Geology, geochemistry, and genesis of orogenic gold–antimony mineralization in the Himalayan Orogen, South Tibet, China

Ore Geology Reviews 58 (2014) 68–90

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Geology, geochemistry, and genesis of orogenic gold–antimony mineralization in the Himalayan Orogen, South Tibet, China Wei Zhai a,b,⁎, Xiaoming Sun a,b,c,⁎, Jianzhou Yi d, Xiangguo Zhang d, Ruwei Mo c, Feng Zhou c, Huixiao Wei c, Qinggao Zeng d a

School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, PR China Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou 510006, PR China Department of Earth Sciences, Sun Yat-sen University, Guangzhou 510275, PR China d Regional Geological Survey Party, Tibet Bureau of Geology and Mineral Exploration, Lhasa 851400, PR China b c

a r t i c l e

i n f o

Article history: Received 3 June 2013 Received in revised form 2 November 2013 Accepted 3 November 2013 Available online 13 November 2013 Keywords: South Tibet Himalayan orogen Orogenic gold–antimony deposit Ore-forming fluid Metamorphic fluid Meteoric water

a b s t r a c t The southern Tibet Au\Sb metallogenic belt in the Himalayan orogen consists of more than 50 gold, gold– antimony and antimony lode deposits, and associated placer gold deposit. The deposits are hosted in a Mesozoic metamorphosed turbidite sequence of the Indian passive continental margin. The Zhemulang Au deposit, Mazhala Au\Sb deposit, and Shalagang Sb deposit are three typical examples of such epizonal orogenic deposits. At Zhemulang, gold-bearing quartz veins occur in the Upper Triassic Songre Formation, consisting of carbonaceous phyllite and slate. Ore minerals are native gold, pyrite, galena, chalcopyrite, and limonite. At Mazhala, the gold- and stibnite-bearing quartz vein orebodies are hosted in Lower to Middle Jurassic slate, interlayered with metastandstone, metasiltstone, and limestone of the Lure Formation. Ore minerals are native gold, stibnite, pyrite, arsenopyrite, and trace amount of cinnabar. At Shalagang, the host rocks are Lower Cretaceous sandstone, siltstone, muddy limestone, and chert of the Duojiu Formation. Orebodies consist mainly of stibnite-bearing quartz veins and locally altered fault breccia. Ore minerals are stibnite, cinnabar, valentinite [Sb2O3], limonite, and trace amount of pyrite, arsenopyrite, and realgar. For the three deposits, the wallrock alteration has produced the minerals silica, carbonates, white mica, sulfide and chlorite. The three deposits have a similar element associations, but with a few slight variations. The Zhemulang, Mazhala, and Shalagang deposits, in order of element enrichments relative to crustal abundance, are anomalous in Au, Sb, Te, Bi, As, Pb, Ag, and W; Sb, Au, Te, As, Pb, Bi, Ag and W, to Sb, Te, As, Au, Hg, W, Pb, and Ag, respectively, and all depleted in Cu, Zn, Sn, and Mo. Various aqueous, carbonic, and hydrocarbon fluid inclusions were recognized in quartz and/or stibnite at the three deposits. These include type 1a one-phase aqueous inclusions, type 1b two-phase aqueous inclusions, type 2a carbonic inclusions, type 2b aqueous-carbonic inclusion, and rare type 3 hydrocarbon inclusions that include two-phase hydrocarbon inclusions (type 3a) and dark one-phase hydrocarbon inclusions (type 3b). The three deposits have the similar low-salinity H2O\CO2\CH4\N2 ore fluids with trace amounts of hydrocarbons. For the Zhemulang, Mazhala, and Shalagang deposits, the salinities of aqueous inclusion range mainly between 3.3 and 6.4 wt.% NaCl equiv., 2.5 and 4.9 wt.% NaCl equiv. and 4.1 and 6.4 wt.% NaCl equiv., respectively. The ore-forming temperatures vary mainly from 180 to 320 °C, 160 to 300 °C and 140 to 240 °C, respectively. The estimated mineralization depths are 4 to 6 km, 3 to 5 km, and 1 to 4 km of the epizonal environment, respectively. The different mineralization temperatures and pressures led to the different element enrichments at the three deposits. For the Zhemulang, Mazhala and Shalagang deposits, ore fluid isotopic compositions are δDH2O −107.5 to −36.7‰ and δ18Ofluid 2.8 to 8.2‰, δDH2O −119.0 to −72.7‰ and δ18Ofluid 7.5 to 16.2‰, and δDH2O −173.4 to −139.2‰ and δ18Ofluid 7.5 to 12.3‰, respectively; δ13Cfluid values are −11.7 to −9.6‰, −3.5 to −2.5‰, and −6.5 to −5.1‰, respectively; and δ34S values are −4.0 to −1.1‰, −0.8 to 2.3‰, and −3.9 to 2.1‰, respectively. The ore-forming fluids were partly derived from metamorphic devolatilization of immediate or deeper level country rocks, with a deposit's corresponding metamorphic degree controlling the fluid PTX. The ore-forming fluid for Zhemulang, Mazhala, and Shalagang consisted of predominantly metamorphic water with minor involvement of meteoric water, a mixture of metamorphic fluid and meteoric water, and predominantly meteoric water, respectively. Ore metals were derived from country rocks, including synsedimentary Sedex-like sulfide

⁎ Corresponding authors at: School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, PR China. E-mail addresses: [email protected] (W. Zhai), [email protected] (X. Sun). 0169-1368/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.oregeorev.2013.11.001

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layers in the Jurassic strata observed at the Mazhala Au\Sb deposit. Among three deposits, the variation of δ13Cfluid and δ34S reflects the fact that the Zhemulang Au deposit formed from a relatively high content of organic carbon and low ƒO2 fluid, the Mazhala Au\Sb deposit from a relatively low content of organic carbon and high ƒO2 fluid, and the Shalagang Sb deposit from an intermediate content of organic carbon and ƒO2 fluid. Fluid immiscibility was the main mechanism for ore metal precipitation at all three deposits. The vertical zonation of Au, Au\Sb, and Sb mineralization suggests that additional gold resources may exist below the antimony or gold–antimony orebodies. Stream sediment and soil geochemical surveys and the occurrence of placer gold prospects are effective for identifying areas of orogenic gold and antimony deposits in the Himalayan and other orogens. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The Himalayan orogenic system was created by the collision of India with Asia starting in the early Cenozoic, and the orogenesis is still ongoing (Wang et al., 2001; Zhang et al., 2004). This makes the orogen one of the outstanding natural laboratories on Earth for studying continental collisions and related geological processes (Chung et al., 2005; Molnar et al., 1993; Yin, 2006). Mineral deposits formed in this orogen are thus relatively young and have undergone limited geological and tectonic overprinting, making it an ideal site for studying metallogenesis accompanying the process of mountain building. Separated by the Indus–Tsangpo suture (ITS), the Lhasa terrane lies to the north of the Himalayan orogen (Chung et al., 2005; Yin, 2006). In the northern part of the Lhasa terrane and parallel to the east–west ITS, the Gangdese polymetallic mineral belt includes mainly Cenozoic porphyry Cu\Mo (\Au) deposits, Cu and polymetallic skarn deposits and graniterelated Ag\Pb\Zn vein deposits formed in a post-collisional extension-related environment (Hou and Cook, 2009; Hou et al., 2009). The southern Tibet Au\Sb metallogenic belt, in the Himalayan orogen to the south of the ITS, contains more than 50 gold- and/or stibnite-bearing vein deposits or occurrences, and associated placer gold deposits (Tibet Institute of Geologic Survey, 2003; Nie et al., 2005; Yang et al., 2009). Typical Au deposits including Mayoumu (or Mayum) (Duo et al., 2009; Jiang et al., 2009; Wen et al., 2006), Bangbu (N20 t Au) (Sun et al., 2010), and Zhemulang (Ai, 2007); Au\Sb include Mazhala (Wang and Zhang, 2001; Yang et al., 2009; Zhang et al., 2000) and Zhegu (Nie et al., 2005; Yang et al., 2009); and Sb include Shalagang (Li et al., 2002; Yang et al., 2000; Yang et al., 2009) and Chequnzhuobu (Wu et al., 2008). Different opinions have been offered for the genesis of the lode deposits, including defining these as epithermal, or orogenic deposits, and suggesting a variety of ore fluid types including magmatic or mixed magmatic–meteoric. In this paper, we select three typical deposits from the Au\Sb belt, the Zhemulang Au, Mazhala Au\Sb, and Shalagang Sb (Fig. 1), and present detailed field descriptions of the deposits, systematic geochemical characterizations, and fluid inclusion and isotope data to help characterize the ore-forming fluids and develop a model of ore genesis. In addition, we synthesize this information and that from former studies into a model that places the southern Tibet Au\Sb deposits into the tectonic framework of the Himalayan orogen. 2. Regional geological setting The Himalayan orogen, located between the ITS fault and Main Frontal fault, can be divided into four belts. These are the Tethyan Himalayan, High Himalayan, Lower Himalayan, and Sub-Himalayan belts according to LeFort (1975) and Yin (2006), which are separated by the South Tibet Detachment fault, Main Central Thrust fault, and Main Boundary Thrust fault from north to south, respectively (Fig. 1). The Tethyan Himalayan belt hosts the Au\Sb deposits. It contains a series of east–west-striking thrust faults and folds comprising the Tethyan Himalayan fold-and-thrust belt, which are cut by numerous, nearly north–south extensional faults. From east to west, seven dome structures are also distributed in the central part

of Tethyan Himalayan belt; the Yalaxiangbo, Ranba, and Kangma domes are shown in Fig. 1. The domes are medium- to high-grade metamorphic core complexes of Precambrian basement rocks, which are intruded by Cenozoic leucogranites. Paleozoic to Mesozoic low-grade to unmetamorphosed sedimentary strata comprise the outer parts of the domes, separated by low-angle detachment faults. The strata in Tethyan Himalayan belt can be divided into four sedimentary sequences (Yin, 2006). These include (1) Proterozoic to Devonian laterally persistent units deposited in an epicratonal setting; (2) Carboniferous–Early Jurassic rift and post-rift sequence that shows dramatic northward changes in thickness and lithofacies; (3) Jurassic–Cretaceous passive continental margin sequence; and (4) latest Cretaceous–Eocene syn-collision sequence (Fig. 1). The Proterozoic rocks consist of schist, gneiss, migmatite, and leptite interlayered with marble, quartzite, and amphibolite, distributed mainly to the south of South Tibet Detachment fault, but also in the core complexes in the Tethyan Himalayan belt. Ordovician rocks consist of quartz-rich marble and banded marble, interlayered with carbonaceous white mica slate and phyllitic slate, and are only exposed in a small area south of Kangma dome (too small to show in Fig. 1). Permian shelf strata, with a thickness of more than 3100 m, consist of conglomerate, sandstone, bioclastic marble, and silty slate interlayered with sandstone and are exposed surrounding the Kangma dome. The Early–Middle Triassic sequence, with a thickness of more than 4990 m, consists of fine-grained metasandstone, pyrite-bearing carbonaceous slate, and silty slate. The Late Triassic, with a thickness of more than 8360 m, consists of metasandstone, carbonaceous slate, silty slate, phyllite, and schist that is interlayered with altered basalt and andesite in its upper part. The Jurassic units, with a combined thickness of more than 9060 m, comprise silty slate, slate, fine-grained metasandstone, metasiltstone, micritic limestone, and sandstone interlayered with basalt, dacite, and volcanic clastic rocks. The Cretaceous sequence, with a thickness of more than 3230 m, consists mainly of micritic limestone, mudstone, siltstone, chert, sandstone, limestone, and argillaceous limestone interlayered with altered basalt (Gao et al., 1994; Li et al., 1995; Liu et al., 2005; Su et al., 2004). The Mesozoic strata comprise mainly post-rift passive margin turbidites and underwent metamorphism during the Himalayan orogeny. The metamorphic grade is greenschist facies in the Triassic rocks, but Cretaceous rocks are almost unmetamorphosed. Data from western Pakistan indicate the initial collision between Indian and Asia was ~65 Ma (Klootwijk et al., 1991, 1992), the peak of ultra-high pressure metamorphism occurred at 47–46 Ma (Foster et al., 2002; Smith et al., 1994), and exhumation of ultra-high pressure rocks to greenschist-facies conditions was accomplished between 46 and 40 Ma (Tonarini et al., 1993; Treloar et al., 2003). The less well-constrained tectonic events in the central Himalaya postdate those in the western Himalaya, with this difference reflecting the diachronous nature of the Indo–Asian collision (Foster et al., 2002). The Himalayan leucogranites outcrop in the core of metamorphic core complexes in the Tethyan Himalayan and High Himalayan. They have high 87Sr/86Sr ratios typical of pelite-derived anatectic magma with crystallization ages of about 35–12 Ma (Aoya et al., 2005; Harrison et al., 1997; Searle and Godin, 2003; Searle et al., 1997; Zeng

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et al., 2009). Nearly E–W-trending diorite and quartz diorite dikes and sills within the Mesozoic strata have whole-rock K–Ar ages of 34– 20 Ma (Liu et al., 2005; Su et al., 2004). In the southern Tibet Au\Sb metallogenic belt, the gold deposits are predominantly hosted in Triassic rocks. These include the Bangbu, Zhemulang, Chalapu, Juqu, Ranba (Fig. 1), and Mayoumu gold deposits in the west (Duo et al., 2009; Jiang et al., 2009). The gold–antimony deposits are hosted in Jurassic rocks, including Mazhala, Gudui, and Zhegu. The antimony deposits are hosted in Jurassic and Cretaceous lithologies, including Shalagang, Wuladui, and Chequnzhuobu. These gold, gold– antimony, and antimony deposits are also hosted in Himalayan diorite or mafic rock. All these deposits occur as veins in faults or fractures striking nearly east–west, or counterparts propagating from orogen-parallel regional faults.

3. Geology of the ore deposits 3.1. Zhemulang gold deposit The Zhemulang is a small gold deposit (gold resource b 5 t) located in Lang County, Tibet Autonomous Region (Fig. 1). The Upper Triassic rocks of the Songre Formation at Zhemulang include carbonaceous phyllite interlayered with metasiltstone and metasandstone; pyrite-bearing carbonaceous phyllite interlayered with graphite muscovite schist and metasandstone; and a unit of slate, carbonaceous slate, and meta-feldspathic quartz sandstone (Fig. 2). Goldbearing quartz veins occur in nearly east–west-striking faults, which dip SES at 30 to 50° (Fig. 3A, C, D). The veins and faults cut the Juqu–Zhemulang brittle–ductile shear zone in which the ductile fabrics include asymmetric folds, S–C fabrics, mortar textures, pressure shadows, and mica-fish. The shear zone strikes WNW and dips

south at 50 to 80°. Its width is variable, from 100–300 m to 2–3 km (Li et al., 1995). To the west, the large (N20 t) Bangbu deposit (Sun et al., 2010) is hosted in the second-order faults or splays of the Juqu–Zhemulang shear zone (Fig. 1). Because of the high altitude (~4000 m) and remote location of the orebodies, only reconnaissance exploration work has been carried out at Zhemulang (Ai, 2007). Three gold-bearing quartz veins have been discovered, the largest being No. 3, with a surface exposure length of ~1.0 km, down-dip extent of N100 m, and width of 0.2–1.0 m. Over an interval of 25 m, four exploration tunnels parallel to the No. 3 orebody have been developed, and oxidized and a small part of primary gold-bearing quartz veins were observed (Fig. 3A, C, D). Our samples were all collected in these tunnels. Ore minerals of the Zhemulang deposit include native gold, pyrite, galena, limonite, and chalcopyrite; the gangue minerals are quartz, with trace amounts of white mica, carbonate. Because of strong oxidization, sulfide content is low in ore. Oxidization of sulfides has caused cellular structure in the quartz veins (Fig. 3D), and also a vuggy texture characterizes the vein with euhedral quartz crystals as much as 10-cm-long, indicating that the quartz formed in an open-space environment. The mineralization can be divided into two stages. There is an early quartz-gold stage and a later sulfide-gold stage, which filled the cracks and vugs in the quartz veins. The wallrock alteration is not welldeveloped at Zhemulang, with the formation of alteration minerals carbonates, chlorite, white mica, and pyrite, and limonite forming during later oxidation.

3.2. Mazhala gold–antimony deposit Mazhala is a small gold–antimony deposit (antimony resource b 0.01Mt; gold resource b 5 t) located in Cuomei County, Tibet Autonomous

Fig. 1. Geological map of south Tibet gold–antimony metallogenic belt. ITS = Indus–Tsangpo Suture fault; MBT = Main Boundary Thrust fault; MCT = Main Central Thrust fault; STD = South Tibet Detachment Fault; TH = Tethyan Himalayan; HH = High Himalayan; LH = Lower Himalayan; SH = Sub-Himalayan; (Modified and compiled after Tibet Institute of Geologic Survey, 2003; Pan et al., 2004; Yin, 2006).

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Fig. 2. Geological map of Zhemulang gold deposit (simplified after Ai, 2007).

Region (Fig. 1). Host rocks for the deposit are those of the LowerMiddle Jurassic Lure Formation, consisting of slate interlayered with metasandstone, metasiltstone, and limestone. The strata comprise the regional Mazhala synclinorium. Thirty-nine orebodies have been discovered at Mazhala, and these cluster into five orebody groups (Fig. 4) (Zhang et al., 2000). The main epigenetic gold-bearing stibnite-quartz veins are controlled by the NW-, EW-, and NE-striking extensional faults or splays off the regional E– W extensional fault. Individual veins are 10- to 400-m-long and as

thick as 2.5 m, with variable dips of 20–80° (Fig. 5). Among the thirty-nine orebodies, most of the near-surface mineralization has been mined out, and only the No. 8-1 and No. 8-2 orebodies are now mined underground, which is where samples were collected. Sedex-like sulfide layers are present in the area. They are from 10 to 30 cm thick hosted in siltstone, are composed of syngenetic pyrite, marcasite, arsenopyrite, quartz and carbonate, and are conformable with the wall-rock strata (Fig. 6A, B). They were overprinted by epigenetic gold–antimony mineralization and only one orebody belong

Fig. 3. Photographs showing gold-bearing quartz vein of Zhemulang gold deposit. A. Outcrop of No. 3 Gold-bearing quartz vein orebody. B. Adits paralleling the No.3 orebody and their number. C. Oxidized gold-bearing quartz veins in adit cutting the foliation of the Juqu–Zhemulang brittle–ductile shear zone. D. Cellular structure of oxidized gold-bearing quartz vein with many cavities after sulfides being oxidized and leached.

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Fig. 4. Geological map of Mazhala gold–antimony deposit (simplified after Zhang et al., 2000).

to this type of overprinted ore, but this mineralization is economically insignificant. The ore minerals in the veins are native gold, stibnite, pyrite, marcasite, arsenopyrite, limonite, and trace amount of cinnabar. The native gold is typically visible in quartz, ranging from 0.01 to 2.0 mm in diameter, and also occurs as inclusions in sulfides, or as free grains at boundaries or in fissures in quartz or sulfides (Fig. 6 C, D). Electron microprobe analysis of nine gold grains shows the gold fineness is 982 to 990 (Table 1), which is higher than typical epithermal gold deposits and similar to most orogenic gold deposits (Groves et al., 1998, 2003). The gangue minerals are quartz, calcite, chlorite, epidote, and white mica. The average antimony grades of individual orebodies are between 25.6 to 67.1%, the average gold concentrations of individual orebodies are between 1.53 to 49.43 ppm, with a maximum value of 800 ppm (Zhang et al., 2000). The wallrock alteration has produced the minerals silica, carbonates, pyrite, arsenopyrite, chlorite, and white mica, with supergene limonite.

Fig. 5. Geological section of No.0 prospecting line of No. 3 orebody at Mazhala gold– antimony Deposit (simplified after Zhang et al., 2000).

The epigenetic mineralization can be divided into two stages. Early vein with the formation of quartz, calcite, and gold, followed by late gold-stibnite with stibnite and gold filling the cracks or vugs in goldbearing quartz veins (Fig. 6C, E, and F). Because the recrystallized syngenetic Sedex-like sulfide layers are more rigid than slate or metasiltstone, they are partly boudinaged before being overprinted by the gold–antimony ore-forming event (Fig. 6B). 3.3. Shalagang antimony deposit Shalagang is a medium-sized antimony deposit (antimony resource 0.01–0.1 Mt) located in Jiangzi County, Tibet Autonomous Region (Fig. 1). The Lower Cretaceous Duojiu Formation that hosts the deposit can be divided into four members. From bottom to top, these are lithic sandstone and siltstone interlayered with quartz sandstone, mud-bearing limestone and pelite interlayered with banded limestone, mud-bearing chert and quartz-feldspar sandstone interlayered with mudstone, and mud-bearing limestone and pelite. Two small Himalayan diabase and diorite intrusive bodies are exposed underground in the No. 7 orebody. A total of twelve orebodies have been discovered, which are all controlled by faults, occur as veins in nearly E–W-striking faults within the chert/quartz-feldspar member and diabase bodies. The orebodies are 40–350 m in length, with thicknesses of 0.5–3 m (Figs. 7, 8). The orebodies consist mainly of stibnite-quartz veins, braccias, and stockworks (Fig. 9). The ore minerals are stibnite, cinnabar, valentinite, limonite, and trace amounts of pyrite, arsenopyrite, and realgar. The pyrite and arsenopyrite commonly occur in altered fault braccia. The gange minerals are quartz, calcite, white mica, chlorite, and epidote. In the stibnite-quartz veins, quartz and stibnite commonly display a vuggy texture in hand specimen and under the microscope, with the euhedral quartz crystals with maximum lengths of 2–5 mm, and thus much smaller than at Zhemulang and Mazhala. Among the twelve orebodies, the antimony grades are between 0.6 to 57.6%, with the average grades of individual orebodies being 1.98 to 29.0% (Yang et al., 2000). The wallrock alteration has produced silica, carbonates, and white mica in the metasedimentary

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Fig. 6. Photographs and photomicrographs of ores from Mazhala gold–antimony deposit. A. Sedex-like sulfide layer in conformable contact with metasiltstone and slate. B. Gold- and stibnite-bearing quartz vein overprinting Sedex-like sulfide layer and the later was boudinaged during tectonic deformation. C. Stibnite and native gold in cracks of quartz under reflected light. D. Native gold in stibnite under reflected light. G. Gold-bearing stibnite quartz-vein with vuggy quartz. H. Stibnite filling in the center of gold- and stibnite-bearing quartz vein showing stibnite formed later than quartz. Au = native gold, Qtz = quartz, Stb = stibnite.

and cinnabar. Late stibnite veins, with quartz and needle-like or longplaty stibnite, fill cracks in the quartz veins, or holes and vug in the center of the veins (Fig. 9B, C), with the maximum length of stibnite crystals reaching 2–5 cm (Fig. 9D).

wallrocks, and the carbonates, epidote, chlorite, pyrite, arsenopyrite and supergene limonite in the diorite and diabase. At Shalagang, the antimony mineralization can be divided into two stages. Early quartz veins contain trace amounts of disseminated stibnite

Table 1 Fineness of Native Gold at Mazhala Gold–antimony Deposit (%). Sample no.

As

Ag

Pb

Cu

Bi

Zn

Fe

Hg

Sb

Au

∑

fineness

09MZ-1-5/1 09MZ-1-5/2 09MZ-1-5/3 09MZ-1-5/4 09MZ-1-5/5 09MZ-1-5/6 09MZ-1-5/7 09MZ-1-13/1 09MZ-1-13/2

0.000 0.000 0.025 0.000 0.000 0.019 0.000 0.000 0.004

0.611 0.677 0.494 0.585 0.164 0.214 0.017 0.495 0.231

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.000 0.026 0.000 0.000 0.000 0.000 0.000

0.835 0.728 0.769 0.937 0.834 0.706 0.729 0.942 0.641

0.000 0.035 0.005 0.000 0.026 0.000 0.056 0.000 0.000

0.000 0.000 0.000 0.002 0.000 0.023 0.020 0.000 0.000

0.000 0.008 0.258 0.137 0.096 0.000 0.525 0.274 0.287

0.000 0.008 0.008 0.037 0.006 0.000 0.472 0.002 0.001

100.530 99.496 100.782 100.174 100.549 99.963 99.257 99.178 98.981

101.97 100.95 102.34 101.90 101.68 100.93 101.08 100.90 100.15

986 986 985 983 989 990 982 983 988

The samples were analyzed in Sun Yat-sen University by JEOL JXA-8800R Electron Probe Micro-analyzer, Accelerating Voltage is 20.0 kV, the Beam Current is 10−8 A, the Beam Diameter is 1 μm.

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Fig. 7. Geological map of Shalagang antimony deposit (Simplified after Yang et al., 2000).

4. Geochemistry of the ore deposits 4.1. Element associations In order to identify the element associations, the element concentrations of 34 selected typical ore samples from the Zhemulang, Mazhala and Shalagang deposits were analyzed in Xinjiang Mineral Experiment Research Institute. Trace elements were measured including Hg and Te by Double-Channel Atomic Fluorescence Spectrometer; Au, Sb, and Ag by Atomic Absorption Spectrometer; Cu, Pb, Zn, W, Sn, Bi and Mo by ICP-MS; and As by X-Ray Fluorescence and Ultraviolet Visible Spectrometer (Table 2). Enrichment factors were calculated using the crustal abundances as the background values and the results are presented in Table 3. At the Zhemulang gold deposit, the element association is Sb, Te, Au, Bi, As, Pb, W and Ag in order of enrichment factors of gold and accessory elements. According to the reconnaissance geological exploration report of the Zhemulang gold deposit (Ai, 2007), the average gold grade is

higher than 2.33 ppm, therefore the association actually should be Au, Sb, Te, Bi, As, Pb, Ag and W, whereas Cu, Zn, Sn, Mo, and Hg are depleted relative to the crustal abundance. Because most of the analyzed samples were collected from the oxidized zone, the selected elements may be diluted by oxidization in varying degrees. At the Mazhala gold–antimony deposit, the element association in gold-bearing antimony–quartz vein orebodies is Sb, Au, Te, As, Pb, Bi, Ag, and W, with depletion in Cu, Zn, Sn, and Mo, in order of enrichment factors of gold, antimony, and accessory elements. At the Shalagang antimony deposit, the element association, in order of enrichment factors of antimony and accessory elements is Sb, Te, As, Au, Hg, W, Pb, and Ag, with depletion in Cu, Sn and Mo, Zn is slightly enriched relative to crustal abundance, the concentrations of Hg and Zn are higher than the Zhemulang and Mazhala deposits. Because the content of cinnabar in the Shalagang antimony ore is relatively high, so the measured Hg concentrations are higher than the values in the Zhemulang and Mazhala deposits. The three epigenetic hydrothermal lode Au and/or Sb deposits (e.g., the Zhemulang, Mazhala and Shalagang deposits) demonstrate a similar element association of Au, Ag, As, Sb, Bi, Te, W and Pb, and are depleted in Cu, Zn, Sn and Mo (Fig. 10), probably indicating the same ore metal origin. It is should be noted that those deposits are classified as different types and show various element enrichment values. The order of element associations of the Zhemulang gold deposit, the Mazhala gold–antimony deposit and the Shalagang antimony deposit vary from Au\Sb\Te\Bi\As\Pb\Ag\W, Sb\Au\Te\As\Pb\Bi\Ag\W and Sb\Te\As\Au\Hg\W\Pb\Ag, respectively. The differences in those element associations may be due to the changes of the physico-chemical conditions. In the Zhemulang gold deposit, the concentrations of Au, Bi, and W are high and those values decreased gradually from the Mazhala gold–antimony deposit to the Shalagang antimony deposit. In contrast, the concentrations of Sb, Hg and Zn increase from the Zhemulang deposit to the Shalagang antimony deposit. 4.2. Fluid inclusions

Fig. 8. Geological section of No.0 prospecting line of No. 1 and 3 orebodies at Shalagang antimony Deposit (after Yang et al., 2000).

Previously, a reconnaissance fluid inclusion investigation was conducted on the Mazhala and Shalagang deposits (Yang et al., 2009). Two samples from the Mazhala gold–antimony deposit were examined. Inclusions contained two-phase aqueous inclusions and three-phase CO2-liquid inclusions with homogenization temperatures from 185 to

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Fig. 9. Photographs and photomicrographs of Shalagang antimony deposit. A. Stibnite-quartz veinlet or stockwork in diabase. B. Late stage quartz and stibnite filling the interspace of euhedral quartz under transmitted light. C. Late stage quartz and stibnite filling the center of the quartz vein formed in early stage. D. Stibnite ore. Qtz = Quartz, Stb = Stibnite.

304 °C and salinities of 0.2 to 8.13 wt.% NaCl equiv. Seven samples were examined from Shalagang antimony deposit and only two-phase aqueous inclusions were identified, with homogenization temperatures from 135 to 367 °C and salinities from 0.5 to 12.0 wt.% NaCl equiv. No previous fluid inclusion investigation has been carried out on the relatively newly discovered Zhemulang deposit. In this study, 32 samples from the three deposits were selected for systematic inclusion study. The fluid inclusion petrographic observation and microthermometric measurements were performed using an Olympus BX51P infrared microscope equipped with Linkam FTIR 600 infrared heating/freezing stage, Mingmei MD15 microviewer infrared camera, and Qimagine MP5.0 color camera allowing the observation on PC monitor. Fluorescence observations were performed using a Zeiss AxioImager A1m microscope equipped with UV light and Qimagine MP5.0 color camera. The analytical precision of Linkam heating/freezing stage is ±0.1 °C. Calibrations were made using synthetic CO2 fluid inclusions and pure-water inclusions. The salinities and densities of inclusions were calculated using FLINCOR (Brown, 1989). Laser Raman spectroscopy was performed in the laboratory of Instrumental Analysis and Research Center, Sun Yat-sen University, China, using a Renishaw 2000 infra Raman microscope at 25 °C. The incident laser wavelength was 514.5 nm. The determination of inclusion paragenesis was made following to the classification of primary, pseudosecondary, and secondary as outlined by Roedder (1984). The fluid inclusions in gold-bearing quartz, gold- and stibnite-bearing quartz, and stibnite-bearing quartz from Zhemulang, Mazhala, and Shalagang, respectively, are mainly primary and pseudosecondary, distributed as clusters, randomly, or in trails. Infrared microscopy was used to study inclusions stibnite in the latter two deposits and similar inclusions were observed. Secondary fluid inclusions in trails clearly cutting the quartz and stibnite grain boundaries are not developed, due to little deformation and metamorphism of these Cenozoic deposits and perfectly preserved vuggy textures in the ore veins.

On the basis of phases present at room temperature, phase changes during heating/cooling, and Laser Raman spectroscopy, three types of fluid inclusions have been recognized. Type 1 are aqueous inclusions with only one phase of liquid at room temperature (type 1a) or twophase liquid–vapor inclusions at room temperature (type 1b), which homogenized to liquid during heating measurements. Type 2 are aqueous-carbonic inclusions including single phase carbonic (type 2a) or two-phase aqueous-carbonic inclusions (type 2b) with a distinct carbonic phase only indicated by the formation of clathrate during cooling cycles. In type 2 inclusions, carbonic phase changes and/or clathrate formation occurred during microthermometric measurements, and the carbonic phase homogenized to liquid in most inclusions, although a few examples homogenized to vapor or the critical phase. The melting temperatures of the carbonic phase are lower than that for pure CO2 and the most of clathrate melting temperatures are higher than 10 °C, indicating that the carbonic phase is not a pure CO2. Laser Raman spectroscopy analyses indicated the carbonic phase belong to the CO2\CH4\N2 system. Type 2b inclusions have variable volume percentages for the carbonic phase, from b50 vol.% that homogenized to a liquid aqueous to N50 vol.% that homogenized to a carbonic phase. Type 3 are hydrocarbon inclusions, including two-phase hydrocarbon inclusions (type 3a) commonly with a light green or pale green liquid hydrocarbon and a black gas bubble, with no fluorescence under a UV light, and dark one-phase hydrocarbon inclusions (type 3b), some of which fluoresce. Among the three types of fluid inclusions, type 1 and type 2 are predominant, whereas type 3 are rare. 4.2.1. Zhemulang Fluid inclusions in quartz from the gold-bearing quartz veins are types 1b, 2a, 2b, and rare 3a (Fig. 11A, B, I). The results of microthermometric measurements are provided in Table 4 and Figs. 12, 13. For all type 1b inclusions, the eutectic temperatures were difficult to measure, with best freezing temperature estimates of −43.1 to −32.4 °C. The final ice melting temperature are −4.9 to −0.4 °C, and the corresponding salinities of

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inclusions are 0.7 to 7.8 wt.% NaCl equiv., with a mode from − 4.0 to − 2.0 °C that corresponds to salinities of 3.3 to 6.4 wt.% NaCl equiv. The homogenization temperatures are 146 to 292 °C, with most from 180 to 260 °C (Table 4, Fig. 12). For type 2 inclusions, the melting temperature of the carbonic phase are −59.7 to −56.4 °C, with a mode at −58.5 to −56.6 °C. The clathrate melting temperatures are 6.2 to 11.1 °C, with most at 10 to 11.1 °C. The homogenization temperatures of the carbonic phase are 12.7 to 23.4 °C. The total homogenization temperatures of aqueouscarbonic inclusions are 184 to 338 °C (Fig. 12). For most type 2 inclusions, the melting temperatures of the solid carbonic phase are depressed below the CO2 triple point of −56.6 °C, and the clathrate melting temperatures are above CO2 clathrate dissociation temperatures of 10 °C, which together indicate the presence of CH4 and/or N2 (Diamond, 1994, 2001). Laser Raman spectroscopy analyses indicate that the carbonic phase consist of 74.7 to 94.4 mol% CO2, 5.1 to 24.5 mol% N2, and undetected to 1.7 mol% CH4 (Fig. 11H, Table 5). Type 3a hydrocarbon inclusions occur in clusters or as individuals in quartz grains or in the gaps between quartz grains (Fig. 11I). Laser Raman spectroscopy analyses indicate that the liquid- and vaporphase hydrocarbons have the same chemical compositions, which consist mainly of alkanes and polycyclic aromatic hydrocarbons. In the Raman spectra (Fig. 11K), the strongest peaks for methane, ethane, propane, and polycyclic aromatic hydrocarbons are obvious (Atamas et al., 2004; Colangeli et al., 1992; Schrötter and Bernstein, 1963; Zhang et al., 2007). Alkanes and polycyclic aromatic hydrocarbons are the main components of oil, but there are no bitumen peaks (Jehlička and Beny, 1999; Shoute et al., 2002) in the Raman spectra, suggesting that the inclusions contain high maturity hydrocarbons (Zhang et al., 2009).

4.2.2. Mazhala Fluid inclusions in quartz and stibnite grains from the goldstibnite deposit are also types 1b, 2a, 2b, and rare 3a (Fig. 11C, D, J). The type 2 inclusions are predominant over type 1b. Among the 13 samples examined for fluid inclusion study, only one of these is suitable for conducting microthermometric measurements of fluid inclusions in stibnite. For type 1b inclusions, the eutectic temperatures were again difficult to measure, with the estimated freezing temperatures ranging from −43.1 to −31.8 °C. Ice melting temperatures for fluid inclusions in quartz are from −3.1 to −0.9 °C, with corresponding salinities of 1.5 to 5.0 wt.% NaCl equiv.. A mode exists at −3.0 to −1.5 °C and corresponds to salinities of 2.5 to 4.9 wt.% NaCl equiv. Ice melts in fluid inclusions in stibnite from − 3.0 to − 1.0 °C, with a peak at − 2.5 to − 1.5 °C, corresponding to salinities of 2.5 to 4.1 wt.% NaCl equiv.. Thus salinities of fluid inclusions in stibnite and quartz are nearly identical. The homogenization temperatures for both quartz and stibnite are 148 to 303 °C, with most homogenization temperatures from 160 to 280 °C (Table 4, Fig. 12). For type 2 inclusions, the melting temperatures of the carbonate phase are − 59.9 to − 56.4 °C. The clathrate melting temperatures are 8.7 to 11.2 °C, with most from 10 to 11.0 °C. The homogenization temperatures of the carbonic phase are 18.7 to 30.9 °C. The total homogenization temperatures of type 2b inclusions are 171 to 320 °C (Table 4, Figs. 12, 13). Laser Raman spectroscopy analyses indicate that the carbonic phase consists of 97.3 to 99.6 mol% CO2, 0.4 to 2.7 mol% N2, and as much as 0.3 mol% CH4 (Table 5). Rare type 3a hydrocarbon inclusions were identified in quartz as being cogenetic with type 1 and type 2, they belong primary or pseudosecondary inclusions. Laser Raman spectroscopy analyses

Table 2 Gold, antimony and accessory elements concentration of ores from Zhemulang, Shalagang and Mazhala deposits (ppm). Sample No.

Ore type

Hg

Zhemulang 2010009 2010017 2010026 2010027 2010028 2010030 2010032 2010034 2010041 2010044

Gold-bearing quartz vein Gold-bearing quartz vein Gold-bearing quartz vein Gold-bearing quartz vein Gold-bearing quartz vein Gold-bearing quartz vein Gold-bearing quartz vein Gold-bearing quartz vein Gold-bearing quartz vein Gold-bearing quartz vein

0.019 0.022 0.024 0.031 0.023 0.032 0.021 0.026 0.018 0.009

Mazhala 09MZL-I-5 09MZL-I-6 09MZL-I-7 09MZL-I-8a 09MZL-I-13 09MZL-I-15 09MZL-2-7 09MZL-2-12 10MZL-121 10MZL-122

Gold–stibnite–quartz vein Gold–stibnite–quartz vein Gold–stibnite–quartz vein Gold–stibnite–quartz vein Gold–stibnite–quartz vein Gold–stibnite–quartz vein Gold–stibnite–quartz vein Gold–stibnite–quartz vein Gold–stibnite–quartz vein Gold–stibnite–quartz vein

0.094 0.061 0.094 0.085 0.067 0.059 0.12 0.13 0.064 0.098

Shalagang 09SL-I-4 Altered fault breccia 09SL-I-7 Stibnite–quartz vein 09SL-I-10 Stibnite–quartz vein 09SL-VIII-2 Stibnite–quartz vein 09SL-VIII-3 Stibnite–quartz vein 09SL-VIII-5 Stibnite–quartz vein 09SL-VIII-6 Stibnite–quartz vein 09SL-IX-2 Altered fault breccia 09SL-IX-5 Altered fault breccia 10SL-099 Stibnite–quartz vein Element abundance of continental crustb a b

Au

Pb

As

Sb

Cu

0.19 1.37 0.34 0.15 0.17 0.22 0.05 0.60 0.03 0.02

51.3 9.6 59.1 2511.0 757.3 678.1 159.1 240.2 5.7 5.0

183.2 29.0 22.9 198.2 231.2 108.5 68.3 42.5 18.1 12.4

12.1 25.4 24.1 31.2 282.1 710 21.7 15.1 32.5 28

7.8 80.7 3.4 380.3 10.4 100.8 5.0 6.6 4.1 2.3

292 52.05 280 4.91 123 72.22 103 168 2.61 15.04

231.8 145.3 227.5 79.6 149.8 395.4 15.8 100.7 20.3 144.5

0.024a 0.015a 0.004a 0.011a 0.006a 0.12a 0.008a 0.011a 0.013a 1.50a

31.18a 10.35a 37.60a 34.68a 33.82a 32.61a 50.52a 49.24a 56.44a 11.16a

85.9 35.5 108.2 106.4 58.7 113.2 55.2 69.1 25.7 33.6

21.8 2.6 9.5 63.0 25.4 14.8 49.5 15.9 21.8 5.9 8.0

0.028a 0.001a 0.001a 0.000a 0.006a 0.004a 0.000a 0.005a 0.005a 0.097a 1.0

0.971 18.051 60.87a 23.40a 8.56a 31.33a 25.90a 13.42a 14.63a 24.19a 0.2

20.5 14.3 31.9 41.4 13.5 31.6 30.8 21.2 18.3 12.5 75

0.074 0.43 0.31 1.00 0.31 0.13 0.11 0.20 0.19 0.29 0.08

wt.%. From Li and Ni (1990), Taylor and McLeannan (1991).

0.01 0.02 0.06 0.10 0.00 0.04 0.05 0.03 0.11 0.31 0.003

W

12.1

14.4 5.1 3.3

1.9 0.9

32.5 1.1 1.4

3.1 1.3 4.7 1.0 5.1 1.0

Sn

Zn

Ag

Te

Mo

Bi

0.2 1.2 0.3 3.5 1.9 2.6 1.5 0.8 0.050 0.048

0.030 0.025 0.027 0.13 0.29 0.32 0.14 0.048 0.030 0.017

0.64 0.65 0.26 0.82 1.05 0.55 0.42 0.30 0.28 0.74

0.56 0.15 2.17 29.90 11.42 9.56 3.39 4.56 0.082 0.035

0.91 1.15 1.02 0.90 0.96 0.93 1.04 0.96 0.84 0.94

6.7 23.0 7.4 17.4 25.0 42.5 63.8 25.5 8.3 4.2

1.60 1.15 1.35 1.15 1.72 2.12 2.34 2.37 5.60 1.36

43.9 23.8 95.3 45.4 21.0 28.7 20.0 27.2 107.8 109.1

0.3 0.4 2.0 0.1 0.4 0.2 0.8 2.9 0.2 0.3

1.48 0.12 2.86 0.90 2.90 1.82 4.89 8.17 2.61 0.50

0.34 0.53 0.24 0.52 0.36 0.42 0.22 0.19 0.29 0.38

1.26 0.55 1.76 1.16 0.33 1.69 0.32 0.72 0.24 0.46

0.94 2.23 6.09 0.96 2.07 0.93 0.97 0.97 1.31 0.82 2.5

12.5 224.6 57.5 427.5 186.3 82.2 30.9 69.5 135.4 1.2 80

0.4 0.1 0.3 0.2 0.056 0.2 0.1 0.2 0.2 0.2 0.08

0.012 0.031 1.61 0.30 0.006 0.66 0.55 0.21 0.12 0.14 0.001

0.60 0.52 0.11 0.36 0.47 0.22 0.26 0.79 0.43 0.26 2.0

0.087 0.022 0.011 0.15 0.20 0.010 0.076 0.012 0.011 0.008 0.06

Au/Ag 0.95 1.14 1.13 0.04 0.09 0.08 0.03 0.75 0.60 0.42

973.33 130.13 140.00 49.10 307.50 361.10 128.75 57.93 13.05 50.13

0.03 0.20 0.20 0.50 0.00 0.20 0.50 0.15 0.55 1.55 0.038

W. Zhai et al. / Ore Geology Reviews 58 (2014) 68–90

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Table 3 Enrichment factors (= element concentration/element abundance of continental crust). Sample no.

Ore type

Hg

Au

Pb

As

Sb

Zhemulang(element association, in order of average enrichment factor: Sb, Te, Au, Bi, As, Pb, Ag, W) 10ZM009 Gold-bearing quartz vein 0.24 63.33 6.41 183.2 10ZM 017 Gold-bearing quartz vein 0.28 456.67 1.2 29 10ZM 026 Gold-bearing quartz vein 0.3 113.33 7.39 22.9 10ZM 027 Gold-bearing quartz vein 0.39 50 313.88 198.2 10ZM 028 Gold-bearing quartz vein 0.29 56.67 94.66 231.2 10ZM 030 Gold-bearing quartz vein 0.4 73.33 84.76 108.5 10ZM 032 Gold-bearing quartz vein 0.26 16.67 19.89 68.3 10ZM 034 Gold-bearing quartz vein 0.33 200 30.03 42.5 10ZM 041 Gold-bearing quartz vein 0.23 10 0.71 18.1 10ZM 044 Gold-bearing quartz vein 0.11 6.67 0.62 12.4 Average 0.3 104.7 56.0 91.4

Cu 60.5 127 120.5 156 1410.5 3550 108.5 75.5 162.5 140 591.1

0.1 1.08 0.05 5.07 0.14 1.34 0.07 0.09 0.05 0.03 0.8

1,559,000 517,500 1,880,000 1,734,000 1,691,000 1,630,500 2,526,000 2,462,000 2,822,000 558,000 17,38000.0

1.15 0.47 1.44 1.42 0.78 1.51 0.74 0.92 0.34 0.45 0.9

Shalagang (element association, in order of average enrichment factor: Sb, Te, As, Au, Hg, W, Pb, Ag) 09SL-I-4 Altered fault breccia 0.93 3.33 2.72 280 48,500 09SL-I-7 Stibnite–quartz vein 5.38 6.67 0.33 10 902,500 09SL-I-10 Stibnite–quartz vein 3.88 20 1.19 10 3,043,500 09SL-VIII-2 Stibnite–quartz vein 12.5 33.33 7.87 0 1,170,000 09SL-VIII-3 Stibnite–quartz vein 3.88 0 3.17 60 428,000 09SL-VIII-5 Stibnite–quartz vein 1.63 13.33 1.85 40 1,566,500 09SL-VIII-6 Stibnite–quartz vein 1.38 16.67 6.19 0 1,295,000 09SL-IX-2 Altered fault breccia 2.5 10 1.99 50 671,000 09SL-IX-5 Altered fault breccia 2.38 36.67 2.73 50 731,500 10SL-099 Stibnite–quartz vein 3.63 103.33 0.74 970 1,209,500 Average 3.8 24.3 2.9 147.0 1,106,600.0

0.27 0.19 0.42 0.55 0.18 0.42 0.41 0.28 0.24 0.17 0.3

Mazhala(element association, in order of average enrichment factor: Sb, Au, Te, As, Pb, Bi, Ag, W) 09MZL-I-5 Gold–stibnite–quartz vein 1.18 97333.33 28.98 240 09MZL-I-6 Gold–stibnite–quartz vein 0.76 17350 18.16 150 09MZL-I-7 Gold–stibnite–quartz vein 1.18 93333.33 28.44 40 09MZL-I-8a Gold–stibnite–quartz vein 1.06 1636.67 9.95 110 09MZL-I-13 Gold–stibnite–quartz vein 0.84 41000 18.73 60 09MZL-I-15 Gold–stibnite–quartz vein 0.74 24073.33 49.43 1200 09MZL-2-7 Gold–stibnite–quartz vein 1.5 34333.33 1.98 80 09MZL-2-12 Gold–stibnite–quartz vein 1.63 56000 12.59 110 10MZL-121 Gold–stibnite–quartz vein 0.8 870 2.54 130 10MZL-122 Gold–stibnite–quartz vein 1.23 5013.33 18.06 15000 Average 1.1 37094.3 18.9 1712.0

indicate that liquid and vapor phase of type 3a inclusions have the same composition as the inclusions at Zhemulang.

4.2.3. Shalagang Fluid inclusions in quartz and stibnite grains from the antimony deposit are types 1a, 1b, 2a, and 2b (Fig. 11E, F, G), as well as black onephase type 3b hydrocarbon inclusions (Fig. 11L). The type 1b inclusions are predominant over type 2, and type 1a are rare, only observed in stibnite. Compared to fluid inclusions in the samples from Zhemulang and Mazhala, those from Shalagang in quartz are relatively small, with diameters of inclusions generally b 10 μm. In contrast, fluid inclusions in stibnite are larger, generally long in shape and parallel to the c axis of stibnite crystal, and reaching dimensions of 10 × 20 μm (Fig. 11G).

W

12.1

14.4 5.1 3.3 8.7

1.9 0.9

32.5 1.1 1.4 7.3

3.1 1.3 4.7 1 5.1 3.0

Sn

Zn

Ag

0.36 0.46 0.41 0.36 0.38 0.37 0.42 0.38 0.34 0.38 0.4

0.08 0.29 0.09 0.22 0.31 0.53 0.8 0.32 0.1 0.05 0.3

2.5 15 3.75 43.75 23.75 32.5 18.75 10 0.63 0.6 15.1

0.64 0.46 0.54 0.46 0.69 0.85 0.94 0.95 2.24 0.54 0.8

0.55 0.3 1.19 0.57 0.26 0.36 0.25 0.34 1.35 1.36 0.7

0.38 0.89 2.44 0.38 0.83 0.37 0.39 0.39 0.52 0.33 0.7

0.16 2.81 0.72 5.34 2.33 1.03 0.39 0.87 1.69 0.01 1.5

Te

Mo

Bi

30 25 27 130 290 320 140 48 30 17 105.7

0.32 0.33 0.13 0.41 0.53 0.28 0.21 0.15 0.14 0.37 0.3

9.3 2.5 36.2 498.3 190.3 159.3 56.5 76.0 1.4 0.6 103.0

3.75 5 25 1.25 5 2.5 10 36.25 2.5 3.75 9.5

1480 120 2860 900 2900 1820 4890 8170 2610 500 2625.0

0.17 0.27 0.12 0.26 0.18 0.21 0.11 0.1 0.15 0.19 0.2

21.0 9.2 29.3 19.3 5.5 28.2 5.3 12.0 4.0 7.7 14.2

5 1.25 3.75 2.5 0.7 2.5 1.25 2.5 2.5 2.5 2.4

12 31 1610 300 6 660 550 210 120 140 363.9

0.3 0.26 0.06 0.18 0.24 0.11 0.13 0.4 0.22 0.13 0.2

1.45 0.37 0.18 2.5 3.33 0.17 1.27 0.2 0.18 0.13 0.98

Among the 12 fluid inclusion samples from Shalagang, only type 1 fluid inclusions are identified in stibnite, and only one sample is suitable for microthermometric measurements of inclusions in stibnite. For type 1 aqueous inclusions in both quartz and stibnite, the freezing temperature are −31.5 to −45.0 °C, the final ice melting temperature are −5.3 to −0.3 °C, and the corresponding salinities of inclusions are 0.5 to 8.2 wt.% NaCl equiv., with a mode from −4.0 to −2.5 °C and salinities of 4.1 to 6.4 wt.% NaCl equiv.. The homogenization temperatures are 140 to 290 °C. The homogenization temperatures and salinities of inclusions in stibnite and in quartz are similar (Table 4, Fig. 12). For type 2 inclusions, the melting temperatures of the carbonic phase are −61.2 to −57.0 °C, although most are from −58.5 to −57.0 °C. The clathrate melting temperature are 9.5 to 11.6 °C, with most values from 10 to 11.0 °C. The homogenization temperatures of the carbonic phase

Fig. 10. Histograms of element enrichment factors for Zhemulang, Mazhala and Shalagang deposits.

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are 18.2 to 25.8 °C, although a few are as low as −11.8 °C. The total homogenization temperatures of the type 2b inclusions are 171 to 290 °C (Table 4, Figs. 12, 13). Laser Raman analyses indicate that the carbonic phase consists of 91.9 to 99.4 mol% CO2, as much as 6.0 mol% N2, and 0.6 to 3.6 mol% CH4 (Table 5). Rare amounts of type 3b dark one-phase hydrocarbon inclusions were identified in quartz grains and are cogenetic with the type 1 and type 2 inclusions (Fig. 11L). Laser Raman analyses (Fig. 11M) indicate that aromatic hydrocarbon, non-hydrocarbon compounds, and bitumen are present in the hydrocarbon inclusions (Zhang et al., 2007). In addition, some of type 3b inclusions have peaks for CO2 and CH4 (Fig. 11M), indicating that the hydrocarbon inclusions are sourced from less saturated hydrocarbon oil or gas with a low maturity (Zhang et al., 2007). For CO2\H2O fluid inclusions, the addition of salts to the water can depress the clathrate dissociation temperature, but volatiles such as CH4 and H2S can increase the clathrate dissociation temperature (Diamond, 1994, 2001). At Zhemulang, Mazhala, and Shalagang, the clathrate melting temperatures for most of type 2b aqueous-carbonic inclusions are from 10 to 11 °C, and above the pure CO2 clathrate melting temperatures, so the salinity calculations can not be carried out accurately. Therefore, we assume the salinities of aqueous inclusions to best approximate the overall salinity of the ore-forming fluids. 4.2.4. Concluding remarks The Zhemulang, Mazhala, and Shalagang deposits have similar fluid inclusions types and fluid inclusion microthermometric data, which imply a regional ore-forming fluid composition and physio-chemical mineralization conditions. The three types of inclusions are cogenetic, and homogenization temperatures of type 1b and type 2b are overlapping (Table 4, Figs. 12, 13), suggesting a heterogeneous trapping environment during the mineralization event at the three deposits. 4.3. Stable isotopes Oxygen isotopic compositions were measured for quartz and calcite from the veins; in addition, hydrogen and carbon isotopes of inclusion fluids extracted from quartz, carbon isotopes for calcite, and sulfur isotopes for ore-related sulfides were also analyzed. Individual mineral separations were made at the Laboratory of Department of Earth Sciences, Sun Yat-sen University. Isotopic measurements were carried out in Analytical Laboratory, Beijing Research Institute of Uranium Geology. The δ18O composition of quartz was determined using BrF5 digestion. The δ13C of carbonates was determined using 100% phosphoric acid digestions. The δD and δ13C compositions of fluid inclusions in quartz were determined by thermal decrepitation to extract the fluid and carbonaceous material in fluid inclusions from ~10 g of sample consisting of mineral fragments that are 0.5 to 1.0 mm in diameter. The extracted water was reduced with carbon to generate H2 and carbonaceous material was oxidized to CO2 using Cu2O at 650 °C for isotope analysis. The δ34S compositions of sulfides were determined using the Cu2O oxidation method to generate SO2 for isotope analysis. The δ18O, δ13C, and δD compositions were determined on a Thermo Fisher MAT 253 mass spectrometer, and δ34S compositions were analyzed on a Finnigan MAT 251 mass spectrometer. The analytical precision is better than ±0.2‰ for δ18O, δ13C, and δ34S, and ±1‰ for δD. The measured results are reported relative to SMOW for hydrogen and oxygen, PDB for carbon, and CDT for sulfur. The oxygen isotope composition of water in equilibrium with the quartz were calculated using the fractionation data of Zheng (1992) and the average homogenization temperatures of fluid inclusions for individual sample (Tables 6, 7). For the Zhemulang gold deposit, ten quartz samples from goldbearing quartz veins were analyzed for hydrogen, oxygen, and carbon isotopes. The δDH2O values of fluid inclusion waters are − 107.5 to − 36.7‰, the δ18Oquartz values are 12.7 to 18.1‰, and the calculated δ18Ofluid values are 2.8 to 8.2‰. The δ13Cfluid values of fluid extracted from fluid inclusions are − 11.7 to − 9.6‰. The

sulfur isotope compositions of five pyrite and galena samples from the veins −4.0 to −1.1‰. For the Mazhala gold–antimony deposit, 16 quartz and 2 calcite samples from the gold- and stibnite-bearing quartz veins were analyzed for hydrogen, oxygen, and carbon isotopes. The δDH2O values of fluid inclusion waters are −119 to −72.7‰, the δ18O values of quartz are 17.9 to 25.7‰, and the calculated δ18Ofluid values in equilibrium with quartz are 7.5 to 16.2‰. The δ18Ocalcite values of two calcite samples are 14.9 and 19.3‰. The δ13Cfluid values of fluids from fluid inclusions of quartz are −3.5 to −2.5‰, and the δ13Ccalcite values of two calcite samples are −4.5 to −3.4‰. Using the homogenization temperatures of the two samples and the fractionation factors between CO2 and calcite (Bottinga, 1969), the calculated δ13CCO2 values of CO2 in equilibrium with calcite in ore-forming fluid are −3.5 and −2.9‰. These values are consistent with δ13C of fluid inclusions in quartz. The δ34S values of two pyrite samples from synsedimentary Sedex-like sulfide layers are 2.6 to 2.7‰, whereas, the δ34S values of 12 stibnite samples range from −0.8 to 2.3‰ For the Shalagang antimony deposit, 14 quartz samples from stibnite-bearing quartz veins were analyzed for hydrogen, oxygen, and carbon isotopes. The δDH2O values of fluid inclusions are −173.4 to −139.2‰, the δ18Oquartz values are 20.4 to 23.4‰, and the calculated δ18Ofluid values are 7.5 to 12.3‰. The δ13Cfluid values of fluid inclusions are −6.5 to −5.1‰. The δ34S values f 21 stibnite samples are −3.9 to 2.1‰ 5. Discussion — genesis of the deposits 5.1. Classification of the gold–antimony deposits in southern Tibet Many researchers have carried out studies on the gold, gold– antimony and antimony deposits in the southern Tibet Au-Sb metallogenic belt. Although these deposits have the same geological and structural setting, their genetic model remains problematic. The Shalagang antimony deposit was suggested to have formed from meteoric fluid driven by intrusive bodies and overprinted the early Sedex-type mineralization (Li et al., 2002). The gold, gold–antimony, and antimony deposits were suggested to be related to Yanshanian and Himalayan alkaline basic and intermediate intrusive dikes (Nie et al., 2005). At the Mayoumu (or Mayum) gold deposit, in the western part of the Au\Sb belt, hydrogen and oxygen isotope data support a predominantly meteoric water ore-forming fluid, with a minor magmatic contribution, consistent with many epithermal gold deposits (Duo et al., 2009; Wen et al., 2006). The ore-forming fluid at Mayum was also attributed to a deep-crustal metamorphic source and a magmatic source, and Mayuma was termed an orogenic gold deposit (Jiang et al., 2009). At the Chalapu gold deposit, northern Longzi county (Fig. 1), the visible gold-bearing quartz and cataclastically altered slate were considered, to show features more characteristic of Carlin-type gold deposit (Zheng et al., 2007). The majority of the Sb- and/or Au-bearing deposits, including Mazhala and Shalagang, are distributed around the thermal domes that consist of metamorphic core complexes and Himalayan leucogranite intrusions in their centers, and display a concentric zoning outwards from Au and Au\Sb to Sb mineralization. The ore-forming fluids were suggested to evolve from magmatic water-dominated to meteoric water-dominated outwards, and the mineralization was assumed to have been controlled by a hydrothermal convection system driven by the leucogranites, with the deposits generally showing characteristics of epithermal deposits (Hou et al., 2009; Yang et al., 2009). On the basis of stable isotope, noble gas, and fluid inclusion evidence, the ore-forming fluid for Bangbu gold deposit was suggested to have derived from metamorphic fluids, with some involvement of a mantle fluid (Sun et al., 2010; Wei et al., 2010). The three studied gold, gold–antimony, and antimony deposits formed during the collision that led to the establishment of the Himalayan orogen. The orebodies are hosted in second- or third-order extensional

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Fig. 11. Fluid inclusions and Laser Raman spectra of type 2 and type 3 inclusions from Zhemulang, Mazhala and Shalagang deposits. A, B. Type 1b and 2b inclusions in quartz from Zhemulang. C. Type 1b and 2b inclusions in quartz from Mazhala. D. Type 2b inclusions in stibnite from Mazhala. E. Type 1b and 3b inclusions in quartz from Shalagang. F. Type 2b and 1b inclusions in quartz from Shalagang. G. Type 1a and 1b inclusions in stibnite from Shalagang. H. Laser Raman spectrum of carbonic phase in type 2 inclusion with peaks of CO2 at 1283 and 1386 cm−1, N2 at 2327 cm−1 and CH4 at 2913 cm−1. I. Type 3a inclusions in quartz from Zhemulang. J. Type 3a inclusions cogenetic with type 2b inclusions in quartz from Mazhala. K. Laser Raman spectra of liquid-phase (L) and vapor-phase (V) Hydrocarbon in two-phase hydrocarbon inclusions. The peaks at 2800 to 3000 cm−1 and about 3066 cm−1 and 1610 cm−1 are the characteristic peaks of alkanes, and polycyclic aromatic hydrocarbons (PAHs), the peaks at about 600 to 1600 cm−1 are mixed peaks of alkanes and PAHs (Atamas et al., 2004; Colangeli et al., 1992; Schrötter and Bernstein, 1963; Zhang et al., 2007). The characteristic peaks of bitumen at about 1340 to 1380 cm−1 and 1530 to 1600 cm−1 (Jehlička and Beny, 1999; Shoute et al., 2002) do not present or has weak intensity, indicating that the content of bitumen is very low and the hydrocarbon inclusions belong to high maturity. L. Type 3b inclusions cogenetic with type 1b inclusions in quartz from Shalagang, and some of type 3b inclusions fluoresce under illustrating fluorescence. M. Laser Raman spectra of type 3b onephase hydrocarbon inclusions. The wide fluorescence peaks at about 1500 to 3000 cm−1 are Raman effect of aromatic hydrocarbon, non-hydrocarbon compound and bitumen in hydrocarbon inclusions (Zhang et al., 2007), in addition to this type of characteristic peak, some of type 3b inclusions have peaks of CO2 and CH4, indicating that the hydrocarbon inclusions belong to less saturated hydrocarbon oil or gas inclusion with low maturity (Zhang et al., 2007). Stb = Stibnite.

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Fig. 11. (continued).

faults or splays of regional faults or shear zones within Triassic to Cretaceous metasedimentary rocks. The geochemical association in the ores is Au, Ag, As, Sb, Bi, Hg, Te, W, and Pb, with depletions in Cu, Zn, Sn, and Mo. Ore-forming fluids were low-salinity (2.5 to 6.5 wt.% NaCl equiv.) H18 2O\CO2\CH4\N2 systems. The δ Ofluid values for the Zhemulang, Mazhala, and Shalagang hydrothermal fluid are 2.8 to 8.2‰, 7.5 to 16.2‰, and 7.5 to 12.3‰, respectively; the homogenization temperatures of fluid inclusions for Zhemulang, Mazhala, and Shalagang are 180 to 320 °C, 160 to 300 °C, and 140 to 240 °C, respectively; and the wallrock alteration has produced alteration minerals such as silica, carbonates, sulfides, and white mica at each deposit. The deposits are consistent with other lode gold deposits (McCuaig and Kerrich, 1998), or epizonal orogenic gold and antimony deposits as described by Groves et al.(1998, 2003) and Goldfarb et al. (2001, 2005), and shear zone-hosted mesothermal Sb deposits classified by Dill (1998). Therefore, we consider these gold, gold–antimony and antimony deposits as epizonal orogenic mineral systems, including Mayoumu, formerly named as epithermal gold (Duo et al., 2009), although the epizonal/mesozonal/hypozonal scheme of the continuum model was considered invalid in high-grade metamorphic domains (Phillips and Powell, 2009, 2010; Tomkins and Grundy, 2009). 5.2. Source of ore-forming fluids and metals Orogenic gold deposits have common geological and geochemical features (Goldfarb et al., 2001, 2005; Groves et al., 1998, 2003), but there is no consensus as to the source of ore fluids and metals. A metamorphic origin is favored by most workers, e.g., Powell et al. (1991), Stüwe (1998), Elmer et al. (2006), Pitcairn and Teagle (2006) and Phillips and Powell (2010) favor ore fluids derived from deeper levels of the ore-hosting metamorphic rock-dominant terranes, and correspondingly a metamorphic devolatilization model for formation of gold deposit was proposed (Phillips and Powell, 2010), as also suggested by Fyfe and Kerrich (1985) and Goldfarb et al. (2007) who argue for a deeper metamorphic fluid source which, in some cases

may be related to devolatilization of, subducted oceanic crust. Some orogenic gold deposits are spatially and temporally associated with granitoids, and some workers have attributed the ore fluids to a magmatic origin, e.g., the Samhwanghak in Korea and Linlong in east China (So and Yun, 1997; Yao et al., 1990). A mantle origin for the Autransporting ore fluid was favored by Rock and Groves (1988) and Cameron (1988). Meteoric water is known to contribute to the formation of mesothermal (orogenic) Au\Sb\Hg deposits in the Cordillera (Nesbitt et al., 1986, 1989) and has also been suggested to be involved in the formation of the Late Archean epizonal Wiluna deposit in the Yilgarn province (Hagemann et al., 1994). The mixing of magmatic and metamorphic fluids, or metamorphic and mantle fluids has been suggested for gold mineralization at Loulo orogen in West Africa and Ailaoshan orogen in China, respectively (Lawrence et al., 2013; Sun et al., 2009). A mixed ore fluid that was primarily of metamorphic origin, but with involvement of magmatic fluid in the deeper levels and meteoric or sea water in the shallower levels of some deposits was favored by Groves (Fig. 2, 1993) and McCuaig and Kerrich (Fig., 25, 1998). 5.2.1. Constraints from hydrogen and oxygen isotopes The Zhemulang Au, Mazhala Au\Sb, and Shalagang Sb deposits have similar ore-forming fluid compositions and metal associations, indicating a similar ore-forming fluid and metal source throughout the metallogenic belt. At Zhemulang the ore fluid δ18O of 2.8 to 8.2‰ is lower than most estimates for orogenic gold deposits which generally vary in δ18Ofluid from 6 to 13‰ (Bierlein and Crowe, 2000; McCuaig and Kerrich, 1998). The δD for the Zhemulang ore fluids ranges widely from −107.5 to −36.7‰, with most values between −80 and −20‰ which are typical of orogenic gold deposits (Bierlein and Crowe, 2000; Hagemann and Cassidy, 2000). These H\O values mainly lie in the overlapping magmatic and metamorphic fields, with a shift to lighter δD for some samples (Fig. 14), indicating that the ore-forming fluid is predominantly a metamorphic or magmatic, with involvement of meteoric water in the samples with the light δD. Because there are no igneous rocks in the Zhemulang deposit region (Figs. 1, 2), and regional stream

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Table 4 Summary of the microthermometric data of the three deposits. Sample No.

Zhemulang 2010007 2010014 2010021 2010029 2010033 2010040 2010043 Mazhala 09MZ-1-5 09MZ-1-6 09MZ-1-8a 09MZ-1-13 09MZ-2-1 09MZ-2-2 09MZ-2-3 09MZ-2-4 09MZ-2-5 09MZ-2-7 2010123 2010140 2010121 Shalagang 09SL-1-5 09SL-1-6 09SL-1-7 09SL-1-8 09SL-1-9 09SL-8-2 09SL-8-3 09SL-8-4 09SL-8-5 10SL103 10SL099 10SL098 10SL098

Host Mineral

Quartz

Quartz

Stibnite

Quartz

Quartz Stibnite

Type 1

Type 2

Tmice

Th

TmCO2

Tmclat

−3.5–−0.4(14) −2.9–−0.7(8) −3.1–−2.1(19) −3.9–−3.3(9) −4.9–−2.7(6) −3.9–−2.6(12) −3.5–−2.1(11)

187–286(9) 169–240(11) 184–282(26) 155–211(11) 146–263(7) 193–288(13) 160–292(31)

−56.6–−57.8(3)

9.0–11.1(8) 7.8–10.3(7) 6.2–11(18) 6.3–10.1(8) 7.8–10.8(10) 8.4–11.0(9) 9.8–11(55)

13.8–17.7(4) 18.3–23.4(3) 12.7–17.2(14)

−58.2–−56.9 (40) −57.9–−56.8 (30) −57.3–−56.6(14) −57.4–−56.8(12) −56.6–−56.4(6) −57.4–−56.7(15) −56.9–−56.6(15) −57.6–−57.3(6) −57.4–−56.6(18) −58.2–−57.0(5) −59.9–−57(16) −57.2(1) −58.3–−57.6(16)

9.0–11.2(34) 9.8–10.8(29) 9.8–10.1(4) 9.6–10.8(12) 8.7–10.3(6) 9.6–11.0(10) 9.6–10.2(13)

18.7–29.4(32) 19–29.6(28) 21.7–29.1(12) 19.6–31.0(10) 25.5–29.3(5) 19.6–28.5(11) 20.1–29.7(10)

9.7–10.5(16) 9.9–10.4(5) 9.7–10.7(14)

25.1–28.9(19) 22.5–28.5(5) 25.5–30.9(13)

9.8–10.3(12)

21.9–29.3(11)

−57.4–−57.3(5) −57.9–−57.1(2) −57.9–−57.3 (4) −58.4–−57.6(3) −57.5–−57.2(7) −58.9–−57.8(7)

9.5(2) 10.1(1) 9.7–10.3(4) 9.9–10.1(2)

21.9–23.1(5) 25–25.8(2) 22.5–23.0(3) 20.5(1) 20.2–24.4(11) 22–25.5(6)

−57.9–−57.5(7) −57.8–−57.0(5)

9.7(1) 9.6–10.1(4) 9.9–10.2(10) 10.2–11.6(4)

23.9–25.0(6)

−2.3(1) −2.1–−2.0(2) −2.9–−2.7(8) −2.3–−0.9(4) −1.9(1) −2.4–−2.0(2) −2.4–−2.0(5)

193–216(2) 180–268(10) 152–269(5) 184(1) 209–283(10) 244–273(3) 189–252(8)

−3.1–−2.1(7)

171–265(8)

−2.1–−3.0(9) −2.5–−1.0(129)

166–246(10) 148–303(75)

−3.5–−2.6(23) −3.4–−2.7(15) −3.6–−3.1(19) −3.6–−1.0(29) −5.3–−0.3(11) −3.8–−3.1(10) −3.8–−2.2 (17) −3.3–−0.3(19) −3.0–−0.4(6) −3.2–0.2(9) −3–−2.8(4) −4.8–−0.3(58) −4.5–−1.0(146)

142–250(21) 150–198(17) 142–235(21) 145–216(34) 160–190(15) 178–228(9) 145–220(16) 157–248(19) 160–270(8) 210–290(10) 160–210(7) 140–228(56) 140–223(70)

−56.4(1) −58.0–−58.4(3) −59.7–−57.8(8) −57.0–−58.2(7) −58.2–−56.6(22)

−61.2–−58.3(9)

Th(CO2)L

Th(CO2)V

Th(CO2) T

19.6(1)

217–338(6) 192–241(6) 187–228(16) 184–256(7) 232–325(6) 245–300(5) 216–338(28)

15.0–18.0(6)

29.3–30.0(2) 24.4–36.6(2) 22.0(2) 25.6–30.1(5)

27.1–30.1(2) 25.4(1) 29.4(1)

257–286(26) 255–320(30) 233–286(5) 266–282(6) 280–298 265–314(7) 224–285(10) 250–300(17) 264–298(5) 196–296(14) 171–231(3)

284–290(2) 200(1) 218–223(3)

18.2(1)

−11.8(1)

309–325(2) 171–215(8) 180–204(4)

Abbreviations: Tmice = ice melting temperature, Th = homogenization temperature of two-phase aqueous inclusion, TmCO2 = CO2 melting temperature, Tmclat = clathrate melting temperature, Th (CO2) L = homogenization temperature of CO2 homogenized to liquid phase, Th (CO2) V = homogenization temperature of CO2 homogenized to vapor phase, Th (CO2) T = total homogenization temperature of aqueous-carbonic inclusion. Number in bracket is the number of measurement.

Fig. 12. Histograms of ice melting temperature (Tmice) and homogenization temperature (Th) of aqueous inclusion and total homogenization temperature (Th(CO2)T) of carbonicaqueous fluid inclusion for Zhemulang, Mazhala and Shalagang deposits.

Fig. 13. Histograms of carbonic-phase melting temperature (TmCO2), clathrate melting temperature (TmCla), carbonic-phase homogenization temperature (ThCO2) of aqueous carbonic fluid inclusions for Zhemulang, Mazhala and Shalagang deposits.

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Table 5 CO2, N2, CH4 molar fractions of carbonic phase in fluid inclusion of type 2 at Zhemulang, Mazhala and Shalagang (mol%). Sample No.

2010007-a -b -c 2010029-a -b -c -d 2010043-a -b -c -d 2010033-a -b -c Average

Zhemulang gold deposit

Sample No.

CO2

N2

CH4

CH4 þN 2 CO2

84.2 91.7 91.7 92.7 88.6 93.2 91.0 89.7 90.8 90.7 92.0 94.4 74.7 92.9 89.9

14.1 8.3 8.0 6.8 11.1 6.0 8.8 10.1 8.6 8.7 7.6 5.1 24.5 6.3 9.6

1.7 0.0 0.3 0.5 0.3 0.9 0.28 0.3 0.5 0.6 0.4 0.5 0.8 0.8 0.6

0.19 0.09 0.09 0.08 0.13 0.07 0.10 0.12 0.10 0.10 0.09 0.06 0.34 0.08 0.11

09mz-2-5a -5b -5c -5d -5e 09mz-1-13a -13b -13c 09mz-1-8a -8b -8c -8d 09mz-1-7a -7b Average

Mazhala gold–antimony deposit CO2

N2

CH4

CH4 þN2 CO2

97.9 97.6 99.1 98.7 97.8 97.3 97.3 99.6 98.4 97.3 98.3 98.4 98.1 98.5 98.2

1.8 2.3 0.7 1.2 2.1 2.4 2.7 0.4 1.4 2.7 1.6 1.4 1.7 1.3 1.7

0.3 0.2 0.2 0.2 0.1 0.3 0.0 0.0 0.2 0.0 0.1 0.2 0.2 0.2 0.2

0.02 0.02 0.01 0.01 0.02 0.03 0.03 0.00 0.02 0.03 0.02 0.02 0.02 0.02 0.02

Sample No.

Shalagang antimony deposit CO2

N2

CH4

CH4 þN2 CO2

09sl-8-3a -3b -3c 09sl-1-6a -6b -6c -6d -6e 09sl-8-5a -5b

96.4 98.1 94.8 99.4 94.4 99.4 92.4 91.9 92.2 97.1

0.00 1.0 2.9 0.0 4.0 0.0 5.1 5.7 6.0 0.0

3.6 1.0 2.3 0.6 1.5 0.6 2.6 2.4 1.8 2.9

0.04 0.02 0.05 0.01 0.06 0.01 0.08 0.09 0.08 0.03

Average

95.6

2.46

1.94

0.05

A The mole fraction was calculated using the equation (Frezzotti et al., 2012): X α ¼ =∑ i , where Xα, Aα, σα and ξα, are the molar fraction, the band area, the Raman cross-section and σ i ξi the instrumental efficiency for gas CO2, N2, CH4 respectively, while ∑Ai,σi, and ξi represents the sum of values for CO2, N2, CH4 in the fluid inclusion. The values of σα and ξα for CO2, N2, CH4 are from Burke (2001) and Frezzotti et al. (2012). Aα σ α ξα

sediment surveys of southern Tibet (Du and Huang, 1992; Yu and Cao, 1991) clearly show no spatial association between Au and/or Sb and leucogranites, a magmatic fluid source can be ruled out for the Zhemulang Au deposit. At the Shalagang Sb deposit, the ore-forming fluids have heavier δ18O and the lightest δD compositions among the three deposits studied, with δDwater values of −173.4 to −139.2‰ and δ18Owater values from 7.5 to 12.3‰. The hydrogen isotope values are consistent with southern Tibet meteoric and/or hot-spring water, but the oxygen isotope values are far heavier than such surface water (Yu et al., 1984; Zheng et al., 1982). This indicates that the ore-forming fluid is predominantly meteoric water with a marked oxygen isotopic shift, resulting from water–rock reaction between meteoric water and metasedimentary wallrocks (Fig. 14). At the Mazhala Au\Sb deposit, isotope compositions have large variations, with δDwater values varying from −119 to −72.7‰ and δ18Owater values from 7.5 to 16.2‰. These values fall between those of the Zhemulang gold deposit, which is predominantly formed from metamorphic water, and the Shalagang Sb deposit, which is predominantly of meteoric water with a marked oxygen isotopic shift (Fig. 14), indicating that the Mazhala ore-forming fluid is a mixture of metamorphic and meteoric water. As the heaviest δDwater values at Zhemulang are similar to those of fluid inclusions water of metamorphic quartz segregations in Songre Formation (Fig. 14)(Sun et al., 2010), given that the maximum and minimum δDwater values of the Zhemulang Au deposit and Shalagang Sb deposit represent the δD of metamorphic water and meteoric water for southern Tibet, respectively, then the calculated metamorphic water proportion for the ore-forming fluid at Zhemulang, Mazhala, and Shalagang is 48 to 100% (average of 78%), 40 to 71% (average of 58%) and 0 to 25% (average of 8%), respectively. Meteoric water involvement in the ore-forming process in the Tibet Au\Sb metallogenic belt is inconsistent with the latitudinal control of the δDwater of meteoric water for orogenic Au deposits and related Sb and Hg deposits in the Canadian Cordillera as presented by Nesbitt et al. (1989). The Zhemulang and Shalagang deposits are situated at nearly the same latitude, but with obviously very different hydrogen isotope values. Goldfarb et al. (2005) argue that meteoric water is not a significant component in the formation of orogenic gold deposits, as it was in all the studies in the 1980s of Korean and Cordillera lode gold deposits (Nesbitt et al., 1989; Shelton et al., 1988). In those studies, δDwater was measured by decrepitating fluid inclusions in quartz or calcite, such that bulk extraction of many generations of post-ore secondary inclusions that were trapped in the host mineral grains during long periods of subsequent deformation and regional uplift masked the signature of the primary ore fluid.

The Himalayan is a young orogen, there is no post-mineralization hydrothermal overprinting at Zhemulang, Mazhala, and Shalagang, vuggy and euhedral textures are developed (Figs. 6E, 9B, D), and fluid inclusions are predominantly of primary or pseudosecondary, so that contributions from secondary inclusions to the ore fluid were ignored in our study. We suggest the variable δDwater values are representative of a variety of different ore-forming fluids. As mentioned above, the δ13C values of CO2 in equilibrium with calcite in the ore-forming fluid calculated from calcite gangue, are consistent with values of fluid inclusions in quartz coeval with calcite at Mazhala, and this also demonstrates that secondary fluid inclusions did not affect the isotopic composition of the ore fluid. An alternative origin for very low δDwater measurements of ore fluids are contributions of organic compounds in sedimentary rocks. Such water formed from the breakdown of organic matter and has δDwater values as low as −200‰ (Sheppard, 1986). The country rocks in southern Tibet are predominantly Mesozoic carbon-bearing black turbidites, and hydrocarbon fluid inclusions were discovered in the vein deposits, therefore an organic water source for the very low δDwater values is possible and has been suggested in other orogenic gold deposits (Craw, 2002; Goldfarb et al., 2004). If this is the case for the Au and Sb mineralization in southern Tibet, then the ore fluids for our studied deposits will have the similar very light δDwater values, and the Zhemulang Au deposit that formed at deeper levels, as supported by higher mineralization temperatures than Mazhala and Shalagang, should have more contribution from devolatilized organic matter that resulted in lighter δDwater. As demonstrated by Peters et al. (1991) at the Hodgkinson gold deposit, Queensland, significantly more δDwater-depleted ore fluids characterize it than other gold deposits in the province because δD-depleted organic matter contributing to ore fluid with very low δ13Cfluid values, but are inconsistent with a latitudinal control for the δD of meteoric water (Nesbitt et al., 1989). But this is not the case for Zhemulang, where ore fluids have heaviest δDwater measurements and lightest δ13Cfluid (Table 6), leading us to favor an ore fluid with very low δDwater values in southern Tibet as reflecting meteoric water involvement. The southern Tibet Au, Au\Sb, and Sb deposits are all hosted in the brittle epizonal environment. The Au deposits are hosted in the deeper level Triassic or older metamorphic rocks, Au\Sb deposits are hosted in shallower metamorphic or slightly metamorphosed Jurassic strata, and the Sb deposits are hosted in shallowest slightly metamorphosed or sedimentary strata. From Zhemulang, to Mazhala and Shalagang deposit, the δ18Oquartz values increase and the δDH2O values decrease, reflecting a change in the source of ore-forming fluids from predominantly metamorphic to predominantly meteoric water.

W. Zhai et al. / Ore Geology Reviews 58 (2014) 68–90

83

Table 6 Hydrogen, oxygen and carbon isotope data of Shalagang, Mazhala and Zhemulang deposits. Sample no.

δDwater

δ18Oquartz(SMOW)‰

δ18OCalcite(SMOW) ‰

δ18Ofluidr(SMOW)‰

δ13CFluid(PDB) ‰

Sample level(m)

268 230a 212 230a 211 228 228 200 231 231a 228 237 238a 238a 239

−11.2 −11.6 −11.3 −10.4 −11.5 – −10.0 −9.8 −10.5 −11.0 −10.1 −10.0 −10.2 −11.7 −9.6

3960 3960 3960 3960 3960 3986 3986 3986 4010 4010 4030 4030 4030 4030 4030

11.5 12.6 12.3 13.6 10.1 12.3 7.5

234 241 232 234 245 236 209

−3.2 −3.1 −2.8 −3.5 −2.8 −3.5 −3.5

16.2 13.7 11.8

239 234 234

5050 5050 5050 5050 5050 5030 5030 5030 5030 5030 5030 5030

19.3 19.5 17.9 21.0 21.3 20.6

7.7 10.7 9.1 11.9 12.2 9.0

200 253 253 247 247 200

20.4 20.8 22.3 22.4 20.9 23.2 23.4 21.8 22.4 21.3 21.7 21.3 21.6 21.5

9.2 7.5 10.7 8.1 9.9 11.1 11.6 10.7 10.5 9.4 9.5 9.4 12.3 11.2

208 176 200 163 211 193 197 209 196 196 203 208 257 236

Analyzed mineral

(SMOW)‰

Zhemulang 2010007 2010008 2010014 2010017 2010021 2010025 2010027 2010029 2010033 2010034 2010036 2010040 2010041 2010042 2010043

Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz

−76.1 −55.3 −36.7 −69.0 −57.9 −98.6 −107.5 −51.3 −56.3 −87.3 −73.7 −48.3 −82.1 −79.5 −54.7

14.5 16.4 14.7 12.7 15.2 15.9 15.9 15.3 15.6 18.1 15.5 15.0 17.2 16.5 14.5

6.3 6.5 3.8 2.8 4.2 5.9 5.9 3.7 5.7 8.2 5.5 5.4 7.7 7.0 5.0

Mazhala 09MZ-1-5 09MZ-1-6 09MZ-1-7 09MZ-1-9 09MZ-1-13 09MZ-2-1 09MZ-2-4 09MZ-2-4C 09MZ-2-7 10MZ-127 10MZ-129 10MZ-129C MZL-1b MZL-2b MZL-2-1b MZL-4b MZL-4-1b MZL-8b

Quartz Quartz Quartz Quartz Quartz Quartz Quartz Calcite Quartz Quartz Quartz Calcite Quartz Quartz Quartz Quartz Quartz Quartz

−86.4 −76.6 −84.6 −92.7 −78.4 −90.1 −85.8

21.2 22 22.1 23.3 19.3 21.9 18.6

−104.0 −72.7 −89.5

25.7 23.4 21.5

−106 −108 −104 -98 −115 −119

Shalagang 09SL-1-5 09SL-1-6 09SL-1-7 09SL-I-8 09SL-8-2 09SL-8-3 09SL-8-4 09SL-8-5 09SL-8-6 09SL-8-7 SLGE-4b SLG-6b SLG-17b SLG-19b

Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz Quartz

−169.6 −171.3 −173.4 −162.9 −173.3 −149.3 −172.8 −162.1 −139.2 −172.5 −150.0 −158.0 −166.0 −160.0

a b

Temperature (°C)

δ13CCalcite(PDB) ‰

−3.4

14.9

−2.5 −2.5 −3.0 −4.5

19.3

−5.1 −5.2 – – −5.8 −5.7 −6.5 −6.2 −5.6 −5.8

4590 4590 4590 4590 4430 4430 4430 4430 4430 4430

The averages of homogenization temperatures of the level sample situated.–= not detected. The data from Yang et al., 2009.

5.2.2. Constraints from carbon and sulfur isotopes For most orogenic gold deposit, δ13C values typically range from near 0 to about −10‰ (Goldfarb et al., 2005), but some deposits are as depleted as −25 to −32‰ (Bierlein and Crowe, 2000; Partington and Williams, 2000). The more negative δ13C data are most readily interpreted as products of devolatilization in a sequence where there is a large component of biogenic carbon (Kontak and Kerrich, 1997). For Zhemulang, Mazhala, and Shalagang, the δ13Cfluid values for fluid inclusions are −11.7 to −9.6‰, −3.5 to −2.5‰, and −6.5 to −5.1‰, respectively, and these values are consistent with most orogenic gold deposits. Carbon in hydrothermal fluids originate mainly from the mantle with δ13C values of −5.0‰ ± 2.0‰, sedimentary carbonate with an average δ13C value of 0‰, and organic carbon with a δ13C value of −25.0‰ (Zheng and Chen, 2000).

In the southern Tibet Au\Sb metallogenic belt, the 3He/4He values for inclusion fluids in sulfides from the Mayoumu and Bangbu Au deposits are 0.0755 to 1.69Ra (Ra is the atmospheric 3He/4He, 1.399 × 10− 6) and 0.01137 to 1.01Ra, respectively (Duo et al., 2009; Wei et al., 2010). Using the crust and mantle two endmember formula (Ballentine et al., 2002), the proportion of mantle He in the ore-forming fluid can be calculated from 0.8 to 23.9% (average of 6.1%) and 0 to 14.8% (average of 7.2%), respectively. At the Mazhala Au\Sb and Shalagang Sb deposits, the 3He/4He values for fluid inclusions in stibnite and quartz are 0.01382 to 0.05642Ra and 0.02385 to 0.11488Ra, with the proportion of mantle He in oreforming fluid are 0 to 0.52% and 0.06 to 1.36% (Zhai et al., unpublished data) respectively. These indicate that the ore-forming fluids for Mazhala and Shalagang were predominantly derived from the crust

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Table 7 Sulfur isotope analyses of sulfides from Zhemulang, Mazhala and Shalagang deposits. Sample No.

Mineral analyzed

δ34Sv-CDT (‰)

Comments on host rocks

Sample level

Data origin

Zhemulang 2010-017 2011-018 2011-027 2012-029 2013-030 Average

Pyrite Pyrite Pyrite Galena Galena

−2.0 −1.1 −2.5 −4.0 −3.7 −2.7

Gold-bearing quartz-vein Gold-bearing quartz-vein Gold-bearing quartz-vein Gold-bearing quartz-vein Gold-bearing quartz-vein

3960 3960 3986 3986 3986

This study This study This study This study This study

Pyrite Pyrite

2.7 2.6 2.65 2.2 2.3 0.4 0.1 −0.2 0.5 −0.1 0.1 −0.3 0.1 −0.2 −0.8 0.34

Sedex-like sulfide layer Sedex-like sulfide layer

5050 5050

This study This study

Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein Gold stibnite quartz-vein

5050 5050 5050 5050 5050 5030 5030

This study This study This study This study This study This study This study Yang et al., 2009 Yang et al., 2009 Yang et al., 2009 Yang et al., 2009 Yang et al., 2009

Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein Stibnite quartz-vein

4590 4450 4430

This study This study This study Yang et al., 2009 Yang et al., 2009 Yang et al., 2009 Yang et al., 2009 Qu et al., 2003 Qu et al., 2003 Qu et al., 2003 Qu et al., 2003 Qu et al., 2003 Qu et al., 2003 Qu et al., 2003 Qu et al., 2003 Li, 2000 Li, 2000 Li, 2000 Li, 2000 Li, 2000 Li, 2000

Mazhala 09MZL-1-9 09MZL-1-14 Average 09MZL-1-5 09MZL-1-6 09MZL-1-7 09MZL-1-8a 09MZL-1-15 09MZL-2-2 09MZL-2-12 MZL-1 MZL-2 MZL-4 MZL-6 MZL-8 Average Shalagang 09SL-I-8 09SL-VII-2 09SL-VII-5 SLG-3 SLG-6 SLG-7 SLG-17 S-1 S-2 S-9 S-11 S-12 S-13 S-14 S-16 I-3a G9-3 G9-3a VIII-9 I-1b G9-3b Average

stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite

Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite Stibnite

1.9 1.2 1.3 −3.0 −3.9 −3.6 −3.6 2.1 0.3 −0.1 −1.6 −1.4 −1.7 −1.4 −0.9 −0.3 −1.6 −0.9 −1.8 −0.5 −1.5 −1.0

while mantle carbon in the ores is unlikely. Because the Zhemulang, Mayoumu, and Bangbu deposits are in the same metallogenic belt, and given that the proportion of mantle He and carbon in the ore fluid for Zhemulang is the same as the mantle He and carbon in the Mayoumu and Bangbu gold deposits (average 6.6%), and that the carbon δ13C values of mantle, sedimentary carbonate, and organic carbon are −5.0‰, 0‰, and −25‰, respectively, the calculated organic carbon proportion of the ore-forming fluids for Zhemulang, Mazhala, and Shalagang are 37 to 45%, 10 to 14%, and 20 to 26%. Coincidently, among the three deposits, the δ13Cfluid values for inclusion fluids and the sample altitude have the positive correlations (Fig. 15A), and this may reflect the different organic carbon content in ore fluid. The Zhemulang Au deposit has the highest organic carbon content and the lowest δ13Cfluid value, whereas the Mazhala Au\Sb deposit has the lowest organic carbon content and the highest δ13Cfluid value. Among different samples from Zhemulang and Shalagang, the same positive relationship also exists (Fig. 15A), thus indicating that in the lower part of deposit, the organic carbon content in the ore fluid is increasing. Different deposits in the metallogenic belt, including Zhemulang, Mazhala, and Shalagang, have different δ13Cfluid and, in an individual deposit, the δ13Cfluid values range narrowly. This features show that the

local country rocks and the corresponding degree of metamorphism of the country rocks to individual deposits control the origin of the carbon in the ore-forming fluids. This is consistent with the ore-forming fluids originating predominantly from metamorphic and meteoric water. Sulfur isotope values are extremely variable for orogenic gold deposits, with most δ34S values for sulfide minerals ranging from 0 to 10‰ (Goldfarb et al., 2005), although there are some examples where the δ34S values are as light as −27.2 (Goldfarb et al., 2004) or as heavy as 25‰ (Kontak and Smith, 1989). Anomalous positive δ34S values, such as the range of 9 to 25‰ in Beaver Dam and other Meguma Group-hosted gold deposits, Nova Scotia, are interpreted as the products of heavy sulfur in source reservoirs such as the Meguma Group metasedimentary rocks. Fluids responsible for negative δ34S values of sulfides, such as in the Golden Mile, Western Australia, are interpreted as having undergone some degree of oxidation during fluid-wall rock interaction (Hagemann and Cassidy, 2000; Phillips et al., 1986). Extremely negative δ34S values, such as the range of −27.2 to −7.3‰ in the Donlin Creek Au deposit, USA, are interpreted as ore-related sulfur derivation from synsedimentary sulfide that formed from reduction of seawater sulfate during deposition of the sedimentary country rocks

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(Goldfarb et al., 2004). The δ34S of sulfides in orogenic Au deposits in Alaska has a temporal variation pattern similar to that of seawater sulfate, and the sulfur source reservoir of the Alaska deposits was the ore-hosting sediments (Goldfarb et al., 1997). The sulfur in similar sedimentary rock-hosted orogenic gold deposits worldwide is ultimately from seawater (Chang et al., 2012). The δ34S values for sedimentary pyrite correlate with the content of oxygen in the atmosphere, sulfate in seawater, and bioturabtion in Earth history, and thus all these factors also affect formation of sulfide-bearing ore deposits (Farquhar et al., 2010). The δ34S for pyrite and galena from gold-bearing quartz veins at the Zhemulang deposit range from −4.0 to −1.1‰. These values are lighter than the δ34S value of 6.5‰ for pyrite from pyrite-bearing phyllite of the Songre Formation, the country rock of Bangbu deposit (Wei et al., 2010) and also at Zhemulang. At the Mazhala Au–Sb deposit, the δ34S values for stibnite ranges from−0.8 to 2.3‰, whereas the δ34S values for synsedimentary pyrites from Sedex-like sulfide layers are 2.6 to 2.7‰. At the Shalagang Sb deposit, the δ34S values for stibnite are−3.9 to 2.1‰, also lighter than the δ34S value of 9.9‰ for pyrite from country rocks of Early Cretaceous Duojiu Formation (Li, 2000). Because ore fluids forming the Zhemulang, Mazhala, and Shalagang deposits are predominantly of metamorphic and/or meteoric water, the sulfur likely originated from the country rocks. This is supported by similar δ34S values for the synsedimentary pyrites and epigenetic stibnite at the Mazhala deposit, indicating that some of the observed early sulfides may have contributed sulfur to the ore-forming fluids. The δ34S values are slightly variable among the three deposits, and the intra-deposit ranges are relatively narrow, which may reflect some degree of provinciality in the source region. Consistent with δ13Cfluid values for ore fluid, the δ34S values and the sample formation depth also have the positive correlations (Fig. 15B). In chemical equilibrium condition, isotopic fractionation of different sulfides follows δ34S values in sequence of stibnite b galena b pyrite (Zheng and Chen, 2000). The δ34S values of galena are lower than the values of pyrite at the Zhemulang deposit. However in the deposit scale, the δ34S values of stibnite from the Mazhala deposit are higher than the values of pyrite and galena from the Zhemulang deposit, and the range of the δ34S values of stibnite from the Shalagang deposit covered the sulfur isotope range of the Mazhala and Zhemulang deposits (Fig. 7). Hence, the isotopic fractionation as a cause for the variation of δ34S values among different deposits could be ruled out. The country rocks of the three deposits have similar redox parameters and these rocks or their protolith were formed in a passive continental margin environment. Thus, more than likely, the low pyrite and galena δ34S values−4.0 to−1.1‰ at Zhemulang could be result of sulfide formation in a relatively low ƒO2 environment with a high content of reduced organic carbon at the site of ore formation. For the Mazhala deposit, the stibnites with high δ34S values of−0.8 to 2.3‰, formed in a relatively high ƒO2 fluid environment with a low content of organic carbon. Sulfides in the Shalagang deposit, with the δ34S values of−3.9 to 2.1‰, covering the entire range of the other two deposits, formed under intermediate ƒO2 conditions. This ƒO2 variation is consistent with the non-aqueous volatile content in fluid inclusions, which for Zhemulang, Shalagang, and Mazhala have contents of CO2 and ratios of (CH4 + N2)/CO2 in the carbonic phase of 74.7–94.4 mol% and 0.06–0.34, 91.9–99.4 mol% and 0.01–0.09, and 97.3–99.6 mol% and 0–0.03, respectively (Table 5). 5.2.3. Constraints from fluid inclusion chemistry The low salinities for the ore-forming fluids at Zhemulang, Mazhala, and Shalagang differ from magmatic fluids that form intrusion-related and porphyry gold deposits at high temperature and from moderate to high salinity (N 10.0 wt.% NaCl equiv.) fluids (Baker, 2002; Duuring et al., 2007). In our present study, trace amount of hydrocarbon fluid inclusions were discovered in all three deposits, and this type of fluid would not likely exsolve from magma during crystallization. There also exist some type 1 inclusions with low salinities, even less than

85

Fig. 14. Plot of δD vs. δ18O for ore fluid from the Zhemulang, Mazhala and Shalagang deposits, Southern Tibet Au, Sb metallogenic belt. Magmatic, metamorphic and organic (e.g., devolatilization of organic matter in sediments) water fields after Sheppard (1986), field for most orogenic gold deposits after Goldfarb et al. (2004), fields for South Tibet meteoric water and hot-spring water after Zheng et al. (1982) and Yu et al. (1984). The details for ore-forming fluid origin see text.

1.0 wt.% NaCl equiv. in three deposits (Fig. 16), this may be the result of local meteoric water mixing with metamorphic fluid heterogeneously. At the Shalagang deposit, the predominant fluid inclusions are type 1b, only small quantity of type 2 inclusions occurred and measured by microthermometry (Table 4, Figs. 12, 13), this is also consistent with the ore fluid being predominantly meteoric. The ore fluid features are obviously different from typical low sulfidation epithermal gold deposits, where the ore-forming fluid is low salinity, mostly b 3.0 wt.% NaCl equiv., and CO2-poor meteoric water (Sillitoe and Hedenquist, 2003), with no gas hydrate and solid CO2 crystallization observed during fluid inclusion freezing experiment (e. g., Axi in Xinjiang, China: Zhai et al., 2009; Hishikari, Japan: Izawa et al., 1990). The three deposits have different fluid sources, but have the same ore-forming fluid compositions, which consist of H2O\CO2\CH4\N2 with a low-salinity ranging from 2.5 to 6.5 wt.% NaCl equiv. (Fig. 16). This suggests that the meteoric water had undergone deep circulation, obtaining the same salinity and reaching a similar temperature to the metamorphic fluid.

5.2.4. Source of ore metals Zhemulang, Mazhala, and Shalagang have the same element associations of Au, Ag, As, Sb, Bi, Te, W, Pb, and Hg, and this indicates the same ore metal source. The metamorphic and deeply circulating meteoric water ore fluid likely obtained their ore metals from country rocks, which may have included the synsedimentary Sedex-like sulfide layers observed locally in Jurassic strata at the Mazhala Au\Sb deposit. The thick Mesozoic flysch sediments deposited along the northern passive continental margin of India plate contain considerable organic matter and synsedimentary to diagenetic sulfides. In redeced continental margin basin settings, the regionally extensive carbonaceous black shale and turbidite sequences are enriched in Au, As Sb, and a range of trace element (Large et al., 2011). Sedimentary successions, deposited on the slope and rise at passive continental margins, are the main site for syngenetic or diagenetic Sedex Pb\Zn deposits (Leach et al., 2010). These types of sulfide-rich metal accumulations in sedimentary rocks are important sources of ore metals in epigenetic deposits.

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5.3. Mineralization mechanism, temperature, pressure, and vertical zonation Depositional mechanisms for gold in orogenic deposits have been summarized by Mikucki (1998). Fluid–wallrock reaction is commonly accepted as the most widespread precipitation mechanism for replacement deposits or deposits dominated by gold disseminated in altered wallrock (Goldfarb et al., 2005; Groves et al., 2003). Even when the gold is primarily in veins, Phillips and Powell (2010) argue for rock dominated gold depositional systems. The sulfidation of wallrocks with high Fe/Fe + Mg ratios, fluid reduction and destabilization of gold complexes by exchange with carbonaceous metasedimentary rock sequences, and resultant changes in ƒO2 and/or pH can facilitate break down of goldtransporting complexes (Böhlke, 1988; Goldfarb et al., 2005; Phillips and Groves, 1983). For quartz-carbonate vein-hosted orebodies, large pressure fluctuations during hydraulic fracturing, to a large degree explained by the fault-valve model of Sibson et al. (1988), leading to phase separation, are the most likely mechanisms for destabilizing sulfur complexes of gold in hydrothermal fluid (Goldfarb et al., 2005; Groves et al., 2003). Most orebodies in our studied deposits are gold- and/or stibnite-bearing quartz veins, but at the Mazhala and Shalagang deposits, a few orebodies exist as hydrothermally altered fault breccias. Cogenetic primary aqueous inclusions (type 1), aqueous-carbonic inclusions (type 2) and hydrocarbon inclusions (type 3) have been identified in quartz and/or stibnite from veins at the Zhemulang, Mazhala, and Shalagang deposits. The two-phase aqueous and aqueous-carbonic inclusions homogenized over a similar range of temperature at each deposit (Figs. 12, 13). These features indicate the trapping of immiscible fluids as summarized by Ramboz et al. (1982). The unmixed fluid inclusions in quartz vein orebodies indicate that phase separation in response to pressure decreases during mineralization were the main mechanism for Au and Sb precipitation. However, for the hydrothermally altered fault breccia ore at Mazhala and Shalagang, which is characterized by sillicification and carbonization, sulfidation of wallrocks and ore fluid reduction may have destabilized gold and antimony as the sulfur-bearing ligands were broken down. The wallrocks at Mazhala and Shalagang are carbonaceous flysch, diabase, or diorite with high Fe/Fe + Mg ratios, and thus they are relatively reactive. Although meteoric and metamorphic fluids mixing occurred at Zhemulang, Mazhala, and Shalagang during mineralization, as suggested by some workers for Archean (e. g., Hagemann et al., 1994) and Phanerozoic (Craw, 2002; Graupner et al., 2001) orogenic gold deposits, this is not the main depositional mechanism for gold and antimony. As mentioned above, because the meteoric water had undergone deep circulation and attained a similar chemical composition and temperature to the metamorphic fluid, the mixing of the two fluids with similar physico-chemical characteristics cannot facilitate ore metal deposition. Fluid inclusion features indicate that the immiscibility occurred during mineralization, and type 1b two-phase aqueous inclusions and type 2b aqueous-carbonic inclusions have the similar total homogenization temperature ranges for three deposits. Therefore, the measured homogenization temperature data can be considered to approximate the temperature of trapping without pressure correction. For the Zhemulang Au deposit, Mazhala Au\Sb deposit, and Shalagang Sb deposit, the majority of the type 1b and type 2b inclusions had total homogenization temperatures between 180 and 320 °C, 160 and 300 °C, and 140 and 240 °C, respectively (Fig. 12), representing the mineralization temperatures of the three deposits. The mineralization pressures can, in principle, be defined by the widespread phase separation, but an actual estimate of pressure is difficult owing to the compositionally complex H2O\CO2\CH4\N2\NaCl ± hydrocarbon fluid at the three deposits. The fluid inclusion assemblage indicates clear evidence of trapping in a two-phase field. Most isochores for type 1 aqueous inclusions at Zhemulang range from 0.89 to 0.93, and using the upper value of 320 °C for the majority of type1 inclusions, the estimated trapping pressure is about 1380 to 1960 bars. Assuming lithostatic conditions, the Zhemulang deposit formed at paleodepths

of approximately 4 to 6 km. In the same way, the estimated paleodepths for formation of the Mazhala and Shalagang deposits are 3 to 5 km and 1 to 4 km, respectively. The estimated mineralizing depths are consistent with characteristics that indicate these deposits occur in extensional faults or splays in the brittle environment. The Zhemulang Au deposit, Mazhala Au\Sb deposit, and Shalagang Sb deposit are hosted in rocks of the Late Triassic Songre Formation, Early to Middle Jurassic Lure Formation, and Early Cretaceous Doujiu Formation, respectively. Mineralization temperatures and pressures decrease vertically, with element associations correspondingly varying from Au, Sb, Te, Bi, As, Pb, Ag, and W at lowest levels 4 to 6 km, to Sb, Au, Te, As, Pb, Bi, Ag and W at intermediate levels 3 to 5 km, and to Sb, Te, As, Au, Hg, W, Pb, and Ag at upper levels 1 to 4 km. As proposed by Groves et al. (Fig. 2, 1998) for orogenic gold deposits that are in hypozonal and mesozonal environments the mineralization is Au\As\Te; in epizonal levels the mineralization is zoned upward from Au\Sb to Hg\Sb to Hg. The southern Tibet Au\Sb metallogenic belt provides an ideal example of such vertical zonation. There is also a mercury occurrence (Gao et al., 1994) in the region. At the Mayoumu deposit, Quaternary palaeo-hot spring sinter or calcareous sinter mounds and valentinite developed at the surface (Duo et al., 2009; Nie et al., 2005); and at the Gudui Au-Sb deposit, boiling hot-springs and sinter accumulations that are almost 100 m high occur in the mine region (Qinghai–Tibet Plateau Comprehensive Scientific Investigation Team of CAS, 1973). These hot springs may represent venting of the orogenic ore-forming fluids after have deposition of Au and Sb at deep levels in the southern Tibet Au\Sb metallogenic belt. 5.4. Ages of the Au\Sb mineralization There are no robust absolute ages for the three deposits because they lack suitable dating minerals. The relative timing of orogenic gold deposit formation worldwide has been summarized by Groves et al. (2000), with emphasis on the Yilgarn craton of Western Australia. They concluded that in evolving metamorphic belts, orogenic gold deposits most commonly form perhaps 20 to 100 Ma after regional metamorphism, plutonism, and early phases of orogenic deformation in the immediate host rocks. Many of the world's orogenic belts evolved over periods of 100 to 200 Ma, with orogenic gold lodes having been emplaced throughout diachronous post-collisional deformation and uplift episodes in the orogen. For the Zhemulang deposit, the gold-bearing veins occur in extensional brittle faults and cut the Juqu–Zhemulang brittle–ductile shear zone (Figs. 2, and 3C, D). This implies that the ore-bearing veins were emplaced during or after uplift of the Himalayan orogen, when the Juqu–Zhemulang brittle–ductile zone was uplifted into the brittle environment. As Klootwijk et al. (1991, 1992) proposed for the western Pakistan Himalaya, the initial contact between the Indian and Asian continents occurred at ~65 Ma, the peak ultra-high pressure metamorphism of coesite-bearing eclogite occurred at 47–46 Ma (Foster et al., 2002; Smith et al., 1994), and exhumation of ultra-high pressure rocks to greenschist-facies conditions was accomplished between 46 and 40 Ma (Tonarini et al., 1993; Treloar et al., 2003). The diachronous collision of India and Asia included tectonic events in the central and eastern Himalaya postdating those in the western Himalaya (Foster et al., 2002). In southern Tibet, there are Himalayan mafic rocks, diorite or quartz diorite dikes, and sills with ages of 34–20 Ma (Liu et al., 2005; Su et al., 2004) trending nearly east–west and intruding the Mesozoic strata. The Himalayan leucogranites have ages of about 35 to 12 Ma and outcrop in metamorphic core complexes in the Tethyan Himalayan and High Himalayan (Aoya et al., 2005; Harrison et al., 1997; Searle and Godin, 2003; Searle et al., 1997; Zeng et al., 2009). The main uplift was initiated at about 35 Ma. Therefore, the Zhemulang gold deposit may form after 35 Ma, during or after uplift of the orogen. At Mazhala, the gold–antimony mineralization overprinted a diorite dike, and at Shalagang the stibnite-bearing quartz veins cut the diabase and diorite.

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Fig. 15. Altitude of sample vs. δ13C (A) and δ34S (B) for Zhemulang, Mazhala and Shalagang deposits. Filled diamond represents the average of δ34S values.

Zhang et al. (2011) dated the diorite using the SHRIMP method on zircons and determined an age of 23.6 Ma at Shalagang, thus constraining mineralization ages of Mazhala and Shalagang to post-23.6 Ma.

may have been emplaced throughout the post-collisional deformation, magmatic, and uplift episodes.

5.5. Tectonic process and formation of gold-antimony deposits

6. Exploration implications for orogenic gold and antimony deposits in the region

For the tectonic evolution process of Himalayan orogen, comprehensive research has been carried out. Chung et al. (2005) and Yin (2006) indicated that during the Mesozoic, southern Tibet was a passive continental margin of India, with deposition of a thick turbidite sequence containing syngenetic sulfides. During the ca. 70 to 60 Ma, rollback of the subducting Neo-Tethyan slab (Fig. 17A), the enhanced gravitational pull of the slab increased the convergence rate, and eventually the subducted oceanic slab broke off owing to gravitational settling. The active Gangdese arc was formed on the northern side of the IndusTsangpo suture. At about 45 Ma, the Neo-Tethyan oceanic slab detached from the Indian continental lithosphere and led to the collision of the lower part of the India and Asia lithosphere, with the continuous northward impingement of Indian mantle lithosphere giving rise to significant contraction and thickening in the crust and lithospheric mantle, and metamorphism in the Himalayan orogen. The north-dipping South Tibet detachment fault, originally a thrust fault, and the Main Central thrust fault initiated the initial Himalayan and Tibet plateau uplift. During this process, initial gold mineralization may have occurred in extensional faults or splays derived from thrust faulting (Fig. 17B). This is consistent with the 40Ar/49Ar age for muscovite formed at the Mayoumu deposit of 49 to 44 Ma (Duo et al., 2009). At about 26 Ma, the thickened lithospheric root was removed followed by asthenospheric mantle upwelling and caused the major uplift of the Himalayan orogen. Basaltic and ultrapotassic magmatism occurred owing to a significant thermal perturbation caused by removal of the lithospheric mantle, and formed the diorite and basaltic dike and sills in southern Tibet, including the diorite and diabase at the Mazhala and Shalagang deposits. The thermal anomaly and the uplift led to the emplacement of the leucogranites in the region. The thermal anomaly also led to deep convection of meteoric water and created a low salinity H2O\CO2\CH4\N2 ± hydrocarbon ore fluid, whereas the coeval metamorphic devolatilization of thickened crustal rocks at deeper levels formed additional ore fluids. Local east–west extension within orogen caused the reactivation of South Tibet detachment fault and formation of E–W extensional faults systems elsewhere in the brittle environment during or after the uplift of the orogen. In deeper level Triassic and older rocks, the metamorphic fluids migrated along extensional faults or splays to form auriferous quartz-veins, such as at the Zhemulang and Bangbu gold deposits. In middle level Jurassic strata, a mixed metamorphic-meteoric fluid formed the gold- and stibnite-bearing quartz-veins, such as at Mazhala and Gudui. In the shallower Jurassic to Cretaceous strata, meteoric water-dominated fluid formed the stibnite-bearing quartz veins, including Shalagang, Wuladui, and Chequnzhuobu (Fig. 17C). In the southern Tibet Au\Sb metallogenic belt, the majority of the gold, gold–antimony and antimony deposits

Because the Cenozoic Himalayan orogen is a relatively young event, erosion is limited and mineral deposits that formed during orogenesis have been well preserved. More than 50 orogenic gold, gold–antimony, and antimony deposits or occurrences have been discovered within the South Tibet Au\Sb metallogenic belt in the Himalayan orogen (Nie et al., 2005; Yang et al., 2009). These allow us to better understand the genesis of orogenic gold–antimony deposits and their distribution patterns, which is also critical for developing successful future exploration strategies to identify other orogenic deposits in the Himalayan and other orogenic belts worldwide. The distribution of the lode deposits and occurrences in the South Tibet Au\Sb metallogenic belt indicates that the Triassic and older strata, which have undergone the most unroofing and erosion, are the most favorable host rocks for gold deposits. Jurassic and Jurassic–Cretaceous strata, which have undergone less erosion, are the best hosts for gold–antimony and antimony deposits in the region, respectively. Stream sediment or soil anomalies showing the element association Au\Ag\As\Sb\Bi\Te\W\Pb\Hg, which represents the geochemical signature of the gold and antimony mineral systems, should assist in the identification of the lode targets. At the Zhemulang gold deposit, a 1:200000-scale stream sediment survey outlined an anomalous target of 44 km2 with N4 ppb Au and N50 ppm As (Du and Huang, 1992). A follow-up 1:50,000-scale stream sediment and mapping survey found the deposit (Ai, 2007). At Shalagang, 1:500,000-scale stream sediment surveys identified widespread anomalous Sb. Subsequent follow-up work discovered an Sb\Hg\As\Au anomaly of about 10 km2 that averaged 3.6 ppm Sb, with some samples containing as much as 226 ppm Sb, 1145 ppb Hg, 1190 ppm As, and 40.9 ppb Au, which led to the discovery of the deposit (Yang et al., 2000). The distribution of placer gold areas should target upstream gold and gold–antimony occurrences. Exploration should be focused in the areas of second- and thirdorder faults or splays from the deep-crustal regional faults, because they host the known orebodies in the belt. Wallrock alteration, including silicification, sulfidation, and carbonization, are well-known guides to lode gold and antimony deposits in orogenic belts (e.g., Eilu and Mikucki, 1998). Zones of gossan, iron-stained quartz vein, valentinite and/or cervantite[Sb3+Sb5+O4] are prospecting indicators at the surface. For the deposits discovered up to now in southern, only shallow, near-surface exploration has been conducted because of the harsh field conditions. Thus, there may exist significant gold resources below some of the antimony or gold–antimony orebodies in southern Tibet, as expected based upon the established vertical zonation. Our study first identified the syngenetic Sedex-like sulfide layers in the Jurassic strata at Mazhala, which suggest such sulfide accumulations within

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Fig. 16. Salinity vs. homogenization temperature of two-phase aqueous fluid inclusions in Zhemulang, Mazhala and Shalagang deposits.

Mesozoic strata in the Himalayan orogen may define areas of abundant metal available for remobilization and concentration during metamorphism, and thus regions in southern Tibet with high resource favorability.

Acknowledgements The authors thank academician Ji Dou, the geological technical staff at the Bangbu gold mine, the Mazhala gold–antimony mine, and the Shalagang antimony mine, as well as Moxiang Han, Yeheng Liang, and Weijian Pan at Sun Yat-sen University for their help during fieldwork. WZ thanks Xianxuan Liu at the Xinjiang Mineral Experiment Research Institute and Hanbin Liu at the Analytical Laboratory, Beijing Research Institute of Uranium Geology for their help with analyses of Au, Sb, other trace elements, and stable isotopes. The authors are particularly indebted to Richard Goldfarb, USGS, for his detailed revision of earlier drafts and constructive comments that improved the manuscript considerably. We thank two anonymous journal reviewers and F. M. Pirajno, the journal editor, for their comments and encouragement. This study was financially supported by the National Natural Science Foundation of China (Nos. 40873034, 40830425, U13022015, 41072070), the National Key Basic Research Program (No. 2009CB421006) from the Ministry of Science and Technology, China, Higher School Specialized Research Fund for the doctoral program funding issue (No. 200805580031), and the Project Supported by Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2011) and IGCP/SIDA-600.

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