conti- nental volcanic rocks located in the northern Great Xing’an Range, such as Siwumuchang, Mo’erdaoga and Aolaqi epithermal gold deposits (Zhao and Wu, 2002; Wang et al., 2008), but are similar to those deposits located in the Mongolia–Okhotsk Orogen, such as the Darason, Khali, Kliuchevskoi and Kirov gold deposits (Duan et al., 1990; Shen, 1998; Zorin et al., 2001; Wu et al., 2008a). The Shabaosi lode gold deposit was initially classified as

    a medium–low temperature hydrothermal-type gold deposit (Jia et al., 2004; Wang et al., 2005) or altered sandstone type of gold deposit (Qi et al., 2000; Zhao et al., 2000). Preliminary studies on the geology, alteration, stable isotopes and fluid inclusions have previously been performed for the Shabaosi deposit (Quan et al., 1998; Qi et al., 2000; Zhao et al., 2000; Jia et al., 2004; Wang et al., 2005; Wu, 2006; Wu et al., 2008b), however systematic isotope studies are still rare. In this study, we report new results for the Shabaosi gold deposit, based on studies of fluid inclusions in quartz, fluid inclusion 40Ar/39Ar dating, and S–Pb isotopes of sulphides. These new

    *Correspondence to: G. Wu, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Baiwanzhuang Street 26, Xicheng District, Beijing, 100037, China. E-mail: wuguang65@163.com

    Copyright © 2014 John Wiley & Sons, Ltd.

    GEOLOGICAL JOURNAL Geol. J. 50: 592–606 (2015) Published online 6 June 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/gj.2577

  • results allow us to characterize the ore genesis of the Shabaosi gold deposit and the associated metallogenic setting, which in turn may provide important insights into further exploration in the northern Great Xing’an Range.

    2. REGIONAL GEOLOGY

    The Shabaosi lode gold deposit is located in the Upper Heilongjiang Basin of the northern Great Xing’an Range, which itself is attached to the southeast of the Mongolia– Okhotsk Orogen (Fig. 1). The Mongolia–Okhotsk Ocean existed in eastern Asia during the Middle–Late Palaeozoic (Zorin et al., 1998; Zorin, 1999). The closure of the Mongolia–Okhotsk Ocean led to the formation of the bordering Mongolia–Okhotsk Orogen during the Late Palaeozoic to the Middle–Late Mesozoic (Zonenshain et al., 1990; Parfenov et al., 2001). The Mongolia–Okhotsk Orogen extends approximately 3000 km from the present- day Sea of Okhotsk southwestwards to central Mongolia, i.e. between the Siberian and Mongolian continents (Fig. 1A). The Upper Heilongjiang Basin lies in a nearly E–W direction along the China–Russia border (Fig. 1B). The southern and eastern margins of the basin are controlled by the Xijilin–Tahe Fault and the Derbugan Fault, respec- tively (Fig. 2). The basement of the Upper Heilongjiang Basin consists of Palaeo-Proterozoic Xinghuadukou Group, Early Cambrian Ergunahe Formation and Early Palaeozoic granites (Wu et al., 2005, 2012). The Xinghuadukou Group is composed of metamorphic rocks, whereas the Ergunahe Formation is dominantly composed of marble and slate rocks. The basement is partly covered by the Lower–Middle Jurassic sedimentary rocks. Voluminous Late Jurassic–Early Cretaceous volcanic and clastic rocks are exposed in the Upper Heilongjiang Basin. The Mesozoic clastic rocks are partly covered by the Devonian crystalline limestone and marl as a klippen. In the Upper Heilongjiang Basin, intru- sions include the Early Palaeozoic monzogranite and quartz diorite (zircon SHRIMP U–Pb age 504–517Ma by Wu et al., 2005) and the Mesozoic alkali feldspar granite and quartz diorite (zircon SHRIMP U–Pb age of 130Ma by Wu et al., 2009), but they have a small exposure area. The Upper Heilongjiang Basin is characterized by the ENE-trending Mohe thrust–nappe belt, which consists of a series of brittle–ductile shearing belts in the Mesozoic clastic rocks.

    3. ORE GEOLOGY

    The gold ore bodies are hosted by sandstone and siltstone of the Middle Jurassic Ershi’erzhan Formation (Fig. 3A, B; Fig. 4A) and are dominantly controlled by three N–S-trending altered fracture zones but without distinct boundaries with the wall

    rocks. Three gold mineralized altered fracture zones, i.e. No. I, II and III, were delineated in the Shabaosi deposit. The al- tered fracture zone I consists of ore body I–1 and I–2. The ore body I–1 is 138m in length and has an average thickness of 11m and an average Au grade of 4.1 × 10�6. The ore body I–2 is up to 75m in length and 5.7–16.8m wide with an aver- age Au grade of 4.0 × 10�6. The altered fracture zone II in- cludes one ore body, which is 263m in length and 3–28m in thickness and has an average Au grade of 4.1 × 10�6. The al- tered fracture zone III includes ore body III-1 and III-2, which are 170–560m in length and 2 to 5m in thickness and have av- erage Au grades of 3.9–5.1 × 10�6.

    The ore minerals consist mainly of pyrite, with minor arsenopyrite, stibnite, sphalerite, chalcopyrite and galena (Fig. 4B, C, D and E). These ore minerals account for less than 2% of the ore volume. Gangue minerals include quartz, sericite, calcite, feldspar, chlorite, graphite and some clay minerals (Fig. 4E, F). The ore minerals are distributed as sparse disseminations, veinlets, or stockwork veins. In contrast, the gangue minerals show a massive, brecciated, or vuggy structure. Hydrothermal alteration surrounding the ore bodies and fracture zones is well developed, however no clear alteration zoning can be identified. In addition to sulphidation that is directly associated with the gold ore, other common alteration types including silicification, sericitization, carbonatization and argillization also occur. According to the ore textures, structures and mineral associ- ation, the ore-forming process is divided into five stages: quartz–pyrite stage (I), quartz–polymetallic sulphide stage (II), quartz–pyrite–clay minerals stage (III), quartz–fine grained pyrite stage (IV), and quartz–carbonate stage (V). Stages II and III are the main ore-forming stages. Native gold grains were found within or between the grains of quartz, pyrite and arsenopyrite (Qi et al., 2000; Wu et al., 2008b).

    4. SAMPLING AND ANALYTICAL METHODS

    4.1. Fluid inclusions

    Samples used in this study were collected from the aurifer- ous quartz–pyrite veinlets (ore body II) of the main ore- forming stage. Doubly polished thin sections (

  • Figure 1. Geological sketch map showing distribution of gold deposits in the Mongolia–Okhotsk metallogenic belt and its adjacent region ((A) modified from Li et al., 2004; (B) modified from Zorin et al., 2001). Major faults: I, main branch of the Mongolia–Okhots