Zircon U-Pb dating of Pubei granite and strontium isotope from sphalerite of the Xinhua Pb-Zn-（Ag）deposit，Yunkai Area of Guangxi Province，South China

Minfang Wang·Xubo Zhang·Daohui Pi·Xiaonan Guo，3

?

Zircon U-Pb dating of Pubei granite and strontium isotope from sphalerite of the Xinhua Pb-Zn-（Ag）deposit，Yunkai Area of Guangxi Province，South China

Minfang Wang1，2·Xubo Zhang1·Daohui Pi1·Xiaonan Guo1，3

The Yunkai Area is located at the southern South China Block and is part of the Qinzhou Bay-Hangzhou Bay Metallogenic Belt，which is a famous polymetallic mineralization belt.The Xinhua Pb-Zn-（Ag）deposit is located in the western part of Yunkai Area，with anabundanceofPubeibatholiths.ZirconU-Pb geochronology of Pubei batholiths shows that crystallization age ranges from 251.9±2.2 to 244.3±1.8 Ma，thus belonging to Indosinian orogeny.Geochemistry and Sr isotopic compositions of the Pubei batholiths show that it is derived from the partial melting of large scale crustal melting during the stage of exhumation and uplifting of the lower-middle crust.In addition，strontium isotope of sphalerite from the Xinhua Pb-Zn-（Ag）deposit，has limited ranges in87Rb/86Sr and87Sr/86Sr，ranging from 0.4077 to 1.0449，and 0.718720 to 0.725245，respectively.The initial87Sr/86Srratiosofsphaleriterangesbetween 0.718720 and 0.725245，which is higher than that of upper continental crust and lower than that of the Pubei batholiths，illustrating the fluid might be derived from the mixing of Pubei pluton and upper continental crust.

Zircon U-Pb·Strontium isotope·Xinhua

Pb-Zn-（Ag）deposit·Yunkai Area·South China

1 Introduction

The Yunkai Area is located at the southern South China Block（SCB）and is part of the Qinzhou Bay-Hangzhou Bay Metallogenic Belt（QHMB）（Fig.1a）.In the past decades，many researchers have discussed the basement nature，regional magmatism，structural style，tectonothermal history，and internal relationship among the mylonite，migmatite，and granite within Yunkai Area（Qiu and Chen 1993；Peng et al.1996；Wang et al.1998a，1999），and they concluded that the Yunkai Area has experienced multistage orogeny from the Neoproterozoic to the Mesozoic，with extensive magmatism（Qiu and Chen 1993；Peng et al. 1996；Li et al.2005b；Wang et al.2007a；Wan et al.2010）. It is an ideal area to study for better understanding the temporal and spatial framework of tectonothermal overprinting.The Yunkai Area has Proterozoic basement unit，and SHRIMP zircon dating for the granite gneiss from the Tiantang Group shows an Archean age of 2702±13 Ma（Qin et al.2006，2007）.In addition，numerous ore deposits and mineral occurrences have been discovered in recent years in the Yunkai Area（Liu 1997；Liang and Zhang 1998；Cai et al.2002b；Mao et al.2011b，2013；An 2012），which makes it one of the important polymetallic（Pb-Zn-Ag-Au）mineralization zones.

Although various granite plutons，widely emplaced throughoutIndosinian subduction and orogeny（Maoet al.2011a），exist，the relationship between ore deposits and granites is weak.The isotopic U-Pb systems in zircons，due to chemical stability of zircon and high temperature（＞900°C）of Pb diffusion closure in a mineral（Lee et al.1997；Cherniak and Watson 2001；Wu and Zheng 2004），are the most reliable systems to estimate the age of the igneous rock crystallization. Therefore，in this contribution，we present new laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）zirconU-PbofPubeibatholithsto constrain the timing of granitoids.In addition，strontium isotope compositions were analyzed on sphalerite from ores in the Xinhua deposit to discuss the sources of metals.Based on the geochemical，geochronological，and strontium isotopic data from the Pubei batholiths and sphalerite from ores in the Xinhua deposit，the metal source and geodynamic framework of the mineral system have been discussed.

? Minfang Wang wang_minfang@163.com Xubo Zhang 1026690223@qq.com Daohui Pi pdaohui@163.com Xiaonan Guo 691387433@qq.com

1Faculty of Earth Resources，China University of Geosciences，Wuhan 430074，China

2State Key Laboratory of Geological Processes and Mineral Resources，China University of Geosciences，Wuhan 430074，China

3The Second Geological Survey of Henan Province，Xuchang 461000，China

Fig.1 a Simplified geological map of the South China Block，showing the location of QHMB，the major faults and important tectonic zone（modified after Chen and Jahn 1998；Wang et al.2007a，c）.b Geological map of the Yunkaidashan Belt（modified after Cai et al.2002；Wang et al.2007a，c）

The Xinhua Pb-Zn-（Ag）deposit is located in the Yunkai Area of Guangxi Province and has estimatedreserves of 46.1 kt of Zn with an average grade of 3.95 wt%Zn，26.6 kt Pb with an average grade of 2.33 wt% Pb，as well as 190 tonnes Ag with an average grade of 79.58 g/tonnes Ag（cf.Huang et al.2008）.Since the discovery of the deposit in 1980s，few studies have addressed the geology of the deposit（Chen 2004；Huang et al.2008；Chen et al.2009），and these are published in Chinese without circulating in international journals.

2 Regional geological setting

The QHMB has attracted much attention due to its interesting tectonic properties，which have been interpreted as a giant tectonic suture between the Yangtze and Cathaysian blocks（insert map of Fig.1a）（Pirajno and Bagas 2002；Shu et al.2011；Zhou et al.2012；Li et al.2013；Wang et al. 2013；Zheng et al.2014）.The suture is about 2000 km long and 70-130 km wide，starting from Hangzhou Bay and ending at Qinzhou Bay.It has experienced multi-stage orogeny from the Mesoproterozoic to the Neoproterozoic，according to the majority ages of the ophiolite suites（1030-860 Ma；Shu 2006；Mao et al.2011b）related to the evolution of the Rodinia supercontinent.

The NE-trending Yunkaidashan Belt lies along the southern segment of the boundary between the Yangtze and Cathaysian blocks of the South China Block（Chen and Jahn 1998），being up to 150 km wide and over 300 km long.It is tectonically divided from east to west into the Wuchuan-Sihui shear zone，the Xinyi-Gaozhou block，and the Fengshan-Cenxi shear zone（Fig.1b）.The Yunkaidashan Belt is traditionally considered to be composed of Archean basement（e.g.，Tiantang Group），Paleozoic and Mesozoic sedimentary cover，and voluminous mid-Paleozoic foliated granites（Qiu and Chen 1993；Peng et al. 1996）.

The Yunkai Area herein means the area located in the Yunkaidashan Belt，bounded to the east by the Wuchuan-Sihui Fault，to the west by the Lingshan-Tengxian Fault，and to the north by the Luoding-Yuecheng Fault.The research area includes two tectonic units：the Yunkai uplift and the Bobai depression，with an area of～12，000 km2（Fig.1b）.In general，the Yunkai Area is within the boundary between Guangxi and Guangdong Provinces，and large amounts of ore deposits have been reported in the Yunkai Area（Lei et al.1992；Cai 2002；Li 2004；Mao et al. 2011b；Zhang 2011）.

The Yunkai Area has experienced two stage orogeny of Hercynian and Indosinian epoch，and consisted of basement of E-W direction fold system.In addition，the subduction of the Pacific plate and Indian plate in Yanshanian epoch happened，with massive volcanic eruptions（Liu 1997；Wang et al.1998b；Cai et al.2002）.There are more than 300 Au-（Ag）and 450 Pb-Zn deposits and mineral occurrences in the Yunkai Area（Liang and Zhang 1998；Liang et al.1998）.It could be divided into four NE-trending mineralization belts：I-Xinxing-Yangchun Cu-Pb-Zn-Au-（Ag）metallogenic belt，characterized by skarn，porphyry and magmatic hydrothermal deposits，such as Shilu Cu-（Mo），Tiantang Cu-（Mo）-Pb-Zn and Bengkeng Pb-Zn deposits，etc.，which are related to the crustalderived granite of Indosinian-Yanshanian epoch，with ages of Late Triassic-Early Jurassic；II-Deqing-Luoding-Gaozhou Au-Pb-Zn-（Ag）metallogenic belt，characterized by quartz vein Au-Ag and quartz vein-type Pb-Zn-Ag deposits，such as Hetai Au and Yunfu Ag-Pb-Zn deposits，etc.，related to mineralization in Late-Early Cretaceous（150-80 Ma）；III-Cenxi-Luchuan Ag-Au-Pb-Zn metallogenic belt，characterized by quartz vein Ag-Au and magmatichydrothermalPb-Zn-Cudeposits， suchas Pangxidong Ag-Au，Jinshan Ag-Au and Fozichong Pb-Zn-Cu deposits，etc.，related to granite of Middle-Late Jurassic（160-140 Ma）；IV-Tengxian-Pubei Pb-Zn metallogenic belt，characterized by hydrothermal vein Pb-Zn-（Ag）deposits，such as Xinhua，Shanming and Xiashui Pb-Zn deposits（Fig.2）.

3 Geology of ore deposit

The Xinhua Pb-Zn-（Ag）deposit is located 109°42＇24＇＇E，22°31＇48＇＇N，with an area of 6.82 km2.It is located in the northwest of Pubei complex anticline，bounded by Lingshan and Bobai fold belt，and it formed in the conjunction of NW and SN direction fracture.The Ar-Ar dating of sericite from the principal stage of sphalerite（unpublished data）provides constraints on the timing of sphalerite mineralization of Yanshanian epoch.The Pubei batholiths，covering an area of 4100 km2，is one of the main units of the Darongshan granitic suite，with similar lithology to the Darongshan batholiths（cordierite-biotite granite），which are situated in the eastern side of the Lingshan-Tengxian Fault and to the west of the Bobai-Wuzhou Fault（Fig.1b）. The Darongshan granitic suite，outcropping an area of approximately 10，000 km2，consists of five major units：Taima（hypersthene granitic porphyry），Nadong（biotite granite）andJiuzhou（cordierite-biotitemonzogranite）plutons，and Pubei and Darongshan batholiths（Chen et al. 2011）.The common existence of Al-rich minerals，such as cordierite and almandine garnet in various amounts，fits the mineralogical criteria of the S-type granite（Chappell and White 1974）.The modal mineral composition of the Pubei batholiths is comprised of 35%quartz，35%K-feldspar，20%plagioclase，8%biotite，and 2%minor cordierite，titanite，apatite.Generally，feldspar group minerals have been altered to sericite.

Fig.2 The metallogenic sketch map of the Yunkai Area，illustrating the distribution of main types of ore deposits（modified after Cai et al.2002；Liang et al.1998）

There are abundant NWW-，NW-，and near NS-trending faults in the Xinhua deposit，and the numerous NWW-trending faults are the ore-hosting structure（Fig.3a）.The Xinhua deposit is comprised of tens of individual ore bodies in veins.No.1，No.2，No.7，and No.7-1 are the largest four ore bodies.The No.1 ore body is located in the compression zone of F1.It is more than 885 m long and 0.14-6.75 m thick，and it extends 480 m down dip，with an averagegradeof2.22 wt%Pb，3.56 wt%Zn，and 71.45 g/tonnes Ag.The No.2 ore body is located in the compression zone of F0.It is more than 880 m long and 0.19-2.90 m thick，and it extends 510 m down dip，with an averagegradeof2.36 wt%Pb，4.89 wt%Zn，and 84.80 g/tonnes Ag.Additionally，the No.7 ore body is located in the compression zone of F7.It is more than 1010 m long and 0.17-3.22 m thick，and it extends 450 m down dip，with an average grade of 2.61 wt%Pb，4.49 wt%Zn，and 77.37 g/tonnes Ag.Finally，the No.7-1 ore body is also located in the compression zone of F7.It is more than 500 m long and 0.53-4.04 m thick，and it extends 330 m down dip，with an average grade of 2.13 wt%Pb，2.85 wt%Zn，and 84.71 g/tonnes Ag.It is concluded that ore bodies were actually controlled by faults，and they had a shape of narrow shrink or swell，thin out or extended（Fig.3b）.

Based on the detailed field and microscopic observation，four ore types have been distinguished：（1）massive quartz-Pb-Zn ores，（2）vein quartz-barite-Pb-Zn ores，（3）disseminated altered granite-Pb-Zn ores and（4）compressive silicification Pb-Zn ores（Fig.4a-d）.

The ore minerals are sphalerite，galena，chalcopyrite，and tetrahedrite（Fig.4e）.The main gangue minerals are quartz，barite，and calcite（Fig.4f）.The typical alteration in the Xinhua deposit is characterized by the silicification and sericitization，and silicification is the most extensive and economically important.In addition，pyrite，siderite，dolomitization，chloritization，and kaolinization are also recognized.The alteration distributed in both sides of the ore bodies and ore-bearing faults，was identified as 2-50 m in width.

Fig.3 a Simplified geological map of the Xinhua deposit，showing the distribution of abundant faults and ore bodies. b No.12 section of the Xinhua deposit，showing ore bodies’distribution in veins（modified after Guangxi Pubei Lead and Zinc，Ltd.2005）

4 Ore mineralogy and paragenesis

The mineralization in Pubei complex（cordierite-biotite granite）can be divided into four stages（Fig.5）：stage 1，quartz-pyrite-sphalerite；stage2，siderite-galena-tetrahedrite；stage 3，quartz-galena；and stage 4，quartz-calcitebarite.The mineral assemblage of stage 1 is dominantly massive quartz and coarse-grained sphalerite，with small amounts of pyrite.Stage 1 mineralization was accompanied by silicification and sericitization（Fig.6a）and was the principal stage of sphalerite.The mineral assemblage of stage 2 is dominantly coarse-grained euhedral galena，siderite，tetrahedrite，and minor sphalerite.This stage is characterized by silicification and carbonatization（Fig.6b）. Galena and siderite are characterized by open-space filling textures，such as miarolitic structure and veinlets（Fig.6c）. The mineral assemblage of stage 3 is dominantly finegrained quartz and galena and minor calcite（Fig.6d），with extensive silicification and carbonatization.The mineral assemblage of stage 4 is dominantly quartz，calcite，and barite，with minor galena.Calcite-barite veins cut sphalerite and galena veins of stage 1 and 2（Fig.6e，f）.

Fig.4 Hand specimens and corresponding photomicrographs of the Pubei batholiths and ores from the Xinhua deposit.a Massive quartz-Pb-Zn ores，with lead gray galena and brown sphalerite appearance.Quartz vein cut the massive mores with sharp boundary.Hand specimen photograph.b Vein quartz-barite-b-Zn ores，with abundant brown sphalerite and minor galena.White barite paragenesis with colourless quartz. Hand specimen photograph.c Typical appearance of the cordierite-biotite granite（Pubei batholiths），with disseminated sphalerite in altered granite.Hand specimen photograph.d Compressive silicification Pb-Zn ores.Silicification compressive rock/granites included by sphalerite and galena，distributed in the compressive zone.-95 m level of underground workings.e The typical ore minerals in the Xinhua deposit.Subhedralanhedral tetrahedrite（steel gray to iron-gray）included in shpalerite.Reflected light image，plain-polarized light.f The typical gangue minerals in the Xinhua deposit.Plain-polarized light.Brt barite，Bt biotite，Cal calcite，Ccp chalcopyrite，Gn galena，Ms muscovite，Pl plagioclase，Qz quartz，Ser sericite，Si silicate，Sp sphalerite，Ter tetrahedrite.Abbreviations based on Whitney and Evans（2010）

Fig.5 Mineral paragenesis of the lead-zinc ores in the Xinhua deposit showing mineral assemblages

5 Samples and analytical methods

5.1 U-Pb geochronology

5.1.1 Sample preparation

The sampling was carried out with the main objective of establishing the crystallization age of the Pubei batholiths，and two samples were taken，medium-fine sized cordieritebiotite granite（XH-5）and coarse-medium sized cordieritebiotite granite（XH-14）（Fig.3）.Each sample weighted approximately 5 kg for analyses.

5.1.1.1 Medium-fine sized cordierite-biotite granite（XH-5）The sample was collected from-55 m level of underground workings around the No.17 ore body.It is granite with small amounts of titanite and apatite.Zircons in medium-fine sized cordierite-biotite granite are about 100-250 μm long and 40-80 μm wide and euhedral，prismatic.They have typical oscillatory zoning，indicating magmatic origin（Fig.7a）.

5.1.1.2 Coarse-mediumsizedcordierite-biotitegranite（XH-14）The sample was collected from the eastern adit. It is a granite with small amount of apatite，yielding numerous large to medium-sized，clear，high-quality zircons（Fig.7b）.Zircons in coarse-medium sized cordieritebiotite granite are more than 100 μm in diameter and exhibit clear and uniform internal structures.Euhedral zircons have a rectangular shape with width/length ratios of～1：2 to 1：3.There are also rounded，flat and squareshaped zircons with typical oscillatory zoning of magmatic origin.

5.1.2 Zircon U-Pb dating

Zircons were separated using conventional heavy liquid and magnetic techniques，in general terms，to that described in detail by Dube′et al.（1996）.The samples were washed，then crushed in a jaw crusher and pulverized to a fine powder.The heavy mineral fraction was concentrated using a Wilfley table and heavy liquids.The concentrate was passed through heavy liquids and a Frantz magnetic separator in a sequence of steps designed to discriminate fractions with different magnetic susceptibility.Generally，the high quality zircons were from the least magnetic highdensity fractions（Romeo et al.2006），which were selected according to criteria of morphology and clarity under a microscope.The zircons were abraded to remove the external surface of the grains in an attempt to minimize Pbloss，following the air abrasion technique of Krogh（1982）. The abraded zircons were cleaned and purified using HF and HNO3，and the ion exchange chemistry was performed in microcolumns following the technique described by Krogh（1973）but instead reducing the volume of the columns and the reagents in a ratio of 1：10.

Fig.6 Macroscopic features of hand specimens selected for ore mineralogy and paragenesis studies.a The typical characteristics of stage 1，with massive quartz and coarse grained sphalerite occurred and extensive silicification and sericitization.b The paragenesis of coarse grained euhedral galena and siderite，which is the principal stage of galena.c Veinlets of galena，quartz and siderite in stage 2，with minor calcite.d Fine grained quartz and galena（stage 3）replace coarse grained sphalerite（stage 1）.e-f Veins of quartz and barite cut the sphalerite and galena of stage 1 and 2.Brt barite，Cal calcite，Gn galena，Qz quartz，Sd siderite，Ser sericite，Sp sphalerite.Abbreviations based on Whitney and Evans（2010）

Selected zircons were mounted in epoxy disks and polished to expose their cores.All grains were photographed under transmitted and reflected light.They were subsequently examined by the cathodoluminescence（CL）imagetechniquetorevealtheirinternalstructures. AQuanta450FEGhigh-resolutionemissionfieldenvironmental scanning electron microscope connected to an Oxford SDD Inca X-Max 50 energy dispersive system and a Gatan Mono CL4+CL system at the State Key Laboratory of Geological Processes and Mineral Resources（GPMR），China University of Geosciences，Wuhan，was used.The imaging conditions were 10 kV with a spot size of 5 nm.

Fig.7 CL images of representative zircon grains，showing internal structures，analyzed locations，and calculated apparent206Pb/238U ages.Yellow solid circles denote U-Pb dating spots，and numbers in circles are equivalent to spot analyses given in Table 1

The clearest，least fractured rims of the zircon crystals were selected as suitable targets for laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）analysis with a combination of the GeoLas2005 laser-ablation system and the Agilent7500a ICP-MS at GPMR.The 193 nm ArF excimer laser，homogenized by a set of beam delivery systems，was focused on the zircon surface with an energyfluenceof15 J/cm2.Theablationprotocol employed a spot diameter of 32 μm at 5 Hz repetition rate for 35 s.Helium was used as the carrier gas to enhance the transport efficiency of the ablated materials.U-Th-Pb isotopic compositions were optimized using zircon 91500 as the reference standard and NIST610 as the internal standard.Temora standard was used as the age standard（206Pb/238U=416.8 Ma）（Black et al.2003）.Results of isotopic measurement on zircons（all errors are given at the 1 σ level）are listed in Table 1 and are shown in conventional U-Pb concordia plots in Fig.8.Detailed operating conditions and procedures followed Liu et al.（2008，2010）.Off-line selection and integration of background and analyze signals，time-drift correction，and UPb dating were performed using ICPMSDataCal（Liu et al. 2008，2010）.Uncertainties on the isotopic ratios were calculated at 2 σ level，considering the uncertainty of the measurements by mass spectrometry，isotopic fractionation，and the proportion of initial common lead and its isotopic composition，according to Stacey and Kramers（1975）.The results are presented as206Pb/238U ages，using the Isoplot program（Ludwig 2003），and uncertainties are reported at the 95%confidence level.The decay constants that were used are those recommended by Steiger and Ja¨ger（1977），and compositions for initial common Pb were taken from the model of Stacey and Kramers（1975）.

5.2 Strontium isotope compositions

Six sphalerite separates were collected from Pb-Zn ores at the-55 and-95 m levels of underground workings（Fig.3a；Table 2），respectively.Pure sphalerite separates（～1 g）were obtained from each sample by crushing，washing，drying，and handpicking under a binocularmicroscope.Chemical separation of Rb and Sr from matrix elements as well as mass spectrometric measurement was accomplished at the Analytical Laboratory of Beijing Research Institute of Uranium Geology（BRIUG）.The analytical procedure for Rb-Sr isotope analyses is similar to that given by Li et al.（2005a）.Spec-Sr exchange resin of special efficiency was used for the separation and purification of Rb and Sr.The whole procedural blank of both Rb and Sr was about 6 pg.Isotopic compositions were measured by the Isoprobe-T Thermal Ionization Mass Spectrometer.An88Sr/86Sr ratio of 8.37323 was used to calibrate mass fractionation of Sr isotope.Repeated measurements of87Sr/86Sr ratio of NBS987 Sr standard solution were 0.710247±17（2σ）.Uncertainties（2σ）were 2%for87Rb/86Sr ratios and 0.005%for Sr isotope compositions.

Table 1 Analytical data of zircon LA-ICP-MS U-Pb dating on the cordierite-biotite granite in the Pubei complex batholiths

Table 1 continued

6 Analytical results

6.1 Zircon U-Pb geochronology

6.1.1 Sample XH-5

Seven zircon grains were chosen for the U-Pb analyzing. The zircons have Th and U contents ranging from 684 to 1754 ppm and from 342 to 825 ppm，respectively，with Th/ U ratios of 0.28-0.63.Except for one older zircon age of 792.5±7.9 Ma（spotXH-5-10）（Table 1），theother206U/238Pb ages are almost consistent with each other，yielding a weighted mean age of 251.9±2.2 Ma（n=6；MSWD=0.15）（Fig.8a），which was interpreted as the crystallization age of the granite.The older zircon age indicates neoproterozoic magmatism，which is consistentwith the global Grenville（Sibaoan）collisional orogeny（Qin et al.2006）.

Fig.8 Concordia diagram for the samples of this study.Error ellipses are plotted with 1σ uncertainties.All the ages are calculated as weighted averages using Isoplot（Ludwig 2003）and uncertainties are reported at the 95%confidence interval.MSWD：mean-squared weighted deviates

Table 2 Location and detailed description of analytical samples from the Xinhua deposit

6.1.2 Sample XH-14

Twenty-six zircon grains were chosen for the U-Pb analysis.The zircons have Th and U contents ranging from 704 to 5310 ppm and from 50 to 446 ppm，respectively，with Th/U ratios of 0.02-0.43.All the206U/238Pb ages almost agree with each other，yielding a weighted mean age of 244.3±1.8 Ma（n=26；MSWD=1.5）（Fig.8b），which was also interpreted as the crystallization age of the granite.

6.2 Strontium isotope compositions

Results of Rb-Sr isotope of sphalerite are presented in Table 3.Sphalerite separates from the Xinhua deposit have limited ranges in87Rb/86Sr and87Sr/86Sr，ranging from 0.4077 to 1.0449，and 0.718720 to 0.725245，respectively. The initial Sr isotope compositions（at 244 Ma）are estimated to vary from 0.718720 to 0.725245（Table 3）.

7 Discussion

7.1 Ages of magmatism

Granites are widely distributed in South China，outcropping an area of approximately 170，000 km2，and 12%are Triassic in age（Sun 2006）.Indosinian granites in the SCB interior are traditionally classified as the Hercynian-Indosinian peraluminous S-type granites，and they are characterized by large amount of peraluminous massive granites and minor gneissic granites（Wang et al.2007c，2013；Mao et al.2011a）.Zircon U-Pb age for theseperaluminous massive granitesfall intoa rangeof 248-202 Ma，with two age-peaks of～239 and～220 Ma for magma emplacement（Deng et al.2004；Peng et al. 2006；Sun 2006；Li and Li 2007；Wang et al.2007b，2013；Luo et al.2010）.Zircon U-Pb ages of minor gneissic granites fall into a range of 250-242 Ma（Peng et al.2006；Zhou et al.2006a，b，2007）.

Table 3 Rb-Sr data ofsphalerite separates from the Xinhua deposit

Fig.9 Comparison of（87Sr/86Sr）iratios among Xinhua Pb-Zn-（Ag）deposit，upper mantle（Faure 1977），upper continental crust（Jahn et al. 1999），basement of Cathaysia Block（Shen et al.2008），and Taima，Jiuzhou，Pubei plutons（Qi et al.2007）

Chen et al.（2011）reported LA-ICP-MS U-Pb zircon and electron microprobe（EMP）monazite ages for the Pubei batholiths as being 260.4±3.1 to 255.5±5.8 Ma，which are older than SHRIMP zircon U-Pb ages（233±5 Ma）by Deng et al.（2004）.Therefore，the Pubei batholiths were the resultofIndosinianorHercynianorogeny.AlthoughCharoy and Barbey（2008）and Deng et al.（2004）have reported and compiled ages of zircon U-Pb and the whole-rock Rb-Sr isochron，thisdatahasreceivedlessconsideration，likelydue to the lack of descriptions on the zircon morphology and structure.In contrast，the Zircon U-Pb dating in this study shows that crystallization age of the Pubei batholiths ranges from 251.9±2.2 to 244.3±1.8 Ma，supporting the conclusion of Indosinian orogeny.

7.2 Sources of ore-forming fluid

Sphalerite from the Xinhua Pb-Zn-（Ag）deposit has initial87Sr/86Sr ratios（ISr）from 0.718720 to 0.725245，higher than the upper mantle and upper continental crust，and these fall into the range of basement of Cathaysian block（Fig.9）.The ISrfrom sphalerite is larger than 0.710，meaning the sources of crust，but not the mantle.Comparing with ISrof Taima，Jiuzhou，and Pubei plutons，the Pubei pluton has the highest ISr（0.7266-0.7295；Qi et al.（2007））（Fig.9），showing that it is derived from higher evolutionofcrustalmaterials.ISrinsphalerite（0.718720-0.725245），which is lower than that of the Pubei pluton and higher than that of upper continental crust，show that the fluid might be derived from the mixing of Pubei pluton and upper continental crust.

7.3 Regional geological significances

The timing of Pubei batholiths has important significance for the understanding of the geotectonic context of the Darongshan granitic suite.In general，two contrasting genetic models have been proposed for the Darongshan granitic suite.Some researchers propose that the Darongshan granitic suite might be genetically related to the NWSE compression due to continent-continent collision of two smaller continental blocks of the Asian Plate（Chen et al.1995；Deng et al.2004）.Other researchers argue that it resulted from various degrees of melting at different crustal levels during crustal thinning in extensional postcollision setting（Charoy and Barbey 2008；Chen et al. 2011；Zhao et al.2012）.

Our zircon U-Pb age for the Pubei batholiths demonstrates the relationship of the Indosinian orogeny in South China，not Hercynian orogeny.According to the strontium，isotopic compositions by previous researchers（Qi et al.，2007）suggest that the Pubei granites resulted from high degrees of the melting of older crust.However，the ISrof sphalerite from the Xinhua deposit，higher than upper continental crustal and lower than that of the Pubei pluton，implies the fluid mixing of these two members.

8 Conclusion

The Xinhua Pb-Zn-（Ag）deposit is located in the western part of Yunkai Area，South China，related to Pubei batholiths.Zircon U-Pb geochronology shows that the crystallization age ofthe Pubei batholiths rangesfrom 251.9±2.2 to 244.3±1.8 Ma，illustrating its Indosinian orogeny.Sphalerite from the Xinhua Pb-Zn-（Ag）deposit has limited ranges in87Rb/86Sr and87Sr/86Sr，ranging from 0.4077 to 1.0449，and 0.718720 to 0.725245，respectively，and it has an ISrranging from 0.718720 to 0.725245. Geochemistry and Sr isotopic compositions of the Pubei batholiths show that it is derived from the partial melting of large scale crustal melting during the stage of exhumation and uplifting of the lower-middle crust.In addition，the ISrof sphalerite from the Xinhua deposit shows that the fluid might derive from the mixing of Pubei pluton and upper continental crust.

AcknowledgmentsThis study was supported by grants by the National Natural Science Foundation of China（No.41272097），the China Geological Survey Project（No.12120114016601），the Special Fund for Basic Scientific Research of Central Colleges，China University of Geosciences（Wuhan）（No.CUG120702），and the Teaching Laboratory Foundation of China University of Geosciences（Wuhan）（No.SKJ2013085，SKJ2014010）.We are indebted to colleagues of Guangxi Pubei Lead and Zinc，Ltd.for their help in the field.Special thanks to Xiaofeng Cao，Yanjun Li and Zhen Yang for the sample collection.

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29 May 2015/Revised：19 July 2015/Accepted：15 November 2015/Published online：25 November 2015?Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2015