Zircon U–Pb, molybdenite Re–Os and muscovite Ar–Ar isotopic dating of the Xitian W–Sn polymetallic deposit, eastern Hunan Province, South China and its geological significance

Highlights

• U–Pb, Re–Os and Ar–Ar dating are systematically used to constrain the magmatism and W–Sn mineralization in the Xitian W–Sn deposit.

• All published ages from various styles of W–Sn deposits in the Nanling region at 160–150 Ma are collected.

• Jurassic magmatism and mineralization are linked to back–arc lithospheric extension in the Nanling region, possibly triggered by a flat–slab subduction of Paleo–Pacific plate.

• High Re contents in molybdenite suggest that the ore–forming components were from mixed mantle and crustal sources.

Abstract

The Xitian tungsten–tin (W–Sn) polymetallic deposit, located in eastern Hunan Province, South China, is a recently explored region containing one of the largest W–Sn deposits in the Nanling W–Sn metallogenic province. The mineral zones in this deposit comprise skarn, greisen, structurally altered rock and quartz-vein types. The deposit is mainly hosted by Devonian dolomitic limestone at the contact with the Xitian granite complex. The Xitian granite complex consists of Indosinian (Late Triassic, 230–215 Ma) and Yanshanian (Late Jurassic–Early Cretaceous, 165–141 Ma) granites. Zircons from two samples of the Xitian granite dated using laser ablation-inductively coupled mass spectrometer (LA-ICPMS) U–Pb analysis yielded two ages of 225.6 ± 1.3 Ma and 151.8 ± 1.4 Ma, representing the emplacement ages of two episodic intrusions of the Xitian granite complex. Molybdenites separated from ore-bearing quartz-veins yielded a Re–Os isochron age of 149.7 ± 0.9 Ma, in excellent agreement with a weighted mean age of 150.3 ± 0.5 Ma. Two samples of muscovites from ore-bearing greisens yielded ⁴⁰Ar/³⁹Ar plateau ages of 149.5 ± 1.5 Ma and 149.4 ± 1.5 Ma, respectively. These isotopic ages obtained from hydrothermal minerals are slightly younger than the zircon U–Pb age of 151.8 ± 1.4 Ma of the Yanshanian granite in the Xitian area, indicating that the W–Sn mineralization is genetically related to the Late Jurassic magmatism. The Xitian deposit is a good example of the Early Yanshanian regional W–Sn ore-forming event (160–150 Ma) in the Nanling region. The relatively high Re contents (8.7 to 44.0 ppm, average of 30.5 ppm) in molybdenites suggest a mixture of mantle and crustal sources in the genesis of the ore-forming fluids and melts. Based upon previous geochemical studies of Early Yanshanian granite and regional geology, we argue that the Xitian W–Sn polymetallic deposit can be attributed to back-arc lithosphere extension in the region, which was probably triggered by the break-off of the flat-slab of the Palae-Pacific plate beneath the lithosphere.

Introduction

China ranks first in the world in terms of tungsten resources and reserves, and has some of the largest tungsten deposits. According to the mineral commodity summaries of the United States Geological Survey (USGS, 2016), China has more than 57% tungsten reserves of the world, and yielded more than 81% of the world’s total tungsten production in 2015. The Nanling region in the central part of South China accounts for more than 92% of the Chinese tungsten resource; thus it is the most important Tungsten–Tin (W–Sn) polymetallic province, and it has a close spatial relationship with the Yanshanian granites (Hsü, 1943, Lu, 1986, Mao et al., 2013a, Mao et al., 2013b). Several large-scale W–Sn polymetallic deposits are distributed within the Nanling region, such as the Shizhuyuan, Furong, Yaogangxian, Xihuashan, and Pangushan deposits (Fig. 1). The Shizhuyuan deposit is the largest among the economically important skarn-, greisen- or vein-type W–Sn polymetallic deposits in South China (Mao and Li, 1995, Mao et al., 1995, Mao et al., 1996a, Mao et al., 1996b, Zaw et al., 2007, Yin et al., 2002). It mainly occurs at the contact between the Late Devonian dolomitic limestone and the Late Jurassic (Yanshanian) granitoid, with 750, 000 t WO₃, 490, 000 t Sn, 300, 000 t Bi, 130, 000 t Mo and 200, 000 t Be (Lu et al., 2003). Mining activities have been carried out in the Nanling region for several tens of years, but there remains an untouched reserve of 1.7 million tons of tungsten and 1.2 million tons of tin (Che et al., 2005, Peng et al., 2006, Zaw et al., 2007, USGS(United States Geological Survey), 2016).

The Xitian W–Sn polymetallic deposit in eastern Hunan Province is one of the largest newly-discovered deposits during the latest phase of exploration (1999–2011) in the Nanling region, an area with a great potential for W and Sn resources (Wu et al., 2004, Fu et al., 2009, Fu et al., 2012) (Fig. 2). It has four types of mineralization; these are skarn-type hosted at the contact zone between the Yanshanian granite pluton and the Devonian dolomitic limestone, and greisen- and vein-types, as well as structurally altered rocks within or near the Yanshanian granite. The deposit has a Sn reserve of 586,000 tonnes with a grade of 0.26–0.36% and a W reserve of 46,300 tonnes with a WO₃ grade of 0.28–0.63% (Mao et al., 2013a). In addition, Pb, Zn, Mo, Nb and Ta are the by-products from the mineralization (Wu et al., 2004, Luo et al., 2005).

Numerous field investigations, together with geochemical, geochronological, and isotopic studies have been sporadically carried out in the area of the Xitian deposit since 1999 and reported in the Chinese literature (Luo et al., 2005, Ma et al., 2005, Zeng et al., 2005, Cai and Jia, 2006, Xu et al., 2006, Chen et al., 2013, Zhou et al., 2013, Niu et al., 2015, Deng et al., 2015). Various geochronologic techniques have been adopted in the studies, including the K–Ar, Rb–Sr, Sm–Nd isochrons, and LA-ICPMS, SHRIMP, and SIMS zircon U–Pb. However, the lack of systematic application of modern techniques has resulted in a limited consensus on the exact timing of the granitic intrusion, although it has been tentatively dated between 175-161 Ma by regional petrographic correlation (HNBGMR, 1988). The intrusion has been subdivided into two stages, 233–227 Ma and 155–150 Ma (Ma et al., 2005, Niu et al., 2015, Su et al., 2015), and three stages, at ~ 230 Ma or Indosinian, at ~ 155 Ma or Early Yanshanian, and ~ 114 Ma (Whole rock Rb-Sr dating) or Late Yanshanian (Chen et al., 2014, Fu et al., 2009). How the Xitian deposit relates to these stages is a matter of debate. A combination of molybdenite Re–Os, muscovite Ar–Ar, and fluid inclusion Rb–Sr dating (Liu et al., 2008a, Liu et al., 2008b, Fu et al., 2009, Fu et al., 2012, Wu et al., 2012, Wang et al., 2015) has yielded an age of 157–150 Ma or Late Jurassic for the deposit. However, Niu et al. (2015) argued the presence of another mineralization in 227–233 Ma, and Deng et al. (2015) even obtained a molybdenite Re–Os age of 225.5 ± 3.6 Ma. It is therefore necessary to undertake a systematic study to date the deposit and the adjacent granite to resolve this conundrum.

This paper aims to accurately date the Xitian W–Sn deposit and the associated granites. Zircon U–Pb, molybdenite Re–Os and muscovite Ar–Ar isotopic dating, together with field, petrographic and geochemical studies, have rigorously been carried out in these years. These data confirm the two stages of the Xitian granitic intrusion and, what is more, the temporal relationship between the deposit and the Early Yanshanian magmatism. This is helpful in better understanding the W–Sn mineralization of the Nanling region. The new data are combined with recently published geochronology of other W–Sn deposits in the Nanling region to accurately and precisely constrain the timing of mineralization and to identify the geodynamic processes that contraolled the metallogenesis.