Geology and geochemistry of the adjacent Changkeng gold and Fuwang silver deposits, Guangdong Province, South China

Ore Geology Reviews 31 (2007) 304 – 318 www.elsevier.com/locate/oregeorev

Geology and geochemistry of the adjacent Changkeng gold and Fuwang silver deposits, Guangdong Province, South China Hua-Ying Liang a,⁎, Ping Xia, Xiu-Zhang Wang a , Jing-Ping Cheng a , Zhen-Hua Zhao a , Cong-Qiang Liu b a

Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry and South China Sea Institute of Oceanography, Chinese Academy of Sciences, 510640 Guangzhou, People's Republic of China b Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002, People's Republic of China Received 12 December 2003; accepted 22 October 2005 Available online 30 June 2006

Abstract The Changkeng Au and Fuwang Ag deposits represent an economically significant and distinct member of the Au–Ag deposit association in China. The two deposits are immediately adjacent, but the Au and Ag orebodies separated from each other. Ores in the Au deposit, located at the upper stratigraphic section and in the southern parts of the orefield, contain low Ag contents (b 11 ppm); the Ag orebodies, in the lower stratigraphic section, are Au-poor (b 0.2 ppm). Changkeng is hosted in brecciated cherts and jasperoidal quartz and is characterized by disseminated ore minerals. Fuwang, hosted in the Lower Carboniferous Zimenqiao group bioclastic limestone, has vein and veinlet mineralization associated with alteration comprised of quartz, carbonate, sericite, and sulfides. Homogenization temperatures of fluid inclusions from quartz veinlets in the Changkeng and Fuwang deposits are in the range of 210 ± 80 °C and 230 ± 50 °C, respectively. Salinities of fluid inclusions from the two deposits range from 1.6 to 7.3 wt. % and 1.6 to 2.6 wt.% equiv. NaCl, respectively. The δDH2O, δ18OH2O, δ13CCO2 and 3He/4He values of the fluid inclusions from the Changkeng deposit range from − 80‰ to −30‰, − 7.8‰ to − 3.0‰, −16.6‰ to −17.0‰ and 0.0100 to 0.0054 Ra, respectively. The δDH2O, δ18OH2O, δ13CCO2 and 3He/4He values of fluid inclusions from the Fuwang deposit range from − 59‰ to −45‰, −0.9‰ to 4.1‰, −6.7‰ to − 0.6‰ and 0.5930 to 0.8357 Ra, respectively. The δDH2O, δ18OH2O, δ13CCO2 and 3He/4He values of the fluid inclusions suggest the ore fluids of the Changkeng Au-ore come from the meteoric water and the ore fluids of the Fuwang Ag-ore are derived from mixing of magmatic water and meteoric water. The two deposits also show different Pb-isotopic signatures. The Changkeng deposit has Pb isotope ratios (206Pb/204Pb: 18.580 to 19.251, 207Pb/204Pb: 15.672 to 15.801, 208Pb/ 204 Pb: 38.700 to 39.104) similar to those (206Pb/204Pb: 18.578 to 19.433, 207Pb/204Pb: 15.640 to 15.775, 208Pb/204Pb: 38.925 to 39.920) of its host rocks and different from those (206Pb/204Pb: 18.820 to 18.891, 207Pb/204Pb: 15.848 to 15.914, 208Pb/204Pb: 39.579 to 39.786) of the Fuwang deposit. The different signatures indicate different sources of ore-forming material. Rb–Sr isochron age (68 ± 6 Ma) and 40Ar–39Ar age (64.3 ± 0.1 Ma) of the ore-related quartz veins from the Ag deposit indicate that the Fuwang deposit formed during the Cenozoic Himalayan tectonomagmatic event. Crosscutting relationships suggests that Au-ore predates Ag-ore. The adjacent Changkeng and Fuwang deposits could, however, represent a single evolved hydrothermal system. The ore fluids initially deposited Au in the brecciated siliceous rocks, and then mixing with the magmatic water resulted in Ag deposition within fracture zones in the limestone. The deposits are alternatively the product of the superposition of two different geological events. Age evidence for the Fuwang deposit, together with the Xiqiaoshan Tertiary volcanic-hosted Ag deposit in the

⁎ Corresponding author. E-mail address: [email protected] (H.-Y. Liang). 0169-1368/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2005.10.007

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same area, indicates that the Pacific Coastal Volcanic Belt in the South China Fold Belt has greater potential for Himalayan precious metal mineralization than previous realized. © 2006 Elsevier B.V. All rights reserved. Keywords: Himalayan; Gold; Silver; Guangdong; Changkeng; Fuwang; China

1. Introduction In the area around the Changkeng–Fuwang district, a series of different types of deposits have been discovered. These include the adjacent Changkeng Au and Fuwang Ag deposits, the Nanpengshan–Hengjiang Pb– Zn–(Cu) deposit, the Chashan Pb–Zn deposit and the Dieping and Xiqiaoshan Ag deposits (Fig. 1). Discovery of the Xiqiaoshan Tertiary volcanic-hosted Ag deposit in the South China Fold Belt (SCFB) suggests that the precious-metal metallogeny of the belt may have developed during four orogenic stages: the Caledonian Orogeny (ca. 400 Ma), the Indosinian Orogeny (ca. 200 Ma), the Yanshanian Orogeny (180 to 90 Ma) (Pirajno and Bagas, 2002), and the Himalayan Orogeny (b90 Ma; Wang and Mo, 1995). The Changkeng deposit is the largest Au deposit hosted in brecciated siliceous rocks in China. The giant Fuwang Ag deposit (N 5000 t Ag; Tu, 2000) is the largest Ag deposit in the SCFB. Gold prospecting in the vicinity

of Changkeng village began in 1991 on the basis of a small outcrop of anomalous gold in brecciated siliceous rocks. Two big Au orebodies were discovered after 2 years of prospecting. The Fuwang Ag orebodies were discovered during this prospecting. Ore-grade Ag was first discovered in the deeper parts of drill holes zk0002, zk0406 and zk1901 in 1992. Three Ag-bearing ore zones were delineated by 1995. The large Changkeng Au and giant Fuwang Ag deposits are not only economically significant but also unusual because, although adjacent to one another, the two deposits are spatially separated. Du et al. (1993) summarized the geological characteristics of the two deposits. The Au and Ag deposits contain fluid inclusions that have overlapping homogenization temperature and salinity values (Guo and Du, 1996), but have distinct H-, O-, Cand He-isotope compositions in their fluid inclusions (Guo and Du, 1996; Sun et al., 1999; Zhang et al., 2000). Isotopic signatures of the fluid inclusions in the Au and Ag deposits indicate that the ore fluids consisted of

Fig. 1. Simplified regional geologic map of the Changkeng Au and Fuwang Ag deposits (after Du et al., 1993).

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evolved meteoric water or formational water (Guo and Du, 1996; Sun et al., 1999; Zhang et al., 2000). Most researchers, including those cited above, argue that the Changkeng and Fuwang deposits were formed by the same geological event during the Yanshanian Orogeny (180 to 90 Ma), although there is no direct geological evidence to support this hypothesis yet. The Xiqiaoshan Tertiary volcanic-hosted Ag deposit in the vicinity of the Fuwang deposit raises the possibility that the Fuwang Ag deposit might have formed during the Himalayan period. Age determination of the Fuwang deposit, therefore, is important to understand the genesis of the deposit. No comprehensive ore genetic models have been proposed to explain the similarities and differences of the Changkeng and Fuwang, including their distinct geochemical signatures. This paper summarizes previous work and presents new data. A new genetic model is proposed for the deposits on the basis of these data. 2. Geological setting The Changkeng Au and Fuwang Ag deposits are located along the southwestern rim of the Shanshui basin (Fig. 1). The mining area is about 75 km SWof Guangzhou City (longitude 112°48′01″ to 112°50′13″E, latitude 23°00′00″ to 23°00′02″N). Rocks in the region are older metamorphic rocks, Palaeozoic and Mesozoic sedimentary rocks, Mesozoic granite, and Late Cretaceous to Cenozoic cover rocks (Fig. 1) (Bureau of Geology and Mineral Resources of Guangdong Province, 1985; Du et al., 1993).

Older metamorphic rocks in the district are Neoproterozoic to Ordovician metasedimentary sequences. The 5000-m-thick sequence of Neoproterozoic metamorphic rocks consists of schist, slate, and quartzose sandstone, intercalated with thin-bedded quartzite and locally outcrops in the western parts of the mining area (Fig. 1). These rocks are the oldest basement rocks in the region of western Guangdong and eastern Guangxi Province (Bureau of Geology and Mineral Resources of Guangdong Province, 1985). The Neoproterozoic metamorphic rocks are Ag-rich (averaging 100–800 ppb, Pang et al., 1996). Unconformably overlying the basement rocks is a sequence of Middle Devonian to Early Carboniferous marine sedimentary rocks that consist largely of sandstone and siltstone. These rocks are locally interbedded with Middle to Upper Devonian Maozifeng Group tuff, sandstone and limestone, Lower Carboniferous Menggongao Group siltstone and sandstone, and Shidenzi-, Ceshuiand Zimenqiao-Group limestone. The Carboniferous formations rest conformably on the Devonian Maozifeng Formation. The Au orebodies are hosted in the brecciated siliceous rocks on the top of the Zimenqiao Group bioclastic limestone (Du et al., 1993). The Ag orebodies are located in the fractures in the Zimenqiao Group bioclastic limestone (Du et al., 1993) (Fig. 2), where karst caves are present. Resting unconformably on the Lower Carboniferous limestone are Mesozoic to Cenozoic continental sedimentary clastic rocks consisting of Upper Triassic sandstone, conglomerate, pellite, and carbonaceous argillite of the Xiaoping Formation (Fig. 1) and Lower

Fig. 2. Profile along No. 4 exploration line in the Changkeng Au and Fuwang Ag deposits.

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Cretaceous gravel and macroclastic rock, overlain by Eocene volcanic rocks. Quaternary river wash, flood deposits and talus sediments outcrop widely in the orefield (Fig. 1). Although no intrusive rocks outcrop in or near the orefield (Du et al., 1993), Eocene volcanic and sedimentary rocks are widespread, and up to 2000 m thick, within the 2000 km2 Sanshui basin (Tang, 1987). The volcanic rocks consist of basalt, rhyolite, tuff, trachytic tuff, and pyroclastic agglomerate. The small Xiqiaoshan Ag deposit (N300 t Ag) was discovered in fractured and altered trachytic tuff near the volcanic vent (Fig. 1). This mineralization style indicates that there was hydrothermal activity in the area during the Himalayan orogenic event. The hydrothermal activity may have a genetic link to the formation of the larger Fuwang Ag deposit. Large faults in the area include the NE-striking Guangzhou–Chonghua (GZ-CH) fault zone, the EWstriking Gaoyao–Huila fault zone and the NW-striking Xijiang fault (Bureau of Geology and Mineral Resources of Guangdong Province, 1985; Du et al., 1993). The Guangzhou–Chonghua fault zone formed during the Caledonian Orogeny and was reactivated in the Mesozoic and Cenozoic periods. Additional movement along the Gaoyao–Huilai fault zone is thought to be related to Eocene uplift and volcanic activity in the Shanshui Basin (Bureau of Geology and Mineral Resources of Guangdong Province, 1985). Movement of large and deep faults in the area provided channels for the transportation of ore fluids.

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limestone and is characterized by disseminated mineralization. The orebodies are ca. 700 m long and 11 m thick; average grade is about 6–8 g/t Au (Du et al., 1993). The genesis of the Au-hosting siliceous rocks has been extensively debated. Du et al. (1993) and Zhang et al. (1998) both argue that the Au-hosting siliceous rocks are jasperoid quartz formed from limestone that was associated with the mineralizing event. However, Zhang

3. Deposit geology The Changkeng Au and Fuwang Ag deposits differ in their geologic characteristics, stratigraphic position, and metal contents. They are located very close to one another (Figs. 1 and 2), forming a composite association, but the Au and Ag ore bodies are spatially separated from one another. The Changkeng Au orebodies are present in the northern part of the orefield and in the upper part of the stratigraphic section (Du et al., 1993), whereas the Fuwang Ag orebodies are located in the southern part of the orefield and in the lower parts of the stratigraphic section (Fig. 2). 3.1. Changkeng Au deposit The detailed geological characteristics of the Changkeng Au deposit were summarized by Du et al. (1993) and Zhang and Xie (1996). The deposit, with total reserves of about 32 t Au, is hosted in brecciated siliceous rocks on the top of rocks of the Zimenqiao Group bioclastic

Fig. 3. Radiolaria in the Changkeng Au-hosting siliceous rocks (under SEM, analyzed by Hao-Ruo Wu), and hand sample from the Changkeng Au deposit. (A) Albaillella sp, (B) Follicucullus sp, (C) carbonaceous siliceous intercalated beds in brecciated rocks, detritus (white color) lies in a formation parallel to the carbonaceous siliceous intercalated beds, it appears to be a bioclastic debris flow.

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Table 1 Average chemical composition of gold ore, silver ore and chert, Changkeng Au and Fuwang Ag deposits, Guangdong Province, China Sample Gold number ore (n = 8)

Massive chert (n = 8)

Major oxides and SiO2 86.48 0.15 TiO2 Al2O3 3.97 Fe2O3 2.36 FeO 0.95 MnO 0.05 MgO 0.14 CaO 0.60 Na2O 0.03 K2O 0.90 P2O5 0.05 H2O+ 1.31 0.52 H2O− CO2 0.40 S 1.37 Total 99.28

elements (wt.%) 91.86 89.96 0.04 0.04 1.85 1.66 2.70 1.17 0.27 0.85 0.07 u.d. 0.1 0.08 0.15 1.28 0.17 0.08 0.38 0.43 0.05 0.09 0.28 0.86 0.04 0.21 no data 0.87 1.31 1.50 99.27 99.08

Trace elements (ppm) Number (n = 8) (n = 8) Gold 5.78 0.06 Silver 10.1 10.1 Cu 49.7 21.3 Pb 45.7 58.4 Zn 133.5 145.8 Ba 573.6 695.4 Co 2.0 3.7 V 17.8 19.7 Ni 16.3 15.9 Cr 39.4 43.6 Sr 49.5 37.7 As 380.0 282.6

Ag-ore between siliceous rocks and siliceous limestone (n = 8)

(n = 5) 0.18 248.2 321.4 2689.1 7895.4 125.5 3.9 9.1 15.9 37.7 35.1 327.3

Ag-ore in limestone (n = 8) 51.99 0.43 2.9 0.64 0.72 u.d. 0.18 21.36 0.07 0.4 0.03 1.84 0.3 17.23 no data 98.09

(n = 4) 0.16 297.8 559.0 2896.6 8796.5 43.4 4.1 5.0 11.6 38.0 45.7 231.6

u.d. = under determination.

and Xie (1996) and Xia et al. (1996) suggest that these rocks are syngenetic cherts, based on the fact that the host rocks are concordant with the underlying neritic bioclastic limestone and overlying argillite and/or siltstone of the Lower Carboniferous Zimenqiao Formation (Fig. 2). Zhang and Xie (1996) also suggest that carbonaceous brecciated fragments in the host rocks are similar to carbonaceous siliceous cement in the economic mineral association. Furthermore, the Au-hosting siliceous rocks have thinly bedded and laminated structures and are therefore similar to chert in their Fe/Ti, (Fe + Mn)/Ti ratios and on Fe–Mn–(Cu + Ni + Co) diagrams introduced by Böstrom (1983) (Xia et al., 1996). Further clues to the genesis of the siliceous rocks are apparent from their sedimentary features. The Au-hosting siliceous rocks dip SSE at 15–30°, averaging 15 m in thickness (thicknesses as much as 50 m are recorded)

(Zhang and Xie, 1996) and are more than 800 m long along strike. They consist of thin-bedded and laminated, massive, and brecciated material and are black in color. The laminated and massive material consists mainly of quartz (b0.01 mm in diameter), carbonaceous matter, framboidal pyrite, clay minerals (illite and dickite), detrital carbonate and less amount of chalcedony. A number of radiolaria (Fig. 3A, B), which suggest an age between Carboniferous and Permian, are present in some of the laminated and massive siliceous material. This suggests that some parts of the Au-hosting siliceous laminated and massive material are radiolarian chert. Some foraminifera and coral are also locally present in the gold-hosting massive siliceous material. Detritus in some brecciated rocks are oriented and they parallel the carbonaceous siliceous intercalated beds (Fig. 3C). These fossil and structural characteristics are compatible with a bioclastic debris flow and the silicification, therefore, must be younger than the sedimentation. The limestone–siltstone contacts are common sites of deep burial diagenetic silicification. Some of the massive zones contain framboidal pyrite and therefore may have formed by diagenetic processes as diagenetic chert. The non-brecciated massive and laminated chert is barren. Syngenetic and diagenetic silicification may not relate to gold deposition, although zones of both syngenetic and diagenetic silicification have high background Au contents (Table 1). The high background abundance of Au in the pre-ore silicification raises the possibility that preexisting Au-enrichment in the stratigraphic section hosting the Changkeng deposit, similar to processes indicated at the Meikle and neighboring Carlin-type deposits in Nevada (Emsbo et al., 2003). Variations in thickness over short distances in the siliceous rocks suggest that silicification is slightly discordant to bedding and therefore these rocks experienced overlaps of epigenetic silicification. The Changkeng Auore is hosted mainly in the breccia and these Au-bearing breccias imply that the gold was introduced later than the diagenetic silicification. Gold mineralization is associated with dissolution of carbonate and deposition of quartzose siliceous carbonate ores in the Changkeng Au deposit that contains between 20.80 and 2.47 wt.% CaO. More than half of the carbonate minerals in the altered limestone have been leached out during decalcification (Peters et al., 2002, 2007-this volume). The Au-hosting rocks, therefore, are formed by multistage silicification that at least includes syngenetic, diagenetic, and hydrothermal silicification. This hydrothermal silicification is related to Au deposition. The Changkeng gold ore is mainly in the breccias and less in the silicified carbonate. The brecciated fragments consist of thin-bedded and laminated chert (Fig. 4A),

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massive chert, as well as mudstone, quartz crystals, minor calcite, and chalcedony. The gold-mineralized breccias contain pyrite veinlets or disseminated pyrite in the matrix (Fig. 4B–H). Ore textures are brecciated and disseminated. The majority of the Au (N 82%) occurs in native form, typically 0.05 to 0.07 μm in diameter (Du et al., 1993). Minerals in the ore that carry this gold are quartz, clay minerals (illite, dickite), and pyrite, with average gold contents of 4.19 ppm, 19.92 ppm, and 15.80 ppm, respectively (Du et al., 1993). The native gold mainly is distributed in microfractures of pyrite and quartz, or along the crystal rims of illite, dickite, pyrite, and quartz crystals (Du et al., 1993). Crosscutting relations and mineral

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assemblages in the ore indicate that gold mineralization consisted of (1) an early quartz-disseminated pyrite–clay mineral (mainly illite and dickite) stage and (2) a late quartz–stibnite–realgar–orpiment–pyrite–clay mineral stage (Fig. 5). The first stage paragenetic association is characterized by brecciated or massive silicification with disseminated pyrite and/or pyrite veinlets. The late stage mineralization filled cracks in the first stage mineralization. Most of the gold is associated with the early stage; the late stage contains only minor amounts of gold. The Ag-content in the gold orebodies is low (generally b 11 ppm). Ore minerals in the gold deposit include disseminated anhedral pyrite, native gold, stibnite, realgar, and orpiment.

Fig. 4. Photomicrographs of polished thin sections illustrating salient mineralized features of the Changkeng Au and Fuwang Ag deposits. (A) Chert with bedded structure, showing the sedimentary characteristics. (B) Pyrite (Py) deposition in pore space in altered, quartz (Qz)–clay mineral microbreccias. (C) Quartz–clay mineral veinlet containing smaller grains of pyrite filling in quartz (Qz) and siliceous breccias (Si) consisting of quartz and clay minerals. (D) Pyrite veinlet cutting massive siliceous rocks (Msr). (E) Matrix with disseminated pyrite filled in massive silicification (Qz + clay minerals). (F) Quartz–pyrite veinlet cutting massive siliceous rock (Msr) and brecciated siliceous rocks (Bsr) with disseminated pyrite (Py). (G) Matrix with fine-grained disseminated pyrite (Py). (H) Pyrite (Py), carbon filling in the altered breccias consisting of quartz and clay minerals. (I) Mineralized vein consisting of sphalerite (Sph), galena (Ga), calcite (Cal) and quartz (Qz) from the Fuwang Ag deposit.

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Gangue minerals include quartz, illite, dickite, calcite, less fluorite, and barite (Du et al., 1993). Alteration associated with the Au-mineralization mainly is silicification, decalcification, and argillization that formed illite and dickite. 3.2. Fuwang Ag deposit The Fuwang deposit, located 20–50 m down-section from Changkeng deposit (Fig. 2) consists of three ore zones with a total reserve of about 6000 t Ag. The Fuwang deposit has distinct characteristics and mineral associations and contains only trace Au contents. The main orebody is about 1200 m long and averages 3.1 m in thickness (Du et al., 1993). The average grade is ca. 268 g/t Ag, 0.3% Pb, and 0.9% Zn; the deposit is poor in Au (b0.2 ppm). The deposit is hosted in Early Carboniferous Zimenqiao bioclastic limestone along the contact fault zones between the limestone and the overlying Changkeng Au-hosting siliceous rocks (Fig. 2). The Fuwang Ag-mineralization is localized by a fault system in Zimenqiao bioclastic limestone with zones of silicification. Crosscutting relations and mineral assemblages indicate that the Ag-mineralization consists of (1) an early stage (sulfide–quartz–calcite–Ag–minerals) and (2) a late quartz–calcite stage (Fig. 5). The early stage paragenetic association is characterized by coarse-grained quartz, calcite, sphalerite, galena, Ag-minerals, and lesser pyrite (Fig. 4I). Silver is present in sulfide–quartz–calcite veins, sulfide–quartz veinlets, and in the adjacent silicified zones. The ore is enveloped by widespread alteration zones composed of quartz, carbonate, sericite–illite and sulfide minerals.

Fig. 5. Paragenetic sequence for the Changkeng Au and Fuwang Ag deposits.

The main Ag-minerals are freibergite [Cu6(Ag, Fe)6Sb4S13], pyrargyrite (Ag3SbS3), andorite (PbAgSb3S6), diaphorite (AgSbS2), argentite (AgS2), as well as minor owyheeite [Ag3+xPb10−2xSb11+xS28], trace polybasite [(AgCu)16Sb12S11], acanthite (Ag2S) (Du et al., 1993), and native silver. The main sulfides are sphalerite and galena, with lesser pyrite and rare arsenopyrite, chalcopyrite, and bornite. The Ag-minerals mainly are distributed along rims or in microfractures of galena and sphalerite. Less commonly, the Ag-minerals are associated with quartz and calcite. In drillhole ZK 4802, at a depth between 235.9 and 236.9 m, where the average Au content is 5.1 ppm and average Ag content is 448.3 ppm, a small sulfide–quartz– calcite–silver vein crosscuts the Au-bearing breccias. This crosscutting relationship indicates that the Au deposit is older than the Ag deposit. 4. Geochemistry 4.1. Methods and analyses Samples of drillcore from different depths in the Au and Ag deposits were selected for analysis for major and trace elements. Whole rock samples collected from the Au orebodies consisted of massive- and bedded-chert, and those from the Ag orebodies were from the contact zone between Au-hosting siliceous rock and limestone, and Ag orebodies in limestone. Major chemical compositions were analyzed by wet chemical methods at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Trace elements were determined at the Nanjing University by X-ray fluorescence methods using lithium tetraborate fused disks and powder pellets. Relative accuracy for this determination is within 1%. Quartz samples from the early stage of Ag mineralization were analyzed for Rb–Sr and Ar–Ar isotope composition. The quartz samples were washed by deionized water before crushing to 40 to 80 mesh and were deemed N 99% pure by examination under a binocular microscope. Selected quartz samples were washed respectively by HCl and HNO3 to clean out the sulfide and carbonate minerals, and then washed by deionized water to eliminate acids. For Rb–Sr isotope analysis, the secondary fluid inclusions in quartz were first driven out by heat-explosion supersonic echo on the basis of the temperature difference between the primary and secondary fluid inclusion, in order to gain the true age of the mineralized quartz. Procedures for the Rb–Sr isochron analyses are discussed by Li et al. (1993). Firstly, 0.1–1 g of pretreated pure quartz was placed in a Teflon container,

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followed by addition of mixed 87Rb+84Sr spike. The quartz samples were dissolved by HF + HCl in a microwave furnace and completely converted to chlorate. Rb and Sr were separated by using the cation exchange technique. The Rb and Sr isotopes were measured on a Finigan Mat-261 multi-collector mass spectrometer at the Yichang Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences. Measured ratios were normalized for instrument mass fractionation based on replicated analyses of the NBS987 for 87Sr/86Sr standard. The total standard variations observed in duplicate analyses at 2σ generally are less than 1–3% for 87 Rb/86Sr, and less than 0.00003 for87Sr/86Sr. For Ar–Ar isotope analysis, the analytical procedures described by Sang et al. (1994) were used. The pretreated quartz samples were wrapped in aluminum foil and placed in the central part of the B8 drillhole site of the 49-2 Type reactor for fast neutron irradiation. The irradiation time is 60 h, with intermittent neutron flux 6.6 × 10 12 n/cm 2 /s and integrated neutron flux of 1.26 × 1018 n/cm2. Standards used to monitor neutron fluxes are ZBH-25 biotite, ZBJ Hornblende from China and GA1 550 Biotite from Australia, whose ages are 132.9 ± 1.32 Ma, 132.8 ± 1.4 Ma, and 97.6 ± 0.6 Ma, respectively. The isotope analyses were conducted on the RSILVER-10 Model gas-source mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The sample weight is 0.3416 g, irradiation parameter J is 0.00965, and the decay constant λ is 5.543 × 10− 10/a. The blanks of the whole system are 40 Ar = 1.6 × 10 − 14 mol and 36 Ar = 1.2 × 10− 16 mol. Analytical errors for Ar are in the range of 0.5–1%. For Pb isotope analyses, pyrite collected from the first stage Au-mineralization, galena collected from the main stage Ag-mineralization and chipped whole rock samples were washed in ultra-pure acetone and subsequently triple-distilled water. Samples were dissolved by HNO3 for pyrite, and HF, HCl and HNO3 for rocks. High pure Pb was separated by using the cation exchange resin with 0.5 mol/l HBr. The Pb was collected with 6N HCl and evaporated to dryness. The precipitated PbCl2 was dissolved in a few milliliters of water. About 0.5–1 μg PbCl2 in water solution was loaded on a single Re-filament with phosphoric acid and silica gel. The isotopic ratios were measured on a solidsource single-collector mass spectrometer at the Beijing Research Institute of Uranium Geology. Measured ratios were normalized for instrument mass fractionation based on replicated analyses of the NBS981 and NBS 982 common lead standard. The total standard variations observed in duplicate analyses at 2σ are generally

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less than 0.1% for 206Pb/204 Pb, 207Pb/204Pb and 0.3% for 208 Pb/204Pb ratios, respectively. Homogenization and ice-melting temperatures of fluid inclusions hosted by quartz were determined in the Linkam THMSG600 Heating/Freezing stage. Precision of the determinations is within ± 0.1 °C. For H- and O-isotopic compositional analyses of fluid inclusions, mineralized quartz vein samples were first crushed into grains with sizes ranging from 40 to 60 mesh. Quartz and sulfide minerals were separated by heavy liquid and then hand-picked under the binocular microscope until purity exceeded 99%. Fluid inclusions in mineral separates were decrepitated at temperatures ranging from 200 to 500 °C, and the extracted water and CO2 were collected in the nitrogen trap. Determination of 18O/16O composition of inclusion water follows the procedure of Epstein and Mayeda (1953). H- and Oisotopic composition was determined on a FinniganMAT 252 ratio mass spectrometer. Reproducibilities of δ18O and δD values are ± 0.2‰ and ± 1‰, respectively. 4.2. Major and trace element composition of the ore Major and trace element data for the ore samples are presented in Table 1, which show that the gold orebodies (breccias) and massive and bedded chert have similar major element and trace element compositions, except that the Au orebodies have elevated concentrations of Al2O3, K2O, Cu, As and Au and lower concentrations of SiO2. This could be explained by the fact that the Au orebodies contain large amounts of siltstone detritus and argillization. Gold mineralization was associated with the introduction of Cu and As. The Au- and Ag-mineralization systems differ in their trace element compositions. The silver orebodies

Fig. 6. Hydrogen and oxygen isotopic composition of fluid inclusions in quartz veins from the Changkeng Au and the Fuwang Ag deposits.

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Table 2 Pb isotope composition of ore sulphide minerals and host rocks from the Changkeng and Fuwang deposits, Guangdong Province, China Sample number

Occurrence

Ore mineral/formation

Location

206

Pb/204Pb

207

Pb/204Pb

208

Pb/ Pb

204

Cks15 CK32 CK38 CK35 CK94 CK95 Ck156 CK157 CK160 Ck194 Ck195 Ck218 Ck292 Ck343 Ch83 Ck329 Ck330 Ck310 Ck311 Ck315 Ck316 Ck322

Changkeng Au-ore

Fuwang Ag-ore

Host rock

Rocks in the orefield

Prite/C1z Pyrite/C1z Pyrite/C1z Stibnite/C1 Galena/C1z Galena/C1z Galena/C1z Galena/C1z Galena/C1z Cherts (C1z) Brecciated siliceous rocks (C1z) Brecciated siliceous rocks (C1z) Chert (C1z) Siliceous limestone (C1z) Breccias limestone (C1z) Siltstone (D) Siltstone (D) Metamorphic sandstone (∈-O) Metamorphic sandstone (∈-O) Metamorphic sandstone (∈-O) Metamorphic sandstone (∈-O) Metamorphic sandstone (∈-O)

Gold orebodies along second prospecting line

Zk1604, − 260.5 m Zk1604, − 290.8 m ZK0411, − 213.3 m Zk0411, −213.6 m Zk0411, −267.4 m Surface Surface Zk0802, − 139.5 m Zk4802, − 235 m Zk1902, − 269 m Surface Surface Surface Surface Surface Surface Surface Surface

19.251 19.092 18.832 18.580 18.820 18.891 18.845 18.851 18.834 18.789 18.733 19.224 18.716 18.756 18.578 18.775 18.727 18.806 19.170 18.955 18.969 19.433

15.801 15.797 15.728 15.672 15.848 15.914 15.902 15.873 15.848 15.770 15.733 15.705 15.702 15.766 15.640 15.763 15.707 15.737 15.775 15.769 15.762 15.752

39.104 39.075 39.002 38.700 39.579 39.786 36.690 39.685 39.657 39.345 39.150 38.925 39.069 39.249 39.108 39.264 39.069 39.256 39.834 39.586 39.455 39.920

C1z, D, and ∈-O represent the Lower Carboniferous Zimenqiao Group bioclastic limestone, Upper Devonian Maozifeng Group siltstone, Cambrian to Ordovician Longtouzhai Group metamorphic sandstone, respectively.

have elevated content of Ag, Cu, Pb, and Zn. The Ag orebodies are present in Early Carboniferous Zimenqiao bioclastic limestone and at the contact fault zones between the limestone and the overlying Changkeng Auhosting siliceous rocks and therefore contain different major element composition, but have somewhat similar in trace element compositions. The Au orebodies are present along the contact fault zones between the limestone and the overlying Changkeng Au-hosting siliceous rocks and the Au orebodies have similar major element composition but different trace element composition. The Ag-mineralization is associated with decarbonization and silicification. 4.3. Characteristics of ore-forming fluid Previous studies on the microthermometry, chemical composition, O-, H-, C- and He-isotopic composition of fluid inclusions from the mineralized quartz veinlets of the Changkeng Au and Fuwang Ag deposits are reported in Guo and Du (1996), Sun et al. (1999), S. Zhang et al. (1998), and W.H. Zhang et al. (2000). The fluid inclusion data from the two deposits show some similarity, but also differences. Fluid inclusions in quartz or in sulfide minerals from the main Au- and Ag-mineralization paragenesis have the following attributes.

(1) Fluid inclusions from the gold orebodies and the silver orebodies usually are small in diameter, ranging from 3 to 8 μm. Most fluid inclusions have been trapped along growth planes and are inferred to be primary inclusions and some of the fluid inclusion are distributed mainly along microfractures and thus represent secondary trapped inclusions (Zhang et al., 1998). The fluid inclusions are widely variable in shape, including negative crystal, rounded, irregular and ellipsoidal morphologies (Zhang et al., 2000). These fluid inclusions can be divided into three types: liquid inclusions, two-phase liquid-rich inclusions, and two-phase vapor-rich inclusions. The liquid inclusions usually are much smaller (b5 μm) than the other inclusion types. Some gas hydrocarbon inclusion and oil–gas–liquid three-phase inclusions have been observed in the Changkeng Au deposit (Zhang et al., 2000). (2) Fluid inclusions in quartz and pyrite contain gases and solutes, such as H2O, K+, Ca2+, Na+, Mg2+, SO42−, Cl− and F− , and CO2, the major gas component. The two deposits have similar major component compositions (Zhang et al., 1998) except that the Au deposit has high K+/ Na+ ratios (2.22 to 8.74) and the Ag deposit has relatively low K+/Na+ ratios (0.46 to 1.57) (Guo and Du, 1996). Homogenization temperatures of fluid inclusions from the deposits are highly variable. The data for the

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Fig. 7. 206Pb/204Pb–207Pb/204Pb plots of Changkeng Au and Fuwang Ag-ores, host rocks and other deposits in Neoproterozoic Formation in Western Guangdong Province. Plumbotectonic framework trends are plotted for reference (U: upper crust, I: orogen, M: mantle; Zartman and Doe, 1981; CR: Crust from Cumming and Richards, 1975; SK: Crust from Stacey and Kramers, 1975).

two deposits overlap in homogenization temperature, in the ranges 210 ± 80 °C and 230 ± 50 °C, respectively. Salinities of the fluid inclusion from the deposits also overlap, ranging from 1.6 to 7.3 wt.% and 1.6 to 2.6 wt. % equiv. NaCl, respectively. (3) Oxygen and hydrogen isotopic data for the Changkeng and Fuwang deposits are shown in Fig. 6. δDH2O values range from −80‰ to −30‰ and δ18OH2O values range from −7.8‰ to −3.0‰ for the Changkeng gold ore. δDH2O values range from −59‰ to −45‰ and δ18OH2O values range from −0.9‰ to −4.1‰ for the Fuwang Ag-ore (Guo and Du, 1996; Sun et al., 1999). These data are interpreted by Sun et al. (1999), Guo and Du (1996) and Zhang et al. (1998) to indicate that ore fluids consisted of evolved meteoric water or formational water. The δDH2O and δ18OH2O values of the Changkeng

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Au deposit increase progressively from the early stage to the late stage mineralization and the δDH2O and δ18OH2O values of the late stage mineralization are near the Tertiary meteoric water value, suggesting that the ore fluid of the late stage consists largely of meteoric water. The Changkeng and Fuwang deposits also differ in their C- and Heisotopic compositions. Limited δ13CCO2 values (2 samples) of fluid inclusions in pyrite from the Changkeng deposit range from −17.0‰ to −16.6‰ (Guo and Du, 1996), with the 3He/4He values ranging from 0.0100 to 0.0054 Ra (Sun et al., 1999). Guo and Du (1996) interpreted the δ13CCO2 data to indicate the involvement of organic carbon. The highly variable δDH2O values (−80‰ to −30‰) of the ore fluids from the Changkeng deposit may be due to the interactions between the ore fluid, clay minerals, and organic material during formation of the deposit. The δ13CCO2 and 3He/4He values for the Fuwang deposit range from −6.7‰ to −0.6‰ and from 0.5930 to 0.8357 Ra, respectively. Sun et al. (1999) interpreted the 3He/4He data as indicating involvement of evolved meteoric water in the ore fluids that formed the Changkeng Au deposit and a mantle source of He in ore fluids that formed the Fuwang Ag deposit. 4.4. Pb isotopes Pb isotopic data on pyrite, galena, stibnite, and whole rocks from the Changkeng Au and Fuwang Ag deposits are listed in Table 2. They also are plotted on a 207Pb/ 204 Pb versus 206Pb/204Pb diagram (Fig. 7), together with the growth curves of Cumming and Richards (1975), Stacey and Kramers (1975), and Zartman and Doe (1981). These data (Table 2; Fig. 7) illustrate that galena from the Fuwang Ag deposit is homogenous and has higher 207Pb/ 204 Pb and 208Pb/204Pb ratios than the host rocks and the sulfide minerals from the overlying Changkeng Au deposit. Fig. 7 also illustrates that the sulfide minerals, ores,

Table 3 Ar–39Ar fast neutron activation analysis data of quartz from the Fuwang Ag deposit, Guangdong Province, China

40

39 Ark10−12mol (40Ar⁎/ Ark 39 Ark)m ± 1σ %

Incremental heating

Heating temperature (°C)

(40Ar/ Ar)m

(36Ar/ 39 Ar)m

(37Ar/ 39 Ar)m

(38Ar/ 39 Ar)m

39

39

1 2 3 4 5 6 7 8 9 10

350 430 500 600 750 900 1050 1200 1350 1500

72.44 21.04 27.83 12.57 22.18 21.25 34.41 26.35 30.56 55.93

0.24 0.06 0.08 0.03 0.06 0.06 0.10 0.06 0.08 0.17

1.21 0.45 0.69 0.38 0.58 0.68 0.71 0.51 0.63 0.92

0.57 0.22 0.29 0.14 0.23 0.26 0.34 0.23 0.29 0.39

0.31 0.89 0.69 1.62 0.90 0.83 0.57 0.87 0.63 0.35

Analyst: Sang Haiqing.

2.82 ± 0.04 1.96 ± 0.01 3.36 ± 0.02 3.33 ± 0.00 3.35 ± 0.01 3.31 ± 0.01 4.44 ± 0.02 6.76 ± 0.01 6.63 ± 0.02 6.97 ± 0.02

4.07 11.60 9.06 21.10 11.70 10.80 7.40 11.30 8.15 4.53

Appearance age (Ma) ± 1σ 54.4 ± 2.3 37.9 ± 0.7 64.7 ± 1.5 64.2 ± 1.0 64.5 ± 1.3 63.8 ± 1.4 84.9 ± 2.2 127.9 ± 2.5 125.4 ± 2.9 131.7 ± 3.9

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Fig. 8. Age spectrum (A) and isochron diagram (B) showing thedata of mineralized quartz from the Fuwang Ag deposit.

and host rocks from Changkeng show a significant spread in isotopic ratios, especially in 206Pb/204Pb ratios. Samples from the Au deposit, the Ag deposit and the host rocks plot near the upper crust growth curve of Zartman and Doe (1981) (Fig. 7). The Pb isotope results, thus, indicate that the ore Pb was derived from preexisting evolved crustal material. Three pyrite specimens and one stibnite specimen from the Changkeng deposit have Pb isotopic compositions within the domain of the host rocks, and underlying limestone, siltstone and sandstone. This general isotopic compositional similarity suggests that the Pb in the Au deposit is derived from local sources. The Pb isotope composition of galena from the Fuwang Ag deposit has higher207Pb/204Pb and 208Pb/204Pb ratios than those of the Au deposit and the host rocks, which suggests that the lead of the Ag and Au deposits were not derived from the same sources. The Neoproterozoic metamorphic basement in the western Guangdong region is characterized by high 207Pb/204Pb ratios (Zhang et al., 1993a,b; Chen et al., 1998). The lead isotope ratios of galena from the Fuwang deposit lies on the same growth curve (μ = 10.7, calculated based on the Stacey and Kramer model) as sulfide minerals from the Da-Jiangping sedex pyrite deposit (Tu, 1988; Zhang et al., 1993a), and the Chadong Ag deposit hosted in the Neoproterozoic metamorphic rocks (Fig. 7) (Zhang et al., 1993b). The Pb isotope signature of the Fuwang Ag deposit indicates that the Pb of the deposit is derived from the Neoproterozoic metamorphic basement. This conclusion is further supported by high background Ag abundances (average of 480 samples= 100 to 800 ppb; Pang et al., 1996), in the Neoproterozoic metamorphic basement in western Guagdong region and lower abundance of Ag (averaging 34 to 70 ppb) in the Phanerozoic rocks (Du et al., 1993) in the Changkeng district. The different Pb isotopic signatures of the Changkeng and Fuwang deposits could also be explained by

the different fluids that transport Au and Ag. Gold is transported by H2S-rich fluids incapable of transporting and introducing much Pb; the Pb isotopic signatures in the Au deposit ought to be dominated by Pb derived from the host rocks (Hofstra and Cline, 2000). Silver is transported dominantly by chloride complexes that are capable of transporting and introducing Pb, and it is reasonable to interpret that the Pb in the Ag deposits was derived from external sources (A. Hofstra, personal communication, 2003). 4.5. Age of the Fuwang silver mineralization 4.5.1. 40Ar/39Ar age of mineralized quartz veins Quartz samples were heated in a closed container and the gas released was analyzed on a mass spectrometer for its argon isotopes (Table 3). The 40Ar/39Ar age spectrum (Fig. 8) is low on the left side and high on the right side. The age spectrum suggests that the quartz contains three sites of 40Ar released at lower (b500 °C), medium (500 to 900 °C), and high temperature (1050 to 1500 °C). The low- and high-temperature 40Ar might be derived from secondary fluid inclusions and from excess Ar in the quartz crystal, respectively. The main plateau is composed of 4 steps with 52.66% of the total 40Ar Table 4 Rb–Sr isotopic data of fluid inclusions in vein quartz from the Fuwang Ag deposit 87

Rb/ Sr

Sample number

Mineral Rb (10− 6)

Sr (10− 6)

ck250 ck262 ck266 ck288 ckl165 ckl197

Quartz

0.0801 10.166 0.8421 2.3150 3.3738 1.9409 1.4183 1.0451 0.906 4.871 1.754 1.554

0.2819 0.6859 2.5124 0.5136 1.528 0.945

87

Sr/86Sr (1σ)

86

0.72602 ± 0.00002 0.71897 ± 0.00002 0.71847 ± 0.00002 0.71774 ± 0.00002 0.72162 ± 0.00009 0.71808 ± 0.00007

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Fig. 9. Rb–Sr isochron of mineralized quartz from the Fuwang Ag deposit.

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The early Tertiary isotopic ages of the Fuwang deposit, together with the nearby Xiqiaoshan Tertiary volcanic-hosted Ag deposit, indicate that the SCFB was affected by the Himalayan mineralization event. The precious-metal metallogeny of the SCFB, therefore, may have developed during four orogenic stages: the Caledonian Orogeny (ca. 400 Ma), the Indosinian Orogeny (ca. 200 Ma), the Yanshanian Orogeny (180 to 90 Ma) (Pirajno and Bagas, 2002), and the Himalayan Orogeny (b90 Ma; Wang and Mo, 1995). This study implies that the Coastal Volcanic Belt in the SCFB has greater potential for Himalayan precious metal mineralization than previously realized. 5.2. Origin of Changkeng Au and Fuwang Ag deposits

released and yields a plateau age of 64.3 ± 0.1 Ma (Fig. 8). The Ar isotopic data of the main plateau also yields isochron age of 64.0 ± 0.1 Ma with an initial 40Ar/36 ratio of 294.5 ± 0.4. The apparent plateau age is consistent with the corresponding isochron age. 4.5.2. Rb–Sr isotopic chronology Samples for dating were collected from different parts of the Fuwang silver–quartz vein orebody. The Rb and Sr isotopic data obtained from six samples are listed in Table 4 and plotted on Fig. 9. The quartz samples have a wide range of 87Rb/86Sr ratios, varying from 1.0451 to 10.166. The samples define an Rb–Sr isochron, with an age of 68 ± 6 Ma, MSWD = 4.14. 5. Discussion 5.1. Relationship of the Fuwang Ag deposit to Tertiary igneous activity Low-sulfidation epithermal deposits are commonly associated with intermediate to silicic magmatism (Henry et al., 1997; John, 2001). The Fuwang Ag deposit is located along the southwestern rim of the Shanshui Tertiary fault-bounded basin (Bureau of Geology and Mineral Resources of Guangdong Province, 1985). The Rb–Sr isochron age (68 ± 6 Ma) and 40 Ar–39Ar plateau age (64.3 ± 0.1 Ma) of the mineralized quartz veins from the Fuwang deposit are concordant with each other within error and correspond to the Himalayan tectono-magmatic event (Himalayan is a period b90 Ma; Wang and Mo, 1995). The Rb–Sr isochron age and the 40 Ar– 39 Ar plateau age are consistent with the age (42 to 67 Ma) of Tertiary volcanic activity in the Shanshui basin (Zhu et al., 1992), indicating that the ages determined from the quartz are geologically reasonable.

The Changkeng Au deposit is hosted in siliceous rocks and limestone and shows similar mineralization and mineral associations to many Carlin-type deposits (Radtke et al., 1980; Hofstra and Cline, 2000; Hu et al., 2002; Peters et al., 2002, 2007-this volume; Hofstra et al., 2003; Khin Zaw et al., 2007-this volume). Peters et al. (2002) classified the Changkeng Au deposit as an unconformity-hosted Au deposit as part of the sedimentary rock-hosted gold deposit family of deposits. Some researchers, therefore, argue that the Au orebodies are a Carlin-type gold deposit, and that the Au and Ag deposits were formed at the same geological event by the same geological processes (Du et al., 1993; Zhang et al., 1998; Sun et al., 1999). Although the Changkeng Au deposit shares some common features with Carlin-type gold deposits in China (Hu et al., 2002; Peters et al., 2002, 2007-this volume), such as an affinity with limestone, disseminated mineralization, a mineral association consisting of stibnite, realgar, orpiment, and pyrite, it has some unique differences, such as strata-bound occurrence in thick silicification zone and close spatial association with the Ag deposit. Silver is not a common association with Carlin-type gold deposits in China, but does occur proximal to Carlin-type deposit in Nevada at the Tuscarora Au–Ag district (Castor et al., 2003). The Au and Ag orebodies are separated from each other; the Changkeng Au deposit is hosted in brecciated siliceous rocks on the top of the Zimenqiao group bioclastic limestone and is characterized by disseminated mineralization. The Fuwang Ag deposit, hosted in the Zimenqiao group bioclastic limestone, has vein and veinlet mineralization. The Au orebodies contain less Ag and base metals and the Ag orebodies are poor in Au. This separation of Au and Ag ores differs from the epithermal Tuscarora

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Au–Ag district, Nevada, where the Au–Ag orebodies overlap (Castor et al., 2003). Although the Changkeng Au and Fuwang Ag deposits overlap in homogenization temperature, salinities of fluid inclusions, the two deposits have distinct lead isotope signatures and ore fluid geochemistry. Galena from the Fuwang Ag deposit has higher 207Pb/204Pb and 208Pb/ 204 Pb ratios than sulfides from the Changkeng Au deposit (Table 2, Fig. 7), suggesting that the Pb in the Au- and the Ag-ores are derived from different sources. Lead isotopic data suggest that the Pb of the Fuwang Ag deposit was most likely derived from rocks in the basement and that the Pb of the Changkeng Au deposit was most likely derived from a local source. The 3He/4He ratios (0.5930 to 0.8357 Ra) of fluid inclusions in sphalerite and quartz from the Fuwang deposit are higher than those of the Changkeng deposit (0.0100 to 0.0054 Ra). The noble gas isotopic characteristics (Stuart et al., 1995) suggest that volatiles of mantle origin were involved in the formation of the Fuwang Ag deposit, whereas volatiles in the Changkeng Au-ore fluids may have come from the atmosphere. Given the close temporal and broad spatial association of the silver deposit with Early Tertiary magmatism, the D-, O-, C-, and He-isotope data are consistent with ore fluids that consisted of a mixture of deeply sourced magmatic and local meteoric water. The Pb isotope data may be due to magmatic assimilation of Neoproterozoic rocks during its ascent or to circulation of ore fluids through the Neoproterozoic rocks. The most plausible explanation is to ascribe these combined mineralized ages and isotopic results for the Fuwang Ag deposit to the Himalayan tectonomagmatic event. During this event, mantle wedge melting, basaltic underplating and magma fractionation resulted in the release of magmatic fluids and a source of heat to drive circulation of meteoric water system. The circulating water system dissolved Ag metal from the Neoproterozoic basement rocks that had undergone enrichment and redistribution during the Yanshanian tectonomagmatic event (Pirajno and Bagas, 2002). The ore-forming fluids were then probably transported to fault systems in the Lower Carboniferous limestone and Ag was precipitated due to the change of pH values caused by the reaction of fluid with the limestone. Due to the lack of minerals suitable for isotopic dating, the age of the Changkeng Au deposit has not been conclusively determined. However, Du et al. (1993) and Sun et al. (1999) and Zhang et al. (1998) suggest that the deposit has a genetic link to the Yanshanian tectonomagmatic event (180 to 90 Ma) (Pirajno and Bagas, 2002). Crosscutting relationship indicates that the Changkeng Au deposit is older than the Fuwang Ag deposit.

The presence of disseminated gold in brecciated siliceous rocks along with epigenetic quartz, orpiment, realgar, and stibnite, together with meteoric D, O, C, and He isotopic values and fluid inclusion salinity and temperature data that overlap those from Fuwang silver deposit, suggests that the gold was deposited by an epigenetic meteoric hydrothermal system. Given that the Fuwang Ag deposits have a Himalayan (Tertiary) age, if the Changkeng Au deposit was considered to have formed by an epigenetic meteoric hydrothermal system during the older Yanshanian (Triassic) period, then this infers that the two adjacent deposits were formed by different geological processes and at different times. If, however, the Changkeng Au and Fuwang Ag deposits were both formed during the Himalayan period, the Au was deposited first in the brecciated siliceous rocks by the circulating meteoric hydrothermal system. This system, then, mixed with the fluids that transporting Ag and base metals, and deposited Ag and base metals in the fracture zone in the Lower Carboniferous Zimenqiao bioclastic limestone at Fuwang. This latter model suggests that the adjacent Changkeng Au and Fuwang Ag deposits could represent a single evolved hydrothermal system where the ore fluids deposited Au first, in the brecciated silicification, and then mixed with the magmatic water and deposited Ag in the fracture zone in the limestone. The alternative genetic model is that the adjacent Changkeng Au and Fuwang Ag deposits may have resulted from the superposition of different geological events. Given the close proximity of the two deposits and the overlap of fluid inclusion salinities and temperatures, we consider that the two deposits were most likely formed by the first model. 6. Conclusions Based on the above evidence and discussion, we conclude the following: 1. The Changkeng Au and Fuwang Ag deposits are located at the same locality but the Au and Ag orebodies are spatially separated from each other. The Changkeng Au deposit is hosted in brecciated siliceous rocks on the top of bioclastic limestone of the Zimenqiao Group and is characterized by disseminated mineralization. The Fuwang Ag deposit, hosted in the Lower Carboniferous, comprises vein and veinlet mineralization. The Auhosting siliceous rocks are formed by multistage silicification that includes at least syngenetic, diagenetic and hydrothermal silicification. Hydrothermal silicification is related to Au deposition. Pre-ore silicification

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has high background Au abundance and may have contributed Au to the deposit. The two deposits have overlapping fluid inclusion salinities and homogenization temperatures, but have distinct mineral association and geochemical characteristics. The gold orebodies contain less Ag and base metals and have Pb isotopic compositions similar to those of the host rocks and underlying formations. The D-, O-, C-, and He-isotopic data indicate that the Au was deposited by an epigenetic meteoric hydrothermal system. The Ag orebodies are poor in gold and have Pb isotopic composition similar to Neoproterozoic rocks and Neoproterozoic rockhosted deposits. Rb–Sr isochron age (68 ± 6 Ma) and 40 Ar–39Ar age (64.3 ± 0.1 Ma) of the mineralized quartz from the Ag deposit indicate that the Fuwang Ag deposit has genetic and temporal links to the Himalayan volcanic rocks. The source of Pb and likely silver in the Fuwang Ag deposit, however, is from the older Neoproterozoic basement rocks. The D-, O-, C-, and He-isotope data suggest that the ore fluids consisted of a mixture of magmatic and meteoric water. The adjacent Changkeng Au and Fuwang Ag deposits could represent a single evolved system or two different hydrothermal systems. Given the close proximity of the two deposits and the overlap of fluid inclusions salinities and temperatures, the two deposits may most likely have formed by an evolved hydrothermal system that deposited Au first in brecciated siliceous rocks, and then mixed with Ag-bearing fluids and deposited Ag in the fracture zones in the Lower Carboniferous Zimenqiao bioclastic limestone. 2. Early Tertiary isotopic ages from Fuwang and the nearby Xiqiaoshan Tertiary volcanic-hosted Ag deposit suggest that the SCFB underwent a Himalayan event of precious metal mineralization. The discovery of the Himalayan Ag-mineralization suggests that the Coastal Volcanic Belt within the SCFB has greater potential for Himalayan precious metal mineralization than has been previously realized. Acknowledgements The cooperation and support of 757 Geological Party, Bureau of Geology and Mineral Resource, Guangdong Province, greatly facilitated the fieldwork for this study. Hu Zhang and Tong-Jing Li are thanked for the help during the field work. The work was support by the Major State Basic Research Program of P.R. China (No.1999043203), the NSFC (No.49872035, 40472049) and the important project of CAS (Nos. KZCXZ-SW-117 and GIGCX-04-03). Albert H. Hofstra, Jean S. Cline, Stephen G. Peters and Khin Zaw are thanked for greatly

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