Fractionation of cadmium isotope caused by vapour-liquid partitioning in hydrothermal ore-forming system: A case study of the Zhaxikang Sb–Pb–Zn–Ag deposit in Southern Tibet

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S0169-1368(19)30139-8 https://doi.org/10.1016/j.oregeorev.2020.103400 OREGEO 103400

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Ore Geology Reviews

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Please cite this article as: D. Wang, Y. Zheng, R. Mathur, M. Yu, Fractionation of Cadmium isotope caused by vapor-liquid partitioning in hydrothermal ore-forming system: A case study of the Zhaxikang Sb–Pb–Zn–Ag deposit in Southern Tibet, Ore Geology Reviews (2020), doi: https://doi.org/10.1016/j.oregeorev.2020.103400

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Fractionation of Cadmium isotope caused by vapor-liquid partitioning in

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hydrothermal ore-forming system: A case study of the Zhaxikang Sb–Pb–Zn–

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Ag deposit in Southern Tibet

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Da Wang1,2, Youye Zheng1,3*, Ryan Mathur2*, Miao Yu4

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1State

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and Resources, China University of Geosciences, Beijing 100083, China

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2Department

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3State

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Resources, China University of Geosciences, Wuhan 430074, China

Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences

of Geology, Juniata College, Huntingdon, Pennsylvania 16652, USA

Key Laboratory of Geological Processes and Mineral Resources, and Faculty of Earth

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4Beijing

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*Corresponding author: Youye Zheng: [email protected]

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Institute of Geology, Beijing, 100120, China

Ryan Mathur: [email protected]

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Abstract

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Here, we conducted a case study of the Zhaxikang Sb–Pb–Zn–Ag deposit to explore Cd

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isotopic fractionation mechanisms in hydrothermal ore-forming system. The δ114/110Cd values of

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sphalerite systematically decrease from ore mineralization stage 1 (–0.30‰ to 1.01‰; average

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value = 0.17‰; n = 4) through stage 2 (–0.51‰ to –0.09‰; average value = –0.23‰; n = 3) to

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stage 3 (–0.34‰ to –0.23‰; average value = –0.285‰; n = 2). A simple Rayleigh distillation

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models this temporally decreasing trend resulted from Cd isotopic fractionation that is most likely

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related to vapor-liquid dynamics of ore-forming fluid. This mechanism for Cd isotopic

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fractionation is further augmented by the general geochemical characteristics and fluid inclusion

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data of sulfides and cogenetic gangue minerals. Firstly, the sphalerite has low Cd concentrations

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(1183 – 2199 ppm), correspondingly high Zn/Cd ratios (248 – 421), and large Cd isotopic variation

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range with relatively lower δ114/110Cd values (–0.51‰ to 1.01‰). Commonly, sphalerite from

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sedimentary exhalative systems possess these characteristics caused by vapor-liquid interactions.

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Secondly, fluid inclusion data from the Pb–Zn sulfides and cogenetic carbonate alteration minerals

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indicates that vapor-liquid two-phase inclusions are in dominance (more than 90%). Meanwhile,

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the vapor-liquid ratios (20% – 50%) of these two-phase inclusions are consistent with those of the

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established Cd (25.48% – 55.07%) and Zn (26.70% – 48.70%) isotopic Rayleigh distillation model.

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To conclude, vapor-liquid partitioning is main cause for observed Cd isotopic variations in

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Zhaxikang deposit, and is demonstrated as an important Cd isotopic fractionation mechanism in

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hydrothermal ore-forming system.

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Keywords: Cd isotope; Rayleigh distillation model; vapor-liquid partitioning; hydrothermal ore-

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forming system; Zhaxikang deposit

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1. Introduction

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One of the fundamental questions associated with ore deposits is to understand where metals

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originated and how these systems became endowed with the metal budget they possess. Relatively

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new techniques of transition metal and metal isotope have begun to further advance our

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understanding of metal source, migration and concentration in the crust (Duan et al., 2016;

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Saunders et al., 2015; Lv et al., 2016; Mathur et al., 2018). The advantage of using these metal

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isotopic systems is that we can derive information directly from the ore minerals themselves, which

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has led to the measurement of significant Fe-Cu-Zn-Mo-Sn isotopic variations in sulfides from

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different types of ore deposits (e.g., Wilkinson et al., 2005; Markl et al., 2006; Mathur et al., 2009,

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2012; Yao et al., 2016, 2018; Wang et al., 2017; Wu et al., 2017; Gao et al., 2018).

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From a geochemical perspective, Cadmium (Cd) is isotopically unique. The reason is that the

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Cd has a large number (8) of naturally formed stable isotopes (Cd106: 1.25‰, Cd108: 0.89‰,

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Cd110:12.5‰, Cd111: 12.8‰, Cd112: 24.1‰, Cd113: 12.2‰, Cd114: 28.7‰ and Cd116: 7.49‰) and

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exhibits a great mass range (108 to 116 amu) on the periodic table (Cloquet et al., 2005; Zhu et al.,

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2015a, b). The relative mass difference of the isotopes and the larger mass range of Cd may allow

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for monitoring the fractionation of a heavy metal within hydrothermal systems (Tu et al., 2004;

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Yao et al., 2018). With the regards to ore deposit investigation, Cd is an economically significant

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metal that may exhibit sulphophile, volatile and lithophile behavior. Because of the difficulty to

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form independent minerals, Cd is found enriched by isomorphic replacement with other elements

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(e.g., replace Zn in sphalerite) within the mineral structure. This is the case in Pb–Zn deposits

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where the element is mined economically. Accordingly, the related research of Cd isotopes has the

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potential to trace metal sources and provide insights into ore-forming processes within Pb–Zn

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deposits.

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The attempts of Cd isotopic measurement can date back to 1970s (Rosman and De Laeter,

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1975, 1976, 1978), the analytical precision was limited at that time yet. In recent years, the

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optimized TIMS (thermal ionization mass spectrometer) double spiked method (e.g., Sands et al.,

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2001; Schmitt et al., 2009) and newly-developed MC-ICP-MS (Multicollector-Inductively

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Coupled Plasma Mass Spectrometer) measuring technique (e.g., Lacan et al., 2006; Gao et al., 2008)

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have much improved the analytical precision (δ114/110Cd: ± 0.10‰; Ripperger et al., 2007).

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Currently, measurable Cd isotopic fractionation has been demonstrated to occur in nature and

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significant Cd isotopic variations have been reported in ores (Wen et al., 2015, 2016; Yang et al.,

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2015; Zhu et al., 2016a, 2017), igneous rocks (Wombacher et al., 2003; Schmitt et al., 2009),

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carbonate and sedimentary rocks (Wombacher et al., 2003; Horner et al., 2011), chondrite and lunar

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samples (Rosman and De Laeter., 1975; Sands et al., 2001; Schediwy et al., 2006; Wombacher et

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al., 2004, 2008), sea and river water (Lacan et al., 2006; Ripperger et al., 2007; Abouchami et al.,

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2011; Gault-Ringold et al., 2012; Yang et al., 2012; Lambelet et al., 2013), soil and sediments (Gao

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et al., 2008; Schmitt et al., 2009; Zhang et al., 2016). Cd isotopic fractionation reported in these

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contributions has been attributed to evaporation and condensation, biological and inorganic

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processes (Wang et al., 2013; Zhu et al., 2013).

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The research of these works focused on factors for Cd isotopic variation related to

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environmental science, marine science and cosmochemistry. Mechanisms for Cd isotopic

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fractionation in hydrothermal ore-forming system are nascently studied, with only the kinetic

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Rayleigh fractionation mechanism related to mineral precipitation (solid-liquid partitioning; Wen

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et al., 2016; Zhu et al., 2017). In hydrothermal ore-forming system, the kinetic Rayleigh

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fractionation related to vapor-liquid partitioning is also considered as an important transition metal

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isotopic fractionation mechanism (e.g., Cu: Graham et al., 2004; Cu and Mo: Yao et al., 2016; Fe

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and Zn: Wang et al., 2018a). Therefore, we carried out a case study of the super-large complex

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polymetallic Zhaxikang deposit in Southern Tibet, which is representative and ideal for Cd isotope

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related study in hydrothermal ore-forming system. The investigation centers on δ114/110Cd values

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of sulfides and establishment of Cd isotopic Rayleigh distillation model to demonstrate the

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influence of vapor-liquid partitioning on observed Cd isotopic variations.

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2. Geological Background

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The North Himalayan Metallogenic Belt (NHMB) is an important component of the Tethys-

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Himalaya Metallogenic Domain (Fig. 1A). There are three main regional mineralization events

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within NHMB that have generated a series of Sb, Sb–Au, Au, W–Sn(Be), Pb–Zn(Ag) and Sb–Pb–

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Zn–Ag deposits (Fig. 1B; Yang et al., 2009; Zheng et al., 2012, 2014; Li et al., 2017). The first

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regional mineralization event is related to multiple seafloor volcanic events during synsedimentary

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period (220 to 130 Ma); the second regional mineralization event is associated with the

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metamorphic fluid system during syn-collision period (60 to 42 Ma); and the third regional

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mineralization event relates to the magmatic-hydrothermal activity during post-collision period (25

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Ma to now; Wang et al., 2018b). As the regional geology of NHMB has been described in detail

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by several previous studies (e.g., Wang et al., 2017; Sun et al., 2018), it is not necessary to be

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repeated here.

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The Zhaxikang deposit, in the location of ~48 km west from Longzi County Town, is the only

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super-large deposit identified within NHMB (Fig. 1B). According to the latest exploration results

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of Huayu Mining Company, the Zhaxikang deposit contains Zn+Pb 2.066 Mt at average grade of

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6.38%, Sb 0.235 Mt at an average grade of 1.14%, Ag 2660.6 t at an average grade of 101.64g/t,

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Au 10.4 t at an average grade of 2.9 g/t, associated Ga 361 t, and 20 Mt Mn–Fe carbonate ores at

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an average grade of 42% for Fe+Mn. The Lower Jurassic Ridang formation consists of epi-

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metamorphic marine clastic rocks and hosts majority of the mineralization within the orefield (Fig.

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1C). Some Upper Jurassic Weimei formation, composed of fine-grained metamorphic quartzose

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sandstone, silty slate, and calcarenite, also outcrops within the orefield. Meanwhile, there is a small

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amount of Quaternary sediments accumulated within valleys (Fig. 1C; Zheng et al., 2012). The

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engineering and geological mapping projects have identified sixteen faults (F1 – F16) that host

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nine orebodies (I–IX; Fig. 1C and D) within the orefield. Among these orebodies, orebody V is the

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largest and richest that hosts more than 80% of the reserves. The orefield records magmatism that

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have formed diabase, porphyritic rhyolite, basalt, leucogranite units and some granite porphyry

114

dykes (Fig. 1C). Various types of alteration associated with mineralization have occurred within

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the orefield, such as the silicification associated with Sb mineralization, carbonatization that is

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related to Pb–Zn mineralization in the form of Mn–Fe carbonate veins, the chloritization, weak

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sericitization and clayization. In addition, ore-forming elements exhibit a vertical sequence that is

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zoned from a lowermost Zn (Pb + Ag) zone through a central Zn + Pb + Ag–(Sb) zone to an

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uppermost Pb + Zn + Sb + Ag zone, whereas there is no horizontal zoning (Wang et al., 2018a).

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The ore paragenetic sequence in Zhaxikang deposit has been divided into six stages of ore

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formation based on hand specimen and microscopic observations (Fig. 2). These six stages are

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assigned to two clear pulses. The first pulse (Pb–Zn mineralization) consists of stages 1 and 2 that

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is primarily dominated by Mn–Fe carbonates and sulfides (Figs. 2 and 3A-D). The second pulse

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(Sb mineralization) includes stages 3 to 6 that is principally characterized by quartz, calcite,

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sulfosalt minerals, and sulfides (Figs. 2 and 3E-I). As the transitional stage between the two pulses

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of mineralization, the sulfides in stage 3 mainly form by replacement of earlier sulfides (Wang et

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al., 2017, 2018a). Additionally, a supergene stage is distinguished by the formation of ferrohydrite,

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smithsonite, sardinianite, valentinite, travertine, malachite and siliceous sinter in the shallowest

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elevation (Fig. 2).

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3. Sampling and Analytical methods

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The most common isomorphic replacement for Cd is to replace Zn and then enter into the

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crystal lattices of sphalerite, hence sphalerite is the most ideal mineral for Cd isotopic research in

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hydrothermal ore-forming system. Due to the absence of Zn-bearing minerals from stages 4 to 6,

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nine sphalerite and four galena samples from stages 1 to 3 are chosen for relevant analyses in

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Zhaxikang deposit. In these samples, there are four sphalerite-galena coexisting mineral pairs.

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These ore samples were crushed to around 100 – 200 mesh for separation of sphalerite and 200 –

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300 mesh for separation of galena, then the separations of sulfide crystals were prepared with

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careful handpicking under a binocular microscope on the basis of size, clarity, color, and

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morphology to achieve a purity of 99.99%.

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The measurement of Zn and Cd concentrations were conducted on an ICP–OES (Inductively

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Coupled Plasma Optical Emission Spectrometer) at Pennsylvania State University. Approximately

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20 to 70 mg of sulfide powders were dissolved in 4 ml of heated (120℃) ultrapure aquaregia for

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12 hours. Then the aliquots from each solution were acidified and diluted in 2% nitric acid for

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chemical analysis. Zn and Cd concentrations were determined with standard calibration curves that

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ranged from 0.6 to 10 ppm and Indium was used as an internal standard for analysis. The errors of

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the concentration data are usually within 5%, which were used to determine the amount of Cd

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needed for isotope analysis.

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The Cd isotopic compositions were measured on the Neptune MC–ICP–MS at Pennsylvania

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State University. Cd was purified using the anion exchange chromatography (Cloquet et al., 2005)

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with volumetric yields for the samples greater than 94% after two rounds of column

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chromatography. In this procedure, 2ml of wet BioRad AG MP-1 resin chloride form (100 – 200

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mesh) was added to a 10ml BioRad chromatography column. The resin was sequentially cleaned

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with 10ml of 2% HNO3, 10ml of MQ water (18.2), 5ml and of 1.2 molar HCl. The sample was

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loaded onto the resin with 1ml of 1.2 molar HCl and the unwanted ions were sequentially eluted

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with another 4ml of 1.2 molar HCl, 15ml of 0.3 molar HCl and 16ml of 0.012 molar HCl. The Cd

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was collected in 17ml of 0.0012 molar HCl in the final elution. This process was repeated with the

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use of new resin for the second column to eliminate Sn. The 116Sn mass was monitored in H4 cup,

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with 107Ag in L4 cup, 109Ag in L2 cup, 110Cd in L1 cup, 111Cd in Ax cup, 112Cd in H1 cup, 113Cd in

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H2 cup, 114Cd in H3 cup (Table 1). Instrumentation setup and introduction was similar to Wasylenki

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et al. (2014). All samples were doped with 100 ppb NIST 987 Ag isotope standard which was used

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to correct for mass bias using the exponential fractionation correction (Marechal et al., 1999). The

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107Ag/109Ag

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at 100 ppb Cd, with on peak blank subtraction in 2 blocks of 15 ratios. The reported values are an

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average of 2 separate measurements, and the data are presented relative to the NIST SRM 3108 Cd

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of the NIST 987 Ag isotope standard is reported at 1.07638. Solutions were measured

standard in per mil notation defined as: δ114/110Cd (‰) =

(

)

( )sample ―1 ( ) NIST SRM 3108 114Cd 110Cd

114Cd 110Cd

× 1000

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(Abouchami et al., 2013). All the cited data from previous literatures in this paper are converted

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relative to NIST SRM 3108 Cd standard according to Cloquet et al. (2005); Abouchami et al. (2013)

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and Zhu et al. (2017). The variation of NIST SRM 3108 throughout the measuring session was 0.04

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‰ (2 n=14) and is considered the error of measurements.

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4. Results

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All the analytical results are listed in Table 2. The galena has Zn concentrations of 0.030% –

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0.153% (average value: 0.084%, n = 4) and Cd concentrations of 14 – 26 ppm (average value: 20

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ppm, n = 4), respectively. The Zn/Cd ratios of galena range from 14 to 82 (average value: 41, n =

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4). Besides, stage 2 galena shows δ114/110Cd values of –2.29‰ and 2.24‰, and stage 3 galena

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exhibits δ114/110Cd values of –0.21‰ and 0.01‰. By contrast, the sphalerite has Zn and Cd

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concentrations of 40.456% – 58.440% (average value: 51.502%, n = 9) and 1183 – 2199 ppm

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(average value: 1596 ppm, n = 9), respectively. Meanwhile, the Zn/Cd ratios of sphalerite are 248

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– 421 (average value: 332, n = 9). The overall δ114/110Cd values of sphalerite range from –0.51‰

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to 1.01‰ with an average value of –0.08‰ ± 0.83‰ (2SD, n = 9). Separately, δ114/110Cd values of

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sphalerite in different mineralization stages are as follow: (1) stage 1: –0.30‰ to 1.01‰ (average

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value: 0.17‰ ± 1.01‰, 2SD, n = 4); (2) stage 2: –0.51‰ to –0.09‰ (average value: –0.23‰ ±

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0.39‰, 2SD, n = 3); (3) stage 3: –0.34‰ to –0.23‰ (average value: –0.285‰ ± 0.11‰, 2SD, n =

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2).

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5. Discussion

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5.1 The occurrence of Cd in sulfides

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The occurrence of Zn and Cd in galena have been studied and used to relate to specific ore

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genetic models (Palero-Fernández and MartínIzard, 2005; Zhou et al., 2011). In this instance, their

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data has been used to understand the Zhaxikang and Fule MVT deposits. Specifically, both the

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Zn2+ and Cd2+ should enter into the galena by directly isomorphic replacement with Pb2+ in

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Zhaxikang deposit, which differs from the situation in Fule MVT deposit that both Zn and Cd

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present as sphalerite micro-inclusions within galena (Zhu et al., 2017). This inference is

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demonstrated by the following comparisons: (1) Zhaxikang deposit: ①No correlation between Zn

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and Cd concentrations for galena samples (Fig. 4A); ②Cd concentrations (14 – 26 ppm; average

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value = 20 ppm, n = 4) of galena samples fall into the range of hydrothermal deposits (10 – 500

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ppm; Schwartz, 2000), but much lower than those of Fule MVT deposit (48 – 1163 ppm; average

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value = 367.3 ppm; n = 6; Fig. 4C) (T test p=0.0548); ③Zn/Cd ratios (14 – 82) of galena samples

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are not within the range of sphalerite (248 – 421); ④The BSE (Back Scattered Electron) image

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(Fig. 5A; Wang et al. 2018a) and photomicrograph (Fig. 5B) show that there is no sphalerite micro-

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inclusions within galena; (2) Fule MVT deposit: ①Cd/Zn ratios of bulk galena samples (159 –

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512) are within the range of sphalerite samples (23 – 588); ②There is a positive correlation (R2 =

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0.94) between Cd and Zn concentrations for galena samples; ③BSE images confirm that sphalerite

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micro-inclusions exist within galena (Zhu et al., 2017). These Cd characteristics suggest that the

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occurrence of Zn–Cd within galena in Zhaxikang and Fule deposits are different.

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On the other hand, two different kinds of isomorphic replacement between Zn2+ and Cd2+

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within sphalerite have been confirmed by LA–ICP–MS (Laser Ablation Inductively Coupled

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Plasma Mass Spectrometer) measurements: (1) Cook et al. (2009) suggested that the Zn2+ is directly

210

substituted by Cd2+ during crystallization; whereas (2) Belissont et al. (2014) considered that both

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Cd2+ and Fe2+ enter into sphalerite by direct substitution with Zn2+. In the study of Fule MVT

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deposit, the sphalerite has high Cd concentrations (5240 – 35000 ppm) but low Fe concentrations

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(365 – 970 ppm), thus Zhu et al. (2017) thought that the Cd2+ enter into sphalerite by direct

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substitution with Zn2+. By comparison, the sphalerite from Zhaxikang deposit has lower Cd

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concentrations of 1183 – 2199 ppm with no correlation between Cd and Zn concentrations (Fig.

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4B), but much higher Fe concentrations (4.05% – 9.44%, Sun et al., 2018), which indicates that the

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Zn2+ is directly substituted by both of Cd2+ and Fe2+ during crystallization of sphalerite in

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Zhaxikang deposit.

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5.2 Comparison with previous Cd isotopic genetic and evolution models By investigating the Cd characteristics of sphalerite that include Cd concentrations, Zn/Cd

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ratios and Cd isotopic compositions, Wen et al. (2016) divided nine Pb–Zn deposits with different

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geneses in China into three categories: (1) The Low-T (temperature) systems (MVT Pb–Zn deposits)

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have high Cd concentrations (2415 – 34981 ppm), relatively higher δ114/110Cd values (–0.10‰ to

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0.59‰) and low Zn/Cd ratios (17 – 201); (2) The High-T systems (porphyry, magmatic

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hydrothermal, skarn, and VMS Pb–Zn deposits) show moderate Cd concentrations (2410 – 4126

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ppm), tight Cd isotopic variation range (–0.25‰ to 0.05‰) and moderate Zn/Cd ratios (155 – 223);

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(3) The Exhalative systems exhibit low Cd concentrations, correspondingly high Zn/Cd ratios and

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large Cd isotopic variation range with relatively lower δ144/110Cd values, which include the SEDEX

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(sedimentary exhalative) Pb–Zn deposits (595 – 996 ppm, 316 – 368, –0.29‰ to 0.22‰) and

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seafloor hydrothermal sulfides (295 – 1174 ppm, 211 – 510, –0.49‰ to 0.35‰). Comparing with

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these three aforesaid categories, the Cd characteristics of sphalerite from Zhaxikang deposit (low

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Cd concentrations: 1183 – 2199 ppm, correspondingly high Zn/Cd ratios: 248 – 421, large Cd

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isotopic variation range with relatively lower δ144/110Cd values: –0.51‰ to 1.01‰) conform to

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Exhalative system, yet are obviously distinct from Low-T and High-T systems (Fig. 6). Therefore,

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these Cd characteristics of sphalerite support a SEDEX genetic model for the Pb–Zn mineralization

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in Zhaxikang deposit.

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This interpretation is consistent with previous geology, mineralogy, elements, isotopes and

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geochronology evidence: (1) the ores from the first pulse of mineralization exhibit lamellar, banded,

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massive, globular, concentric annular, disseminated, brecciated, fine-grained layered and colloform,

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net-veined and Dal Matianite ore textures, as well as syndepositional structure and even ancient

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hydrothermal vent (Zheng et al., 2012, 2014), which are similar to those of the ores from Red Dog

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SEDEX-type ore district in Alaska with typical submarine hydrothermal sedimentation

244

(metasomatism) origin (Moore, et al., 1987; Leach et al., 2005); (2) the Fe–Zn isotopic

245

compositions of the first pulse of ore-forming fluid (δ56Fe: –1‰ to –0.5‰, δ66Zn: –0.28‰ to 0‰)

246

overlaps those of seafloor hydrothermal fluids, which are calculated from Fe–Zn isotopic Rayleigh

247

fractionation models (Wang et al., 2017, 2018a); (3) the Sm–Nd isochron age of Mn–Fe carbonate

248

(173.7 ± 7.4 Ma) and Rb–Sr isochron age (147 ± 3.2 Ma) of stage 2 sphalerite are in keeping with

249

the age of marine volcanic rocks (220 – 130 Ma) within regional strata, which are dominated by a

250

set of Late Triassic-Early Cretaceous flysch formations formed by turbidity sediment and

251

carbonaceous-siliceous-argillaceous rock series related to hydrothermal sedimentation (Wang et al.,

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2019).

253

On the basis of unique characteristics for different ore-forming systems, Wen et al. (2016)

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selected different initial δ114/110Cd values and fractionation factors (α) to establish the Cd isotopic

255

evolution models for Low-T, High-T and Exhalative systems (Fig. 7). These evolution models can

256

illustrate the Cd isotopic variations during the deposition of aqueous Cd in different hydrothermal

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fluids. The aforementioned comparisons about genetic models have proved that SEDEX genesis is

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most plausible for the Pb–Zn mineralization in Zhaxikang deposit, which can well explain why

259

neither of the evolution models for Low-T and High-T systems is suitable for Cd isotopic

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compositions of sphalerite in Zhaxikang deposit (Fig. 7). However, the δ114/110Cd values of

261

sphalerite in Zhaxikang deposit are also still not in exact consistency with the evolution model for

262

Exhalative system (Fig. 7). This phenomenon reveals that the metallogenic model in Zhaxikang

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deposit is not simple deposition of aqueous Cd, and the Cd isotopic variations in Zhaxikang deposit

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haven’t absolutely induced by the kinetic Rayleigh fractionation related to mineral precipitation

265

(Wen et al., 2016; Zhu et al., 2017). In the following section, we will discuss about the Cd isotopic

266

fractionation mechanisms in Zhaxikang deposit in detail.

267 268

5.3 Background on Cd isotopic fractionation and possible causes of Cd isotopic variations

269

At present, the mechanisms for Cd isotopic fractionation in hydrothermal ore-forming

270

system is still not clear. Nevertheless, a diverse set of hypotheses have been presented to explain

271

isotopic values in ores derived from geochemical reactions during ore formation. In general, two

272

hypotheses regarding the interpretation of the transition metal isotopic data from hydrothermal

273

mineralization exist: (1) the physiochemical nature of the fluids evolve during the mineralization

274

process which cause isotopic changes in the minerals of the system; (2) the addition/subtraction of

275

different metal sources leads to mixing before which causes isotopic changes in the minerals of the

276

system (Yao et al., 2016). The processes that could operate during these two scenarios include

277

disequilibrium chemical diffusion or changes in temperature, salinity and pH (e.g., Fe: Huang et

278

al., 2010; Zn: Toutain et al., 2008; Ag: Mathur et al., 2018; Cu: Maher et al., 2011), electron transfer

279

induced by redox reactions (e.g., Cu: Mathur et al., 2005; Fe: Kavner et al., 2005; Sn: Yao et al.,

280

2018; Ag: Fujii and Albarede, 2018), equilibrium fractionation (e.g., Fe: Polyakov et al., 2000,

281

2007, 2011), kinetic Rayleigh fractionation related to solid-liquid partitioning (mineral

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precipitation; e.g., Cd: Zhu et al., 2017; Zn: Wilkinson et al., 2005; Kelley et al., 2009) or vapor-

283

liquid partitioning (e.g., Cu: Maher et al., 2011; Sn: Mathur et al., 2017; Cu and Mo: Yao et al.,

284

2016), heterogeneity metal source and mixing of isotopic reservoirs (e.g., Zn: Wilkinson et al.,

14

285

2005; Fe: Hou et al., 2014). Nevertheless, if all these factors are appropriate for Cd isotope and

286

which one or several are the main causes of Cd isotopic variations in Zhaxikang deposit? Next, we

287

will discuss one by one.

288

Firstly, the disequilibrium chemical diffusion or changes in temperature, salinity and pH can’t

289

lead to the significant Cd isotopic variations in Zhaxikang deposit. The Cd isotope should show a

290

smaller fractionation at high temperature, thus the temperature decreasing of ore-forming fluids

291

could lead to heavier Cd isotopes enriched in earlier precipitated sphalerite (Yang et al., 2015).

292

However, both the high- and low-temperature sphalerite has heavier δ114/110Cd values, although the

293

fluids temperature decreased from 300℃ (bottom) to 150℃ (top) in the Tianbaoshan deposit (Zhu

294

et al., 2016a). Meanwhile, based on the study of Cd isotopic fractionation during inorganic calcite

295

growth, Horner et al. (2011) discovered that the fractionation factor (αCalcite–Cd(aq)) is insensitive to

296

temperature, ambient [Mg2+] and precipitation rate, and the Cd isotopic fractionation related to

297

kinetic isotope effects during largely unidirectional incorporation of Cd at the mineral surface will

298

become more pronounced at high salinity. In Zhaxikang deposit, Cd is mainly enriched by

299

isomorphic replacement with Zn in sphalerite. According to the previous fluid inclusion data

300

trapped on sphalerite and cogenetic Mn–Fe carbonate (Yu, 2015), from stage 1 (203 – 273℃, 9.0

301

– 14.5 wt% NaCl equiv.) through stage 2 (215 – 265℃, 8.9 – 16.7 wt% NaCl equiv.) to stage 3

302

(237 – 259℃, 12.0 – 16.1wt% NaCl equiv.), the temperature and salinity show little changes.

303

Notably, the disequilibrium chemical diffusion or changes in temperature and salinity of ore-

304

forming fluid can’t result in the significant Cd isotopic variations of sulfides (sphalerite: –0.51‰

305

to 1.01‰; galena: –2.29‰ to 2.24‰) in Zhaxikang deposit. Furthermore, significant Cu (Maher et

15

306

al., 2011; Mathur et al., 2013) and Ag (Mathur et al., 2018) isotopic fractionation have been linked

307

to the changes in pH. Whereas, currently, no data exists to support the pH induced Cd isotopic

308

fractionation.

309

Secondly, there is no electron transfer caused Cd isotopic fractionation during ore-forming

310

process with respects to Zhaxikang deposit. In Pb–Zn deposits, Cd is mainly involved in sphalerite

311

by directly substitutions with Zn, the equation is ZnSsolid + Cd2+ Liquid = CdSSolid + Zn2+ Liquid

312

(Mookherjee, 1962; Tsusue and Holland, 1996; Sverjensky et al., 1997; Wen et al., 2016).

313

Meanwhile, Zn2+ and Cd2+ should enter into the galena by directly isomorphic replacement with

314

Pb2+ in Zhaxikang deposit as discussed in Section 5.1, too. Specifically, most of the Zn2+ and Cd2+

315

are subordinated and form the aqueous complexes with ligands of Cl−, HS− and OH− in

316

hydrothermal fluids (Bazarkina et al., 2010; Tagirov and Seward, 2010). Over a wide range of

317

temperatures (20℃ – 450℃), acidities (pH: 1 – 8) and chloride concentrations (mCl: 0.04 – 18

318

mol/kg H2O), aqueous Cd speciation is dominated by chloride species [CdClm(H2O)2−m n]

319

(Giordano, 2002; Mei et al., 2015; Wen et al., 2016). Accordingly, Cd ions exist in the valence

320

state of +2 both in the ore-forming fluids and sulfides, there is no redox reactions occurred during

321

ore formation.

322

Thirdly, heterogeneity metal source and mixing of isotopic reservoirs are common in

323

hydrothermal ore-forming system. Zhu et al. (2016a) thought that the ore-forming fluids are

324

heterogeneous and heavy Cd isotopes were enriched in original ore-forming fluids, which result in

325

heavier Cd isotopic enrichments in early precipitated sphalerite in Tianbaoshan deposit. The

326

Zhaxikang deposit is a complex polymetallic deposit that has experienced two pulses of

16

327

mineralization. Therefore, heterogeneity metal source and mixing of isotopic reservoirs are more

328

likely to occur. However, these two factors still not the dominated causes induced the Cd isotopic

329

variations in Zhaxikang deposit. The evidences are as follow: (1) There is no correlation between

330

Cd concentrations and δ114/110Cd values of sulfides (Figs. 8A and B), and the previous H–O isotopic

331

data (Xie et al., 2017) of cogenetic Mn–Fe carbonate are almost the same and all fall into the

332

formation water area (Fig. 9), which indicates that there is no heterogeneity in the metal source for

333

Pb–Zn mineralization (stages 1 and 2). (2) According to the commonsense metallogenic element

334

associations characteristics in ore deposits, the Cd should mainly source from the first pulse of

335

mineralization (Pb–Zn), rather than the latter second pulse of mineralization that is dominated by

336

Sb mineralization, which implies that the Cd is single-source and there is no mixing of Cd isotopic

337

reservoirs.

338

Fourthly, the transition metal isotopic fractionation between coexisting mineral pairs have

339

attracted extensive attentions. For instance, Fe isotopic fractionation between magnetite and pyrite,

340

magnetite and pyrrhotite, pyrrhotite and pyrite (Zhu et al., 2016b), Mn–Fe carbonate and pyrite

341

(Wang et al., 2017), as well as Zn isotopic fractionation between Mn–Fe carbonate and sphalerite

342

(Wang et al., 2017). In this paper, we compare Cd isotopic fractionation within four sphalerite-

343

galena coexisting mineral pairs. The results show there is Cd isotopic fractionation between

344

sphalerite and galena (Fig. 10A), yet these mineral pairs have not attained Cd isotopic equilibrium

345

(Fig. 10B). The galena has very large Cd isotopic variation range (–2.19‰ to 2.24‰) with low Cd

346

concentrations (14 – 26 ppm), which can be well explained by Cd isotopic fractionation between

347

sphalerite and galena. According to the mass balance theory, as Cd concentrations of sphalerite

17

348

(1183 – 2199 ppm) are dozens higher than galena in Zhaxikang deposit, little Cd isotopic variation

349

in sphalerite will cause large Cd isotopic variation in galena within same mineral pair. From the

350

above, the Cd isotopic fractionation between sphalerite-galena mineral pairs must have some

351

contribution to the Cd isotopic variations in Zhaxikang deposit, yet still can’t well decipher the

352

significant Cd isotopic variations of sphalerite (–0.51‰ to 1.01‰).

353

Lastly, the kinetic Rayleigh fractionation mechanism is most commonly used to explain the

354

transition metal isotopic variations in hydrothermal deposits. The research of Schmitt et al. (2009)

355

and Horner et al. (2011) indicate that the minerals preferentially enrich lighter Cd isotopes than

356

related fluids. Similarly, based on the density functional theory, Yang et al. (2015) calculated the

357

fractionation properties of Cd species in hydrothermal fluids, and then suggested that the light Cd

358

isotopes preferentially fractionate into solid phase relative to relevant fluids. The Zn and Cd have

359

shown similar geochemical behavior and experience similar hydrothermal evolution histories in

360

Pb–Zn deposits (Schwartz, 2000; Schmitt et al., 2009). Consequently, this kinetic Rayleigh

361

fractionation mechanism is suitable for Zn isotope, too (Gagnevin et al., 2012). This equilibrium

362

induced fractionation because of partitioning into solid-liquid phases has been modeled with

363

Rayleigh distillation to decipher increasing δ114/110Cd (e.g. MVT: Zhu et al., 2017) and δ66Zn (e.g.

364

Irish-type: Wilkinson et al., 2005; SEDEX: Kelley et al., 2009) values with time in Pb–Zn deposits.

365

With the regards to Zhaxikang deposit, it is an entirely different situation. The ore samples for Zn

366

and Cd analyses are both collected from the same location in orebody V, hence the Cd and Zn

367

isotopic values still have good correlation although they are not from the same suite of samples.

368

Both the δ114/110Cd and δ66Zn values of sphalerite display a gradually decreasing trend from stage

18

369

1 (δ114/110Cd: –0.30‰ to 1.01‰, average value = 0.17‰, n =4; δ66ZnAA-ETH: –0.12‰ to 0.07‰,

370

average value = 0‰, n = 4) through stage 2 (δ114/110Cd: –0.51‰ to –0.09‰, average value = –

371

0.23‰, n = 3; δ66ZnAA-ETH: –0.38‰ to –0.05‰, average value = –0.189‰, n = 14) to stage 3

372

(δ114/110Cd: –0.34‰ to –0.23‰, average value = –0.285‰, n = 2; δ66ZnAA-ETH: –0.32‰ to –0.06‰,

373

average value = –0.193‰, n =7) (Fig. 11A and B; Wang et al., 2017, 2018a). The number of

374

measurements for Cd isotope are too limited to do statistically analysis (T Test ①stages 1 and 2:

375

p = 0.138475, ②stages 2 and 3: p = 0.377581). Yet, the number of measurements for Zn isotope

376

are enough to do the T Test and there are statistically differences in Zn isotopic compositions for

377

each stage (①stages 1 and 2: p = 0.001814, ②stages 2 and 3: p = 0.472327). Meanwhile, the

378

decreasing trend in δ66Zn and δ56Fe values have also been found within concentric ore samples

379

(Fig. 12; Wang et al., 2018a), which enhances the authenticity for the temporally decreasing trend

380

in transition metal isotopic compositions. In brief, this temporally decreasing trend indeed exists

381

both macroscopically and microscopically in Zhaxikang deposit. This is in accord with the

382

inference in Section 5.2 that the metallogenic model in Zhaxikang deposit is not simple deposition

383

of aqueous Cd, and the Cd isotopic variations in Zhaxikang deposit haven’t absolutely induced by

384

the kinetic Rayleigh fractionation related to solid-liquid partitioning (mineral precipitation).

385

This different situation in Zhaxikang deposit has also been found in other ore deposits (e.g.,

386

δ114/110Cd values of sphalerite in MVT deposit: Zhu et al., 2016a; δ65Cu values of chalcopyrite in

387

porphyry deposit: Graham et al., 2004, Yao et al., 2016). Zhu et al. (2016a) have proposed that

388

there must be a potential mechanism resulting in early precipitated minerals enriched in heavy

389

isotopes. We argue that the potential mechanism in Zhaxikang deposit should be the kinetic

19

390

Rayleigh fractionation related to vapor-liquid partitioning. This inference is supported by the

391

following evidence: (1) The Pb–Zn mineralization is demonstrated to have the SEDEX genesis,

392

hence the degasification is ubiquitous in this kind of ore-forming system. (2) The fluid inclusion

393

data from the Pb–Zn sulfides and cogenetic carbonate alteration minerals indicates that vapor-

394

liquid two-phase inclusions are in dominance (more than 90%) with the vapor-liquid ratios of 20%

395

– 50% (Yu, 2015). (3) The evaporation (vapor-liquid partitioning) has been used to explain the

396

measurable Cd isotopic fractionation occurs in nature in previous studies (Wang et al., 2013; Zhu

397

et al., 2013). (4) Transition metal isotopic fractionation initiated by vapor-liquid partitioning could

398

be the cause for fractionation factors used in Rayleigh distillation models, which can result in the

399

temporally decreasing trend of transition metal isotopic values (e.g., Cu: Graham et al., 2004; Yao

400

et al., 2016). (5) We have confirmed that the metallogenic model in Zhaxikang deposit is not simple

401

deposition of aqueous Cd in Section 5.2. Meanwhile, the Fe–Zn–Cd should experience similar

402

evolutionary process during Pb–Zn mineralization, and the kinetic Rayleigh fractionation related

403

to vapor-liquid partitioning have been confirmed to cause the decreasing trend of Fe–Zn isotopic

404

compositions with time in Zhaxikang deposit (Wang et al., 2018a). This Rayleigh distillation

405

mechanism models the temporally decreasing trend as follow: During the ore formation, there is

406

vapor-liquid partitioning within ore-forming system and the ratios change with the temperature

407

decreasing. Initially, the increment of vapor phase preferentially enriched in light isotopes relative

408

to ore-forming fluid, which lead to the early minerals precipitated from the ore-forming fluid

409

enriched heavy isotopes. Then with the removing of precipitated mineral from the ore-forming

410

system and condensation of vapor phase back to ore-forming fluid, the isotopic values of residual

20

411

ore-forming fluid and subsequent minerals have lower values (Fig. 13; Graham et al., 2004; Yao et

412

al., 2016; Wang et al., 2018a).

413

On the other hand, Wang et al. (2017) have found that the higher δ56Fe values and lower δ66Zn

414

values coincide with an increase in alteration for stage 3 minerals in Zhaxikang deposit. Meanwhile,

415

there is obvious temporally decreasing trend in δ114/110Cd and δ66Zn values from stages 1 to 2, yet

416

the δ114/110Cd and δ66Zn values are almost same in stages 2 and 3 (Fig. 11A and B). In view of the

417

fact that stage 3 is a transitional stage between the two pulses of mineralization, the overprint by

418

the second pulse of mineralization have also changed the Cd isotopic compositions of sulfides in

419

stage 3 to some extent although there is no mixing of Cd sources.

420

On the whole, we propose that the kinetic Rayleigh fractionation related to vapor-liquid

421

partitioning is the main cause for Cd isotopic variations in Zhaxikang deposit, overprint by the

422

latter second pulse of mineralization and Cd isotopic fractionation between sphalerite-galena

423

mineral pairs also have some contribution.

424 425

5.4 Cd isotopic Rayleigh distillation model for vapor-liquid partitioning mechanism

426

To further verify the correctness of kinetic Rayleigh fractionation mechanism related to vapor-

427

liquid partitioning in section 5.3 and constrain some aspects associated with this cause for observed

428

Cd isotopic fractionations, we established a simple open system Cd isotopic Rayleigh distillation

429

model for Zhaxikang deposit. The objective of this model is to demonstrate that vapor-liquid

430

partitioning induced Cd isotopic fractionation can explain the measured δ114/110Cd values and the

431

temporally decreasing trend. For this discussion, we assume that the simplest explanation for Cd

432

isotopic fractionation that would lead to the temporally decreasing trend in δ144/110Cd values would 21

433

be a physiochemical change in hydrothermal fluid. The rationale for Cd isotopic fractionation

434

model is described below. The Rayleigh distillation equations are found in Sharp. (2007) and Yao

435

et al. (2016):

436

―1 – 103 δXvap= (δX0vap+ 103) × Fαmin

437

δXaq = (δXvap + 103) × α – 103

438

where X = Cd isotope ratio of interest, vap = vapor, aq = aqueous, 0 = original/starting Cd isotopic

439

composition, F = fraction of X/X0, α = fractionation factor defined as Xp/Xs (p = product of reaction,

440

s = substrate of reaction).

441

These two equations are used to predict Cd isotopic compositions given different fractions of

442

vapor and liquid phases. The following geological facts and rationale assumptions were applied in

443

construction of the Cd isotopic Rayleigh distillation model:

444

(1) In Zhaxikang deposit, the sphalerite is the dominated carrier for Cd, the Cd concentrations of

445

sphalerite are dozens higher than galena (Table 2). Accordingly, this Cd isotopic fractionation

446

model just compares with δ114/110Cd values of sphalerite;

447

(2) The above discussion has demonstrated that SEDEX genesis is most plausible for the Pb–Zn

448

mineralization in Zhaxikang deposit, consequently the initial δ144/110Cd value (–0.11%; Fig. 7) of

449

exhalative system (Wen et al., 2016) should be regarded as the δX0vap;

450

(3) Fractionation factors must be chosen accurately to fit the decreasing trend in δ144/110Cd values

451

so that the Rayleigh distillation model would generate values seen in the natural samples. Horner

452

et al. (2011) have studied the Cd isotopic fractionation for calcite in ocean environment, and found

453

that the αCalcite–Cd(aq) (0.99955 ± 0.00012) is insensitive to temperature, ambient [Mg2+] and

22

454

precipitation rate (across the range studied). In light of that the Pb–Zn mineralization has marine

455

genesis, the sphalerite exhibits a great Cd isotopic variation range (–0.51‰ to 1.01‰) and there

456

are large amounts of cogenetic carbonatite (Mn–Fe carbonate) in Zhaxikang deposit, the lowest

457

fractionation factor (0.99943; Horner et al., 2011) is selected in the construction of this model.

458

Fig. 14 is the established Cd isotopic Rayleigh distillation model that illustrates how δ114/110Cd

459

values of sphalerite change due to different proportions of vapor-liquid partitioning in the ore-

460

forming system (variable defined by F in equations above), which is suitable for the δ144/110Cd

461

values of sphalerite in Zhaxikang deposit (Fig. 14A). The total δ144/110Cd values (–0.51‰ to 1.01‰)

462

cover the F (vapor-liquid ratios) ranging from 5.23% to 74.23% (Fig. 14A), and the concentrated

463

δ144/110Cd values (–0.34‰ to 0.10‰) define the F range of 25.48% – 55.07% (Fig. 14A).

464

Importantly, these F ranges calculated from Cd isotopic Rayleigh distillation model are in keeping

465

with previous Zn isotopic Rayleigh distillation model for vapor-liquid partitioning (Fig. 14B; Wang

466

et al., 2018a) and fluid inclusions data of Zhaxikang deposit (Fig. 14C; Yu, 2015). When the mean

467

δ66Zn value of bulk earth (0‰; Chen et al., 2013) is considered as the initial value (δ66Zni), the

468

concentrated δ66ZnAA-ETH values (–0.38‰ to 0.07‰) of sphalerite cover the F range of 26.70% –

469

48.70% in the Zn isotopic Rayleigh distillation model (Fig. 14B; Wang et al., 2018a). Furthermore,

470

the vapor-liquid two-phase fluid inclusions are in dominance (more than 90%) with the vapor-

471

liquid ratios ranging from 10% to 70% and concentrating on 20% – 50% (Yu, 2015), which are

472

also in line with those of Cd isotopic Rayleigh distillation model (Fig. 14C). The aforesaid

473

comparisons further evidence the reasonability of vapor-liquid partitioning mechanism for Cd

474

isotopic fractionation in Zhaxikang deposit.

23

475 476

6. Conclusions

477

In the case study of Zhaxikang deposit, the kinetic Rayleigh fractionation related to vapor-

478

liquid partitioning has been proved as an important Cd isotopic fractionation mechanism in

479

hydrothermal ore-forming system, which is the main cause for observed Cd isotopic variations in

480

Zhaxikang deposit. The liquid-vapor transitions are processes associated directly with formation of

481

ore deposits, genesis of igneous rocks, and meteorite condensation processes. Consequently, this

482

paper is beneficial to further identify the Cd isotopic fractionation mechanism in ore, rock,

483

sediments and solar system forming processes, which will contribute to future research efforts.

484 485 486 487

Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the submitted work.

488 489

Acknowledgments

490

We would like to express our gratitude to three anonymous reviewers, for their constructive

491

comments and suggestions that have largely improved this manuscript, and Associated Editor Dr.

492

Yanbo Cheng and Editor-in-Chief Prof. Huayong Chen for the efficient editorial handling. Mr.

493

Matthew Gonzalez (Pennsylvania State University) are also thanked for aid in measuring and

494

access to the Neptune instruments. We acknowledge the financially support by the Deep Resources

495

Exploration and Mining, National Key R&D Program of China (2018YFC0604104,

24

496

2017YFC0601506), the China Postdoctoral Science Foundation Funded Project (2019M650785)

497

and the Open Research Project from the State Key Laboratory of Geological Processes and Mineral

498

Resources, China University of Geosciences (GPMR201811).

25

499

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721

Figure captions

722

Fig. 1. (A) Tectonic framework of the Himalayan Terrane (Yin, 2006); (B) Regional geological map of the North

723

Himalayan Metallogenic Belt (modified from Zheng et al., 2014; Wang et al., 2018a); (C) Geological map of the

724

Zhaxikang Sb–Pb–Zn–Ag deposit (modified from Zheng et al., 2012; Wang et al., 2018a); (D) Cross-section

725

along the Exploration Line 8 (modified from Wang et al., 2019).

726

IYZS: the Indus-Yarlung Zangbo Suture Zone; STDS: the South Tibet Detachment System; MCT: the Main

727

Central Thrust Fault; MBT: the Main Boundary Thrust Fault; MFT: the Main Frontal Thrust; NH: the North

728

Himalayan Tethys sedimentary fold belt; HH: the High Himalayan crystalline rock belt; LH: the Low Himalayan

729

fold belt; SH: the Sub-Himalayan tectonic belt.

730 731

Fig. 2. Ore paragenetic sequence within the Zhaxikang deposit (modified from Wang et al., 2018a).

732 733

Fig. 3. Hand specimen photographs of representative samples from the Zhaxikang deposit. (A) Stage 1 lamellar

734

sphalerite-pyrite-arsenopyrite and stage 2 massive and banded sphalerite-pyrite hosted by fine-grained Mn–Fe

735

carbonate. The mineral assemblage is in turn cross-cut by stage 4 quartz-boulangerite veins (cited from Wang et

736

al., 2018a). (B) Stage 1 lamellar sphalerite-pyrite-arsenopyrite and stage 2 massive sphalerite-pyrite hosted within

737

fine-grained Mn–Fe carbonate (cited from Wang et al., 2018a). (C) Stage 2 massive sphalerite-galena-pyrite

738

hosted by coarse-grained Mn–Fe carbonate. (D) Stage 2 massive, globular and concentric annular sphalerite-

739

pyrite hosted by coarse-grained Mn–Fe carbonate (cited from Wang et al., 2018a). (E) Stage 3 quartz veins cross-

740

cut the pyrite-sphalerite to form the banded and brecciated texture. The sample also contains minor amounts of

741

early Mn–Fe carbonate. (F) Stage 3 brecciated sphalerite within stage 3 quartz-calcite (cited from Wang et al.,

32

742

2018a). (G) Stage 3 sphalerite-galena veins cross-cut by stage 6 quartz-calcite veins (cited from Wang et al.,

743

2018a). (H) Stage 4 boulangerite-quartz (cited from Wang et al., 2018a). (I) Elongate stage 5 stibnite hosted by

744

stage 5 quartz.

745

Abbreviations are as follows: Mcar1 = stage 1 fine-grained Mn–Fe carbonate; Apy1 = stage 1 lamellar

746

arsenopyrite; Py1 = stage 1 lamellar pyrite; Sp1 = stage 1 lamellar sphalerite; Mcar2 = stage 2 coarse-grained

747

Mn–Fe carbonate; Py2 = stage 2 pyrite; Sp2 = stage 2 sphalerite; Gn2 = stage 2 galena; Py3 = stage 3 pyrite; Sp3

748

= stage 3 sphalerite; Gn3 = stage 3 galena; Ccp3 = stage 3 chalcopyrite; Qtz3 = stage 3 quartz; Cal3 = stage 3

749

calcite; Blr4 = stage 4 boulangerite; Qtz4 = stage 4 quartz; Stb5 = stage 5 stibnite; Qtz5 = stage 5 quartz; Cal6 =

750

stage 6 calcite; Qtz6 = stage 6 quartz.

751 752

Fig. 4. (A) Plot of Cd vs Zn concentrations for galena from Zhaxikang deposit; (B) Plot of Cd vs Zn

753

concentrations for sphalerite from Zhaxikang deposit; (C) The comparison in Cd concentrations of galena

754

between Zhaxikang and Fule MVT deposits (the data of Fule MVT deposit is cited from Zhu et al., 2017).

755 756

Fig. 5. The (A) BSE image (cited from Wang et al. 2018a) and (B) photomicrograph for ore samples from

757

Zhaxikang deposit. Abbreviations are as Fig. 3.

758 759

Fig. 6. Distribution of Zn/Cd ratios versus δ144/110Cd values for sphalerite in different mineralization systems,

760

compared with Zhaxikang deposit (modified from Wen et al., 2016).

761 762

Fig. 7. Evolution of δ144/110Cd values during the deposition of aqueous Cd in different hydrothermal fluids,

763

compared with Zhaxikang deposit. Dashed lines represent the evolution of the deposited minerals, solid lines 33

764

represent the evolution of residual aqueous Cd, grey fields represent the observed fractionation range of the

765

sphalerite samples (modified from Wen et al., 2016).

766 767

Fig. 8. (A) Diagram of δ114/110Cd values versus Cd concentrations of sphalerite in Zhaxikang deposit; (B) Diagram

768

of δ114/110Cd values versus Cd concentrations of galena in Zhaxikang deposit.

769 770

Fig. 9. The δD–18O H2O diagram of Mn–Fe carbonate in Zhaxikang deposit (the H–O isotopic data are cited

771

from Xie et al., 2017; and the base map are modified from Yang et al., 2009).

772 773

Fig. 10. (A) The Cd isotopic fractionation within galena-sphalerite coexisting mineral pairs in Zhaxikang deposit;

774

(B) The Cd isotopic equilibrium fractionation diagram of galena-sphalerite coexisting mineral pairs in Zhaxikang

775

deposit.

776 777

Fig. 11. (A) The temporally decreasing δ114/110Cd values from stages 1 to 3 for sphalerite in Zhaxikang deposit;

778

(B) The temporally decreasing δ66Zn values from stages 1 to 3 for sphalerite in Zhaxikang deposit.

779 780

Fig. 12. The temporally decreasing trend in δ66Zn and δ56Fe values within concentric ore samples from Zhaxikang

781

deposit (modified from Wang et al., 2018a).

782 783

Fig.13. Conceptual model illustrating how Cd partition among mineralizing vapor and liquid phases. Box

784

indicates the minerals formed, pentagon represents the vapor and ellipse indicates the starting hydrothermal fluid

34

785

(modified from Yao et al., 2016).

786 787

Fig.14. (A) The Cd isotopic Rayleigh distillation model for vapor-liquid partitioning, comparing with the total

788

and concentrated δ144/110Cd values of sphalerite in Zhaxikang deposit; (B) The Zn isotopic Rayleigh distillation

789

model for vapor-liquid partitioning, comparing with the concentrated δ66Zn values of sphalerite in Zhaxikang

790

deposit (cited from Wang et al., 2018a); (C) Comparison between the F ranges of sphalerite in Cd isotopic

791

Rayleigh distillation model for vapor-liquid partitioning and the vapor-liquid ratios of vapor-liquid two-phase

792

fluid inclusions in Zhaxikang deposit (Yu, 2015).

793 794 795 796

Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the submitted work.

797 798 799

Highlights

800

(1) Vapor-liquid partitioning is main cause for observed Cd isotopic variations in Zhaxikang

801

deposit.

802

(2) The SEDEX genesis is most plausible for the Pb–Zn mineralization in Zhaxikang deposit.

803

(3) Kinetic Rayleigh fractionation related to vapor-liquid partitioning is demonstrated as an

804

important Cd isotopic fractionation mechanism in hydrothermal ore-forming system.

805

(4) Cd isotopic fractionation induced by vapor-liquid partitioning can be used to identify ore, rock,

806

sediments and solar system forming processes. 35

807 808 809

810 811 812

813 814

Table 1 The cup configuration of the MC-ICP-MS for Cd isotopic analyses. Cup H4 L4 L2 L1 Ax H1 H2 Number 116Sn 107Ag 109Ag 110Cd 111Cd 112Cd 113Cd Isotope

H3 114Cd

Table 2 The Zn and Cd concentrations and δ114/110Cd values of sulfide samples from the Zhaxikang deposit. Sample Number Mineral Zn (%) Cd (ppm) Zn/Cd δ114/110CdNIST SRM 3108 2σ (‰) ZXK-PD9-B2-1 Sp1 40.456 1183 342 1.01 0.04 Stage 1 lamellar spha sphalerite-pyrite hos D52-3 Sp1 58.440 1606 364 0.10 0.04 Stage 1 lamellar spha and banded sphale carbonate. The mine quartz-boulangerite v 14-6 Sp1 55.308 1543 358 –0.13 0.04 Stage 1 lamellar spha sphalerite hosted by 14-9 Sp1 50.618 1202 421 –0.30 0.04 Stage 1 lamellar spha sphalerite hosted by also contains a Mn–F columnar quartz and ZXK-PD9-B2-2 Sp2 49.599 1581 314 –0.10 0.04 The same as ZXK-PD 9-4 Sp2 53.821 1437 375 –0.09 0.04 Stage 2 globular and 9-4 Gn2 0.030 14 22 –2.19 0.04 coarse-grained Mn–F ZXK-12-B134 Sp2 50.325 1961 257 –0.51 0.04 Massive ore contain ZXK-12-B134 Gn2 0.030 22 14 2.24 0.04 stage 2 coarse-graine of quartz grains. ZK007-B4 Sp3 54.433 2199 248 –0.34 0.04 Stage 3 brecciated an ZK007-B4 Gn3 0.121 26 46 0.01 0.04 calcite, and the sam hosted by sphalerite. ZXK-XC-3 Sp3 50.521 1655 305 –0.23 0.04 Stage 3 quartz vein ZXK-XC-3 Gn3 0.153 19 82 –0.21 0.04 banded and brecciate Abbreviations: Sp1= stage 1 sphalerite, Sp2 = stage 2 sphalerite, Sp3 = stage 3 sphalerite, Gn2 = stage 2 galena, Gn3 = stage 3 galena.

815

36