Hydrothermal processes in the Edmond deposits, slow- to intermediate-spreading Central Indian Ridge

    Hydrothermal processes in the Edmond deposits, slow- to intermediatespreading Central Indian Ridge Hong Cao, Zhilei Sun, Shikui Zhai, Zhimin Cao, Wei Huang, Libo Wang, Yongjun He PII: DOI: Reference:

S0924-7963(16)30380-3 doi: 10.1016/j.jmarsys.2016.11.016 MARSYS 2913

To appear in:

Journal of Marine Systems

Received date: Revised date: Accepted date:

26 April 2016 4 October 2016 12 November 2016

Please cite this article as: Cao, Hong, Sun, Zhilei, Zhai, Shikui, Cao, Zhimin, Huang, Wei, Wang, Libo, He, Yongjun, Hydrothermal processes in the Edmond deposits, slowto intermediate-spreading Central Indian Ridge, Journal of Marine Systems (2016), doi: 10.1016/j.jmarsys.2016.11.016

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Hydrothermal processes in the Edmond deposits,

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slow- to intermediate-spreading Central Indian

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Ridge

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Hong Caoa,b,c, Zhilei Suna,b*, Shikui Zhaib,c, Zhimin Caob,c, Wei Huanga,b, Libo Wanga,b, Yongjun Hea a

Key Laboratory of Marine Hydrocaobon Resources and Environment Geology MLR, Qingdao Institute of Marine

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Geology, Qingdao, China b

Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology,

Qingdao, China c

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China Key Laboratory of Submarine Geosciences and Technology, Ministry of Education, Department of Marine

Geoscience, Ocean University of China, Qingdao, China

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Corresponding author: Zhilei Sun, [email protected], Tel.: +86 (532) 85723759, Fax: +86 (532) 85720553.

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ABSTRACT

The Edmond hydrothermal field, located on the Central Indian Ridge (CIR), has a distinct mineralization history owing to its unique magmatic, tectonic, and alteration

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processes. Here, we report the detailed mineralogical and geochemical characteristics of hydrothermal metal sulfides recovered from this area. Based on the mineralogical investigations, the Edmond hydrothermal deposits comprise of high-temperature Fe-rich massive sulfides, medium-temperature Zn-rich sulfide chimney and low-temperature Ca-rich sulfate mineral

assemblages.

According to

these

compositions, three distinctive mineralization stages have been identified: (1) low-temperature

consisting

largely

of

anhydrite

and

pyrite/marcasite;

(2)

medium-high temperature distinguished by the mineral assemblage of pyrite, sphalerite and chalcopyrite; and (3) low-temperature stage characterized by the mineral assemblage of colloidal pyrite/marcasite, barite, quartz, anglesite. Several lines of evidence suggest that the sulfides were influenced by pervasive low-temperature diffuse flows in this area. The hydrothermal deposits are relatively

ACCEPTED MANUSCRIPT enriched in Fe (5.99–18.93 wt%), Zn (2.10–10.00 wt%) and Ca (0.02–19.15 wt%), but display low Cu (0.28–0.81 wt%). The mineralogical varieties and low metal content of sulfides in the Edmond hydrothermal field both indicate that extensive

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water circulation is prevalent below the Edmond hydrothermal field. With regard to trace elements, the contents of Pb, Ba, Sr, As, Au, Ag, and Cd are significantly higher

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than those in other sediment-starved mid-ocean ridges, which is indicative of contribution from felsic rock sources. Furthermore, the multiphase hydrothermal activity and the pervasive water circulation underneath are speculated to play

understanding

about

the

complex

mineralization

process

in

slow-

to

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intermediate-spreading ridges globally.

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important roles in element remobilization and enrichment. Our findings deepen our

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1. Introduction

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field; Central Indian Ridge

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Keywords: hydrothermal deposits; mineralogy; geochemistry; Edmond hydrothermal

Since the discovery of active hydrothermal venting on the East Pacific Rise (EPR) at 21°N in 1977 (Corliss et al., 1979), more than 300 high-temperature hydrothermal

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venting sites have been located in the global ocean. However, most of these sites are located in the Pacific and Atlantic Ocean (Baker and German, 2013), only a few hydrothermal fields have been identified in the Indian Ocean (Gamo et al., 2001; Hashimoto et al., 2001; Van Dover et al., 2001; Nakamura et al., 2012; Tao et al., 2012). The few explorations into hydrothermal systems in the Indian Ocean, culminated in year 2000 with the report of active hydrothermal fields in ridges (Herzig and Plüger, 1988; Plüger et al., 1990; Jamous et al., 1992; Gamo et al., 1996a; German et al., 1998). Exploration for hydrothermal activity in the Central Indian Ridge (CIR), a branch of the Indian Ocean, started with the GEMINO program in 1983 (Herzig and Plüger, 1988, Plüger et al., 1990). Since then, several investigations aiming at hydrothermal plumes or hydrothermal sulfides (Plüger et al., 1990; Halbach et al.,

ACCEPTED MANUSCRIPT 1996; Halbach et al., 1998) led to the discovery of the first vent field, Edmond, in the Indian Ocean.. In April 2001, the R/V Knorr (Voyage 162-13) and ROV Jason discovered the Edmond field at 23°S, 69°E (Von Damm et al., 2001). In 2005, the R/V

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Dayangyihao of China visited the Edmond field and successfully captured polymetallic sulfide samples by using a TV grab (Tao et al., 2007;

Zhu et al., 2008).

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The fluids from Edmond are extremely hot, with a temperature of up to 382°C, and very rich in Cl (up to 70% greater Cl content than the local seawater) (Gallant and Von Damm, 2006). This suggests a shallow magma chamber or deep magmatic

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intrusions that provide the thermal driving force for hydrothermal circulation. These lava flows (volatile) result in enrichment of precious metals (e.g., Au and Ag), which

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are probably the source for the hydrothermal mineralization system (Butterfield and Massoth, 1994; Yang and Scott, 1996).

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Hydrothermal deposits from different geological settings are generally expected

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to have different mineralogical and geochemical characteristics. Thus, the CIR sites provide an excellent opportunity to further understand the seafloor mineralization

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process. In addition, on slow spreading ridges, the tectonic environment is relatively stable and unique, as evidenced by the development of transform faults, low-frequency tectonic events, long episodic hydrothermal activity, large magma

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chamber. As the hydrothermal fluid reacts with the surrounding rocks for a prolonged period of time, a favorable environment is set for the formation of large hydrothermal polymetallic deposits (Rona, 1986; 1993; Charlou et al., 1998; Münch et al., 2001; Nakamura et al., 2015). Previous hydrothermal research studies on CIR focused primarily on fluid chemistry (Gallant and Von Damm, 2006; Kumagai et al., 2008), microbial mineralization (Peng et al., 2007), rare earth element (REE) (Zeng et al., 2015), and isotopes (Wang et al., 2012). However, much additional research is required in this region to level the understanding with other oceanic regions. Particularly, the mineralogical and geochemical characteristics of sulfides, the ore-forming mechanism, and metal sources on the CIR still need to be further investigated. Therefore, the principal goals of our research are to discuss ore genesis and the

ACCEPTED MANUSCRIPT environmental distribution of the ore-forming elements of sulfide on the CIR. We create a metallogenic model of this hydrothermal field with its tectonic background by studying the mineralogical and geochemical properties of the sulfides in detail. In

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order to address these questions, the polarizing microscope, X-ray diffraction (XRD), electron microprobe chemical analysis (EPMA), X-Ray Fluorescence (XRF),

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inductively coupled plasma mass spectroscopy (ICP-MS), and as well as inductively coupled plasma atomic emission spectroscopy (ICP-AES) have been employed.

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2. Geological setting

The CIR forms the southern extension of the Carlsberg Ridge and terminates at the Rodriguez Triple Junction (RTJ) The RTJ is a slow-to intermediate-spreading

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mid-ocean ridge with a full spreading rate of 50 mm yr-1(Demets et al., 1990),which progressively decreases from the RTJ (56.4 mm yr-1) towards the equator (36 mm yr-1)

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(Munschy and Schlich, 1989). The NW-to-NNW trending ridge (152°) is frequently

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segmented by transform faults and non-transform discontinuities, and the segments are between 25 and 85 km in length. The central valley of the CIR is 10–25 km wide

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and varies in water depth between 3,200 and 4,000 m (Briais, 1995). In particular, the third and fourth segments are characterized by steeper scarps than the other parts of the CIR axial valley. Further, there is evidence for hydrothermal convection in the

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subsurface driven by shallow axial magma chambers (Münch et al., 1999). The expansion of the mid-ocean ridge leads to thinning of the oceanic lithosphere, while the strong fault in the horizontal direction is a good channel for upwelling of the deep mantle; these two phenomena can lead to considerable magma upwelling and the formation of oceanic lithosphere in a short period of time (Nakamura et al., 2015). The Edmond hydrothermal field, located at 23°52.68′S, 69°35.80′E on the northern end of segment S3 of the CIR at a water depth of 3,290–3,320 m, is on a small protrusion from the eastern rift valley wall that is ~6 km away from the ridge axis (Gallant and Von Damm, 2006). This section of the CIR has a full spreading rate of about 50 mm yr-1 (from 20°30′S to 25°30′S), which increases slightly from the North to the South (Munschy and Schlich,1989). The morphological characteristics of

ACCEPTED MANUSCRIPT the CIR are generally similar to those of a slow-spreading center, despite the intermediate spreading rate at these latitudes (Briais et al., 1995). The area of the hydrothermal field is about 6,000 m2, and black chimney fluid can still be observed

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erupting slowly through the deep-sea submersible vessel (Von Damm et al., 2001). In this region, magmatic activity is frequent, which results in the exposure of abundant

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fresh, hyaline pillow- and sheet-lava (N-MORB), always containing mainly olivine basalt and porphyritic basalt (Georgenet al., 2001).

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3. Sampling and analytical methods

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The hydrothermal samples (see Fig. 1 for sampling locations) were recovered using a TV-controlled grab during the R/V Dayangyihao DY105-17A cruise in 2005, from within a depth of approximately 3,292 m. The sampling records show that the

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temperature of the samples when taken to the deck was as high as 60°C.The

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temperature of the samples indicates that high-temperature fluid eruption was still prevalent in the hydrothermal area (Chen, 2012). The sulfide samples were porous,

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which indicated that they were relatively fresh;in addition, pyrites with a small grain size were also visible.

Polished thin sections of the sulfides were examined under a reflected and

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transmitted light polarizing microscope (Zeiss Axioskop 40 Pol) at the Qingdao Institute of Marine Geology to study the mineral occurrences and textural relationships. The XRD analysis was performed at the Ocean University of China using a Thermo ARL X’TRA diffractometer with CuKα radiation generated at 45 kV and 40 mA. Electron microprobe chemical analyses (EPMA) of individual mineral phases were carried out with a JEOL JXA-8100 at the First Institute of Oceanography, State Oceanic Administration. The quantitative analyses were carried out under the following conditions: accelerating voltage, 25 kV; beam current, 10 nA; focused beam, 5 μm. Each spot analyzed by the electron microprobe was marked on a back-scattered electron image for subsequent trace element analysis of the same

ACCEPTED MANUSCRIPT mineral grain. The standard minerals (USA SPI Supplies) used in the EPMA analyses included pyrite (FeS2), sphalerite (ZnS), marcasite (FeS2), chalcopyrite (CuFeS2), anhydrite (CaSO4), barite (BaSO4), covellite (CuS), anglesite (PbSO4), quartz(SiO2),

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as well as the pure metals native sulfur (S), nickel (Ni), cadmium (Cd), antimony (Sb), and selenium (Se).

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Bulk chemical analyses were carried out using XRF, ICP-AES, and ICP-MS. First, the products were cleaned using ultrapure water, dried at low temperature and ground into a powder (less than 200 meshes in size). Then, the powder was dried at

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105°C for 3 h in an oven. Finally, it was cooled for 24 h in a dryer. Powder samples weighing 0.04 g were placed in a digestion tank where they were digested in

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approximately 2 ml of an acid solution (HNO3:HF=10:1). Then, the mixture of sample and acid solution was heated at 150 °C for 48 h in a closed vial. The solutions

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were transferred into a Polytetrafluoroethylene (PTFE) crucible and then heated on an

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electric heating plate, and when they were almost dry, 2 mL of 2 mol/L HCl was added and heated again for 12 h in closed vials. Next, the solution was transferred into

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vials, and then diluted to 20 g with 2% HNO3 (solution A). After shaking, 5 g of the solution was separated and diluted to 20 g with 2% HNO3 again(solution B). Finally, solution A and solution B were analyzed by ICP-AES (OPTIMA4300, Perkin Elmer)

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and ICP-MS (Agilent 7500c, Agilent Technologies), respectively, at the Element and Isotope Analysis Laboratories of Marine Geosciences, Ocean University of China.

4. Results 4.1. Mineralogical characterization of the hydrothermal sulfides The samples from the Edmond hydrothermal field can be classified into three groups: Fe-rich massive sulfide (Fig. 2a-d), Zn-rich sulfide chimney (Fig. 2e) and Ca-rich sulfate (Fig. 2f). 4.1.1.

Fe-rich massive sulfides

Fe-sulfides (pyrite and marcasite) (~40 vol.%) and chalcopyrite (~10 vol.%) are

ACCEPTED MANUSCRIPT the major components, sphalerite (~4 vol.%), and barite (~2 vol.%) are the minor components of the massive sulfides (TVG7-3a, TVG7-4, TVG7-5, TVG7-17). Pyrite is mainly found in the form of fine-grained aggregates (Fig. 3a), and occasionally, a

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small amount of euhedral grains are visible in the mineral pores (Fig. 3b). In some cases, pyrite and marcasite occur as multiple crystallizations in oolites (Fig. 3c, d). A

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prominent thin chalcopyrite vein is observed in the inner part of the Fe-rich massive sulfides, replacing pyrite along its edge and fissure. Sphalerite always coexists with or rims chalcopyrite, with a sharp contact boundary observed between the two sulfides

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(Fig. 3a). Locally, rare fine-grained covellite is observed (Fig. 3f). In addition, barite

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often occurs as blades, filling voids or interpenetrating early-stage sulfides. 4.1.2. Zn-rich chimney

Sample TVG7-9 is a fragment of chimney and mainly consists of sphalerite (50–60

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vol.%), Fe-sulfides (pyrite and marcasite: 30–40 vol.%), and minor chalcopyrite (~1

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vol.%). Sphalerite mainly occurs as (1) colloform textures (Sp2) on the exterior

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surface of the chimney and as (2) fine-grained grains (Sp1) in the inner zone of the chimney, with minor pyrites filling in its vugs (Fig. 4a). Colloform sphalerite (Fig. 4b) commonly rims granular sphalerite, exhibits a wide range of color from

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yellow-orange to dark red, and is variably opaque to translucent under transmission light (Fig. 4c). These differences reflect variations in the Fe content. The line-scanning spectrum of sphalerite (as shown in Fig. 4c, in the direction of the arrow) shows a wide range of Fe content (Fig. 4d). Moreover, a rhythmic texture (Fig. 4e) and corroded vugs (Fig. 4e, f) are found in sphalerite. Certain portions of coarse-grain sphalerite spotted chalcopyrite or pyrite, so-called “chalcopyrite “disease” , or pyrite “disease”, the chalcopyrite “disease” with a length less than 50 μm (Fig. 4g), which has been documented previously (e.g., Barton and Bethke, 1987). Early-stage pyrite (Py1) is present as coarse grains, whereas euhedral crystals are rarely formed in open space (Fig. 4h). Late-stage pyrite (Py2) generally forms “framboids”, which subsequently coalesce to give rise to larger forms and recrystallize to form bigger euhedral grains (Fig. 4i). The pyrite “disease” (Py3) texture ranges in size from 1 to

ACCEPTED MANUSCRIPT 15 µm (Fig. 4j). Occasionally, a small amount of marcasite surrounds colloform sphalerite as a shell (Fig. 4k). Locally, marcasite forms euhedral crystals (Fig. 4l) or a radial pyrite-marcasite sphere, with obvious alternating growth zones (Fig. 4m).

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Fossil worm tubes (120×350 µm) can be observed outside the sulfides (Fig. 4n), most of which arefilled with amorphous silica and/or Fe-sulfides and a few of which are

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filled with sphalerite. Additionally, minorlate-stage minerals, such as barite (~5 vol.%) (Fig. 4o), quartz (Fig. 4p), rare anglesite (Fig. 4q), and native sulfur are confined to vugs or fractures (Fig. 4r); however, quartz, anglesite and native sulfur only associate

Ca-rich sulfate

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4.1.3.

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with Zn-rich chimney.

Sample TVG7-11 is divided into Ca-rich sulfate, predominantly composed of anhydrite (~50 vol.%), pyrite and marcasite (both accounting for ~30 vol.%). In this

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sample, anhydrite occurs as a blade or radial texture (Fig. 5a), often replaced by pyrite

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along natural fissures. Pyrite is generally observed to have:(1) a colloform

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morphology, commonly showing intergrowth with marcasite (Fig. 5b), or (2) a grained morphology, occurring as veins in the fissure of anhydrite (Fig. 5c). Intriguingly, a small amount of biogenic framboidal pyrite can also be observed (Fig.

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5d).

4.2. Mineral chemistry Pyrite also contains Cu (up to 0.69 wt%), Zn (up to 2.14 wt%), Pb (up to 1.49 wt%) and Mo (0.70–1.00 wt%), as well as trace amounts of Ni, Co, Cd, Au and Ag (Table 1, details see Table S1). The As content is significant at about 0.44 wt%. The average S/Fe ratio in pyrite is 2 and 1.86 in Fe-rich massive sulfides and Zn-rich chimney, respectively. Therefore, the Zn-rich chimney is formed in a relatively sulfur-poor environment, as the sulfur content is lesser than that stipulated in its stoichiometric formula. With regard to trace elements, Zn-rich chimney is much more enriched with Pb than Fe-rich massive sulfides. The average content of Cu in generation I and generation II Zn-rich chimney is 0.15 wt% and 0.02 wt%, respectively; Zn, 0.12 wt%

ACCEPTED MANUSCRIPT and 1.33 wt% respectively; and Pb, 0.24 wt% and 0.52 wt%, respectively. Thus, the generation II pyrite has a lower Cu content and higher Zn and Pb content than the generation I pyrite.

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The average S/Fe atomic ratio of marcasite is 1.87 (Table 2), which shows that the sulfur content is poor. With regard to trace elements, the Zn (up to 1.80wt%), Pb (up

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to 0.30wt%) and Mo (up to 0.95 wt%) content is considerably higher. Compare with the symbiotic pyrite, the As content of marcasite is higher (up to 0.30 wt%, while it is below detection limit in the pyrite), while the Zn content is lower (the average Zn

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content in marcasite and pyrite is 0.83 wt% and 1.29 wt%, respectively) than that in the pyrite.

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Sphalerite is found both in Fe-rich massive sulfides and Zn-rich chimney, but its chemical composition is distinct in both samples (Table 3, details see Table S2).

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Sphalerite in Fe-rich massive sulfides generally contains 11.10 to 27.23 wt% Fe, and

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is therefore classified as high Fe-bearing sphalerite and even super-high Fe-bearing sphalerite. In Zn-rich chimney, sphalerite has a much lower Fe content (from 2.06 to

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6.92 wt%, based on 20 analyses). At the same time, sphalerite at difference stages has different chemical compositions in Zn-rich chimney: early-stage sphalerite (Sp1) is characterized by higher Fe and Cu content (4.23–6.92 wt% and 0–0.52 wt%

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respectively) than late-stage sphalerite (Sp2) (2.29–4.42 wt% and 0–0.12 wt% respectively). In addition, in Zn-rich chimney, the Fe and Cu content in chalcopyrite disease or pyrite disease sphalerite is much higher than that in normal forms of sphalerite at the same stage. The Cu/Fe atomic ratio of chalcopyrite (CuFeS2) varies from 0.89 to 1.01 (Table 4, details see Table S3), which differs from its theoretical value of 0.2 at % (Lafitte and Maury, 1983), but it is still in the range of the Cu/Fe ratio of inland chalcopyrite, not belonging to stoichiometric Fe-rich chalcopyrite type, which is a typical high-temperature mineral found in seafloor sulfide deposits (Caye et al., 1988). The Zn content of chalcopyrite is all below detection limit. Chalcopyrite is sometimes enriched in Mo(up to 0.77 wt%), Pb (up to 0.31wt%) and Ag (up to 0.35wt%). Fine-grained anglesite (PbSO4) in the Zn-rich chimney contains 64.01–68.15 wt%

ACCEPTED MANUSCRIPT Pb, 10.50–11.14 wt% S, and 19.09–20.11 wt% O, and the Pb/S atomic ratio ranges from 0.89 to 1 (Table 5), which is close to the theoretical value.

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4.3. Bulk chemistry of the deposits

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The bulk chemical compositions of the Edmond deposits are presented in Table 6.

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The high variability of the Fe (5.99–18.93 wt%), Zn (2.10–10.00 wt%), and Ca (0.02–19.15 wt%) content reflects the mineralogical

heterogeneity. These

hydrothermal deposits are, in general, characterized by relatively high Fe, Zn, and Ca

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content and low Cu content, which reflects the dominance of pyrite, sphalerite and anhydrite.

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Most of the samples have high concentrations of Pb (210–1,606 ppm), Mn (192–530.4 ppm), As(131–490 ppm), Co (56.93–455.6 ppm) and Sr (338.5–916.2 ppm), and the variations in the concentration of Cd (23.24–221.9 ppm) and Ba

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(14.49–1943 ppm) are remarkable. Furthermore, the high concentrations of precious

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metals (up to 8.44 ppm Au and more than 115.5 ppm Ag) in individual samples are

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notable.

5. Discussion

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5.1. Typical mineral structures Chalcopyrite “disease” is prevalent in sphalerite grains. Regarding its genetic formation, there are two alternative viewpoints. While some scientists believe it is a result of the replacement of Zn-sulfide by chalcopyrite (Barton, 1978; Barton and Bethke, 1987), others argue that it is an exsolution of chalcopyrite, which means that it is sphalerite which has been contaminated by copper (Ramdohr, 1980; Peng and Zhou, 2005). In this study, EMPA analysis of the sphalerite grains showed that the “diseased” sphalerite has higher Fe and Cu content (4.48–5.88 wt% and 0.24–0.53 wt%, respectively) and lower Zn, Cd and Ag content than other forms of sphalerite. Elemental mapping analysis also showed that the Fe content is much higher around

ACCEPTED MANUSCRIPT the solution vugs (Fig. 6a, b), indicating the existence of an Fe-rich rim. Based on these findings, we believe that introduction and contamination by late-stage hydrothermal fluid enriched in Fe and Cu along the weak zone and mineral fracture of

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sphalerite aggregates are responsible for the significantly higher Fe and Cu content in sphalerite.

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Sphalerite spotted pyrite also has higher Fe and Cu content (4.29–5.41 wt% and 0.14 to 0.38 wt%, respectively) than normal forms of sphalerite, whereas the pyrite “disease” is depleted in Fe and enriched in Zn (24.56–43.69 wt% and 5.59–36.65 wt%

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respectively). Thus, it seems that the cause of pyrite “disease” is the same as that for chalcopyrite “disease”: that is, the invasion and consequent contamination or leaching

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of primary sulfides by later hydrothermal fluid enriched in Fe results in the distinct pyrite disease texture.

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5.2. Sequence and temperature evolution during mineralization

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The mineralogical analyses indicate that the hydrothermal minerals display distinct

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changes in texture according to the generation they belong to, and different generations are characterized by obviously different elemental compositions. With regard to the Edmond (69.6°E) hydrothermal area, the juxtaposition of Ca-rich sulfate,

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Fe-rich massive sulfides, and Zn-rich chimney reflects a relatively complete mineral precipitation sequence of the early-middle-late stage, namely, from (1) anhydrite + colloform pyrite/marcasite, to (2) pyrite + sphalerite + chalcopyrite, and to (3) colloform pyrite/marcasite + barite + quartz (Fig. 7). According to the mineral structure and spatial relationship between each mineral, we have tentatively reconstructed the formation of hydrothermal sulfide as follows: First, venting of hot hydrothermal fluids, which immediately mix with ambient cold seawater, resulting in the precipitation of anhydrite and colloform pyrite/marcasite and formation of an embryonic edifice of chimney (Haymon and Kastner, 1981; Goldfard, 1983; Hekinian, 1983). Such mass quantities of anhydrite are speculated to be related with the mixing of high-temperature hydrothermal fluid and the infiltrated seawater (Gallant and Von Damm, 2006). During this stage, colloform

ACCEPTED MANUSCRIPT pyrite/marcasite replaces the fossil worm tubes, which are generally present on the outer wall of the edifice. As the porosity of the outer wall decreases gradually, the fluid temperature increases inside sulfide chimney. As a result, high-temperature

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granular pyrite and sphalerite are formed (Tivey and Delaney, 1986).

Second, as the temperature rises once again, a large number of chalcopyrite

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precipitates accompanied by a small amount of sphalerite replace the early pyrite along the mineral cracks and margins. At this point, chalcopyrite and pyrite inclusions occurred within the sphalerite grains (Barton, 1987). This sphalerite characterized by

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a high Fe content (average 20.26 wt%) is formed as a consequence of impregnation by late-stage hydrothermal fluid enriched in Cu and Fe. The variation in the elemental

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composition of sphalerite grains (mainly Fe) indicates that the hydrothermal temperature or chemical composition may periodically change (as evident from the

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rhythmic texture of sphalerite). However, the possibility of intermittent eruption

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cannot be ruled out merely on the basis of what is currently known about this process. Finally, the hydrothermal activity or temperature is reduced, and the external

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seawater gradually permeates into the interior of the deposit, triggering a turbulence state in the physical and chemical environment. Colloform pyrite and marcasite precipitate as alternations (according to previous studies, the favorable conditions for

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marcasiteis pH<4.5 and temperature <200°C, while for pyrite it is pH>4.5) (Schoonen and Barnes, 1991). Further, the penetration of seawater leads to an increase in oxygen fugacity. Consequently, barite, quartz, covellite and other low-temperature hydrothermal minerals (e.g., anglesite) are precipitated gradually. Under this condition, quartz infills in open spaces, which promotes the consolidation of the chimney (Tivey and Delaney, 1986; Halbach et al., 1997). Moreover, as the fluid temperature decreases, excess sulfur also precipitates as native sulfur in relatively closed spaces (e.g., Halbach et al., 1989; Zhai et al., 2001). 5.3. Hypothetical source of metals in the Edmond sulfides The Edmond field represents a typical hydrothermal system on the sediment-starved mid-ocean ridge. The composition of sulfides on Edmond is similar

ACCEPTED MANUSCRIPT to that of hydrothermal sulfides recovered from other sedimented ridges and/or back-arc basins (Table 8, Fig. 10). Therefore, the reason for the depletion of certain metals (for instance, the Cu + Fe + Zn content ranges only from 15.33 to 22.63 wt%)

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and the high concentration of Pb (presumably due to the presence of a small amount of anglesite, which was identified microscopically), Ba, Sr, Cd, As, Au and Ag needs

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to be explored. In general, the bulk chemical composition of submarine hydrothermal deposits is mainly influenced by the original hydrothermal fluid and rock types from which the metals were leached (Doe, 1994).

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With respect to the difference between the sulfides on the Edmond and other hydrothermal systems, the characteristics of the hydrothermal fluid are presumably

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the most influential factors. The fluid from Edmond is characterized by a very high temperature (up to 382°C) and an enrichment of chlorine (up to ~70% more than that

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in local seawater) and transition metals (Cu, Fe, Zn) (Gallant and Von Damm, 2006).

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This suggests the existence of shallow magma chambers or magma intrusions in the deep basement, which probably provide the thermal energy for hydrothermal

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circulation. Moreover, the lava flow (volatile) enrichment of precious metals (e.g., Au and Ag) can potentially provide the basic material for the hydrothermal mineralization system (Butterfield and Massoth, 1994; Yang and Scott, 1996). However, the

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composition of sulfides on Edmond is distinctly inconsistent with the hydrothermal fluid therein. We speculate that the low metal content is a result of the presence of abundant gangue minerals (anhydrite, barite, and silicates)that dilute the Cu, Fe and Zn content, or a result of the pervasive water circulation, which is mainly influenced by the transform faults and the absence of transform faults underneath the hydrothermal deposit (Von Damm et al., 2001). Gallant and Von Damm (2006) previously proposed that the mass of anhydrite is related with the mixing of high-temperature hydrothermal fluid and the infiltrated seawater. In addition, an increase in the mixing between hydrothermal fluid and ambient water is believed to lead to sequestration of large amounts of metal elements in the form of sulfide stockworks, a decrease in the concentration of metal elements in the residual hydrothermal fluid, and subsequently, alteration in the composition of metal sulfides (e.g.,

ACCEPTED MANUSCRIPT Humphris et al., 2015). Concerning the enrichment of some metals, the nature of the wall rocks is presumably the critical factor. In the mid-ocean ridge system, the dissolution of

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primary sulfides and the leaching of ferromagnesian minerals in basalts are the major sources of Fe, Cu and Zn. However, other elements, such as Pb and Ba, are mainly

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derived from the replacement of feldspar, which is particularly abundant in felsic volcanoes (Halbach et al., 1993; Hannington et al., 2013). Correspondingly, the sulfides from the Edmond are characterized by high concentrations of Pb, Ba, Sr, As,

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Au, Ag, and Cd, which are rather unusual for a modern mid-ocean ridge basaltic environment. Therefore, we inferred that some felsic rocks may have influenced the

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composition of hydrothermal sulfide on the Edmond on the basis of the compositions of sphalerite and pyrite in this area.

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The Zn/Cd ratio of sphalerite has been accepted as an indicator of the

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classification of volcano–sedimentary, hydrothermal and skarn–hydrothermal deposits as well as metamorphosed sedimentary deposits in several studies (Jonasson and

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Sangster, 1978; Viets et al., 1992; Zaw and Large, 1996; Eyuboglu et al., 2015). Gottesman and Kampe (2007) suggested that high Zn/Cd ratios of over 500 are derived from basaltic rocks, whereas moderate ratios (330–430) are derived from

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andesitic rocks. Likewise, Zn/Cd ratios lower than 250 are an indication of felsic rock sources. Demir et al. (2015) also emphasized that the lower Zn/Cd ratios (<250) in sphalerite minerals from the hydrothermal Köstere deposit (Gümüşhane, NE-Turkey) were related to granitic intrusions. The Zn/Cd ratios of sphalerite in the Edmond region display a wide range (57–2,621), but the ratio is lower than 250 in most of the samples (n=22), and it exceeds 500 in only a few samples (n=6). These findings imply that the hydrothermal sulfides on the Edmond are distinctly related to felsic magmatism. Additionally, the Co and Ni contents and the Co/Ni ratio of pyrite have also been considered as indicators of ore genesis and formation conditions in many studies. Hydrothermal pyrite usually contains significantly lower Co and Ni contents in felsic rock than in mafic rock (Güleç and Erler 1983; Bajwah et al., 1987; Blevin and Chappell, 1991; Ho et al., 1995). Based on the depletion in Co and Ni in the

ACCEPTED MANUSCRIPT pyrites (<0.09 wt%), it is possible that the distinctive sulfides on the Edmond have felsic sources. Christie and Sinton (1981) have suggested that in oceanic spreading centers, because of the propagation of the segments into older oceanic crusts,

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extensive magma fractionation results in the production of intermediate to felsic melts, which may occur in isolated bodies. Rocks of granitic composition have also been

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found on the CIR near the Agro Fracture Zone (Engel and Fisher, 1975). Hence, we presume that in the Edmond field, there exist some felsic rocks that served as the sources of Pb, Ba, Cd, Ag, and As.

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Furthermore, as shown by the positive correlations of Pb with As, Cd, Ag and Ba (RPb-As=0.90, RPb-Cd=0.64, RPb-Ag=0.66, RPb-Ba=0.79) (Table 3), it seems that zone

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refining of the deposit through remobilization of elements has resulted in the enrichment of Pb, Ba, Sr, As, Ag, and Cd. These obvious correlations not only reflect

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the similar geochemical behavior of these elements in the medium-low temperature

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hydrothermal fluid, but also indicate a later leaching process in which the primary sulfide was altered by late-stage fluid under low-temperature conditions (probably

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less than 100°C) (Zeng, et al., 2011). Apart from these features, the concentration of Ag (up to 115.5 ppm) and Au (up to 8.44 ppm) on the Edmond is also considerable. An obvious relationship has been observed between Ag and several other elements,

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such as Pb, Cd, Mo and Ni (RAg-Cd=0.92, RAg-Mo=0.88, RAg-Ni=0.88, RMo-Ni=0.99, RMo-Cd=1.00, RNi-Cd=0.98), which indicates that they had undergone a similar geochemical process. Alternatively, it is possible that the Ag content is mainly controlled by Cd, Mo and Ni. In regard to Au, in previous studies, it was found that most Au-rich samples belong to either Zn-rich sulfides or Cu-rich sulfides (Murphy and Meyer, 1998; Fouquet et al., 2010; Zakharova et al., 2010). The Au/Ag ratio (0.001–0.190; mean, 0.001) of sulfides from the Edmond, except for TVG7-9b, conspicuously exceeds the range found in other global hydrothermal sites (0.001–0.023) (Hannington et al., 1991), hinting at a remarkable enrichment in Au at this site. Wu et al. (2013) also observed the Au enrichment in the Edmond field and inferred that this might be due to secondary enrichment or recrystallization of the early-stage “invisible Au.” In this study, Au and Sr show a significant positive

ACCEPTED MANUSCRIPT correlation with each other (RSr-Au=0.98), and Au is positively correlated with Cu, Zn and Ca (RCu-Au=0.72, RZn-Au=0.62, RCa-Au=0. 64) to some extent, yet shows a weak negative correlation with Fe and Pb (RFe-Au=-0.73, RPb-Au=-0.68). Based on these

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observations, we speculate that the high concentration of Au could be attributed to the late-stage leaching of Au from primary minerals (chalcopyrite, sphalerite and calcite)

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by upwelling hydrothermal fluid and pervasive seawater (Von Damm et al., 2001). Furthermore, the very high chlorine content in the hydrothermal fluid and the significant albitization below the Edmond field may have affected the concentration

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of Au, which can migrate in the form of the AuCl2- complex in hydrothermal fluid (Gallant and Von Damm, 2006; Hu, 2004). However, this theory needs to be explored

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further in the future.

5.4. Interpretation of the distribution of trace elements in sulfides

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The trace element distribution of the main sulfide minerals (Fig. 9) indicates that

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all the sulfide minerals are relatively enriched in Pb and Mo. However, the Pb content

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successively decreases in the order of from pyrite, to sphalerite and to chalcopyrite; this is consistent with the finding that Pb is usually present in low-temperature minerals (Hannington et al., 1999; Smith and Huston, 1992). The metals Cd and Ag

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are related to sphalerite. In fact, sphalerite is the main host mineral of Cd, and it is believed that Cd2+is substituted for Zn2+ in this mineral (Fig. 10a) (Cook et al., 2009). In addition, the Cd content is controlled by Pb to some extent (Fig. 10c), and the Ag content is partly controlled by Cd (Fig. 10b), which is also indicated by the close correlation between Cd and Ag (RAg-Cd =0.92) in the bulk chemistry observations. This reflects the leaching effect on the primary sulfide by late-stage hydrothermal fluid. In all sphalerites, Fe and Zn show a negative relationship (Fig. 10d), which is indicative of the substitution of Zn by Fe. Moreover, the substitution ratio of Fe to Zn increases with increase in temperature, as previously reported (Pan, 1994). Hence, the Fe content of sphalerite has been widely used as a temperature barometer, despite some limitations to the applicability of this method (Cook et al., 2009). The high

ACCEPTED MANUSCRIPT variation in the Fe content of sphalerite reflects the wide range of ore-forming temperatures. Moreover, the sphalerite precipitation temperature in Fe-rich massive sulfides is significantly higher than that in Zn-rich sulfide chimney. Several elements,

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including As, Fe and Mo, show positive correlations with Pb (Fig. 10e, f), which

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points out that they were all subjected to a similar geochemical process. 5.5. Illustration of mineralization

Based on the mineralization characteristics and elemental geochemistry discussed

in the Edmond field is proposed (Fig.11):

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in the previous subsections, the following conceptual model of sulfide mineralization

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The Edmond hydrothermal field is not located in the neovolcanic zone in the center of the ridge axis (the MESO hydrothermal field develops in this location). Instead, it is > 6 km away from the spreading center, towards the eastern wall of the

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axial valley, where there also exist shallow magma chambers or mafic magma

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intrusions below the basement of this hydrothermal field (or nearby rift), 1–2 km deep

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(Yang and Scott, 1996). These features represent a unique driving force and sources of precious metals (e.g., Au and Ag) for hydrothermal circulation in this field. Moreover, a large number of transform faults and non-transform faults provide

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channels for seawater circulation in this setting. Under the driving force of the shallow magma chamber, the infiltrated water is heated, as a result of which plentyof metal elements are leached out from the surrounding rock (mainly basalt), leading to the formation of hot brine enriched in metal elements (>300°C). The fluid from Edmond is characterized by a very high temperature (up to 382°C) and enrichment of transition metals (Cu, Fe, and Zn) (Gallant and Von Damm, 2006). In addition, the presence of felsic rocks increases the content of Pb, Ba, As, and other related elements, and the substantial subsurface mixing of hot brine with the infiltrated seawater, which supported by significant amount of diffuse flow (Van Dover et al., 2001) and the data from hydrothermal fluid (Gallant et al., 2006), results in abundant anhydrite precipitation as well as sequestration of large amounts of metal elements in the form of underlying stock works, and a decrease in the concentration of metal

ACCEPTED MANUSCRIPT elements in the residual hydrothermal fluid, and subsequently, alteration in the composition of metal sulfides (e.g., Humphris et al., 2015) (the Cu + Fe + Zn content only ranges from 15.33 to 22.63 wt%). The pervasive low-temperature diffuse flow

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provides a variety of favorable niches for microorganisms (Van Dover et al., 2001), leading to the development of a large number of benthic organisms, such as tube

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worms.

With regard to a single hydrothermal cycle, the increase in the fluid temperature inside the deposit during the waxing stage would trigger successive precipitation of

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pyrite, sphalerite and chalcopyrite, which would then replace the early-stage minerals. With the arrival of the waning stage of hydrothermal activity, external seawater

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gradually permeates into the interior of the chimney, as a result of which abundant low-temperature minerals, such as colloform pyrite, covellite, barite, anglesite and

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amorphous silica, subsequently precipitate and infill open spaces. Following this, a

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sulfide deposit ultimately forms in this hydrothermal cycle. Under the slow- to intermediate-spreading conditions on the CIR, it is conceivable that similar

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hydrothermal cycles would reoccur episodically. Therefore, the preformed hydrothermal deposit would undergo constant leaching over multiple stages by hydrothermal fluids. These processes would bring out the formation of a series of

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mineralogical crystallizations and elemental remobilizations and enrichments (such as Au) on the Edmond ridge.

6. Conclusions The CIR Edmond (69.6°E) hydrothermal field is characterized by the coexistence of high-temperature Fe-rich massive sulfides, medium-temperature Zn-rich chimney and low-temperature Ca-rich sulfate. Based on our observations, three mineralization stages have been identified: (1) a low-temperature stage characterized by the mineral assemblage of anhydrite and pyrite/marcasite; (2) a medium-high temperature stage with the mineral assemblage of pyrite, sphalerite and chalcopyrite; and (3) a low-temperature stage with the mineral assemblage of colloidal pyrite/marcasite,

ACCEPTED MANUSCRIPT barite, quartz, anglesite, etc. The sulfides on the Edmond are characterized by lower metal content (the Cu + Fe + Zn content is only 15.33 to 22.63 wt%) than other sediment-starved mid-ocean

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ridges; on the contrary, the concentrations of some elements, such as Pb, Ba, Sr, As, Cd, Au, and Ag, are significantly higher than those in other areas. These

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mineralogical and geochemical features indicate that the sulfides have undergone multiphase hydrothermal activity, which was clearly influenced by pervasive water circulation underneath the Edmond hydrothermal field. The low Zn/Cd ratios in

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sphalerites and the very low Co and Ni contents in pyrite are both indicative of felsic rock sources. That is, in the Edmond field, some felsic rocks serve as the source of Pb,

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Ba, Cd, Ag, and As.

Correlation analysis revealed that the high concentration of Ag is mainly

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controlled by Cd and partly by Pb. With regard to the high concentration of Au, it

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could be attributed to the late-stage leaching of Au from the primary mineral by upwelling hydrothermal fluid and pervasive seawater in this unique geological setting.

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Furthermore, the very high chlorine content of the hydrothermal fluid and the significant albitization below the Edmond field may also affect the concentration of Au; however, this needs to be explored further before it can be confirmed.

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In this study, we have integrated these findings to propose a conceptual model for this hydrothermal mineralization system, by underlining the important role of shallow chambers shifting from a central spreading axis, the distinct source of wall rocks in deep hydrothermal systems, and the pervasive seawater circulation under the developed transform faults, which jointly make the sulfide deposits on the Edmond ridge unique.

Acknowledgements We gratefully acknowledge the support by the captains, crew, and scientific parties who participated on board the R/V Dayangyihao on DY105-17A and DY115-19 cruises for indispensable cooperation in investigation and sampling the

ACCEPTED MANUSCRIPT Edmond hydrothermal field. This study was supported by National Key Basic Research Program of China (No.2013CB429703), Strategic Priority Research

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Science Foundation of China (No. 40872063, No. 41376077).

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Program of the Chinese Academy of Sciences (No. XDB06020204) and Natural

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Jilin Univ. (Earth Sci. Ed.). 42(S2), 69–80).

Wu Z.W., Sun X.M., Dai Y.Z., Shi G.Y., Wang Y., Lu Y., Liang Y.H., 2011. The discovery of

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native gold in massive sulfides from the Edmond hydrothermal field,Central Indian Ridge and its significance.Acta Petrol.Sin. 27(12):3749–3762.

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Yang, K., Scott, S.D., 1996.Possible contribution of a metal–rich magmatic fluid to a sea–floor

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hydrothermal system.Nature.383(6599), 420–423. Zakharova, N.A., Dosymbaeva, Z.D., Koiyzhanova, A.K., Suleimenov, E.N., Beisembaeva, G.Z.,

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2010. Study of gold and silver leaching processes from sulfide ores.In Proceedings of the XI International Seminar on Mineral Processing Technology.2(11), 854–858. Zaw, K., Gemmell, J.B., Large, R.R., Mernagh, T.P., Ryan, C.G., 1996. Evolution and source of

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ore fluids in the stringer system, Hellyer VHMS deposit, Tasmania, Australia: evidence from fluid inclusion microthermometry and geochemistry. OreGeol Rev. 10(3), 251–278. Zeng X, Zhang Z, Li X., Jebbar, M., Alain, K., Shao, Z., 2015. Caloranaerobacterferrireducens sp. nov., an anaerobic, thermophilic, iron (iii)-reducing bacterium isolated from deep-sea hydrothermal sulfide deposits.Int J SystEvolMicrobiol.65(6): 1714–1718. Zeng, Z., Ma, Y., Yin, X., Selby, D., Kong, F., Chen, S., 2015. Factors affecting the rare earth element

compositions

in

massive

sulfides

from

deep–sea

hydrothermal

systems.Geochem.Geophys.Geosyst.16(8), 2679–2693. Zeng, Z., 2011. Submarine hydrothermal geology.Science press. 401. Zhai S.K., Chen L.R., Zhang H.Q., 2001. Magmatism and submarine hydrothermal activities of Okinawa trough.Beijing: Ocean press. 171–17.

ACCEPTED MANUSCRIPT Zhu, J., Lin, J., Guo, S., Chen, Y., 2008. Hydrothermal plume anomalies along the Central Indian

AC

CE P

TE

D

MA

NU

SC R

IP

T

Ridge.Chin. Sci. Bull. 53(16), 2527–2535.

SC R

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ACCEPTED MANUSCRIPT

Table 1

Element S

Fe

Cu

Zn

Pb

Mo

As

(wt %) Fe-rich sulfide

NU

Micro-chemical compositions (average of random points) of pyrite Ni

Co

Cd

Au

Ag

(S/Fe)Atom

(S+Fe)Atom

Total

Py

min

49.56

42.04

-

0.02

0.02

0.70

-

-

-

-

-

-

1.88

97.22

95.43

max

54.27

49.61

0.37

2.14

0.74

1.00

0.18

0.01

0.01

0.05

0.11

0.06

2.20

99.62

104.57

avg.

51.64

45.06

0.11

0.60

0.31

0.85

0.05

-

-

0.01

0.03

0.02

2.00

98.91

98.78

n=9

Py1

min

49.16

45.87

-

-

max

53.19

49.82

0.69

avg.

51.33

47.76

n=4

Py2

min

48.17

44.72

max

51.41

47.94

avg.

50.31

46.49

n=3

"Py disease"

min

40.67

24.56

max

49.69

avg.

45.08

TE

D

Zn-rich sulfide

MA

n=23

0.72

-

-

0.01

-

-

-

1.76

97.27

97.24

0.37

0.41

0.95

0.45

0.06

0.05

0.01

0.13

0.12

1.96

99.52

103.54

0.15

0.12

0.24

0.84

0.19

0.02

0.02

-

0.05

0.03

1.87

99.01

100.85

-

0.76

0.12

0.72

-

-

0.01

-

0.01

-

1.77

98.33

96.53

0.05

1.72

1.49

0.94

-

0.01

0.01

-

0.03

0.03

1.94

98.51

101.53

0.02

1.33

0.38

0.82

-

-

0.01

-

0.02

0.01

1.84

98.45

99.09

-

5.59

0.15

0.59

-

-

-

-

-

-

1.98

74.85

97.01

43.69

0.29

36.65

1.90

0.95

0.01

-

0.01

0.16

-

0.16

2.88

95.76

103.29

46.49

0.11

20.50

0.77

0.75

-

-

-

0.09

-

0.10

2.49

85.55

100.31

AC

CE P

0.06

S

Fe

Cu

Zn

As

Sb

SC R

IP

T

ACCEPTED MANUSCRIPT

Ag

Pb

Au

Mo

(S+Fe)Atom

(S/Fe)Atom

1

52.80

47.67

0.14

0.19

0.30

-

-

0.20

0.13

0.95

99.15

1.93

2

45.73

47.41

0.03

1.80

-

0.10

0.04

0.19

0.04

0.70

98.21

1.68

3

51.29

46.67

-

0.94

0.01

-

-

0.27

-

0.88

98.93

1.91

4

52.76

46.67

0.10

0.37

0.06

-

-

0.30

-

0.88

99.18

1.97

avg.

50.64

47.11

0.07

0.83

0.09

0.02

0.01

0.24

0.04

0.85

98.87

1.87

Table 2

MA

D TE CE P AC

Point

NU

Micro-chemical compositions (average of random points) of marcasite (wt %)

Table 3

SC R

IP

T

ACCEPTED MANUSCRIPT

Micro-chemical compositions (average of random points) of sphalerite S

Fe

Cu

Zn

Si

Se

min

31.2

11.1

-

31.86

0.01

-

max

42.12

21.63

0.23

54.34

0.25

avg.

37.64

20.29

0.08

39.53

0.18

D

Element As

Fe-rich sulfide

min

31.82

4.23

-

56.85

max

33.37

6.92

0.52

62.6

avg.

32.59

5.88

0.15

59.7

n=12

Sp2

min

28.93

2.06

max

33.5

4

31.68

3.36

31.25

max

32.03

avg.

31.59

n=5

Au

Mo

Ba

Total

-

0.02

-

0.13

-

0.57

-

97.23

0.06

0.06

-

0.24

-

0.73

0.02

100.15

-

0.07

0.02

0.03

-

0.19

-

0.66

0.01

98.70

-

-

-

-

0.23

0.06

0.06

-

-

-

98.6

0.04

0.07

0.03

0.01

0.66

0.37

0.4

0.07

0.66

0.19

102.26

0.01

0.01

0.01

0.01

0.49

0.19

0.17

0.02

0.57

0.04

99.79

-

95.59

59.43

-

-

-

-

0.03

-

0.06

-

0.27

65.33

0.05

0.04

0.09

0.06

0.6

0.37

0.48

0.12

0.61

0.12

102.89

0.07

63.75

0.02

0.01

0.01

0.02

0.36

0.2

0.23

0.02

0.55

0.02

100.18

0.14

62.55

-

0.03

-

-

0.56

0.27

0.18

-

0.57

-

100.61

5.41

0.38

65.51

0.01

0.12

0.01

0.08

0.57

0.45

0.55

0.06

0.66

0.13

104.54

4.75

0.25

64.04

0.01

0.05

-

0.03

0.56

0.34

0.32

0.03

0.61

0.06

102.66

diseased “pyrite”

min

CE P

n=3

Pb

-

4.29

AC

avg.

Ag

0.1

TE

Sp1

Cd

-

Zn-rich sulfide n=8

0.04

MA

n=4

Sb

NU

(wt %)

diseased “chalcopyrite”

min

32.34

4.48

0.24

62.37

-

-

-

0.02

0.7

0.5

0.21

-

0.45

-

103.74

max

33

5.88

0.53

66.97

0.01

0.07

0.05

0.07

1.11

0.86

0.37

-

0.54

0.06

108.46

avg.

32.71

5.25

0.42

64.37

-

0.01

0.02

0.06

0.87

0.67

0.28

-

0.49

0.02

105.2

Table 4

SC R

IP

T

ACCEPTED MANUSCRIPT

Micro-chemical compositions (average of random points) of chalcopyrite Element Cu

Fe

S

Zn

Si

Se

As

Sb

Cd

-

Fe-rich sulfide n=9

Ag

Pb

Au

Co

Mo

Ba

Ni

Mn

Cu/Fe(Atom)

Total

0.05

-

-

0.62

-

-

-

0.91

98.75

NU

(wt %)

33.18

29.74

33.92

-

-

-

-

-

-

max

36.51

33.24

36.03

-

0.04

0.09

0.09

0.03

0.05

0.03

0.21

0.06

-

0.77

0.16

0.01

0.01

1.01

106.48

aver.

34.65

31.56

34.91

-

0.01

0.02

0.02

0.01

0.01

0.01

0.13

0.01

0.01

0.67

0.07

-

-

0.96

102.08

-

-

0.21

0.10

-

-

0.47

-

-

-

0.89

100.87

MA

min

Zn-rich sulfide

D

n=5 33.24

32.25

33.86

-

-

-

-

max

34.42

33.22

34.54

-

0.01

0.09

0.01

0.01

-

0.35

0.31

0.03

0.01

0.55

0.08

0.01

0.02

0.93

102.47

avg.

33.75

32.73

34.22

-

0.01

0.03

-

-

-

0.27

0.19

0.01

-

0.51

0.03

-

0.01

0.91

101.78

AC

CE P

TE

min

IP

T

ACCEPTED MANUSCRIPT

SC R

Table 5

Micro-chemical compositions (average of random points) of anglesite Element O

Se

As

Sb

Cd

Pb

S

-

0.14

64.01

11.14

-

0.07

67.05

10.77

0.04

0.01

68.15

10.50

-

0.01

0.07

66.40

10.80

Zn

Fe

Ba

Total

Pb/S(Atom)

3.57

0.7

-

103.11

0.89

4.49

0.38

0.13

105.12

0.96

0.38

0.87

0.06

99.88

1.00

2.81

0.65

0.06

102.70

0.95

19.09

-

-

2

20.11

0.01

-

3

19.88

-

-

avg.

19.69

-

Element

Cu

Sr

1

0.02

4.45

2

-

2.11

3

-

avg.

0.01

D

TE 3.28

CE P AC

MA

1

NU

(wt %)

ACCEPTED MANUSCRIPT

IP

T

Table 6

Ridge (CIR). Fe

Cu

Zn

Ca

Pb

Ti

Co

Mn

Mo

Ni

As

Cd

Sr

Sample (wt %)

SC R

Chemical composition of the hydrothermal sulfide samples from Edmond, Central Indian

(ppm)

18.93

0.65

3.05

1.46

1606

30.42

455.6

218.3

39.71

31.35

489.8

78.39

896.5

TVG7-17AIR3b

17.72

0.43

2.1

1.51

1229

33.41

245.5

234.6

37.52

32.72

414.2

55.65

862.6

TVG7-17AIR9a

5.99

0.76

8.58

19.15

209.8

3.4

56.93

192.3

2.699

2.035

130.8

23.24

916.2

TVG7-17AIR9b

6.34

0.81

9.04

20.5

747.4

7.03

59.53

198.6

3.194

1.879

145

24.59

469

NU

TVG7-17AIR3a

6.11

0.76

8.64

0.02

704.2

5.56

57.24

192

2.884

2.234

140.5

24.67

453.4

8.32

0.69

10

0.75

158

31.42

246.5

311.3

169.8

319.2

463.4

221.9

398.5

TVG7-17AIR11b

10.95

0.28

4.25

0.62

608.7

35.33

97.29

530.4

64.79

86.59

293.9

94.8

338.5

avg.

10.62

0.63

6.52

6.29

955.3

20.94

174.08

268.21

45.8

68.00

296.8

74.75

619.24

Ag

Au

Ba

Sn

Rb

Sb

Zr

Nb

In

V

Se

Au/Ag

D

TE

Sample (ppm)

MA

TVG7-17AIR9c TVG7-17AIR11a

44.3

8.44

18.54

1.261

0.284

13.83

<0.01

0.02

6.213

8.224

0.2

0.191

TVG7-17AIR3b

43.54

7.18

14.49

1.062

0.435

13.95

<0.01

0.039

6.47

8.654

0.09

0.165

7.82

15.71

0.999

0.667

15.22

<0.01

0.103

7.057

8.492

0.13

0.171

0.13

1396

100.7

17.04

<=

93.07

1.093

9.016

<=

2.315

0.001

0.1

1285

44.63

15.38

<=

88.44

0.883

3.526

24.83

0.951

0.002

0.42

1943

44.99

17.44

<=

74.73

0.785

6.518

12.86

0.871

0.005

0.38

1537

33.54

17.26

<=

91.73

0.819

4.607

11.38

0.609

0.011

3.5

887.11

32.455

9.787

14.33

86.99

0.534

6.201

12.407

0.738

0.058

45.82 115.5

TVG7-17AIR9c

57.91

TVG7-17AIR11a

78.61

TVG7-17AIR11b

33.96

avg.

AC

TVG7-17AIR9a TVG7-17AIR9b

CE P

TVG7-17AIR3a

59.95

ACCEPTED MANUSCRIPT

IP

T

Table 7 The sulfide chemical composition of different hydrothermal field Cu

Zn

As

Pb

(wt %)

(ppm)

CIR Edmond

10.62

0.63

6.52

296.80

955.3

CIR MESO

37.2

6.2

0.8

-

0.05

22.8

6.5

113

1.40

3.04

155.36

SWIR63.6°E

37.53

4.27

0.03

26.28

EPR9-10°N

22.27

24.98

0.61

10.00

EPR13°N

25.96

7.83

8.17

154

EPR21°N

12.44

0.58

19.76

TAG

38.30

1.40

Mariana

2.39

Okinawa Lau Basin

0.03

Ag

Mn

Au

Sr

Ref.

887.11

59.95

268.21

0.74

619.24

1

-

22.4

-

0.6

-

2

12

67

-

5.28

-

3

64.62

44.23

20.26

188.43

43.63

5.08

4

4.93

2.22

0.58

721.22

<0.01

5.67

4

20.78

250000

24.00

58

0.02

-

5

500

800

49

100

0.26

-

5

296

2100

1500

98

246

0.15

-

5

0.03

102.1

265.7

-

24.80

-

-

2.30

5

1.15

9.96

126

74000

333300

184

175

0.78

-

5

7.33

1.77

22.00

537

142700

27600

2100

1567

4.60

-

5

17.10

4.56

17.10

2213

3300

115600

256

542.00

1.40

-

5

NU

20.0 38.26

TE

D

MA

CIR Kairei SWIR49.6°E

Ba

SC R

Fe Field

Data sources: 1-this study. 2-Münch et al., 1999; 3-Wang et al., 2014; 4-Cao, unpublished

AC

CE P

data; 5-Fouquet, et al., 1993

ACCEPTED MANUSCRIPT

IP

T

Table 8 The elements correlation coefficient matrix of hydrothermal sulfides Fe

Pb

Ti

Fe

1.00

Cu

-0.53

1.00

Zn

-0.91

0.73

1.00

Ca

-0.50

0.54

0.44

1.00

Pb

0.64

-0.08

-0.30

-0.58

1.00

Ti

0.73

-0.78

-0.65

-0.70

0.66

1.00

Co

0.84

-0.16

-0.56

-0.49

0.87

0.65

Mn

0.08

-0.80

-0.26

-0.41

-0.04

0.63

Mo

0.09

-0.24

0.14

-0.49

0.62

0.65

Ni

-0.06

-0.11

0.30

-0.40

0.54

0.51

As

0.78

-0.41

-0.53

-0.64

0.90

0.88

Cd

0.10

-0.21

0.14

-0.49

0.64

Sr

-0.83

0.73

0.73

0.68

-0.69

Ag

0.05

0.08

0.26

-0.37

Au

-0.73

0.72

0.62

Ba

0.86

-0.61

-0.70

Co

Mn

Mo

Ni

As

Cd

Sr

Ag

Au

-0.07

1.00

0.37

0.46

1.00

0.26

0.39

0.99

1.00

0.90

0.24

0.68

0.56

1.00

0.41

0.45

1.00

0.98

0.69

1.00

-0.97

-0.75

-0.47

-0.57

-0.43

-0.92

-0.57

1.00

0.66

0.45

0.51

0.25

0.88

0.88

0.63

0.92

-0.39

1.00

-0.68

-0.99

-0.70

-0.58

-0.68

-0.55

-0.91

-0.68

0.98

-0.52

1.00

0.94

0.86

0.40

0.57

0.43

0.96

0.58

-0.97

0.49

-0.96

0.64

TE

0.64

-0.67

Ba

NU

MA

1.00

D

Ca

CE P

Zn

AC

Cu

SC R

Element

0.79

1.00

ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Bathymetric map of the Central Indian Ridge showing the location of the Edmond

T

hydrothermal field. SWIR-South West Indian Ridge; SEIR-South East Indian Ridge;

IP

RTJ-Rodriguez Triple Junction.

SC R

Fig. 2. Representative hydrothermal sulfide samples collected from Edmond hydrothermal field. (a) Massive Fe-rich sulfide sample TVG7-3a, mainly consist of pyrite/marcasite and chalcopyrite. (b) Massive Fe-rich sulfide sample TVG7-4. (c) Massive Fe-rich sulfide sample

NU

TVG7-5. (d) Massive Fe-rich sulfide sample TVG7-7. (e) Zn-rich chimney sample TVG7-11, composed of sphalerite, pyrite/marcasite and barite. (f) Massive Ca-rich sulfate sample

MA

TVG7-9, mainly consist of anhydrite and pyrite/marcasite. Fig. 3. Polished-mounted photomicrographs of Fe-rich sulfides: reflected light, plane

D

polarized. (a) Pyrite (Py) and marcasite (Mar) form fine-grained aggregates, which are

TE

replaced by chalcopyrite (Cp) along the edges and the fissure. (b) A small amount of euhedral pyrite (Py) grains is visible in the mineral pores. (c) Pyrite (Py) and marcasite (Mar) occur in

CE P

the oolitic form, replaced by chalcopyrite (Cp). (d) Oolitic pyrite-marcasite. (e) A thin chalcopyrite (Cp) conduit lining is prominent in the inner area of the sulfide. (f) Fine-grained covellite can be observed near the outer part of chalcopyrite (Cp), with barite (Ba) in blade

AC

form interspersed with sulfide.

Fig. 4. Polished-mounted photomicrographs of Zn-rich chimney. (a) Early-stage sphalerite (Sp1) is fine-grained granular, integrated, and dendritic in form. (b) Late-stage colloform sphalerite (Sp2) surrounds early-stage sphalerite (Sp1). (c) Different color zones from pale yellow-orange to dark red of sphalerite (Sp) under transmission light. (d) Line-scanning spectrum of sphalerite (as shown in Fig. 4c, in the direction of the arrow). (e) The rhythmic texture of sphalerite. (f) Fine-grained chalcopyrite (Cp) precipitates around the edge of serrated solution vugs of sphalerite aggregates. (g) Coarse-grain sphalerite (Sp) shows the “chalcopyrite disease” texture. (h) A small amount of euhedral crystal exists in the sulfide pores. (i) Pyrite (Py) “framboids” precipitate in the solution vugs of sphalerite (Sp), and are subsequently joined to form larger structures that recrystallize to form bigger euhedral grains.

ACCEPTED MANUSCRIPT (j) Coarse-grain sphalerite (Sp) exhibits “pyrite disease” (Py) texture. (k) A small amount of marcasite (Mar) surrounds colloform sphalerite as a shell. (l) Euhedral marcasite (Mar) is visible in the sulfide pores. (m) Radial pyrite-marcasite (Py+Mar) sphere, with obvious

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alternating growth zones. (n) Fossil worm tubes found outside the sulfide. (o) Blade-shaped barite (Ba) distribution between coarse pyrite and sphalerite. (p) Bright white anglesite

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(Ang) particle distribution in the pore of sphalerites (Sp). (q) Small automorphic quartz (Qtz) precipitation in the pore between mineral grains. (r) Native sulfur (S) precipitates among

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mineral grains. (r) Wave spectrum of native sulfur.

Fig. 5. Polished-mount photo micrographs of Ca-rich sulfate fragments. (a) Most anhydrite

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(Adr) crystals show blade and sometimes radial assembly. (b) Colloform symbionts of pyrite (Py) and marcasite (Mar). (c) The grained pyrite (Py) occurs as veins/fractures filling in the anhydrite (Adr) fissure. (d) Locally, a small amount of biogenic framboidal pyrite (Py) can

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Fig.6. Elemental mapping analysis of Fearound the solution vugs. (a) CP electronic photo of

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the test area. (b) Elemental mapping of Fe in the same horizon. Cp-chalcopyrite. Fig. 7. Sequence of mineralization of the hydrothermal sulfide in the Edmond field. The thickness of the horizontal bars indicates the relative abundance of the minerals.

fields.

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Fig. 8. Distribution of the major elements of sulfide minerals from different hydrothermal

Fig. 9. Trace element distribution of various sulfide minerals. Fig. 10. Correlation between the trace elements in sphalerite. Fig. 11. Conceptual model of sulfide mineralization in the Edmond hydrothermal field. See text for detailed descriptions.

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