Two Periods of Skarn Mineralization in the Baizhangyan

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bs_bs_banner doi: 10.1111/rge.12066 Resource Geology Vol. 65, No. 3: 193–209

Original Article Two Periods of Skarn Mineralization in the Baizhangyan W–Mo Deposit, Southern Anhui Province, Southeast China: Evidence from Zircon U–Pb and Molybdenite Re–Os and Sulfur Isotope Data

Chunhai Li,1,2 Yaohui Jiang,1 Guangfu Xing,2 Kunyi Guo,2 Chao Li,3 Minggang Yu,1,2 Peng Zhao1 and Zheng Wang2 1State Key Laboratory of Mineral Deposit Research, School of Earth Science and Engineering, Nanjing University, Nanjing, China, 2Nanjing Institute of Geology and Mineral Resources, China Geology Survey, Ministry of Land and Resources of the People’s Republic of China, Nanjing, China and 3Re-Os Laboratory, National Research Center of Geoanalysis of Chinese Academy of Geosciences in Beijing, Beijing, China

Abstract

The recently discovered Baizhangyan skarn-porphyry type W–Mo deposit in southern Anhui Province in SE China occurs near the Middle–Lower Yangtze Valley polymetallic metallogenic belt. The deposit is closely temporally-spatially associated with the Mesozoic Qingyang granitic complex composed of granodiorite, monzonitic granite, and alkaline granite. Orebodies of the deposit occur as horizons, veins, and lenses within the limestones of Sinian Lantian Formation contacting with buried fine-grained granite, and diorite dykes. There are two types of W mineralization: major skarn W–Mo mineralization and minor granite-hosted disseminated Mo mineralization. Among skarn mineralization, mineral assemblages and cross-cutting relationships within both skarn ores and intrusions reveal two distinct periods of mineralization, i.e. the first W–Au period related to the intrusion of diorite dykes, and the subsequent W–Mo period related to the intrusion of the fine-grained granite. In this paper, we report new zircon U–Pb and molybdenite Re–Os ages with the aim of constraining the relationships among the monzonitic granite, fine-grained granite, diorite dykes, and W mineralization. Zircons of the monzonitic granite, the fine-grained granite, and diorite dykes yield weighted mean U–Pb ages of 129.0 ± 1.2 Ma, 135.34 ± 0.92 Ma and 145.3 ± 1.7 Ma, respectively. Ten molybdenite Re–Os age determinations yield an isochron age of 136.9 ± 4.5 Ma and a weighted mean age of 135.0 ± 1.2 Ma. The molybdenites have δ34S values of 3.6‰–6.6‰ and their Re contents ranging from 7.23 ppm to 15.23 ppm. A second group of two molybdenite samples yield ages of 143.8 ± 2.1 and 146.3 ± 2.0 Ma, containing Re concentrations of 50.5–50.9 ppm, and with δ34S values of 1.6‰–4.8‰. The molybdenites from these two distinct groups of samples contain moderate concentrations of Re (7.23–50.48 ppm), suggesting that metals within the deposit have a mixed crust–mantle provenance. Field observation and new age and isotope data obtained in this study indicate that the first diorite dyke-related skarn W–Au mineralization took place in the Early Cretaceous peaking at 143.0–146.3 Ma, and was associated with a mixed crust–mantle system. The second fine-grained granite-related skarn W–Mo mineralization took place a little later at 135.0–136.9 Ma, and was crust-dominated. The fine-grained granite was not formed by fractionation of the Qingyang monzonitic

Received 13 September 2014. Accepted for publication 5 March 2015. Corresponding author: C. Li, State Key Laboratory of Mineral Deposit Research, School of Earth Science and Engineering, Nanjing University, Nanjing 210016, China. Email: [email protected]

© 2015 The Authors Resource Geology published by Wiley Publishing Asia Pty Ltd on behalf of The Society of Resource Geology. 193 This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. C. Li et al.

granite. This ﬁnding suggests that the ﬁrst period of skarn W–Au mineralization in the Baizhangyan deposit resulted from interaction between basaltic magmas derived from the upper lithospheric mantle and crustal material at 143.0–146.3 and the subsequent period of W–Mo mineralization derived from the crust at 135.0– 136.9 Ma. Keywords: Baizhangyan W–Mo deposit, molybdenite Re–Os, southeast China, sulfur isotopes, zircon U–Pb.

1. Introduction present within the W–Mo oreﬁeld in the southern Anhui Province. The oreﬁeld is associated with the More than 13 W–Mo and W deposits have been recog- Qingyang (or Jiuhuashan) composite granitic pluton nized in recent years near the Middle–Lower Yangtze which is composed of granodiorite, monzonitic River Valley Fe–Cu–Au–Mo metallogenic Belt (YRB) granite, and alkaline granite (Chen, 1985; Zhang & Xu, (Fig. 1) (He et al., 2004; Wan, 2004; Xu et al., 2008; Mao 1997; Bureau of Geology and Mineral Resources of et al., 2011; Song et al., 2013, 2014) in southeast China. Anhui Province, 2006). Several polymetallic W–Mo ore We call the Jiangxi–Anhui–Zhejiang adjoining area the deposits are found along the contact between this W–Mo metallogenic belt in this paper. They may be pluton and the surrounding country rocks (Fig. 2a). different in ages and geodynamic settings compared Several studies have discussed the geological charac- with the W–Sn and W deposits within the Nanling teristics of the Baizhangyan deposit (Zhang, 1989; Zhao district in South China. Among these deposits, et al., 2007; Wang, 2008). The deposit consists of a the Baizhangyan skarn-porphyry type W–Mo(–Au) number of orebodies hosted in the limestone units of the deposit is a typical example of the mineralization Sinian (Neoproterozoic) Lantian Formation which was

Fig. 1 (a) Distributions of Jurassic and Cretaceous granitic rocks and related W (Mo) deposits in Jiangxi-Anhui-Zhejiang adjoining areas and (b) their tectonic position (modiﬁed from He et al., 2004; Wan, 2004; Xu et al., 2008; Mao et al., 2011).

Fig. 2 (a) Geological map of Qingyang complex (Bureau of Geology and Mineral Resources of Anhui Province, 2006) and (b) simple map of the Baizhangyan W(Mo) deposit (Zhao et al., 2007) and sample position (b Section A,B,C). intruded by monzonitic and ﬁne-grained granites and isotope data with the aim of constraining the timing of diorite dykes(Fig. 2b). Song et al. (2013) reported the mineralization and to gain a better understanding of the molybdenite Re–Os and zircon U–Pb ages, temperature relationships between W–Mo mineralization and the and hydrogen-oxygen isotope data of ore-forming ﬂuid intrusions in the Baizhangyan deposit. of Baizhangyan deposit. However, the relationships between these intrusions and the W–Mo(–Au) mineral- 2. Regional geology ization and the petrogenesis of the Qingyang pluton, are still poorly understood. In this study, we report new Geologically, the Qingyang area is located along the zircon U–Pb ages, molybdenite Re–Os ages, and sulfur boundary between the Lower Yangtze sub-region and

Jiangnan sub-region of the Yangtze stratigraphic Cambrian Lantian, Piyuancun, and Huangboling For- region. The main strata in the Qingyang area include mations which were intruded by the monzonitic metamorphosed Late Precambrian and Paleozoic granite, the Baizhangyan ﬁne-grained granite, and a clastic sedimentary sequences and basin-hosted Meso- series of diorite dykes traced by drilling at depth. zoic clasitic-volcanic sequences. The metamorphic Quartz-molybdenite vein W–Mo, W–Mo skarn, and Mesoproterozoic basement of the Yangtze Craton granite-hosted disseminated Mo ore represent the formed during the ﬁrst phase (1000–800 Ma) of main types of the ore district. The orebodies of the the Neoproterozoic Jinning Orogeny. Assembly of the deposit occur as veins, and lenses within the lime- Yangtze and Cathaysia cratons occurred during the stones of the Sinian Lantian Formation. Seventeen second episode (800–700 Ma) of the Jinning Orogeny. W–Mo orebodies were explored with average grades of

Continuous deposition of sedimentary cover sequences 0.334 wt.% WO3 and 0.136 wt.% Mo. Moreover, three in southern Anhui Province lasted from the Sinian to Au orebodies were also discovered in the deposit. the Cretaceous. The geological evolution of the region The largest orebody trends N–S, dips 2°–18° to the can be divided into three stages: cover sedimentation west, and has a strike length of 1600 m and depth of and Caledonian orogenesis; Eurasia continental growth 50–230 m. The thickness of the largest orebody ranges and Indosinian orogenesis; and continental margin– from 1.2 m to 27.7 m (Zhao et al., 2007). Mineralization interplate deformation and Yanshanian orogenesis within the orebodies is zoned from pyrite alteration (Chang et al., 1996). Two episodes of mantle–crust inter- zones near the intrusion, through skarn and silicified action occurred during two Mesozoic post-collisional zones into greisen zones within wall-rocks surround- to post-orogenic magmatic events in southern Anhui ing the intrusion. Province. The first episode (145–125 Ma) involved There are two types of W–Mo mineralization in the intensive interaction of the middle to lower crust with Baizhangyan deposit: major skarn W–Mo mineraliza- underplated basaltic magmas derived from upper litho- tion and minor granite-hosted disseminated Mo spheric mantle while the second episode (125–105 Ma), mineralization. Among skarn mineralization, mineral show slight interaction of the middle to lower crust with assemblages and cross-cutting relationships within basaltic magmas derived from the lower lithospheric both skarn ores and intrusions reveal two distinct mantle (Du et al., 2007b). periods of mineralization. The first one, which pro- The Qingyang composite pluton was emplaced into duced minor skarn W–Au mineralization, was associ- the core of an anticline formed during the Indosinian ated with diorite dykes. Diorite dykes, with widths of Orogeny, and consists of a granodiorite with an Ar–Ar 2–20 m, have been intercepted in >14 drillholes and are age of 137.6 ± 1.4 Ma (Chen et al., 1985), a monzonitic either intersect or are parallel to sedimentary bedding. granite, and an alkaline granite with an Ar–Ar age Diorite dykes contain plagioclase (30%–50%) and horn- of 122.7 ± 1.2 Ma (Chen et al., 1985) (Fig. 2a). Syenite blende (20%–30%), and are strongly chloritized and dikes in the area were emplaced after the granite epidotized. The ore is present in disseminated and (Bureau of Geology and Mineral Resources of Anhui of stockwork forms, and is dominated by scheelite, Province, 2006). The Qingyang composite pluton molybdenite, pyrrhotite and gold (in sulfide), in a intruded into the Sinian to Lower Paleozoic sand- gangue of hornblende, plagioclase, quartz, diopside, stones, shales, and limestones. Intrusion–country rock garnet, calcite, tremolite, actinolite, chlorite and contact zones between the Qingyang pluton and epidote. More significantly, W–Mo mineralization was Sinian, Cambrian, Ordovician, and Silurian sedi- related to the fine-grained granite which accompanies ments host the Longtoushan (W), Chenjiazhan (Au), W–Mo skarn (and minor granite-hosted disseminated Baizhangyan (W–Mo), Yinjiazha (Au), Yangmeiqiao Mo mineralization). The fine-grained granite is located (Mo–Cu), Longhuchong (Au), and Gaojiabang (W–Mo) below the lowermost orebodies and contains plagio- deposits from east to west (Fig. 2a). clase (30%–38%), quartz (25%–32%), K-feldspar (25%– 30%), biotite (2%–4%), hornblende (<1%) and other 3. Deposit geology accessory minerals. Ore is present in both disseminated and laminar forms, and is dominated by scheelite, The Baizhangyan W–Mo deposit (Fig. 2a, b) is located molybdenite, pyrite, pyrrhotite, and chalcopyrite, in a ∼20 km SE of Qingyang county, and is a medium-scale gangue of diopside, garnet, tremolite, actinolite, skarn- porphyry type deposit. Locally E–W and calcite, quartz, chlorite, epidote, plagioclase and horn- NE–SW striking faults cut through the Sinian to Lower blende (Fig. 3).

Fig. 3 Paragenesis sequence of minerals of the Baizhangyan deposit.

4. Sample preparation and analytical expose half of the selected zircons prior to analysis. procedures Zircons were imaged using reﬂected and transmitted light microscopy and cathodoluminescence (CL) using 4.1 Sampling and zircon LA–ICP–MS U–Pb a scanning electron microscope to characterize internal analysis structures and to choose potential target sites for U–Pb Zircons were obtained from samples of the monzonitic dating. U–Pb isotope analysis was undertaken using granite (sample 09BZ-2-6, Figs 2a, 4a) that were an Agilent 7500a Inductively Coupled Plasma Mass obtained about 20 km distant from any of the orebodies Spectrometer (ICP–MS) equipped with a New Wave in the area, from the Baizhangyan ﬁne-grained granite Research 213 nm laser ablation (LA) system at the State (sample 10BZ-2-6; Figs 2b, 4b) and from diorite dykes Key Laboratory of Mineral Deposit Research, Nanjing (samples 10BZ-2-1 and 10BZ-1-8; Figs 2b, 4c, d). The University, China, following the procedures described monzonitic granite is dominated by K-feldspar (40%– by Jackson et al. (2004) and He et al. (2009). Analyses 32%), quartz (25%–30%), plagioclase (23%–30%), and employed a beam diameter of 18–25 μm, a repetition biotite (<3%). rate of 5 Hz, and a laser energy of 10–20 J/cm2. All data Zircon separation involving standard crushing, were processed using GLITTER 4.4.1 (Van et al., 2000) heavy liquid, and paramagnetic techniques was carried and the ISOPLOT/Ex macro of Ludwig (2001). The raw out at the Chengxin Geology Service Ltd, Langfang, ICP–MS data were exported in an ASCII format and Hebei, China. Representative zircons from each sample processed using GLITTER 4.4.1 (Van et al., 2000), an were selected using a binocular microscope before in-house data reduction program. Common Pb concen- mounting in epoxy resin within a 1.4 cm diameter cir- trations were evaluated using the method of Andersen cular grain mount. The mount was then polished to (2002), and age calculations and Concordia diagram

Fig. 4 Three stages of magmatism and two epochs of mineralization in the Baizhangyan deposit. MG: monzonitic granite; FG, ﬁne-grained granite; DD, diorite dyke; Act, actinolite; Bi, biotite; Cal, calcite; Hb, hornblende; K-fs, K-feldspar; Mol, molybdenite; Pl, plagioclase; Q, quartz; Sh, scheelite. construction were undertaken using ISOPLOT (ver. average U and Th concentrations of 230 and 15 ppm, 2.49) (Ludwig, 2001). The concentrations of U and Th respectively. A weighted mean 207Pb/206Pb age of 610 ± during each analysis were determined by comparing 11 Ma (2σ) for the GJ standard was obtained during background-corrected count rates with the mean count this study, corresponding to a highly precise 207Pb/206Pb rates on the GJ standard, which has well-known age of 608.5 ± 0.4 Ma obtained by Thermal Ionization

Mass Spectrometer (TIMS) analysis (Jackson et al., tions, yielding Th/U ratios between 1.04 and 5.80, 2004). suggesting a magmatic origin (Th/U > 0.4) (Rubatto & Gebauer, 2000; Moller et al., 2003). Twelve of these 206 238 4.2 Sampling, molybdenite Re–Os dating, and analyses yielded Pb/ U ages of 125–132 Ma with a weighted mean age of 129.0 ± 1.2 Ma (Mean Standard sulfur isotope analytical methods Weight Deviation (MSWD) = 0.98; Fig. 6a), this age is Molybdenite is present in disseminated and laminar interpreted to be the emplacement age of the monzo- skarns in the study area, and molybdenite samples nitic granite. One spot yielded a 206Pb/238U age of 120 ± (Figs 2b, 4e, f) for Re–Os dating and sulfur isotope 2 Ma, which may record the timing of emplacement of analysis were collected from orebodies. A sulfide sepa- the alkaline granite, and another three spots yielded rate was obtained by gravitational and magnetic sepa- 206Pb/238U ages of 109–113 Ma, with a weighted mean ration before handpicking of molybdenite, pyrite, and age of 110.4 ± 2.9 Ma (MSWD = 0.64), which may > pyrrhotite (purity 99%) separates under a binocular record the timing of emplacement of the late syenite microscope at the Chengxin Geology Service Ltd. dykes. A final two spots yielded 206Pb/238U ages of 152– Re–Os isotope analysis was undertaken at the Re–Os 155 Ma, considerably older than the main group of Laboratory, National Research Center of Geoanalysis, ages of between 125 and 132 Ma. Chinese Academy of Geological Sciences, Beijing, Twenty analyses were conducted on 17 zircons of the China, using a Thermo Electron Corporation TJA sample 10BZ-2-6 from the Baizhangyan fine-grained X-series ICP–MS and following the analytical proce- granite. These zircons have U and Th concentrations of dures of Qu and Du (2003) and Du et al. (2004). Model 66–1419 and 180–2123 ppm, respectively, yielding = + 187 ages were calculated as follows: t {ln (1 Os/ Th/U ratios between 0.58 and 3.19, indicative of a mag- 187 λ λ 187 Re)}/ , where is the decay constant of Re and is matic origin (Th/U > 0.4) (Rubatto & Gebauer, 2000; × −11 −1 equal to 1.666 10 year (Smoliar et al., 1996), here Moller et al., 2003). All 20 spots yielded 206Pb/238U ages “t” is defined as model age. of 131–139 Ma with a weighted mean age of 135.34 ± Sulfur isotope analysis was undertaken at the Isotope 0.92 Ma (MSWD = 0.85; Fig. 6b), interpreted as the Laboratory, Institute of Mineral Resources, Chinese crystallization age of this granite. This age is much Academy of Geological Sciences, Beijing, China, and at more consistent with the SHRIMP U–Pb age reported the Isotope Laboratory, Wuhan Institute of Geology by Qin et al. (2010) and Song et al. (2013) for the fine- and Mineral Resources, China. The molybdenite, grained granite. pyrite, and pyrrhotite used during sulfur isotope The majority of zircons separated from one of the analysis were manually separated before analysis as Baizhangyan diorite dykes (sample 10BZ-2-1) have low follows. Oxygen and sulfides were added to a copper U (86–284 ppm) and Th (75–489 ppm) concentrations, crucible before heating under vacuum conditions to yielding Th/U ratios of 0.56–1.83, suggesting a mag- produce sulfur dioxide. This was then analyzed using a matic origin (Th/U > 0.4) (Rubatto & Gebauer, 2000; Micromass MAT-251 Isotope mass spectrometer, yield- Moller et al., 2003). Nine spots yielded 206Pb/238U ages δ ing sulfur isotope compositions using notations as of 141–149 Ma with a weighted mean age of 145.3 ± 1.7 per mil differences relative to Canyon Diablo Troilite Ma (MSWD = 0.81; Fig. 6c), interpreted as the crystal- ± (CDT). Total analytical errors are approximately 0.2‰ lization age of the vein. A single spot yielded the oldest σ ata2 uncertainty level. age encountered during this study (2545 Ma), an age that is either unreliable or suggests that Neoarchean to 5. Results Paleoproterozoic South China craton material occurs in Mesozoic igneous rocks. Three spots yielded older ages 5.1 LA–ICP–MS U–Pb dating of 700–865 Ma, which may record the Neoproterozoic Zircon U–Pb analysis spots and CL images are shown Jinning Orogeny or may be inherited from the sur- in Figure 5, and the analytical results for the Qingyang rounding wall-rocks. Another two spots yielded an age monzonitic granite, the Baizhangyan fine-grained of 168 ± 6 Ma, considerably older than the magmatic granite, and the diorite dykes are listed in Tables 1–4. ages of 141–149 Ma. A total of 18 spots within zircons from the Qingyang The majority of zircons separated from the other monzonitic granite (sample 09BZ-2-6) were analyzed Baizhangyan diorite dyke sample (10BZ-1-8) also have during this study. These zircons have uniform U (205– low U (38–561 ppm) and Th (36–746 ppm) concentra- 1049 ppm) and variable Th (221–5833 ppm) concentrations, yielding Th/U ratios between 0.57 and 2.15,

© 2015 The Authors Resource Geology published by Wiley Publishing Asia Pty Ltd on behalf of The Society of Resource Geology 199 200 Li C. tal et . eoreGooypbihdb ie ulsigAi t t nbhl fTeSceyo eoreGeology Resource of Society The of behalf on Ltd Pty Asia Publishing Wiley by published Geology Resource 05TeAuthors The 2015 ©

Fig. 5 Zircon CL images and analysis spots. (a) Qingyang MG (09BZ-2-6), (b) Baizhangyan FG (10BZ-2-6), (c) Baizhangyan DD (10BZ-2-1) and (d) Baizhangyan DD (10BZ-1-8). Baizhangyan Skarn mineralization

Table 1 LA–ICP–MS zircon U–Pb isotopic data of Qingyang monzonic granite (09BZ-2-6) Spot Element (ppm) Th/U Isotope rate Age(Ma) Th U 207Pb/235U ± 1σ 206Pb/238U ± 1σ 206Pb/238U ± 1σ 1 866 704 1.23 0.1358 0.0029 0.0208 0.0003 132 2 2 679 264 2.57 0.1654 0.0095 0.0244 0.0005 155 3 3 391 320 1.22 0.1397 0.0043 0.0205 0.0003 131 2 4 364 261 1.39 0.1516 0.0054 0.0196 0.0003 125 2 5 5833 1006 5.80 0.1487 0.0041 0.0201 0.0003 128 2 6 357 234 1.53 0.1616 0.0112 0.0238 0.0005 152 3 7 592 365 1.62 0.1267 0.0047 0.0188 0.0003 120 2 8 1331 1049 1.27 0.1398 0.0022 0.0204 0.0003 130 2 9 552 303 1.82 0.1400 0.0039 0.0205 0.0003 131 2 10 625 518 1.21 0.1262 0.0072 0.0171 0.0004 109 2 11 794 369 2.15 0.1315 0.0099 0.0177 0.0005 113 3 12 234 205 1.14 0.1517 0.0143 0.0197 0.0006 126 4 13 464 401 1.16 0.1356 0.0054 0.0203 0.0003 130 2 14 397 321 1.24 0.1403 0.0061 0.0200 0.0004 127 2 15 221 211 1.04 0.1165 0.0122 0.0173 0.0005 111 3 16 639 248 2.58 0.1346 0.0058 0.0203 0.0003 129 2 17 284 238 1.19 0.1377 0.0055 0.0201 0.0003 128 2 18 529 377 1.40 0.1365 0.0035 0.0203 0.0003 129 2

Table 2 LA–ICP–MS zircon U–Pb isotopic data of the Baizhangyan ﬁne-grained granite (10BZ-2-6) Spot Element (ppm) Th/U Isotope rate Age (Ma) Th U 207Pb/235U ± 1σ 206Pb/238U ± 1σ 206Pb/238U ± 1σ 1 1269 1041 1.22 0.1492 0.0040 0.0211 0.0003 135 2 2 684 323 2.12 0.1449 0.0051 0.0213 0.0003 136 2 3 875 325 2.70 0.1391 0.0057 0.0214 0.0003 137 2 4 1731 1101 1.57 0.1544 0.0060 0.0212 0.0004 135 2 5 787 1363 0.58 0.1450 0.0029 0.0213 0.0003 136 2 6 1095 807 1.36 0.1445 0.0035 0.0214 0.0003 136 2 7 350 275 1.27 0.1480 0.0124 0.0215 0.0005 137 3 8 1023 832 1.23 0.1478 0.0032 0.0214 0.0003 136 2 9 1610 1163 1.39 0.1425 0.0027 0.0213 0.0003 136 2 10 180 66 2.72 0.1392 0.0222 0.0206 0.0007 131 4 11 595 259 2.29 0.1444 0.0105 0.0206 0.0005 131 3 12 1672 525 3.19 0.1517 0.0067 0.0214 0.0004 136 2 13 593 511 1.16 0.1465 0.0038 0.0208 0.0003 132 2 14 402 176 2.29 0.1441 0.0081 0.0212 0.0004 135 2 15 477 402 1.19 0.1509 0.0043 0.0208 0.0003 133 2 16 1972 1279 1.54 0.1421 0.0029 0.0211 0.0003 134 2 17 898 544 1.65 0.1424 0.0044 0.0217 0.0003 138 2 18 554 263 2.11 0.1384 0.0071 0.0218 0.0004 139 2 19 2123 1419 1.50 0.1425 0.0027 0.0209 0.0003 133 2 20 822 407 2.02 0.1448 0.0041 0.0213 0.0003 136 2

again suggesting a magmatic origin (Th/U > 0.4) be an unreliable age or may indicate the existence of (Rubatto & Gebauer, 2000; Moller et al., 2003). Nine Paleoproterozoic zircons of the South China Craton spots yielded 206Pb/238U ages of 141–149 Ma with a within Mesozoic igneous rocks. Another ﬁve spots in weighted mean age of 145.7 ± 1.7 Ma (MSWD = 0.72; three zircons yielded an older age of 754–847 Ma, Fig. 6d), interpreted to be the crystallization age of this which may record the Neoproterozoic Jinning Orogeny dyke. One spot yielded an age of 1992 Ma, which may or may have been inherited from wall-rocks.

Table 3 LA–ICP–MS zircon U–Pb isotopic data of the Baizhangyan diorite dyke (10BZ-2-1) Spot Element (ppm) Th/U Isotope rate Age (Ma) Th U 207Pb/235U ± 1σ 206Pb/238U ± 1σ 206Pb/238U ± 1σ 1 142 190 0.75 10.97244 0.14217 0.48406 0.00576 2545 25 2 164 91 1.81 0.17908 0.01714 0.02648 0.00062 168 4 3 122 96 1.27 0.16057 0.01218 0.02279 0.00042 145 3 4 98 86 1.15 0.18056 0.02645 0.02640 0.00091 168 6 5 144 97 1.48 0.15620 0.01261 0.02295 0.00043 146 3 6 159 117 1.36 0.15732 0.01261 0.02241 0.00047 143 3 7 280 168 1.67 0.15846 0.01103 0.02263 0.00046 144 3 8 197 128 1.54 0.14584 0.01284 0.02209 0.00048 141 3 9 130 104 1.26 0.15605 0.01357 0.02331 0.00048 149 3 10 351 283 1.24 1.03632 0.01898 0.11473 0.00143 700 8 11 273 150 1.83 0.15448 0.00938 0.02281 0.00039 145 2 12 489 284 1.72 0.15170 0.00775 0.02315 0.00039 148 2 13 199 181 1.10 1.16570 0.02532 0.12902 0.00169 782 10 14 75 135 0.56 1.38190 0.02741 0.14369 0.00184 865 10 15 137 99 1.37 0.15327 0.01149 0.02259 0.00040 144 3

Table 4 LA–ICP–MS zircon U–Pb isotopic data of the Baizhangyan diorite dyke (10BZ-1-8) Spot Element (ppm) Th/U Isotope rate Age (Ma) Th U 207Pb/235U ± 1σ 206Pb/238U ± 1σ 206Pb/238U ± 1σ 1 37 38 0.96 1.2515 0.0459 0.1404 0.0022 847 12 2 36 41 0.88 1.2478 0.0400 0.1377 0.0020 831 11 3 746 561 1.33 1.2264 0.0177 0.1337 0.0016 809 9 4 659 493 1.34 1.1652 0.0170 0.1240 0.0015 754 8 5 141 130 1.08 0.1567 0.0086 0.0229 0.0004 146 2 6 160 134 1.19 0.1576 0.0093 0.0231 0.0004 147 2 7 124 91 1.35 0.1481 0.0138 0.0225 0.0005 144 3 8 260 417 0.62 6.1916 0.0795 0.3620 0.0042 1992 20 9 198 146 1.36 0.1623 0.0108 0.0231 0.0005 147 3 10 338 157 2.15 0.1495 0.0094 0.0231 0.0004 147 3 11 70 53 1.32 0.1504 0.0196 0.0221 0.0005 141 3 12 59 103 0.57 1.2327 0.0265 0.1347 0.0017 815 10 13 241 193 1.25 0.1431 0.0127 0.0225 0.0004 144 2 14 48 41 1.15 0.1586 0.0300 0.0232 0.0008 148 5 15 151 104 1.44 0.1632 0.0111 0.0234 0.0004 149 3

5.2 Re–Os ages and sulfur isotope analyses Du et al. (2007a) indicate that the molybdenite used for Re–Os dating during this study has not undergone The results of molybdenite Re–Os dating are given in decoupling of Re and 187Os. From skarn ore, two Table 5, and the results of sulfur isotope analysis of molybdenite samples (09BZ-3-1-1, 09BZ-3-1-2) yielded sulﬁdes from the Baizhangyan deposit are given in Re–Os model ages of 143.8–146.3 Ma, with another Table 6. 10 molybdenite samples yielding a relatively narrow Concentrations of Re and 187Os within molybdenite range of Re–Os ages from 133.3 to 136.8 Ma. These data are 7.75–50.94 ppm and 14.62–78.10 ppb, respectively. were processed using the ISOPLOT/Ex program, The decoupling of Re and 187Os in molybdenite has yielding an isochron age of 136.9 ± 4.5 Ma (MSWD = been reported by Stein et al. (1998, 2001, 2003), and 0.51) with an initial 187Os of −0.19 ± 0.43 ppb and a Du et al. (2007a) reported methods whereby this weighted mean age of 135.0 ± 1.2 Ma (MSWD = 0.37; decoupling could be reduced, although the criteria of Fig. 7). These ages are consistent with a molybdenite

Fig. 6 ICP-MS U–Pb concordia diagrams and weighted mean age diagrams of zircons from (a) the Qingyang MG (09BZ-2-6), (b) Baizhangyan FG (10BZ-2-6) and (c, d) DDs (10BZ-2-1, 10BZ-1-8).

Re–Os isochron ages reported by Qin et al. (2010) and sulfur isotope diffusion between molybdenite and Song et al. (2013), indicating that the model and iso- pyrite (or pyrrhotite) should yield a positive value. δ34 chron ages presented here are reliable. However, molybdenites have SCDT values (1.6‰ and δ34 The SCDT values of sulfides from the Baizhangyan 4.8‰) that are lower than those of pyrrhotite that δ34 deposit are between 1.6‰ and 8.1‰, with SCDT formed during the first period of mineralization values of molybdenite, pyrite, and pyrrhotite varying (8.1‰), and second period of molybdenites have δ34 ∼ between 1.6‰ and 6.6‰, clustering at 7.2‰, and clus- SCDT values ( 6.6‰) that are lower than the values of tering at 8.1‰, respectively. The fractionation factors of pyrite (∼7.2‰) that formed during the same period. sulfur isotope between pyrite and hydrogen sulfide, These data indicate a sulfur isotopic disequilibrium pyrrhotite and hydrogen sulfide, and molybdenite and and suggest that these sulfides formed at different hydrogen sulfide (Ohmoto & Rye, 1979) indicate that stages of mineralization.

Table 5 Results of Re–Os isotope analyses of molybdenite from the Baizhangyan deposit Sample No. Re ± 1σ 187Re ± 1σ Common ± 1σ 187Os ± 1σ Age ± 1σ (ppb) (ppb) Os (ppb) (ppb) (Ma) 09BZ-3-1-1 50478 419 31726 264 0.0337 0.0083 76.12 0.64 143.8 2.1 09BZ-3-1-2 50936 383 32014 240 0.0090 0.0259 78.10 0.67 146.3 2.0 09BZ-3-2 7755 57 4874 36 0.1006 0.0085 10.99 0.09 135.2 1.9 09BZ-3-5 8989 77 5650 48 1.4315 0.0231 12.90 0.10 136.8 2.0 09BZ-3-6 11838 92 7441 58 0.0967 0.0084 16.59 0.14 133.7 1.9 09BZ-3-9 10299 95 6473 59 0.2532 0.0140 14.62 0.12 135.4 2.0 10BZ-1-1-1 9736 77 6119 48 0.0486 0.0616 13.74 0.12 134.6 1.9 10BZ-1-1-2 9532 96 5991 60 0.0053 0.0240 13.41 0.13 134.3 2.1 10BZ-1-5-1 7236 58 4548 36 0.0479 0.0486 10.11 0.08 133.3 1.9 10BZ-1-5-2 7056 62 4435 39 0.0295 0.0422 9.962 0.09 134.7 2.0 10BZ-2-12-3 14502 113 9115 71 0.3333 0.0300 20.59 0.26 135.5 2.3 10BZ-2-12-4 15237 116 9577 73 0.1622 0.0423 21.85 0.21 136.8 2.0

Table 6 Results of sulfur isotope analyses of sulﬁde from the Baizhangyan deposit

δ34 Sample No. Mineral Host SCDT (‰) 09BZ-3-1-1 molybdenite skarn 1.6 09BZ-3-1-1 pyrrhotite skarn 8.1 09BZ-3-1-2 molybdenite skarn 4.8 09BZ-3-1-2 pyrrhotite skarn 8.1 09BZ-3-2 molybdenite skarn 6.2 09BZ-3-6 molybdenite skarn 3.6 09BZ-3-9 molybdenite skarn 4.8 10BZ-1-1 molybdenite skarn 6.2 10BZ-1-5 molybdenite skarn 5.9 10BZ-2-11 pyrite skarn 7.2 10BZ-2-12-3 molybdenite skarn 6.6 10BZ-2-12-3 pyrite skarn 7.2 10BZ-2-12-4 molybdenite skarn 6.6

Fig. 7 Re–Os isotopic isochron diagram and weighted 6. Discussion mean age diagram of molybdenite from the Baizhangyan deposit. 6.1 Implications of zircon U–Pb geochronology and regional magmatism for the geodynamic Datuanshan quartz monzodiorites, and the Huchen processes controlled the Baizhangyan deposit granodiorite at Shizishan, Tongling, Anhui Province Mesozoic diorites are exposed over a wide area near yielded ages of 138.21 ± 0.82, l39.9 ± 1.1, 139.3 ± 1.2, and the Qingyang complex, and zircon LA–ICP–MS U–Pb 140.9 ± 1.2 Ma, respectively (Wu et al., 2008). These ages dating of diorite dykes related to W–Au mineralization suggest that diorite was located beneath the Qingyang within the Baizhangyan deposit yielded ages of 145.3 ± complex. The majority of regional contemporary basic 1.7 and 145.7 ± 1.7 Ma. SHRIMP U–Pb zircon dating of rocks were derived from an enriched region of the a pyroxene diorite from the Chaoshan gold deposit, a lithospheric mantle (Chen et al., 2001; Li, 2001; Yan quartz diorite within the Xinqiao Cu–Fe–Au deposit, et al., 2003; Yuan et al., 2008). Also Zhou et al. (2005) and a quartz diorite at Xiaotongguangshan within the suggested that high-K calc-alkaline diorites in this area Tongling mineral district in Anhui Province yielded were formed by assimilation and fractional crystalliza- ages of 142.9 ± 1.1 (Wang et al., 2004a), 140.4 ± 2.2 tion (AFC) processes involving 30% crustal material (Wang et al., 2004a), and 139 ± 3Ma(Wanget al., 2004b), and 70% mantle-derived magma. respectively. Zircon SHRIMP dating of the Baimanshan Zircon LA–ICP–MS U–Pb dating of the monzonitic pyroxene monzodiorite, the Jiguanshan and granite within the Qingyang complex, a ﬁne-grained

© 2015 The Authors 204 Resource Geology published by Wiley Publishing Asia Pty Ltd on behalf of The Society of Resource Geology Baizhangyan Skarn mineralization granite related to W–Mo mineralization within the mantle–crust mixtures (dozens of ppm), and to crustal- Baizhangyan deposit yielded ages of 129.0 ± 1.2 and only molybdenite (several ppm) (Mao et al., 1999; Selby 135.34 ± 0.92 Ma, respectively. These ages suggest that & Creaser, 2001; Stein et al., 2001; Berzina et al., 2005; mineralization-related fine-grained granite could not Xie et al., 2007). Molybdenite that formed during the have formed from the Qingyang monzonitic granite, first period of mineralization (143.8–146.3 Ma) of the but is more likely to have formed from interaction Baizhangyan deposit contain 50.48–50.94 ppm Re, between the Qingyang granodiorite (∼137.6 Ma) and similar to deposits associated with mixtures of mantle Neoproterozoic Lantian Formation limestones during and crustal material. In comparison, molybdenite that mineralization. However, more evidence is needed formed during the second period of mineralization to support this claim. The low εNd (–10.06 to −4.90) (135.0 ± 1.2 Ma) contain 7.75–11.84 ppm Re, similar to and moderate 87Sr/86Sr (0.7074–0.7086) values of the crustal-derived deposits. The εNd(t) values (–16 to Qingyang granodiorite (∼137.6 Ma) (Chen et al., 1985, −12.3) of the scheelite from the Baizhangyan deposit 1993) are distinct from mid-oceanic-ridge basalt also indicated that the ore-forming materials in (MORB) values (Defant & Drummond, 1990), indicat- the W–Mo deposit was mainly derived from crustal ing that the granodiorite could not have been directly sources (Song et al., 2014). These concentrations derived from partial melting of a subducted slab, but suggest that the first phase of skarn type of W-Au min- more likely originated from lithospheric mantle of the eralization at 143.8–146.3 Ma associated with the Yangtze Craton that interacted with crustal material. diorite dykes was a mixed derivation between crust This suggests that the Qingyang granodiorite is likely and mantle and the second one of skarn type of W–Mo to be a magnetite-series (Ishihara, 1977) or I-type mineralization at 135.0 ± 1.2 Ma associated with the granitoid (Chappell & White, 1974). Some previous fine-grained granite formed from a crustal-dominated researches have suggested that the majority of the system. regional contemporaneous intermediate–acidic rocks The δ34S composition of magmatic sulfides in uncon- were generated by crust–mantle mixing (Chang et al., taminated mafic and felsic magmas is normally 0 ± 1991; Tang et al., 1998; Tao et al., 1998; Chen et al., 2001; 3.0‰ (Ohmoto, 1986; Chaussidon & Lorand, 1990; Zhou et al., 2005), although others suggest that some of Ohmoto & Goldhaber, 1997). The isotopic composition the intermediate–acid rocks are adakitic and formed by of magmatic hydrothermal fluids derived from ∼0‰ partial melting of the lower crust (Wang et al., 2001, I-type granitic magmas can be 3–5‰ heavier than the 2003; Xu et al., 2001; Zhao & Tu, 2003). δ34S of the melt (Ohmoto, 1986; Chaussidon & Lorand, 1990). Thus, the positive δ34S value for molybdenites formed during the first and the second period of 6.2 Implications of molybdenite Re–Os and mineralization in this study is +1.6 to +4.5‰ and +3.6 sulfur isotopic data for genesis of the to +6.6‰, respectively. The positive δ34S value for Baizhangyan deposit molybdenites and narrow range is agreement with The Re–Os isotopic system can determine the timing of derivation of sulfur from a relatively large scale granitic mineralization, provides information on metal sourc- magma system with fairly homogeneous δ34S values ing, and is a highly sensitive indicator of possible and similar fluid separation history at geographically crustal involvement during ore formation (Foster et al., varying localities. Magmatic fluids are believed to play 1996; Lambert et al., 1999; Mao et al., 1999; Selby & a major role in the formation of two periods of W–Mo Creaser, 2001; Stein et al., 2001). The Re–Os ages mineralization. obtained during this study can be divided into Assimilation of sulfur from country rocks also has two groups that identify two different periods of min- been regarded as an important process in the two eralization, consistent with the geology and mineral periods of skarn-type W–Mo mineralization in the assemblages. These data indicate that the Baizhangyan Baizhangyan deposit. The δ34S value (+14.61‰) of deposit formed during two periods of mineralization: sulfide (dominantly pyrite) in the country rock diorite dyke-related W–Au mineralization at 143.8– (Lantian Formation) was reported by (Yan et al., 1994). 146.3 Ma, and fine-grained granite-related W–Mo min- In this study, the δ34S composition for Fe-sulfide con- eralization at 135.0 ± 1.2 Ma. centrates (dominantly pyrite and pyrrhotite) hosted in Previous studies have shown that Re concentrations skarn ores range from +7.2 to +8.1‰. These δ34S values in molybdenite decrease progressively in sulfides suggest a contribution from country rocks sources (e.g. derived from mantle sources (hundreds of ppm), to sulfur derived from evaporites of Lantian Formation).

These indicate that increasing amounts of crustal mineralization, whereas W–Mo mineralization formed material and/or conditions of oxygen fugacity were during the second stage, both of which are consistent involved during crystallization and the emplacement with stages of formation of Cu–Mo–Au deposits within of late-stage granitic magmas; alternatively, these the Tongling and Anqing uplifts. This suggests the late-stage granitic magmas may have contained Baizhangyan deposits formed in a similar geological more crustal material than the mineralization-related setting to the Cu–Au deposits within the Tongling and intrusions. Anqing uplifts. The crust in southern Anhui Province underwent intensive interaction with underplated basaltic 6.3 Links between W–Mo mineralization and magmas derived from the upper portion of the litho- geodynamic events spheric mantle during the Mesozoic (145–125 Ma) (Du Previous studies have reported isotopic ages of et al., 2007b). This basaltic lithospheric mantle material Cu–Mo–Au(–Fe) deposits from southern Anhui Prov- upwelled along a weak part of the crust within the ince (Sun et al., 2003; Yu & Mao, 2004; Chen et al., 2005; Yangtze Plate at some point before 146.3 Ma, with Zhou et al., 2005, 2008; Ma et al., 2006). Mao et al. (2004) mantle-derived basaltic magmas undergoing crustal suggested that the majority of deposits in the contamination and forming diorites immediately Middle–Lower Yangtze River Valley Fe–Cu–Au–Mo prior to the first phase of W–Au mineralization metallogenic belt formed at 143.7–134.7 Ma, although (143.8–146.3 Ma). The Qingyang granodiorite intruded others have suggested that Cu–Au deposits within the along NE–SW- and NNE–SSW-striking faults, forming Tongling and Anqing uplifts formed at 145–135 Ma a second phase of mineralization, associated with (Wang et al., 2004c; Zhou et al., 2005, 2007). Fe deposits the formation of the majority of ore within the in the Luzong and Ningwu volcanic basins formed at Baizhangyan deposit along contact zones between 135–127 Ma (Zhou et al., 2007), whereas U–Au deposits granodiorite, evolved fine-grained granite, and related to A-type granitoids in uplifted areas formed Neoproterozoic Lantian Formation limestones at at 126–124 Ma. W–Au mineralization within the 137.6–130.0 Ma (Fig. 8). The Baizhangyan deposit Baizhangyan deposit formed during the first stage of formed during an Early Cretaceous magmatic–

Fig. 8 Evolution of Qingyang intrusion, Baizhangyan FG and Formation of deposit during 146.3–129.0 Ma.

© 2015 The Authors 206 Resource Geology published by Wiley Publishing Asia Pty Ltd on behalf of The Society of Resource Geology Baizhangyan Skarn mineralization hydrothermal event related to intraplate extensional Mineral Resources for assistance in the ﬁeld and for magmatism, potentially associated with subduction of helpful suggestions that have improved this manu- the Paleo-Paciﬁc Plate (Maruyama et al., 1997; Li, 2000; script. We are grateful for constructive reviews from Sun et al., 2007). Ken-ichiro Hayashi and GX Song.

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