Micro-textural and fluid inclusion data constraints on ...

GEXPLO-05767; No of Pages 11 Journal of Geochemical Exploration xxx (2016) xxx–xxx

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Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China Yi Zheng a,b,⁎, Yuejun Wang a, Huayong Chen c, Zhenwen Lin a,b, Weisheng Hou a, Dengfeng Li c a b c

School of Earth Sciences and Geological Engineering, Sun Yat-sen University, Guangzhou 510472, China Guangdong Provincial Key Lab of Geological Processes and Mineral Resource Survey, Sun Yat-sen University, Guangzhou 510472, China Key Lab of Mineralogy and Metallogeny, Guangzhou Institute of Guangzhou, Chinese Academy of Sciences, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 29 April 2015 Revised 21 June 2016 Accepted 29 June 2016 Available online xxxx Keywords: Ore textures Fluid inclusions Metallic remobilization Ashele Cu-Zn deposit Altay Orogenic Belt

a b s t r a c t The Ashele volcanogenic massive sulfide (VMS) Cu-Zn deposit is placed in the Devonian volcanic-sedimentary basin of the Chinese Altay Orogen, NW China. It occurs in the form of well-developed lensoid orebodies within the Devonian Ashele Formation. The orebodies display analogous deformational characteristics with respect to their host rocks. Two periods of ore formation are distinguishable in the Ashele VMS Cu-Zn deposit: the first one is related to the genesis of banded and massive ores, whereas the second one is characterized by the formation of earlier Cu-dominated- and later Pb-Zn dominated-polymetallic quartz veins. In most cases, the primary banded textures of ores are still preserved. However, some veins cross cut the earlier generated ores with brecciated, durchbewegung, recrystallization and pressure-solution textures produced, indicating that substantial remobilization of metals has occurred due to intense reworking of the VMS. Four types of fluid inclusions were recognized in Cu-dominated quartz veins encompassing CO2-H2O (C-type), aqueous water (W-type), daughter mineral-bearing (S-type) and carbonic (PC-type) fluid inclusions. The homogenization temperatures of fluid inclusions mainly vary between 220 and 280 °C, whereas their salinities focus on 4 to 8 wt.% NaCl equiv. Gases in fluid inclusions identified using Raman spectroscopy consist of CO2, CH4 and N2. Characteristics such as mesothermal temperature, low salinity and elevated CO2 content indicate that the ore-forming fluid might have been of metamorphic origin. Based on the major C-type FIs, the trapping pressures are greater than 8.5 MPa, corresponding to minimum ore-forming depths are deeper than 3.1 km. Taking into account the regional geodynamic evolution, it is concluded that the early formed ore layers were generated in an arc-related geotectonic setting; while deformation and metamorphism that were responsible for metal remobilization in the ore system happened in a post-subduction regime. Based on the micro-textural observation coupled with fluid inclusion data, we conclude that the Ashele Cu-Zn deposit is a typical remobilized VMS deposit intensely overprinted by subsequent deformation and metamorphism. © 2016 Elsevier B.V. All rights reserved.

1. Introduction In the recent decades, the investigation of volcanogenic massive sulfide (VMS) deposits has attracted the attention of many economic geologists as a potential target for various metals of economic importance (Rona, 1984; Ulrich et al., 2002; Hannington et al., 2005; Franklin et al., 2005). Meticulous studies based on various ancient and modern seafloor VMS deposits have revealed some of their most significant textural, mineralogical and genetic features including: (1) their exclusive association with extensional geotectonic environments, e.g., midocean ridges (MOR), back-arc basins (BAB) and continental rifts; (2) Their composite structures are generally composed of an upper unit exhibiting a typical layered morphology and a lower section ⁎ Corresponding author at: Building of Geology and Environment, Sun Yat-sen University, Guangzhou 510275, China. E-mail address: [email protected] (Y. Zheng).

characterized by the predominance of stockwork structure; (3) Their ore-forming fluids are related to heated seawater that has leached the surrounding volcanic rocks. They are characterized by mesothermal to hypothermal temperatures, low salinities, and low CO2 or CH4 abundances; (4) Their mineralization depth is relatively shallow and generally less than 5 km below the ocean floor; (5) Lastly, the ore-forming metals are derived from the host rocks mostly with only negligible contribution from the upper mantle (Ulrich et al., 2002; Franklin et al., 2005). Several studies have shown that some ancient VMS deposits are strongly deformed because of their affection by orogenic process succeeding their formation, and this can lead to metal remobilization and considerable ore grade enhancement (Cook et al., 1994; Cartwright and Oliver, 2000; Marshall et al., 2000; Gu et al., 2007; Tomkins, 2007; Theart et al., 2010; Zheng et al., 2013a; Zhang et al., 2014; Zhong et al., 2015). However, the mechanism of elemental remobilization in the VMS deposits overprinted by tectono-metamorphism is not yet sufficiently understood.

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Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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The Chinese Altay Orogen forms the main part of the Central Asian Orogenic Belt (CAOB) which is considered as an accretionary orogen with episodic oceanic plate subduction and continental growth from the Neoproterozoic to the Permian, and continent-continent collision which occurred during the Late Carboniferous to Early Triassic (Sengor and Natal'in, 1996; Chen, 2000; Xiao et al., 2003, 2008, 2009; Chen et al., 2009; Chen et al., 2012a, 2012b, 2012c). The geodynamic evolution of the Altay Orogenic Belt resulted in the formation of numerous sizeable ore deposits with substantial reserves, including abundant VMS deposits (Yang et al., 2013; Lobanov et al., 2014). Some of them are adjacent to significant orogenic gold deposits, e.g., the Hongtoushan Cu-Au deposit in the Liaoning Province (Gu et al., 2007; Zhang et al., 2014), the Hugeqi Cu-Pb-Zn deposit in the Inner Mongolia Province (Zhong et al., 2011, 2015), the Au-Cu-Pb-Zn deposit in the Kelan basin, Xinjiang Province (Xu et al., 2011; Zhang et al., 2012; Zheng et al., 2012), and the Pingshui Cu-Zn-Au deposit in the Zhejiang Province (e.g., Chen et al., 2015; Ni et al., 2015). Fluid inclusion studies on the VMS deposits in the Chinese Altay Orogen were carried out and disputed between seafloor and metamorphic hydrothermal fluids (Zhang et al., 2012; Zheng et al., 2012, 2014, 2016), which implies that subsequent metamorphism and deformation played a vital role in the remobilization of elements in pre-existing VMS deposits. Therefore, more detailed studies on the mechanism of remobilization, migration and reprecipitation of metal are needed to explain the mineralization evolution of the VMS deposits hosted in the CAOB. The Ashele Cu-Zn deposit was discovered in 1986 and is currently mined by the NW branch company of the Zijin Mining Group. Systematic studies on the mining field structures, ore geology, isotopic geochemistry and geochronology of the Ashele deposit were applied to constrain its VMS origin since its discovery (Chen et al., 1996; Li et al., 1998; Wang

et al., 1998; Niu et al., 2006; Wan et al., 2010a; Yang et al., 2014). However, more recent exploration proved that some Cu-Pb-Zn lodes are situated in the second belt cross cutting the host rocks and layered ores, which is quite different from the typical VMS deposits (Yang et al., 2014). Additionally, Chang (1997) and Zheng et al. (2015a) investigated the fluid characteristics of the Ashele and concluded that the compositional signatures of some primary fluid inclusions cannot be possible for sea-floor hydrothermal systems (Luders et al., 2001; Chen et al., 2007; Pirajno, 2009), but is in accordance with similar results from orogenic gold deposits instead. Therefore, the Ashele VMS deposit may be a composite of both primary and completely modified orebodies, and provides a unique opportunity to study the effects of deformation and metamorphism on massive sulfide in terms of their textural appearance and compositional signatures. This contribution reports new data obtained from the field investigation, mineralogical examination and fluid inclusions study of the Ashele VMS Cu-Zn deposit. The aim of the current study is to identify some special epigenetic characteristics which may be rather unusual compared to those commonly recognized in VMS deposits, and may assist us to clarify the ore-forming evolution of the Ashele Cu-Zn deposit. In addition, the effect of metals remobilization due to subsequent deformation and metamorphism and the potential for future exploitation of Cu, Pb, Zn and other metals in the region are also discussed. 2. Regional geological framework The term Chinese Altay Orogen refers to the area north of the Tarim Basin, located in central Asia (Fig. 1) (Chen, 2000; Xiao et al., 2008). The Chinese Altay Orogenic Belt has experienced an episodic geotectonic evolution, including consecutive episodes of accretion, subduction,

Fig. 1. Sketch map showing the tectonic framework of North Xinjiang (Chen et al., 2012a, 2012b, 2012c). Siberia plate: S1, Nurt Late Devonian-Early Carboniferous volcanic basin; S2, Keketuohai Paleozoic magmatic arc; S3, Kelan Devonian-Carboniferous fore-arc basin; S4, Armantay-Irtysh accretionary wedge. Kazakhstan plate: K1a, Zharma-Sawur island arc; K1b, Western Junggar accretionary complex; K1c, Eastern Junggar accretionary complex; K1d, Junggar Mesozoic-Cenozoic basin; K1e, Yelianhabirga Late Paleozoic back-arc basin; K1f, Bogada Late Paleozoic aulacogen; K1g, Harlike Paleozoic island arc; K1h, Dananhu island arc; K1i, Turpan Mesozoic-Cenozoic basin; K2a, Sailimu Massif; K2b-Wenquan terrane; K2c, Boloholo Paleozoic arc-basin system; K2d, Yamansu-Jueluotag Paleozoic arc-basin system; K2e, Ili Carboniferous-Permian rift; K2f-central Tianshan Early Paleozoic island arc with Precambrian fragments; K2g, Nalati massif. Tarim Plate: T1, Tarim Mesozoic-Cenozoic basin; T2a, Kuruktag Precambrian massif; T2b, Muzart massif; T3a, Beishan CarboniferousPermian aulacogen; T3b, Kalatierek Late Paleozoic passive marginal sediments; T4a, Southwest Tianshan Late Paleozoic fold-thrust bel; T4b, Southern Tianshan (or Kumishi) Paleozoic accretionary complex.

Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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opening and closure of small basins and final orogenesis (Goldfarb et al., 2003). The area comprises of four major tectonic units from north to south (Fig. 1, Chen et al., 2012a, 2012b, 2012c): (1) the Late Devonian– Early Carboniferous Nurt volcanic basin developed on a pre-Devonian crystalline basement; (2) the central Altay terrane dominated by the Paleozoic Keketuohai magmatic arc with a small amount of Precambrian high-grade metamorphic rocks, Neoproterozoic to earliest Triassic granites and the giant Keketuohai pegmatite; (3) the Devonian-Carboniferous Ashele, Chonghuer, Kelan and Maizi basins developed on the southern margin of the pre-Devonian metamorphic rocks; and (4) the ArmantayErtix accretionary unit that contains high-grade metamorphic rocks with the metamorphic age of 280–260 Ma (Chen et al., 2006), Devonian–Carboniferous fossiliferous sedimentary rocks and numerous granitic intrusions with formation ages of 460 Ma, 408 Ma, 375 Ma and 265 Ma (e.g., Wang et al., 2006). The Devonian–Carboniferous Ashele, Kelan and Maizi basins are metallogenically fertile, and host the giant Ashele Cu-Zn, Keketale PbZn and Mengku Fe deposits (e.g., Wang et al., 1998; Wan et al., 2010a, 2010b, 2011, 2012; Zheng et al., 2013a), and the Abagong polymetallic belt (e.g., Zhang et al., 2012; Zheng et al., 2012, 2013b, 2014, 2015a, 2015b, 2016). These basins constitute a sequence of low-grade metamorphosed terrestrially-sourced turbidites (e.g., Habahe Group), volcanic rocks (e.g., Altay Formation) and volcanic-sedimentary rocks (e.g., Kangbutiebao Formation) and have been considered to form in either a magmatic arc or a rift-related setting (e.g., Wang et al., 1998; Chai et al., 2009; Wan et al., 2011, 2014). The Ashele Cu-Zn deposit is hosted in the Ashele basin hosting the Devonian Ashele Formation dominated by volcanic strata (Fig. 2). The exposed strata in the mining area belong to the Early to Middle Devonian Ashele Formation, which is unconformably underlying the Late Devonian Qi'ye Formation. The Ashele Formation comprises a series of bimodal volcanics including basalt, rhyolite, dacitic tuff, dacitic breccia and agglomerate, subjected to greenschist facies metamorphism (Wei et al., 2007). Metamorphosed basaltic and rhyolitic tuff yielded corresponding zircon U-Pb ages of 388 Ma and 387 Ma, respectively (Yang et al., 2014). The Ashele basin is composed of a series of multiple overturned synclines that strike 50 km with axial planes dipping to northwest. The regional-scale Markakuli Fault and its secondary faults

Fig. 2. Geological map of the Ashele Cu-Zn mining area. (Modified after Chen et al. (1996)).

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and ductile shear zones cross cut the mining area. The ore-controlling structures are dominated by several big folds accompanying NStrending faults. The intermediate to felsic intrusions and basic dike swarm are widespread in the Altay Orogenic Belt, ranging from Ordovician through Permian to Triassic and even to Early Jurassic age. Except for the Ashele Cu-Zn deposit, the Ashele Formation hosts the Sarsuk Cu-rich polymetallic deposit and the Keyingde Cu deposit, as well as the Saidu and Duolanasayi orogenic gold deposits (Yang et al., 2014). 3. Field observation and ore textures The Ashele Cu-Zn deposit, discovered in 1984 by the 5th Geological Team of the Xinjiang Bureau of Geology and Mineral Exploration, contains Cu (1.08 Mt. @ 2.46%), Zn (0.43 Mt. @ 0.41%) and Au (27.2 t @ 0.36 g/t; Yang et al., 2014). Its geotectonic regime is supposed to be a Paleozoic arc-basin system (Wan et al., 2010a). It consists of 4 main orebodies located in the mineralization zone entitled No. 1 (Fig. 3). The majority of the orebodies is blind and distributed at depths ranging from 888 m to 600 m. The No. 1 ore body in the No. 1 mineralization zone is 950 m long and 5 to 120 m wide, contributing more than 98% of the identified Cu reserves. This orebody occurs as “hook-shaped” and is enveloped by the uniformly deformed basalt in the hanging wall and rhyolite in the footwall (Fig. 3). The Ashele Formation and Qi'ye Formation crop out in a 5.0 km × 5.2 km mining area locally intruded by quartz diorite, basaltic andesite, dacite and rhyolite porphyry (Fig. 2). The Ashele Formation is interpreted as a series of bimodal volcanics consisting of basalt, rhyolite, sandstone and limestone (Yang et al., 2014). The Qi'ye Formation contains agglomerate, volcanic breccia, breccia-bearing tuff, agglomerate lava, tuffaceous sandstone and tuffaceous siltstone units with minor rhyolite, dacite and sedimentary tuff (Fig. 2; Wan et al., 2010a). The above two strata have been folded and metamorphosed to greenschist facies as it is indicated by the mineral assemblage consisting of chlorite-epidote-biotite-sericite (Fig. 3; Wei et al., 2007). The wall rocks are substantially altered displaying silicification, baritization, chloritization, carbonatization and intensive sulfidization. A major deformational event is recorded by a series of folds and associated thrust faults, resulting in the general deformation of orebodies (Figs. 4 and 5). In addition, some sulfide veins locally cut across the primary layered orebodies and their surrounding rocks (Figs. 4 and 5). Therefore, two ore-forming periods can be distinguished in the Ashele VMS Cu-Zn deposit: an early period represented by banded- and massive-structured VMS ores and a second hydrothermal period represented by the intrusion of earlier Cu-dominated and later Pb-Zn-dominated polymetallic quartz veins (Figs. 4, 5 and 6). The ore structures include massive, semi-massive, banded, ribbon, mottled, disseminated, vein, and veinlet etc. (Figs. 4 and 5). The main textural varieties exhibited by the studied ores are (in decreasing order of abundance) euhedral-subhedral granular, anhedral granular, interstitial, corona texture, exsolution, pseudomorphic, pressure crack, cataclastic, crystalloblastic, durchbewegung structure and pressure solution replacement (Figs. 4 and 5). The main ore minerals in general for the deposit include pyrite, chalcopyrite, sphalerite, with subsidiary galena and silver-bearing tetrahedrite (Fig. 6). The gangue minerals are represented by quartz, sericite, chlorite, barite, calcite and dolomite (Fig. 6). Most of the orebodies are consistently deformed with the host rocks (Fig. 3), whereas they are mostly found in overturned synclines displaying maximum thickness and highest grades in the saddle of the folds (Fig. 3). In addition, deformation and metamorphism separate the primary ores to several equidimensional lenses, which are evenly distributed within the surrounding rocks (Fig. 3; Yang et al., 2014). Locally, the sulfide veins cut across the wall rocks and the hosted layered orebodies (Fig. 4 A). In hand specimen, the primary orebodies display massive, semi-massive, banded, ribbon, mottled, disseminated, vein and veinlet structure (Fig. 4). A progressive evolution from the

Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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Fig. 3. Geological profiles of No. 1 orebody, the Ashele Cu-Zn deposit. (Modified after Chen et al. (1996)).

Fig. 4. Photographs showing remobilization of the Ashele Cu-Zn deposit. A—the sphalerite-galena ± chalcopyrite vein ores cross cutting the banded chalcopyrite-pyrite ores with the production of massive ores between the contact zone of both; B—typical banded chalcopyrite-pyrite ores; C—typical massive ores with quartz breccias; D—semi-massive chalcopyrite ores coexisting with quartz; E—chalcopyrite-quartz veins cross cutting the host rocks; F—the oriented chalcopyrite distributed in the foliation of greenschists. Abbreviations: Gn = galena, Sph = sphalerite, Qtz = quartz, Cpy = chalcopyrite, Bi = biotite, Chl = chlorite.

Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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Fig. 5. Photomicrographs showing remobilization of the Ashele Cu-Zn deposit. A—the initially brecciated pyrite with cataclastic textures filled and replaced by minor chalcopyrite; B—Brecciated pyrite filled and replaced by larger amounts of chalcopyrite; C—the typical durchbewegung structure with oriented rounded pyrite grains in a matrix of chalcopyrite and sphalerite; D—the oriented pyrite distributed in the quartz veins; E—the newly generated sphalerite-chalcopyrite infilling the brecciated pyrite; F—the chalcopyrite-quartz vein cross cutting the host rocks with sericitization; G—chalcopyrite participating in the particles of biotite and chlorite grains; H—the disseminated chalcopyrite surrounding pyrite skeleton in quartz veins; F-chalcopyrite distributed in the quartz vein. Abbreviations: Gn = galena, Sph = sphalerite, Qtz = quartz, Cpy = chalcopyrite, Bi = biotite, Chl = chlorite, Ser = Sericite.

disseminated ores to more densely disseminated-textured ones to their massive equivalents is apparent in the drill cores (Fig. 6; Chen et al., 1996). It is also obvious that the laminated ores gradually convert to massive ores (Fig. 4A; Yang et al., 2014). In spite of the gradual transitions, most of the primary exhalative textures are still preserved

(Chen et al., 1996). However, the microscopic and macroscopic observations demonstrates that some veins cut across the earlier banded ores (Figs. 4 and 5), which are then accompanied by coarser (annealed) grains and pressure-solution fabrics, brecciation, durchbewegung and replacing textures (Fig. 5).

Fig. 6. Mineral sequences of the Ashele Cu-Zn deposit. The red lines represent the mineral with economic interests, and the blue line with a five-pointed star is the quartz used for microthermometric measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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Obviously, these epigenetic in origin textures are different compared to those of the primary VMS (Frankin et al., 2005; Hannington et al., 2005). The wide variety of ore textures indicates that the studied primary VMS ores were overprinted by deformation and metamorphism that potentially caused considerable metal remobilization within the ore system.

Laser Raman spectroscopic analysis of the FIs was carried out using a Renishaw RW-1000 Raman micro-spectrometer at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. An argon ion laser with a wavelength of 514.5 nm and a source power of 20 mW × 100% was used in detection. The spectral range falls between 50 and ~4000 cm−1 with an accumulation time of 30 s for the analyses of CO2, N2 and CH4 in the vapor phase.

4. Fluid inclusion study 4.2. Petrography and classification 4.1. Sampling and analytical methodology Samples for fluid inclusion study were taken from the orebody No. 1 (Fig. 3). Fourteen quartz samples collected from Cu-dominated veins (Fig. 4D and E), as well as Cu-bearing brecciated and nodular-textured occurrences (Fig. 4C) were studied for fluid inclusions (FIs), petrography, microthermometry and Raman spectroscopy. Microthermometric measurements on FIs were performed using a Linkam THMS MDS600 heating-freezing stage (from −196 to 600 °C) at the fluid inclusions laboratory of Sun. Yat-sen University. The precision of temperature measurements is approximately ± 0.1 °C on cooling runs and ± 2 °C on heating runs. The heating/freezing rate was generally 0.2–5 °C/min, but reduced to ≤ 0.2 °C/min near phase transformation. Salinities of aqueous FIs were estimated using the reference data of Bodnar (1993) for the NaCl-H2O system. Salinities of CO2-rich FIs were calculated using the equations of Collins (1979). Densities were calculated using Flincor software according to the microthermometry data (Brown and Lamb, 1989).

Fluid inclusion types are identified based on their phases (L-V-S) at room temperature, phase transitions observed during heating and cooling, and Laser-Raman spectroscopy. Four compositional types of FIs were identified, including in decreasing order of abundance: mixed aqueous-carbonic (C-type), aqueous (W-type), pure carbonic (PCtype) and daughter mineral-bearing (S-type) FIs (Fig. 7). All the four types of FIs are locked in the Cu-rich and Pb-Zn-rich polymetallic quartz veins, and show no significant difference with respect to types and population, indicating they were possibly formed at the same time. 4.2.1. C-type FIs These FIs vary between 4 and 12 μm in size, but rarely can be up to 20 μm across. They consist of three-phases, including VCO2, LCO2 and LH2O (Fig. 7A and B). The carbonic phases (VCO2 + LCO2) account for 25–60 vol.%. C-type FIs commonly occur isolated or as trails within quartz grains and therefore, are interpreted as primary or pseudosecondary FIs.

Fig. 7. Photomicrographs of typical fluid inclusions in the Ashele Cu-Zn deposit. A—the distributions of all types of fluid inclusion in Cu-polymetallic quartz veins; B—the C-type fluid inclusion; C—the W-type fluid inclusion; D—the S-type fluid inclusion; E—the association of PC-type fluid inclusion.

Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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4.2.2. W-type FIs W-type inclusions account for 10%–50% of the total population of FIs. They are typically two-phase FIs with sizes ranging from 4 to 12 μm in diameter that contain 5–25 vol.% vapor and show negative crystal, elliptical and irregular shapes. Primary FIs occur completely isolated or as clusters (Fig. 7B and C).

4.3.3. PC-type FIs Melting temperatures of solid CO2 in the PC-type FIs range from − 59.7 to − 56.8 °C. Homogenization temperatures of the CO2 phase range from 20.2 to 29.1 °C, with densities varying from 0.87 to 0.98 g/ cm3. These data imply that the CO2 phase contains a certain amounts of CH4 or N2.

4.2.3. PC-type FIs They account for approximately 10% of the total FIs, consisting of almost pure carbonic fluid at room temperature, including monophase (vapor or liquid) CO2. The liquid CO2 FIs are semitransparent (Fig. 7A and E). These FIs occur in planar distribution, suggesting that they are primary or pseudosecondary in origin. Their size and shape are almost similar to those of the C-type FIs, but the PC-type FIs are mostly less than 10 μm in size.

4.3.4. S-type FIs The S-type FIs yielded final melting temperatures of solid CO2 ranging from −60.0 to −56.8 °C. Bubbles in the S-type FIs disappear at homogenization temperatures between 168 and 268 °C before halite dissolution in the heating run. Total homogenization temperatures range from 257 to 299 °C. Estimated salinities range from 34.7 to 38.2 wt.% NaCl equiv., and the calculated densities vary between 0.88 and 1.18 g/cm3.

4.2.4. S-type FIs Only the daughter mineral of halite (NaCl) can be observed as daughter phase in the S-type FIs locally at the Ashele deposit (Fig. 7A and D). The S-type inclusions are 6 to 30 μm in size, accounting for about 5% of the total number of encountered FIs. The transparent daughter minerals of halite occur in cubic form and vary in size between 1 and 4 μm.

4.4. Laser Raman spectroscopy Laser Raman spectroscopy confirms that H2O is the main component of the W-type FIs (Fig. 9A), with only traces of CO2 (Fig. 9B). Consistent with the microthermometric results the carbonic phase in C- and PCtype FIs contains variable contents of N2 (Fig. 9C and D). The identification of CO2 in C- and PC-type FIs, as well as its detection even in minor abundances in W-type FIs, imply that the fluid system was carbonicrich.

4.3. Microthermometry 4.5. Pressure and depth estimates of mineralization The microthermometric data (e.g., first melting temperature of carbonic phase, CO2 clathrate melting temperature, CO2 homogenization temperature and total homogenization temperature), with calculated parameters (e.g., salinity), are summarized in Table 1 and illustrated Fig. 8. 4.3.1. C-type FIs The final melting temperatures of solid CO2 in the C-type FIs range from − 58.0 to − 56.8 °C. These temperatures are slightly lower than the triple point of pure CO2 at −56.6 °C, indicating that a small amount of CH4 and/or N2 is present. Clathrate melting temperatures are between 2.5 and 7.5 °C, whereas homogenization temperatures of CO2 in the liquid phase are between 8.2 and 22.1 °C. All C-type FIs were finally homogenized at temperatures between 180 and 279 °C. The calculated bulk salinities range from 3.52 to 10.19 wt.% NaCl equiv. The CO2 densities range from 0.58 to 0.97 g/cm3 and bulk densities vary between 0.97 and 1.11 g/cm3. Some C-type FIs decrepitated and leaked at a temperature range between 252 and 274 °C before homogenization. 4.3.2. W-type FIs The W-type FIs yielded first ice melting temperatures of approximately −20.3 °C, implying a Na+ rich system. They displayed final ice melting temperatures between −7.9 and −3.5 °C, with corresponding salinities of 2.7 to 8.6 wt.% NaCl equiv. These FIs were mostly homogenized to liquid and only rarely to vapor, at temperatures ranging from 191 to 307 °C. The calculated densities vary between 0.58 and 0.96 g/ cm3.

As the C-type FIs are the major type in the Ashele Cu-Zn deposit, their assemblages permit a reliable estimation of the pressure conditions of fluid entrapment during ore formation. The procedure that has been employed here to estimate the pressure is as follows: (1) taking into account the minimum and maximum densities, the range of isochors for C-type FIs can be defined on the P-T diagram using the FLINCOR software (Brown and Lamb, 1989); (2) then the common range of homogenization temperatures of C-type FIs are selected to constrain the relative pressure of entrapment. The isochors for C-type FIs showing the minimum and maximum densities and temperatures are used to constrain the possibly minimum trapping pressure range in which metallogenesis has occurred (Zhang et al., 2012; Zheng et al., 2012). On the basis of the above method, the C-type FIs yielded a pressure range from 85 to 370 MPa at homogenization temperatures between 191 and 279 °C (Fig. 10). The minimum end-member of estimated pressures (85 MPa) represents the minimum trapping pressure at the Ashele, which correspond to the minimum ore-forming depth of 3.1 km in lithosphere with the density of rocks of 2.7 g/cm3. Therefore, the actual depth of metallogenesis is greater than 3.1 km. 5. Discussion 5.1. Ore texture evolution The classic and undeformed VMS deposits are generally composed of an upper layered unit and a lower stockwork portion, with locally chimney-like morphologies preserved (Ulrich et al., 2002; Yang

Table 1 Microthermometric data of fluid inclusions locked in the earlier Cu-dominated quartz veins. Type

Tm/°C

Tm,cla/°C

Th,CO2/°C

C W S PC

−58 to −56.8 −27 to −21 −60 to −56.8 −59.7 to −56.8

2.5 to 7.5 – – –

8.2 to 22.1 – – 20.2 to 29.1

Tm,ice/°C −7.9 to −3.5 –

Th/°C(numbers)

Salinity/wt.% NaCl equiv

180 to 279 (24) 191 to 307 (16) 257 to 299 (4) –

3.52 to 10.19 2.74 to 8.55 34.68 to 38.16 –

Notation: Tm—first melting temperature of carbonic phase; Tm,cla—CO2 clathrate melting temperature; Th,CO2—CO2 homogeneous temperature; Tm,ice—ice point; Th—total homogenous temperature.

Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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Fig. 8. Histograms of microthermometric measurements of fluid inclusions.

and Scott, 2002; Franklin et al., 2005). However, the deformed orebodies exhibit textural features, such as mylonitic, cataclastic and press-solution textures, replacing rim and durchbewegung textures, make them deviate from the primary ones and illustrate that elements remobilization has occurred in the VMS deposit (Cook et al., 1994; Cartwright and Oliver, 2000; Marshall et al., 2000; Gu et al., 2007; Tomkins, 2007; Theart et al., 2010; Zheng et al., 2013a; Zhang et al., 2014; Zhong et al., 2015). The following textural evolution can be outlined for the Ashele VMS Cu-Zn deposit: (1) the primary VMS orebodies with banded structures were the first to form (Fig. 4A); 2) subsequent deformation and metamorphism resulted into metals remobilization in these primary VMS lenses, accompanied by the development of coarser grains and pressure-solution structures, as well as cataclastic, durchbewegung and replacing textures at the microscopic scale (Fig. 5A–F). The variety of ore textures indicates that the metals budget in the primary VMS ores was modified because of subsequent deformation and metamorphism

(Cook et al., 1994; Cartwright and Oliver, 2000; Marshall et al., 2000; Zheng et al., 2013a). Similar phenomena have also been recognized at other metamorphosed VMS deposit, such as the VMS deposits in the Iberian pyrite belts in Spain, the Matchless deposit in Africa, the Keketale Pb-Zn at Chinese Altay, the Hongtoushan Cu-Au deposit in the Liaoning Province and the Pingshui Cu-Au deposit in the Zhejiang Province, China (Cook et al., 1994; Marshall et al., 2000; Zheng et al., 2013a, 2013b; Gu et al., 2007; Ni et al., 2005, 2015; Chen et al., 2015). All these deposits may be considered as typical examples of VMS with disturbed elemental abundances due to remobilization. 5.2. Ore-forming fluids Ore-forming fluid characteristics is the key to determine the genetic processes that are involved in the formation of a hydrothermal deposit (Fan et al., 2003; Chen et al., 2007). Modern seafloor hydrothermal

Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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Fig. 9. Representative Raman spectra of fluid inclusions. A—W-type fluid inclusion mainly containing H2O; B—the C-type fluid inclusion mainly containing CO2, in addition to N2; C—the Ctype fluid inclusion containing minor N2, in addition to CO2; D—the PC-type fluid inclusion containing minor N2, in addition to CO2.

systems (Luders et al., 2001; Yang and Scott, 2002) and ancient VMS deposits that have experienced merely weak metamorphism, such as the Kuroko in Japan (Pisutha-Arnond and Ohmoto, 1983), Ural in Russia (Bailly et al., 1999) and Hellyer in Australia (Zaw et al., 1996), are characterized by low salinity, limited CO2-dominat inclusions, hydrostatic pressure conditions and generally low to moderate formation temperatures ranging from 140 to 280 °C. However, some deformed VMS deposits host carbonic-rich fluid inclusions and even some FIs of pure CO2-phase have been identified in them (Christian et al., 2003; Xu et al., 2008, 2011). The estimated depth of ore formation as deduced from the carbonic FIs can reach up to 15 km, which is quite unrealistic for primary VMS systems (Xu et al., 2008, 2011). In addition, Christian et al. (2003) reported carbonic fluids represented by CO2 and CO2-H2O that corresponded to pressures of 500 MPa and depths more than 15 km and concluded that the VMS deposits of the entire Iberian belt have undergone metamorphism-induced remobilization, which contributed greatly to the augmentation of ore reserves. Similar cases have been reported from the Keketale VMS-type Pb-Zn deposit in the neighboring Maizi basin (Zheng et al., 2013a).

Fig. 10. P-T diagram of C-type FIs used for estimating the mineralization pressure.

The Cu-rich polymetallic quartz veins of the Ashele deposit contain large amounts of FIs including C-type, PC-type and W type FIs, with minor S-type FIs (Fig. 7), having homogenization temperatures of 220–280 °C and salinities of 4–8 wt.% NaCl equiv. (Fig. 8). Raman spectroscopy data indicate that fluids are carbonic-rich (Fig. 9). The characteristics of mesothermal temperatures, low salinities and carbonic-rich nature of the fluid inclusions are consistent with those from orogenic gold deposits, which are interpreted to originate from the circulation of metamorphic fluids in the crust (Groves et al., 1998; Kerrich et al., 2000; Goldfarb et al., 2001, 2013; Fan et al., 2003; Chen et al., 2001, 2004, 2012a, 2012b; Li et al., 2011; Zhao et al., 2013; Xu et al., 2015, 2016). The estimated depths greater than 3.1 km are certainly not representative of sea floor systems (Franklin et al., 2005), but indicate the impact of metamorphic devolatilization instead. 5.3. Ore genesis and further exploration (targets) The characteristics of mining field structures, ore geology, isotopic geochemistry and geochronology of the Ashele deposit indicate the Ashele deposit generally comprises a VMS ore-forming system, including: (1) the host rocks are dominated by a series of volcanics encompassing basalt, rhyolite and pyroclastic (agglomerate); (2) the orebodies display well-developed laminated as well as massive structures; (3) are overlaid by barite; (4) some ores exhibit δ34S values of less than − 30‰, which are interpreted as resulting from bacteria reduction of sulfates; and (5) the ore-forming geotectonic setting is an extended back-arc basin of Paleozoic age (Chen et al., 1996; Li et al., 1998; Wang et al., 1998; Niu et al., 2006; Wan et al., 2010a). However, recent exploration and research uncover some characteristics at the Ashele that differ from the typical VMS deposits. Yang et al. (2014) reported some Cu-Pb-Zn lodes that are situated in the No. 2 mineralization belt cross cutting the host rocks and layered ores. Chang (1997) investigated the fluid inclusions locked in mineralized quartz at Ashele and concluded based on pressure estimate of some primary fluid inclusions yielded a depth of approximately 15 km, which cannot be possible for sea-floor hydrothermal systems producing massive sulfide deposits (Luders et al., 2001; Chen et al., 2007; Pirajno, 2009), but is in accordance with similar results from orogenic gold deposits. Our presented ore textural and fluid inclusion data in this contribution strengthen more epigenetic characteristics. Therefore, the Ashele VMS

Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015

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deposit presents a combination of both primary and completely modified orebodies. In fact, the Ashele Cu-Zn deposit is adjacent to some very significant orogenic Au deposits (Duolanasayi and Saidu deposits). Although large areas of the deposit preserve primary textural and mineralogical features, the pronounced structural dismemberment, folding and recrystallization of the ores resulted from the impact of succeeding metamorphism coupled with deformation. Therefore, two key factors can be considered vital to the formation of the economic-grade banded pyrite ores in the Ashele Cu-Zn deposit. In the Late Paleozoic, the early-staged seafloor hydrothermal system that was characterized by tabular orebodies and banded pyrite with abundant Cu-Zn sulfides, was developed in an extensional subduction-related back-arc basin accompanied by simultaneous rhyolitic-tuff eruption and sedimentation (Wang et al., 1998; Wan et al., 2010a). In the Permian to Triassic the subsequent closure of the ancient Asian Ocean and inversion of the back-arc basin coupled with collision resulted in largescale deformation and metamorphism (Wei et al., 2007; Xiao et al., 2009). The pre-existing VMS mineralization was overprinted as a result of deformation-induced metamorphism. Thus, the Cu and Zn contents of the banded ores were intensely affected. Copper and Zn were transported in metamorphic fluids and precipitated in dilatational structures (e.g., saddle of synclines) along with remobilized quartz. The above study can broaden our horizon regarding the mineralogical exploration in the Chinese Altay Orogen. Although most of the primary VMS deposits are hosted in the Late Paleozoic volcanic, the link between ancient volcanic rocks and folds is of critical importance to the development of a petrological model targeting to the recognition of similar Ashele-type giant deposits in the broader Chinese Altay Orogen and even CAOB. 6. Conclusions 1) Several VMS exposures in the Ashele Cu-Zn deposit show textural characteristics that differ from the primary texture in VMS orebodies, thus implying an overprint of later process. 2) The fluid inclusions in the Cu-dominated polymetallic quartz veins show that the fluids were characterized by mesothermal temperature, low-salinities and elevated contents of carbon-rich compounds, implying that the primary ores were enriched by the circulation of metamorphic fluids. 3) The Ashele Cu-Zn deposit is an enriched VMS deposit overprinted by deformation and metamorphism, and the occurrences of folded volcanic rocks may be considered as potential targets for the finding of other Ashele-type deposits in the CAOB. Acknowledgements This contribution was financially supported by the NSFC (No. 41502068), Chinese National Basic Research 973 Program (Nos. 2014CB440802) and open fund of the Key Lab of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (No. KLMM20150203). Office of Project-305 and Geological Team 706 of Xinjiang Bureau of Nonferrous Metals are thanked for helping in the field investigation. We are indebted to Prof. Pei Ni, Dr. Thomas Ulrich and one anonymous reviewer for constructive suggestions and comments. References Bailly, L., Orgeval, J.J., Tesalina, S., Zaykov, V., Maslennikov, V.V., 1999. Fluid inclusion data of Alexandrinka massive sulfide deposit, Urals. In: Stanley, C.J. (Ed.), Mineral Deposits: Processes to Processing. Balkema Publishers, Rotterdam, pp. 13–16. Bodnar, R.J., 1993. Revised equation and table for determining the freezing point depression of H2O–NaCl solutions. Geochim. Cosmochim. Acta 57, 683–684. Brown, P.E., Lamb, W.M., 1989. P-V-T properties of fluids in the system H2O ± CO2 ± NaCl: new graphic presentations and implications for fluid inclusion studies. Geochim. Cosmochim. Acta 53, 1209–1221.

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Please cite this article as: Zheng, Y., et al., Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China, J. Geochem. Explor. (2016), http://dx.doi.org/10.1016/j.gexplo.2016.06.015