Mineralium Deposita https://doi.org/10.1007/s00126-018-0804-6
ARTICLE
Mineralogical, textural, sulfur and lead isotope constraints on the origin of Ag-Pb-Zn mineralization at Bianjiadayuan, Inner Mongolia, NE China Degao Zhai 1,2 & Jiajun Liu 1,2 & Nigel J. Cook 3 & Xilong Wang 4 & Yongqiang Yang 1 & Anli Zhang 5 & Yingchun Jiao 5 Received: 8 December 2016 / Accepted: 26 March 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract The Bianjiadayuan Ag-Pb-Zn deposit (4.81 Mt. @157.4 g/t Ag and 3.94% Pb + Zn) is located in the Great Hinggan Range Pb-Zn-Ag-Cu-Mo-Sn-Fe polymetallic metallogenic belt, NE China. Vein type Pb-Zn-Ag ore bodies are primarily hosted by slate, adjacent to a Sn ± Cu ± Mo mineralized porphyry intrusion. The deposit is characterized by silver-rich ores with Ag grades up to 3000 g/t. Four primary paragenetic sequences are recognized: (I) arsenopyrite + pyrite + quartz, (II) main sulfide + quartz, (III) silver-bearing sulfosalt + quartz, and (IV) boulangerite + calcite. A subsequent supergene oxidation stage has also been identified. Hydrothermal alteration consists of an early episode of silicification, two intermediate episodes (propylitic and phyllic), and a late argillic episode. Silver mineralization primarily belongs to the late paragenetic sequence III. Freibergite is the dominant and most important Ag-mineral in the deposit. Detailed ore mineralogy of Bianjiadayuan freibergite reveals evidence of chemical heterogeneity down to the microscale. Silver-rich sulfosalts in the late paragenetic sequence III are largely derived from a series of retrograde and solid-state reactions that redistribute Ag via decomposition and exsolution during cooling, illustrating that documentation of post-mineralization processes is essential for understanding silver ore formation. Sulfur and lead isotope compositions of sulfides, and comparison with those of local various geological units, indicate that the ore-forming fluids, lead, and other metals have a magmatic origin, suggesting a close genetic association between the studied Ag-Pb-Zn veins and the local granitic intrusion. Fluid cooling coupled with decreases in fO2 and fS2 are the factors inferred to have led to a decrease of silver solubility in the hydrothermal fluid, and successively promoted extensive Ag deposition. Keywords Freibergite . Sulfur isotopes . Lead isotopes . Ag-Pb-Zn deposit . Bianjiadayuan . NE China
Introduction Editorial handling: R. Moritz Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00126-018-0804-6) contains supplementary material, which is available to authorized users. * Degao Zhai
[email protected] 1
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
2
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
3
School of Chemical Engineering, University of Adelaide, Adelaide, SA 5005, Australia
4
Earthquake Administration of Liaoning Province, Shenyang 110034, China
5
Lituo Mining Company, Chifeng 024000, China
Silver-lead-zinc vein mineralization has historically contributed significantly to world production of these commodities, and the close temporal, spatial, and genetic relationship with porphyry Cu deposits (PCD) has been well documented (e.g., Guilbert and Park 1986; Muntean and Einaudi 2000; Sillitoe 2010; Sillitoe and Mortensen 2010; Catchpole et al. 2015). Some Ag-Pb-Zn veins, however, do not appear to be genetically linked to PCDs and their genesis remains controversial. Normally, the Ag-Pb-Zn veins are discordant and epigenetic, generally formed by open-space filling. In those deposits, silver is the principal commodity, or occurs as a co-product of lead-zinc ore in polymetallic veins. Such vein-type deposits are clearly hydrothermal in origin, although the source of oreforming fluids and metals may be either magmatic or nonmagmatic (Kissin and Mango 2014). In many cases, both fluid and metal sources are unknown or controversial, and the
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transport and deposition processes of metals from the hydrothermal solutions are still greatly debated. The Great Hinggan Range (GHR) Metallogenic Belt, NE China, hosts a number of porphyry Mo(Cu), skarn Fe(Sn), epithermal Au-Ag, and hydrothermal vein Ag-Pb-Zn ore deposits (Zeng et al. 2009, 2011; Wu et al. 2011a, b; Shu et al. 2013, 2016; Zhai et al. 2013, 2014a, b, c, 2015; Zhai and Liu 2014). Ag-Pb-Zn vein deposits are particularly common in the southern segment of the GHR, including the Shuangjianzishan, Bianjiadayuan, Bairendaba, and Weilasituo deposits (Wang et al. 2013; Ouyang et al. 2014; Ruan et al. 2015; Liu et al. 2016a, b). Among the most important of the new discoveries is the Bianjiadayuan deposit with reserves exceeding 4.8 Mt of ore at an average grade of 157.4 g/t Ag and 3.94% (Pb + Zn). Compared to other similar deposits in this region, this deposit is particularly silver rich with grades of up to 3000 g/t. The well-preserved vein deposit allows for a systematic investigation of silver mineralogy, fluid origin, and deposition mechanisms. Previous studies on the Bianjiadayuan deposit, mostly published in Chinese, have addressed ore deposit geology (Wang et al. 2014d), mineralogy (Wang et al. 2014a), whole rock geochemistry and Sr-Nd isotope compositions (Wang et al. 2013, 2014b, 2016; Ruan et al. 2015), geochronology of intrusive rocks (Ruan et al. 2013, 2015; Wang et al. 2013, 2014b, 2016), stable isotope (S, C, H, O), Pb isotope geochemistry (Ruan et al. 2013, 2015; Wang et al. 2014c), fluid inclusions (Ruan et al. 2015), and geotectonic settings (Wang et al. 2013, 2014b, 2016; Ruan et al. 2015). In contrast, issues related to metal source, physicochemical conditions of ore formation, and precipitation mechanisms remain poorly constrained. For example, Wang et al. (2014d) considered that hydrothermal fluids were sourced from a local quartz porphyry, whereas Wang et al. (2014c) and Ruan et al. (2015) argue that the hydrothermal fluid was a mixture of magmatic and meteoric fluids. The sets of evidence for a magmatic contribution to base metal vein mineralization are mainly derived from the occurrence of Ag-PbZn vein near porphyry and breccia, alteration and metal zonation (Cu-Mo-Sn, Sn-Zn to Ag-Pb-Zn), fluid inclusions, and stable isotope (D-O-C-S) data (Wang et al. 2014c, d; Ruan et al. 2015). Regarding ore deposition, Wang et al. (2014a) suggested that variations in temperature and logfS 2 and reduction of the fluid favored ore deposition, whereas Ruan et al. (2015) proposed that a decrease in temperature, salinity, and pressure contributed to ore formation. Most recently, Wang et al. (2016) advocated for fO2 and pH variation in the fluid as vital for ore deposition. In this contribution, we present the results of a comprehensive investigation of the Bianjiadayuan Ag-Pb-Zn deposit, involving ore mineralogical, textural, and S and Pb isotopic methods. These integrated data are used to determine the
source, nature, and evolution of the hydrothermal fluid, metal source, and the controls on ore mineral deposition. The study also examines the genetic classification of vein-type Ag-PbZn mineralization, and emphasizes the importance of potential post-mineralization processes when aiming to reconstruct silver ore formation in a wide variety of settings.
Regional geological setting The Bianjiadayuan Ag-Pb-Zn deposit occurs in the GHR Metallogenic Belt, which lies in the easternmost part of the Central Asian Orogenic Belt (CAOB, Fig. 1a). The CAOB developed during the Neoproterozoic to Phanerozoic, and is rimmed by the Siberian, Tarim, and North China Cratons (Fig. 1a). It formed via successive accretion of arc complexes, accompanied by emplacement of immense volumes of granitic magmas (Jahn et al. 2000), and is believed to have been the world’s largest site of juvenile crust formation in the Phanerozoic era (Jahn 2004; Shi et al. 2010). The region is characterized by widespread Mesozoic volcanic and intrusive rocks (Fig. 1b), including I- and A-type granitoids (Xiao et al. 2004; Wu et al. 2005, 2011c), which make up > 50% of the surface area in the mountainous regions (HBGMR 1993). Based on a large dataset and a precise geochronological framework for the regional intrusion ages, these volcanics and granitoids have been broadly divided into two groups, which were emplaced in two geotectonic episodes (Table 1). The first group belongs to a Permian and Triassic episode (275 to 210 Ma, zircon U-Pb method), whereas the second group is Jurassic to Cretaceous in age (160 to 130 Ma, zircon UPb method) (Wu et al. 2004; Wei et al. 2008; Zhang et al. 2010). Magmatism from the two episodes is characterized by different rock types, associations, and magma sources; the tectonic settings are also distinct (Table 1). It was proposed that early Paleo-Pacific subduction in the Jurassic caused subsequent lithospheric delamination or rollback and extension of the back-arc area, successively enabling emplacement of Cretaceous granitoids (Wang et al. 2006; Zhang et al. 2008, 2010). Compared with other areas in the CAOB, NE China was significantly affected by Paleo-Pacific subduction, and can be considered one of the most metallogenically important areas of the eastern Asian active continental margin during the Mesozoic (Wu et al. 2011c). The southern segment of the GHR Metallogenic Belt hosts porphyry Cu-Mo, skarn Fe-Sn, and polymetallic (Ag-Pb-Zn-Cu) epithermal and hydrothermal vein deposits (Fig. 1c, Zhai et al. 2014b). The southern segment of the GHR is an important vein Ag-Pb-Zn metallogenic belt in northern China. The ore deposits in this district are mostly related to Jurassic to Cretaceous magmatism
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Fig. 1 a Tectonic scheme of the Central Asian Orogenic Belt (CAOB, modified from Jahn et al. 2000; Xiao et al. 2009; Shen et al. 2015). b Geological map of the Great Hinggan Range (GHR), NE China, showing
the distribution of the Mesozoic granites and volcanics (modified from Zhai et al. 2015). c Geological map of the southern GHR showing the locations of major ore deposits (modified from Zhai et al. 2014b)
(Zhao and Zhang 1997; Mao et al. 2005; Chen et al. 2007; Zhai et al. 2013, 2014b, 2018). Regionally, mineralization and Jurassic to Cretaceous granites demonstrate a close spatial distribution (Fig. 1c). Geochronological studies show that magmatic-hydrothermal deposits in the area formed during two distinct metallogenic events (Li et al. 2012): an early event in the Late Permian (272 ± 3 to 256 ± 7 Ma, zircon U-Pb and molybdenite Re-Os); and a later event in the Cretaceous and Jurassic (167 ± 2
to 129 ± 3 Ma, zircon U-Pb, molybdenite Re-Os and sericite 40Ar- 39Ar). Most porphyry Cu-Mo and skarn Fe-Sn deposits formed during the second metallogenic event, associated with intrusion of A-type granites (Mao et al. 2005; Wu et al. 2005, 2011c). Precise ages for AgPb-Zn mineralization in the area are generally lacking, largely due to the lesser focus on those deposits in previous research programs and the difficulties in obtaining direct age data for Ag-Pb-Zn mineralization.
Miner Deposita Table 1
Summary and comparisons between early and late magmatic episodes in the Great Hinggan Range district in NE China
Divisions
Group 1
Group 2
Ages
275~210 Ma
160~130 Ma
Rock types
I- and S-type granitoids
I- and A-type granitoids
Rock affinity Rock associations
Calc-alkaline Diorite, granodiorite, monzogranite, syenogranite
Alkaline Granite, granodiorite, adamellite, syenite
Related mineralization
Porphyry Mo, greisen type Mo-W, and Cu-Pb-Zn veins
Porphyry Cu-Mo, skarn Fe-Sn, epithermal Au-Ag, and polymetallic veins Lower crust
Magma source
Mantle and recycled ancient crust
Tectonic settings
The Paleo-Asian Ocean closure, post-orogenic extension, and plate subduction Chen and Jahn 2001; Wu et al. 2003, 2004; Li et al. 2012
Relevant references
Ore deposit geology and Ag-Pb-Zn mineralization The strata exposed in the ore district belong mainly to the Permian Zhesi Formation, comprising slates and siltstones that dip to NNW at 50–55°, and Quaternary paleoplacers (Fig. 2a). The dominant igneous rocks exposed in the ore district are gabbro, quartz porphyry, and numerous NEand NW-trending diorite and granite dikes (Fig. 2a). Their U-Pb ages, and crosscutting relationships with mineralization are summarized in Table 2. Gabbro is the main igneous rock, accounting for > 20% of the surface outcrop across the mine area. The gabbro pluton has a length of 1 km, a width of ~ 300 m, and strikes 300° (Fig. 2a). It displays a typical gabbroic texture and is composed of plagioclase (~ 55–60%), pyroxene (~ 30%), hornblende (~ 5%), and biotite (~ 5%) (Wang et al. 2013). The zircon U-Pb age is 133.2 ± 0.9 Ma (Wang 2014). Several base metal (Cu-Pb-Zn) veins are hosted by the gabbro (Fig. 2a). Quartz porphyry is present below surface mainly in the western part of the ore district with a small occurrence in the southeast (Fig. 2a, b). These rocks consist of quartz (~ 45–50%), K-feldspar (~ 40–45%), plagioclase (~ 5– 10%), and minor biotite with a zircon U-Pb age of 140.2 ± 1.2 Ma (Wang 2014). Contact relationships between gabbro and quartz porphyry are not clear (Fig. 2a). Numerous N-E- and rare N-S-trending, dioritic to granitic sills and dikes intruded Permian slate and the main gabbro body (Fig. 2a). These dikes have variable widths, up to 10 m, and U-Pb zircon ages between 129.6 ± 0.4 and 130.5 ± 0.8 Ma (Wang 2014; Wang et al. 2016). Regionally, emplacement of igneous rocks appears principally controlled by NE-NNE-oriented faults (Fig. 1c). Within the orefield, however, the Bianjiadayuan Ag-Pb-Zn veins are structurally controlled by NW-oriented faults (Fig. 2a). Those faults and fractures, with dip angles of 65–80°, normally have lengths of 220–600 m, and widths of 1–10 m. Open
Plate subduction, lithospheric delamination, and extension Wu et al. 2003, 2005; Wang et al. 2006; Wei et al. 2008; Zhang et al. 2008, 2010
faults and fractures are the most favorable structures for Ag-Pb-Zn veins. The major faults cut Permian slate, gabbro, and quartz porphyry (Fig. 2a). The Bianjiadayuan Ag-Pb-Zn vein system is primarily hosted in Permian slate (Fig. 2a, c). More than 20 major veins, numerous stockworks, and minor disseminations have been identified during mine-scale exploration. The Ag-Pb-Zn ores are mainly hosted by N-W and N-E trending open-space filling veins with a length between 50 and ~ 200 m, and extend vertically at least as much as 300 m. Their maximum widths vary from 25 to ~ 50 m, with an average of ~ 9 m. The veins are generally gently-dipping (25–45°) (Fig. 2c), although some dip more steeply (~ 75°) (Fig. 3a). Numerous stockworks, veinlets, and disseminations occur on the margins of the major veins in altered slate (Fig. 3b, c), and some shallow mineralized veins are oxidized at outcrop (Fig. 3a). Exploration identified Sn ± Cu ± Mo mineralization within the porphyry intrusion, and Sn-Pb-Zn mineralization in breccia above the porphyry, both of which are located in the western part of the main Ag-Pb-Zn veins (Fig. 2a). The concealed porphyry type mineralization mainly occurs as veins, stockworks, veinlets, and disseminations in altered porphyry. The breccia is spatially related to the quartz porphyry intrusion (Fig. 2b), and dominantly comprises fragments of slate and quartz porphyry with a cement mainly composed of fine rock fragments and sulfides. Formation of the breccia was considered to relate to a deep concealed quartz porphyry (Wang et al. 2014d). Relationships between Ag-Pb-Zn mineralization and quartz porphyry are also poorly constrained due to absence of crosscutting relationships. However, the Ag-Pb-Zn veins occur adjacent, within a few hundred meters, to porphyry-type mineralization (Fig. 2a). Underground mining identified several Cu-Pb-Zn veins within the gabbro intrusion (Fig. 2a), although this type of mineralization is relatively small in scale compared to the Ag-Pb-Zn veins hosted in slate (Fig. 3c). Hydrothermal alteration is widespread at Bianjiadayuan, with the most intense alteration occurring in and around
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Fig. 2 Simplified geologic map (a) and representative cross-sections (b, c) of the Bianjiadayuan ore deposit
mineralized Ag-Pb-Zn veins. Key components of alteration assemblages are quartz, sericite, chlorite, and epidote. Distinct episodes of alteration are recognized: an early episode of silicification, two intermediate episodes (propylitic and phyllic), and a late argillic episode (Fig. 4). Alteration halos are distributed asymmetrically on either side of the mineralized veins, and are typically widest around the thickest veins as discontinuous borders and envelopes. Silicification is the most widespread expression of alteration type in the deposit, as fine silica within silicified slate (Fig. 3c, d). Silicification predates the main sulfide mineralization and coexists with early-formed sulfides (arsenopyrite and pyrite). Early alteration and mineralization were followed by propylitic alteration, characterized by an assemblage of quartz, chlorite, epidote, and minor sericite and fluorite. Propylitic alteration is spatially associated with deposition of the main sulfide assemblage (galena, sphalerite, chalcopyrite, and pyrrhotite; Fig.
Table 2
3b). Silicification and propylitic alteration are both overprinted by phyllic alteration, which consists of quartz, sericite (Fig. 3d), and subordinate chlorite and epidote. This alteration appears closely related to silver deposition. The final stage of alteration is argillic alteration, expressed as an assemblage of illite and calcite, which overprinted all previous alteration types and occurs predominately along fractures (Fig. 3d). Argillic alteration postdates formation of the main sulfides and silver minerals, but appears to be associated with the formation of subordinate boulangerite. There is no apparent spatial zonation of alteration types as, in most places, alteration assemblages are superimposed upon one another. Alteration types for porphyry type mineralization involve an early potassic, and subsequent phyllic and prophylitic alterations. Hydrothermal alteration associated with gabbro-hosted Cu-Pb-Zn veins is distinct and composed of an assemblage of quartz, chlorite, and calcite.
Summary of various igneous rock types in the Bianjiadayuan Ag-Pb-Zn deposit
Rock types
Quartz porphyry
Gabbro
Diorite-granite
Occurrence Age Crosscutting relationship
Intrusion 140.2 ± 1.2 Ma Host porphyry Cu-Mo-Sn mineralization
Intrusion 133.2 ± 0.9 Ma Host Cu-Pb-Zn veins and cut by diorite-granite dikes
Sill and dike 129.6 ± 0.4 to 130.5 ± 0.8 Ma Cut both Ag-Pb-Zn veins and gabbro
Miner Deposita Fig. 3 Representative mineralization and ore types of the Bianjiadayuan Ag-Pb-Zn deposit. a Oxidized mineralized veins hosted by slate (outcrop). b Galena-sphalerite-quartz vein mineralization associated with chlorite and epidote alteration. c Ag-Pb-Zn vein accompanied by silification; stockworks and veinlets occur on the margins of major veins. d Chalcopyrite-bearing vein associated with quartzsericite alteration assemblage in slate, which is crosscut by a carbonate vein. e Galena and Agbearing sulfosalts replacing earlyformed pyrrhotite. Cal calcite, Ccp chalcopyrite, Gn galena, Po pyrrhotite, Qz quartz, Sp sphalerite
Fig. 4 Summary of the paragenetic sequence for the Bianjiadayuan Ag-Pb-Zn mineralization
Miner Deposita Fig. 5 Photomicrographs of ore mineral textures and assemblages from the Bianjiadayuan deposit. a Euhedral arsenopyrite coexisting with quartz and replaced by late stage boulangerite. b Pyrrhotite replaces or cements early-formed pyrite and arsenopyrite. c, d Pyrrhotite replaces or cuts galena and sphalerite, respectively. e Freibergite occurs as cuspateshaped and irregular inclusions within galena. f, g Freibergite is spatially associated with galena, sphalerite, pyrrhotite, and chalcopyrite. h Boulangerite occurs as needle-like, columnar, and euhedral crystals coexisting with calcite. All images were taken in reflected light. Apy arsenopyrite, Cal calcite, Ccp chalcopyrite, Blg boulangerite, Frb freibergite, Gn galena, Po pyrrhotite, Py pyrite, Qz quartz, Sp sphalerite
Sampling and analytical methods Over 60 samples were collected from different mining levels and drill holes. More than 120 polished thin sections were examined in reflected and transmitted light. Mineral compositions of sulfides and sulfosalts were determined by wavelength-dispersive analysis using the CAMECA SX50 electron microprobe at Indiana University (Bloomington, USA), and a JEOL 8230 Superprobe equipped with wavelength- and energy-dispersive X-ray detectors and back-scatter electron detector at the Microprobe Center, Chinese Academy of Geological Sciences (CAGS), Beijing, China. The accelerating voltage was 20 kV for both instruments. Beam current and count times for major elements were 20 nA and 20 s, respectively (10 s for background measurement), and beam size was 1 μm. Natural and synthetic mineral standards of chalcopyrite, sphalerite, galena, pyrrhotite, InAs, and native Ag, Sb, Sn were utilized for calibration. X-ray lines measured were Ag Lα, Sb Lα, As Lα, Pb Lα, Zn Lα, Sn Lα, Fe Kα, Cu Kα, and S Kα. X-ray element mapping was performed using an energy-dispersive X-ray detector on the JEOL 8230 Superprobe at CAGS. The dwell time was set to 20 μs to provide the highest possible resolution. Field emission scanning electronic microscopy (FESEM) of Zeiss Supra 55 Sapphire at China University of Geosciences Beijing was
utilized for micro-scale element line scanning and mapping. Element line scanning was undertaken for Ag Lα, Sb Lα, Pb Mα, Fe Kα, Cu Kα, and S Kα using an accelerating voltage of 20 kV and working distance of 15 mm. An in-lens detector for secondary electron imaging was used. Fifteen samples for sulfur isotope analysis were collected from different mining depths and drill cores, and are representative of different types of sulfide mineralization in the Ag-PbZn veins. Sulfides were drilled from polished blocks under microscopic examination using a 0.3 mm carbide drill bit. Approximately 0.1 mg of sulfide sample powder was loaded in a tin cup with 1 to 1.5 mg of V2O5 and combusted in an elemental analyzer to produce SO2. Samples were analyzed by continuous flow in a Finnigan Delta V stable isotope ratio mass spectrometer at Indiana University (Bloomington, USA), following Ripley et al. (2010). Reference materials NBS-127, IAEA S-1, IAEA S-2, and IAEA S-3, with δ34S values of + 20.3‰, − 0.3‰, + 21.7‰, and − 31.3‰, respectively, were used as standards. All sulfur isotopic data are reported relative to VCDT in standard δ notation. Analytical precision was ± 0.05‰ and sample reproducibility was within ± 0.2‰ (2σ). Lead isotope compositions of sulfides were determined at CAGS. Analysis was carried out using an England Nu Plasma High Resolution type MC-ICP-MS, employing a NBS 981
Miner Deposita Fig. 6 BSE images of sulfosalt minerals from the Bianjiadayuan deposit. a Freibergite intergrown with galena, and relative bright and dark parts in freibergite are associated with high and low Ag contents as indicated by EPMA data. b Boulangerite occurs as intergrowths with galena c Freibergite is associated with galena, sphalerite, pyrrhotite, and chalcopyrite. d Freibergite occurs as irregular blebs at galena grain boundaries or as fine inclusions in galena. e Pyrargyrite intergrown with galena and pyrrhotite. f Boulangerite occurs as needlelike and dotted inclusions in freibergite or as anhedral grains with sphalerite and quartz. Apy arsenopyrite, Ccp chalcopyrite, Blg boulangerite, Frb freibergite, Gn galena, Po pyrrhotite, Py pyrite, Pyr pyrargyrite, Qz quartz, Sp sphalerite
standard. Long-term repeated measurement of lead isotopic ratios of standard NBS 981 yielded 206Pb/204Pb = 16.9397 ± 0.0111, 207Pb/204Pb = 15.4974 ± 0.0089, and 208Pb/204Pb = 36.7147 ± 0.0262 (± 2σ).
Results Ore mineralogy and paragenesis More than 20 ore and gangue minerals occur in the main mineralization stage and a subsequent supergene oxidation stage, based on the nature of the mineralization and mineral assemblages (Fig. 4). Paragenetic sequences in the mineralization stage demonstrate element associations changing from Fe-As-S, through Pb-Zn-Fe-Cu-S and Ag-Pb-Zn-Sb-S, to PbSb-S from early to late. The early ore mineral assemblage (sequence I) is dominated by arsenopyrite and pyrite, which commonly form coarse, euhedral grains, showing rhombic and cubic habit, respectively (Fig. 5a), regularly coexist with quartz, and are typically crosscut and replaced by chalcopyrite, sphalerite, and boulangerite (Fig. 5a).
The successive ore assemblage in sequence II dominantly comprises intergrowths of galena, sphalerite, chalcopyrite, pyrrhotite, pyrite, and minor stannite. Sulfides are accompanied by quartz, chlorite, epidote, sericite, and locally fluorite (Fig. 4). Most sulfide-bearing veins contain intergrowths of galena, sphalerite, chalcopyrite, and pyrrhotite (Fig. 3b, c), although some are essentially monomineralic, e.g., chalcopyrite (Fig. 3d) and sphalerite veins in altered slate. Pyrrhotite in mineralized veins commonly replaces pyrite (Fig. 5b), and is seen corroding, to variable degrees, both galena (Fig. 5c) and sphalerite (Fig. 5d). Overall, the veins are sulfide-dominant, resulting in massive ores within the largest veins (Fig. 3b, c). The sulfides are commonly crosscut and replaced by freibergite and boulangerite (Fig. 5e). The next ore mineral assemblage in sequence III is dominated by silver-bearing sulfosalts, accompanied by relatively low volumes of sulfides. Sulfosalt species include freibergite, pyrargyrite, and boulangerite, which commonly coexist with galena, replacing early formed chalcopyrite, sphalerite, and pyrrhotite (Fig. 3e). Freibergite is the most abundant and widely distributed Ag-bearing sulfosalt in the deposit, making up roughly 90 vol.% of the total Ag-bearing ores. Freibergite
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Fig. 7 Representative metallic element variations in heterogeneous freibergite from the Bianjiadayuan deposit. a, b Micro-scale EDS element line scanning and mapping of heterogeneous freibergite. Silver and copper contents vary inversely in the bright and dark domains in freibergite grains based on element mapping and line scanning. c, d Metallic element ratio plots on the basis of EPMA data for heterogeneous freibergite
is commonly associated with sulfides, particularly galena. Most freibergite occurs as small cuspate-shaped and irregular inclusions within galena (Fig. 5e), or as independent intergrowths with galena (Figs. 5f, g and 6a), and as irregular blebs at the grain boundaries of galena (Fig. 6d). Drill hole geochemical data reveal that Ag correlates closely with Pb (ESM 1), further implying a close association between freibergite and galena at the deposit scale. Freibergite also coexists with sphalerite (Fig. 5g), chalcopyrite (Fig. 6c), pyrrhotite (Fig. 5f), and boulangerite (Fig. 6f). Pyrargyrite commonly occurs as intergrowths with galena, sphalerite, and pyrrhotite (Fig. 6e). Boulangerite usually occurs as needle-like, columnar, and dotted inclusions in chalcopyrite, sphalerite, and freibergite (Fig. 6f), or as intergrowths with galena (Fig. 6b). Minor amounts of galena, sphalerite, chalcopyrite, and pyrrhotite coexist with the sulfosalts. Gangue minerals are quartz, sericite, and minor chlorite and epidote. The late paragenetic sequence IV only includes boulangerite, which mainly occurs as needle-like, columnar, and euhedral crystals (Fig. 5h) coexisting with calcite and
illite. This mineral assemblage occurs as late veins cutting all previous assemblages. The supergene stage includes anglesite, malachite, limonite, and hematite, which formed due to the oxidation of primary sulfides.
Chemical compositions of freibergite, pyrargyrite, and boulangerite Analytical results for sulfosalts and sulfides are listed in ESM 2. Calculated formulae for freibergite compare, within analyti c a l u n c e r t a i n t y, t o t h e i d e a l f a h l o r e f o r m u l a : (Ag,Cu)10(Fe,Zn)2(Sb,As)4S13. Measured compositions show significant variations of Ag and Cu contents, from 16.7 to 34.3 wt% Ag and 14.2 to 24.6 wt% Cu, which span the compositional range from argentian tetrahedrite to freibergite (ESM 2). Analyses also demonstrate significant variation in the Fe and Zn contents, from 4.2 to 7.1 wt%, and from 0.1 to 1.7 wt%, respectively. The calculated number of atoms per formula unit (based on Σapfu = 29) reveals Ag + Cu from 9.36 to 10.55, and Fe + Zn from 1.83 to 2.43. Sulfur and Sb
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Fig. 8 Reflected light (a), BSE image (b, c), and EPMA element mapping (d–i) of a heterogeneous freibergite mineral grain. Silver contents in (c) are based on EPMA results
contents in freibergite range from 13.75 to 12.40 apfu and 3.64 to 4.24 apfu, respectively. Good correlations between molar Ag and Cu (Fig. 7a–c), and between molar Fe and Zn (Fig. 7d) support substitution within the fahlore structure (Sack et al. 2003). Almost all analyzed grains are Sb-dominant, with only trace amounts of As present. There are also trace to minor amounts of Pb and Sn. One single microprobe spot showed the presence of argentian tennantite, with 6.5 wt% As, 10.7 wt% Ag, and 21.4 wt% Cu. The compositional data are consistent with the reported incompatibility of Ag in tennantite relative to tetrahedrite (Gallego Hernández and Akasaka 2010). Detailed EPMA and FESEM investigations demonstrate that although some freibergite grains are relatively homogeneous compositionally (Fig. 6c), a large proportion of them are remarkably heterogeneous (ESM 3 and Fig. 8). In back-scattered electron (BSE) images, individual freibergite grains display bright and dark areas (Figs. 6a, 7a, b, and 8c). This is consistent with element mapping of individual grains, the bright parts corresponding to higher Ag-contents and the dark parts with relatively low Ag-contents (Fig. 8). The (brighter) Agrich freibergites are commonly Zn-rich relative to low Ag freibergites (Fig. 7d). Interestingly, no spatial zoning of freibergite compositions across different veins and
depths was observed. Instead, a conspicuous relationship between Ag/(Ag + Cu) ratios in freibergite and the abundance of associated galena was commonly identified (Fig. 3e). The second most common Ag-bearing mineral is pyrargyrite, which showed an almost constant chemical composition (54.5 to 56.0 wt% Ag, 22.8 to 23.4 wt% Sb, 19.0 to 20.2 wt% S; ESM 2). Pyrargyrite is characterized by minor As (0.1–0.3 wt%), Fe (0.4–1.1 wt%), Zn, Pb, and Cu. The calculated formula for pyrargyrite is almost identical to ideal stoichiometry, Ag3SbS3. Boulangerite is characterized by a relatively constant chemical composition (52.5 to 56.0 wt% Pb, 25.0 to 27.8 wt% Sb, and 18.2 to 19.9 wt% S; ESM 2). The mineral contains measurable As, Cu, and Fe. Calculated formulae closely resemble ideal stoichiometry, Pb5Sb4S11.
Sulfur isotope compositions A total of 38 micro-scale sulfur isotope analyses were performed on sulfide minerals in polished blocks from 15 samples (Table 3 and ESM 4). Sulfides analyzed include galena, pyrrhotite, sphalerite, pyrite, chalcopyrite, and marcasite, and span both the paragenesis and the vertical extent of the deposit. The majority of selected sulfides are, however, from the main sulfide sequence, in which sulfides are most abundant.
Miner Deposita Table 3 Sulfur isotope compositions of sulfides from the Bianjiadayuan Ag-Pb-Zn deposit
Sample
Elevation (m)
Ore type
1
BJ52-1
735
Massive Gn-Py-Po ore
Gn
0.57
2 3
BJ52-2 BJ52-3
735 735
Py Po
1.70 1.64
4
BJ52-4
735
Gn
0.57
5
BJ52-5
735
Po
1.87
6 7
BJ52-6 BJ52-7
735 735
Gn Sp
0.92 2.43
8
BJ52-8
735
Po
1.95
9 10
BJ5-1-1 BJ5-1-2
775 775
Gn Po
0.71 1.98
11 12
BJ5-1-3 BJ5-1-4
775 775
Gn Sp
0.69 2.53
13
BJ04-1
775
Vein type Py-Sp ore
Py
2.15
14 15
BJ04-2 BJ86-1
775 730
Vein type Gn-Sp-Po-Ccp-Qz ore
Sp Sp
2.53 2.62
16 17 18
BJ86-2 BJ86-3 BJ86-4
730 730 730
Ccp Gn Po
2.65 0.58 2.10
19 20 21 22 23
BJ86-5 BJ86-6 BJ12-45-1 BJ12-45-2 BJ12-45-3
730 730 735 735 735
Sp Gn Gn Po Ccp
2.47 0.85 − 0.70 1.23 0.91
24 25 26 27
BJ12-45-4 BJ4-2-1 BJ4-2-2 BJ4-2-10
735 710 710 710
Po Py Gn Mrc
1.04 1.91 0.74 2.19
28 29 30 31 32
BJ4-2-3 BJ4-2-4 BJ4-2-5 BJ4-2-6 BJ4-2-7
710 710 710 710 710
Ccp Gn Sp Sp Gn
1.73 1.19 2.40 0.06 0.89
33 34 35 36 37 38
BJ4-2-8 BJ4-2-9 BJ1-1 BJ1-2 BJ1-3 BJ1-4
710 710 770 770 770 770
Gn Mrc Gn Sp Po Sp
0.83 2.06 1.30 2.62 2.31 2.70
Massive Gn-Sp-Po ore
Vein type Po-Ccp-Gn-Sp ore
Massive Mrc-Gn-Sp-Py-Ccp ore
Vein type Sp-Gn-Po-Qz ore
Sulfide
δ34SV-CDT (‰)
Number
Ccp chalcopyrite, Gn galena, Mrc marcasite, Py pyrite, Qz quartz, Sp sphalerite, Po pyrrhotite
The overall sulfur isotope dataset show that the sulfides have a relatively uniform δ34S composition (Fig. 9), which range from − 0.7 to + 2.7‰ with a mean of + 1.6‰. It is significant that a relatively small but similar variation of sulfur isotopes is identified both in sulfide ores from different vertical levels, and at the mm-scale in multiple mineral pairs from the same polished block (ESM 4). In general, sphalerite (mean = + 2.3‰), pyrrhotite (mean = + 1.8‰), pyrite (mean = + 1.9‰), chalcopyrite (mean = + 1.8‰), and marcasite (mean
= + 2.1‰) demonstrate slightly higher δ34S values compared to galena (mean = + 0.7‰) (Table 3 and Fig. 9).
Lead isotope compositions Lead isotope compositions were determined from 15 sulfides (galena, sphalerite, pyrrhotite, chalcopyrite, and arsenopyrite) from different mining levels (Table 4). Sulfides from the Bianjiadayuan deposit have similar lead
Miner Deposita
Fig. 9 Histogram of sulfur isotope compositions of sulfides with a typical textual equilibrium sulfide pair
isotope values. For example, the Pb isotope ratios of sphalerite, chalcopyrite, pyrrhotite, and arsenopyrite are within the range of Pb isotope values of galena. In detail, sulfides with different ore textures and from different mining levels and paragenetic sequences also show essentially similar Pb isotopes. The data show that 206 Pb/204Pb, 207Pb/ 204Pb, and 208Pb/204Pb ratios range from 18.155 to 18.372, 15.518 to 15.797, and 38.100 to 39.008, respectively. Table 4
Discussion Sulfur and lead sources The δ 34 S values of sulfides from Ag-Pb-Zn veins at Bianjiadayuan display a narrow range (− 0.7 to + 2.7‰), with only minor variation with respect to either vertical location of the veins or from grain to grain within a sample (ESM 4). This indicates a single, homogeneous sulfur
Lead isotope compositions of sulfides from the Bianjiadayuan Ag-Pb-Zn deposit
Number
Sample
Elevation
Description
Sulfides
206/204
Pb
1sigma
207/204
Pb
1sigma
208/204
Pb
1sigma
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
BJ-15-1 BJ-50 BJ-52-1 BJ-12-19 BJ-12–40 BJ-08 BJ-52-2 BJ-08-2 BJ-01 BJ-54 BJ-15-2 BJ-12-05 BJ-12-31 BJ-04 BJ-15-3
775 m 735 m 735 m 775 m 730 m 735 m 735 m 725 m 770 m 735 m 775 m 725 m 775 m 710 m 775 m
Ag-rich vein type Pb ore Massive Pb-Zn ore Massive Cu-Pb-Zn ore Vein type Pb-Zn ore Massive Pb-Zn ore Vein type Pb-Zn ore Cu-Pb-Zn ore Vein type Pb-Zn ore Massive Pb-Zn ore Cu-Pb-Zn ore Ag-rich vein type ore Massive Pb ore Massive Pb-Zn ore Massive Cu-Pb-Zn ore Ag-rich vein type Pb ore
Galena Galena Galena Galena Galena Galena Sphalerite Sphalerite Sphalerite Sphalerite Pyrrhotite Pyrrhotite Arsenopyrite Arsenopyrite Chalcopyrite
18.1740 18.1550 18.2180 18.2530 18.2080 18.1625 18.1560 18.1612 18.1756 18.226 18.1940 18.2840 18.2130 18.1772 18.2700
0.0010 0.0010 0.0020 0.0020 0.0020 0.0007 0.0020 0.0009 0.0008 0.0020 0.0020 0.0020 0.0020 0.0011 0.0020
15.5460 15.5210 15.6030 15.6440 15.5880 15.5595 15.5180 15.5586 15.5570 15.6140 15.5660 15.6810 15.5940 15.5586 15.6670
0.0010 0.0010 0.0020 0.0020 0.0020 0.0007 0.0010 0.0008 0.0007 0.0020 0.0020 0.0020 0.0030 0.0008 0.0030
38.1890 38.1090 38.3700 38.5010 38.3220 38.2198 38.1000 38.2177 38.2056 38.4110 38.2540 38.6340 38.3750 38.2100 38.5940
0.0030 0.0030 0.0050 0.0060 0.0040 0.0017 0.0030 0.0022 0.0018 0.0050 0.0040 0.0050 0.0070 0.0020 0.0070
Miner Deposita Fig. 10 Lead isotope plots of the Bianjiadayuan sulfides compared to local magmatic-hydrothermal deposits and different rock units. A large dataset (n = 188) has been collected for comparisons (ESM 5). Those data include local Jurassic to Cretaceous granite (Linxi pluton, ~ 20 km from Bianjiadayuan), Cretaceous andesite and basalt, Permian tuff and marble, Hercynian granite, and sulfides from several nearby magmatic-hydrothermal (porphyry and skarn) deposits. Note Pb isotope compositions of the studied deposit show similarities to the local Jurassic to Cretaceous granite, with evident distinctions from other ore deposits and rocks on both 206Pb/204Pb vs. 207 Pb/204Pb (a) and 206Pb/204Pb vs. 208Pb/204Pb (b) plots. The Pb isotope curves for the mantle, orogene, and crust are taken from Zartman and Doe (1981)
source, most likely magmatic. Such an interpretation is consistent with previous studies considering that a narrow range of δ34S values near zero (0 ± 2‰) is best attributed to a single magmatic sulfur source (Seal 2006), rather than by homogenization of sulfur from country rocks with diverse isotopic compositions. The S isotope data for sulfides in the Bianjiadayuan deposit are very similar to sulfur isotopic data for sulfides in nearby hydrothermal ore deposits (e.g., Hashitu Mo, Baiyinnuoer Pb-Zn, Dajing Ag-Pb-Zn-Cu, Haobugao Pb-Zn-Cu, Huanggangliang FeSn, and Bairendaba Ag-Pb-Zn deposits), which generally yield uniform δ 34 S values of − 2 to + 2‰ (Shao and Zhang 2001; Yao et al. 2010; Zhai et al. 2014b).
Lead isotopes of the main sulfides (galena, sphalerite, and pyrrhotite) are compared with data for local Jurassic to Cretaceous granite, Cretaceous andesite, gabbro and basalt, and Permian strata, as well as with Pb isotope data for sulfides from several nearby porphyry and skarn type deposits (ESM 5). Based on 206Pb/204Pb vs. 207Pb/204Pb, and 206Pb/204Pb vs. 208Pb/204Pb plots, the Pb isotope ratios for the Bianjiadayuan ores are typically comparable to local Jurassic-Cretaceous granite and Cretaceous gabbrobasalt compositions (Fig. 10), although a proportion of the Pb isotope data shows relatively more radiogenic 207 Pb/204Pb and 208Pb/204Pb ratios. The local quartz porphyry is suggested to contribute large amounts of metals
Miner Deposita
have affected the Pb isotope composition. This is consistent with regional Pb isotope mapping, which showed that different magmas in the region experienced variable degrees of crustal contamination that led to distinct radiogenic Pb ratios, and the Pb isotopic provinces in terms of different isotope ratios at the regional scale (Guo et al. 2010). The 206Pb/204Pb, 207Pb/204Pb plots show that possible sources of Pb in the Bianjiadayuan sulfides could have been derived from both the crust and mantle (Fig. 10), suggesting a complex Pb source.
Temperatures of ore formation
Fig. 11 Molar Ag/(Ag + Cu) and Zn/(Zn + Fe) ratios of primary freibergite in the Bianjiadayuan deposit. The isotherms are calculated on the basis of Sack (2005). These isotherms are terminated at low Zn/ (Zn + Fe) and high Ag/(Ag + Cu) ratios as a consequence of saturation with respect to pyrrhotite (Po) (Balabin and Sack 2000)
for mineralization, which is supported by Ag-Pb-Zn veins near porphyry and breccia, alteration and metal zonation (Cu-Mo-Sn, Sn-Zn to Ag-Pb-Zn) (Wang et al. 2014d), fluid inclusions (Ruan et al. 2015), stable isotope (D-OC-S) data (Wang et al. 2014c; Ruan et al. 2015), and geochronological constraints (sericite 40Ar/39Ar age of 138.7 ± 1.0 Ma for Ag-Pb-Zn vein mineralization and molybdenite Re-Os age of 140.0 ± 1.7 Ma for porphyry mineralization, Zhai et al. 2017). Shown in the Pb isotope plots (Fig. 10), sulfides and granites have similar Pb isotope systematics, indicating a most likely metal source from Jurassic to Cretaceous granite. Such a conclusion is consistent with previous geochronological, stable isotope (S, H, C, O), and fluid inclusion studies of ore deposits in the region (Zeng et al. 2009; Zhang et al. 2009; Shu et al. 2013, 2016; Ouyang et al. 2014; Zhai et al. 2014b). However, the Bianjiadayuan ores are distinguished from sulfides from other magmatic-hydrothermal deposits with respect to their Pb isotope compositions (Fig. 10), suggesting a distinct metal source for the Bianjiadayuan deposit. In detail, samples collected from different geographic locations reveal different Pb isotope ratios where deposit located in the southern part displays relatively low 206/ 204 Pb ratios, whereas ore deposits in the northern part show relatively high 206/204Pb ratios. One hypothesis is that fertile magma in the region may have experienced different processes, and possibly with distinct origins, which would
Published fluid inclusion microthermometric data reveal that homogenization temperatures for main sulfide mineralization at Bianjiadayuan show a relatively narrow range between ~ 250 and 275 °C (Ruan et al. 2015). Fluid inclusion types are dominated by two-phase liquid-vapor inclusions (L-V), and monophase vapor or liquid aqueous inclusions (type V or L) without CO2-rich or daughter mineral three-phase fluid inclusions (Wang 2014; Ruan et al. 2015). Fluid salinities for formation of main sulfides range from 5.3 to 9.7 wt% NaCl equivalent, and the ore fluid responsible for mineralization was characterized by a boiling/phase separation process, and subsequent mixing with meteoric water during the waning stage (Wang 2014; Ruan et al. 2015). To constrain temperatures for main silver mineralization, we have applied the thermodynamic database for the system Ag2S-Cu2S-ZnS-FeS-Sb2S3-As2S3 and mineral compositions to calculate crystallization temperatures for freibergite (Sack 2005). Several freibergite grains, which have not undergone solid-state reaction/post-mineralization process, were selected as the primary phase to constrain their formation temperature, as suggested by Sack et al. (2002). Isotherms based on Ag/(Ag + Cu) and Zn/(Zn + Fe) ratios in the selected freibergites suggest they formed at 160 to 250 °C (Fig. 11), comparable with fluid inclusion data for the same assemblage (Ruan et al. 2015). This is also consistent with the calculated equilibration temperature for the Fe-Zn exchange reaction between freibergite and sphalerite, which ceased at ~ 176 °C (Sack et al. 2002). Finally, fluid inclusion homogenization temperatures for late calcite range from 130 to 170 °C, averaging ~ 150 °C (Wang 2014), suggesting cooler fluids as the hydrothermal system waned. In summary, based on previously fluid inclusion data and the new mineral geothermometric data, temperatures are interpreted to have evolved from ~ 250–275 °C for the main sulfide formation to ~ 160–250 °C for the Agdominant sulfosalt formation, and then < 150 °C for the final calcite formation, demonstrating gradual cooling during mineral deposition.
Miner Deposita
Fig. 12 A logfS2 versus temperature diagram showing the relative sulfidation state and the evolutionary path of hydrothermal fluids in the Bianjiadayuan deposit. Temperatures were estimated from fluid inclusion and sulfur isotope studies, and logfS2 values from equilibrium mineral assemblages. Sulfidation state determinations are from Einaudi et al. (2003); sulfidation reactions are from Barton and Skinner (1979) and Myers and Eugster (1983)
(both average temperature for each mineral assemblage), respectively. For the purposes of estimating stability relationships among the minerals, each was considered to be ideal solid solution. On the constructed T-fS2 phase diagram (Fig. 12), the Bianjiadayuan ore assemblage belongs to the intermediate-low sulfidation type, within the stability fields of pyrite, chalcopyrite, and tetrahedrite at ~ 265 °C for the main sulfide formation, and within the fields of the freibergite-pyrargyrite-pyrrhotite assemblage at ~ 200 °C for the Ag-sulfosalt formation (Fig. 12). Evolution of ore deposition in the Bianjiadayuan veins therefore was initiated at moderate fS2 conditions, but progressively evolved to a lower fS2 state, coeval with deposition of silver sulfosalts, such as freibergite and pyrargyrite. The constrained pH range for Ag-sulfosalt deposition varies from 5.1 to 7.0, and the logfO2 values range from − 46.1 to − 40.7 (Fig. 13). The above conditions support an interpretation in which the silver bisulfide species (AgHS°) was the sole species responsible for transport.
Mechanisms of Ag-rich ore formation Silver, like gold, occurs dominantly as Ag+ in hydrothermal fluids (Williams-Jones and Migdisov 2014), and speciation for silver transport in hydrothermal fluids is dominated by bisulfide and chloride species (Stefansson and Seward 2003). Experimental studies demonstrate that, at lower temperatures (< 200 °C), silver bisulfide species (AgHS°) predominate at near-neutral conditions, whereas silver chloride species (AgCl2 − ) are dominant under mildly acidic to acidic conditions (e.g., Seward 1976; Gammons and Barnes 1989). In contrast, at relatively high temperatures (> 400 °C), silver is transported dominantly as a chloride complex in most hydrothermal fluids, whereas silver bisulfide species are only stable at mildly basic to basic conditions (e.g., Gammons and Williams-Jones 1995; Stefansson and Seward 2003). Although there has been some debate over the capacity of hydrothermal fluids to transport metals (WilliamsJones and Heinrich 2005), the recent experimental studies of Migdisov and Williams-Jones (2013), Migdisov et al. (2014), and Hurtig and Williams-Jones (2014a, b) have shown that Ag and other metals (e.g., Cu, Au, and Mo) can all be transported in appreciable concentrations by vapors. Physicochemical conditions during precipitation of ore and alteration minerals were estimated from stability relationships among sulfides and silicates using SUPCRT92 (Johnson et al. 1992). As discussed above, the main sulfide and Ag-dominant sulfosalt formation is estimated to have taken place at temperatures of ~ 265 and ~ 200 °C
Fig. 13 A logfO2-pH diagram showing the speciation and solubility of silver in an aqueous fluid containing 1 m NaCl and 0.001 m ΣS at 200 °C and 500 bars, overlain on predominance boundaries of aqueous sulfur species (pink dashed) and mineral stabilities in the Fe–Cu–S–O–H system (black solid). The silver solubility contours were marked with blue dashed lines. The kaolinite–sericite and K–feldspar–sericite stability (black dashed), and barite and calcite solubility lines (gray dashed) were also constructed, assuming in equilibrium with a solution containing 0.001 m K+, 0.001 m Ba2+, and 0.1 m Ca2+. The constrained yellow zone is the stability field for chalcopyrite, and the defined blue part represents approximate stability field for Ag-sulfosalt minerals. The arrow direction represents the evolved pathway for minerals formed in later paragenetic sequence. Calculated using SUPCRT92 (Johnson et al. 1992) and thermodynamic data by Gammons and Williams-Jones (1995), Stefansson and Seward (2003), Akinfiev and Zotov (2001, 2010), and Williams-Jones and Migdisov (2014)
Miner Deposita
As the hydrothermal fluid evolved to lower temperature, the hydrothermal system transformed to decreasing logfO2 conditions (Fig. 13), which significantly reduced AgHS° solubility in the fluid, favoring silver precipitation. Such physicochemical evolution is consistent with the variation in mineral assemblage from Ag-Pb-Zn-Sb-S to Pb-Sb-S associations, e.g., chalcopyrite and pyrite prior to pyrrhotite, and deposition of late-stage calcite. We thus conclude that decrease in temperature, logfO2 , and logfS2 of the hydrothermal fluid were important triggers for silver ore deposition.
Ore mineralogical and textural implications The composition of fahlore, the most important and widespread Ag-host in the Bianjiadayuan ores, corresponds to both argentian tetrahedrite and freibergite. Freibergite is more abundant in the Bianjiadayuan ores but both highand low-Ag components in different grains, and within single mineral grains (Figs. 6a and 8). The existence of compositionally heterogeneous domains in natural freibergite has been rarely reported. Barroso et al. (2003) considered that tetrahedrites that are compositionally zoned with respect to Ag were related to a late influx of Ag-rich fluids coeval with the late stages of freibergite growth. Sack et al. (2003), on the other hand, reported that freibergite with heterogeneous Ag was originally a single grain that exsolved into regions of Ag-rich and Ag-poor compositions. Sack et al. (2003) also suggested that this process resulted in exsolution on remarkably fine scales. The Bianjiadayuan freibergite (Figs. 6a and 8) displays similarities with those reported by Sack et al. (2003) and are distinct from those characterized by Barroso et al. (2003). We, therefore, speculate that the freibergites are attributable to a series of retrograde reactions which result in diverse decomposition and exsolution of primary phases during cooling. Freibergite likely initially crystallized as single grains partially encased in, or surrounded by galena, pyrrhotite, and sphalerite (Fig. 5f, g), and then decomposed into Agrich and Ag-poor domains (Figs. 6a, 7a, b, and 8). Additionally, freibergite in the assemblage galena + boulangerite + pyrrhotite shows apparent Cu ↔ Ag and Zn ↔ Fe variations both among different minerals and individual mineral grain (Fig. 7), respectively, also suggesting the possibility of solid-state reactions during cooling. The evidence for existence of a close association between silver-bearing minerals and galena (ESM 1) possibly reflects a genetic model involving a Agrich galena precursor, then breaking down to a mixture of Ag-poor galena (PbS) and Ag-rich sulfosalts, such as freibergite. This is consistent with empirical studies
(Lueth et al. 2000; Chutas et al. 2008; George et al. 2015), and the low Ag contents in galena (often below minimum limit of detection), probably due to subsequent Ag removal (ESM 2). Alternatively, the possibility also exists that Ag-sulfosalts are late compared to galena deposition, and reflect late stage mineralization at low temperature. Although the key factors critical for initial ore deposition have been evaluated, post-formational redistribution processes are also clearly important, and have impacted on the ore. Freibergite decomposition, as described here, has rarely been reported previously. Evidence of analogous processes may exist, albeit undocumented, in other silver deposits. Our findings, therefore, indicate the importance of potential post-mineralization processes for accurate modeling of silver ore formation in a wide variety of settings.
Genetic type and implications for exploration The classification of Ag-Pb-Zn vein-type mineralization is controversial and a matter of considerable debate. In a recent review, Kissin and Mango (2014) proposed that vein-type Ag-Pb-Zn deposits should be classified as Cordilleran-type vein deposits and Ag-Pb-Zn veins hosted in clastic metasedimentary rocks. The studied Ag-Pb-Zn veins are quite comparable to the Cordilleran-type vein deposits, i.e., sulfide-rich character (commonly massive), primary occurrence as open-space fillings, deposition mostly under epithermal conditions at shallow levels, hydrothermal fluids of moderate to low salinity and temperatures below 375 °C, and the close spatial, temporal, and genetic relationship with a porphyry-related system (e.g., Einaudi 1981; Baumgartner et al. 2009; Fontboté and Bendezú 2009 and references therein). However, the distinctions also exist, including (1) a metal suite of Cu-ZnPb-Au-Ag-(Bi-Sb) is common in the Cordilleran-type veins (Bendezú and Fontboté 2009), in which Cu is an important and Au is a common metallic constitution, whereas the studied veins contain Ag-Pb-Zn with only minor Cu and without Au, and (2) a common feature for the Cordilleran-type veins is that a well-developed metal and alteration zonation existed in deposit scale (e.g., zoned from inner Cu-bearing to outer Zn-Pb-bearing ores, Fontboté and Bendezú 2009; Bendezú and Fontboté 2009), though this has been suggested not a necessary character (Fontboté and Bendezú 2009); by contrast, there is no metal and alteration zonation in the Bianjiadayuan Ag-Pb-Zn mineralization. In an earlier review, Beaudoin and Sangster (1992), based on information from a large number of Ag-Pb-Zn ore districts, proposed that Ag-Pb-Zn veins hosted in clastic metasedimentary rocks be considered to form a distinct
Miner Deposita
class of ore deposits. The Bianjiadayuan deposit is similar in many respects to the Ag-Pb-Zn veins studied by Beaudoin and Sangster (1992). This similarity includes (1) the nature of the ore (e.g., galena, sphalerite, and a variety of Ag-bearing sulfosalts) and gangue (e.g., quartz and calcite) mineral assemblages, (2) the occurrence of the ore minerals in epigenetic veins associated spatially with sericitization and silicification, (3) an ore-fluid temperature of 220 to 300 °C and salinity of 5.3 to 9.7 wt% NaCl equivalent, (4) evidence for ore deposition in response to fluid mixing and boiling during the waning stages of a meteoric water-dominated hydrothermal system, (5) a magmatic source for the fluids (Wang 2014), (6) structural control of the mineralization by late orogenic regional faults, and (7) ore formation during crustal extension. However, the Bianjiadayuan veins differ in several important respects from the veins considered by Beaudoin and Sangster (1992). The most significant difference is the origin of the hydrothermal fluids and the metals. Whereas, the Ag-Pb-Zn veins hosted in clastic metasedimentary terranes typically show no spatial or genetic links with felsic intrusions, and their S and Pb isotopes suggest that the sulfur and metals originated from the country rocks (Beaudoin and Sangster 1992), the Bianjiadayuan Ag-Pb-Zn veins show a clear spatial relationship to the adjacent Sn ± Cu ± Mo mineralized intrusion. Moreover, the sulfur and lead isotope data presented in this study show that these elements are of magmatic origin. The geochronological results (e.g., zircon U-Pb, molybdenite Re-Os, and sericite 40Ar/39Ar data) indicate that the local granitic porphyry, its host Sn ± Cu ± Mo mineralization, and the Ag-Pb-Zn veins formed concurrently at 140–138 Ma (Zhai et al. 2017). Thus, it is reasonable to relate the Ag-Pb-Zn veins genetically to the adjacent porphyry Sn ± Cu ± Mo system. This interpretation is consistent with that of previous studies proposing a close genetic relationship between porphyry Cu-Mo and hydrothermal vein Ag-Pb-Zn systems in other ore districts (Valencia et al. 2008; Sillitoe 2010; Catchpole et al. 2015). The close genetic relationship between porphyry Sn ± Cu ± Mo mineralization and Ag-Pb-Zn veins at Bianjiadayuan provides an important guide for mineral exploration in the region.
Conclusions The important conclusions from this study can be summarized as follows: (1) Sulfur and lead isotopes indicate the origin of oreforming fluids and metals from a felsic magma.
(2) Decreases in temperature, logfO2, and logfS2 of the oreforming fluid were likely the key triggers for silver ore deposition. (3) Mineralogical and textural evidence indicate that freibergite is the most important silver host, and largely derives from decomposition and exsolution processes associated with retrograde and solid-state reactions during cooling. (4) Compositional heterogeneity in freibergite down to the sub-micron scale demonstrates that post-ore processes are important to understand silver ore formation in diverse geological settings. (5) The studied Ag-Pb-Zn veins are genetically related to the adjacent Sn ± Cu ± Mo mineralized intrusion. Acknowledgements We thank Ed Ripley and Ben Underwood (Indiana University, Bloomington) for the sulfur isotope analyses, Chusi Li (Indiana University, Bloomington) and Zhenyu Chen (CAGS) for the EPMA analyses, and Dongjie Tang (CUGB) for the FESEM analyses. This research was supported financially by the National Natural Science Foundation of China (Grants 41672068, 41272110), the Fundamental Research Funds for the Central Universities (Grant 2652015045), the Open Research Funds for GPMR (Grant GPMR201513), and the Chinese B111^ project (Grant B07011). Dr. Anthony E. Williams-Jones is thanked for final reading this paper. We thank Paul Spry and Antoine De Haller for their critical reviews, which considerably improved this paper. Associate Editor Robert Moritz and Editor-in-Chief Georges Beaudoin are thanked for their editorial help and useful suggestions.
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