Geochemical Investigations of Fe-Si-Mn Oxyhydroxides Deposits in Wocan Hydrothermal Field on the Slow-Spreading Carlsberg Ridge , Indian Ocean : Constraints on Their Types and Origin

We have studied morphology, mineralogy and geochemical characteristics of Fe-oxyhydroxide deposits from metal-enriched sediments of the active (Wocan-1) and inactive (Wocan-2) hydrothermal sites (Carlsberg Ridge, Northwest Indian Ocean). Fe-oxyhydroxide deposits on the Wocan-1 site are reddish-brownish, amorphous and subangular. They occur in association with sulfides (e.g., pyrite, chalcopyrite and sphalerite) and sulfate minerals (e.g., gypsum and barite). The geochemical composition shows enrichment in transition metals (Σ (Cu + Co + Zn + Ni) = ~1.19 wt. %) and low (<0.4 wt. %) values of Al/(Al + Fe + Mn) ratio. The Wocan-2 samples show poorly crystallized reddish brown and yellowish Fe-oxyhydroxide, with minor peaks of goethite and manganese oxide minerals. The mineral assemblage includes sulfide and sulfate phases. The geochemical compositions show two distinct types (type-1 and type-2). The type-1 Fe-oxyhydroxides are enriched in transition metals (up to ~1.23 wt. %), with low values of Fe/Ti vs. Al/(Al + Fe + Mn) ratio similar to the Wocan-1 Fe-oxyhydroxides. The type-2 Fe-oxyhydroxides are depleted in transition metals, with Al/(Al + Fe + Mn) ratio of 0.003–0.58 (mean value, 0.04). The ridge flank oxyhydroxides exhibit an extremely low (mean value ~ 0.01) Fe/Mn ratio and a depleted concentration of transition metals. Our results revealed that the Wocan-1 Fe-oxyhydroxides and type-1 Fe-oxyhydroxides of the Wocan-2 site are in the range of Fe-oxyhydroxides deposits that are precipitated by mass wasting and corrosion of pre-existing sulfides. The type-2 Fe-oxyhydroxides are precipitated from sulfide alteration by seawater in an oxygenated environment relative to type-1. The association of biogenic detritus with the oxyhydroxides of the ridge flanks and the low Fe/Mn ratio suggests hydrogenous/biogenic processes of formation and masked hydrothermal signatures with distance away from the Wocan hydrothermal field.

Fe-oxyhydroxides are precipitated into the near active and inactive vent sediments due to the erosion of dispersed coarse-grained oxyhydroxide particles that are deposited a few hundreds of meters from hydrothermal sites.Whereas, fine-grained particles are liable to be dispersed by bottom currents to tens of kilometers into the ridge flanks as plume fall-out [2].
Studies on Fe-oxyhydroxide deposits in the Indian Ocean Ridge systems are few.Recent studies are the work of [18] from Southwest Indian Ridge (SWIR).Hydrothermal plume particles and polymetallic samples that are enriched in metal content were discovered within the Carlsberg Ridge, northwest Indian Ocean during the Chinese DY 28th cruise in 2013 (near 6 • 22 N/60 • 31 E and referred to as Wocan Hydrothermal Field (WHF) [19]).These particles indicate the presence of an active hydrothermal system (Wocan-1), together with the inactive sulfide chimney (Wocan-2) that are related to widespread past venting [19].Additionally, Reference [19] reported the occurrence of iron-oxyhydroxides together with atacamite in the form of vugs lining in the interstitial spaces of the Cu-rich chimney on the Wocan-1 hydrothermal site.However, studies on the origin, morphology and mineral chemistry of Fe oxides and hydroxide deposits of the Wocan field had not been conducted.This present study examines the morphology and chemical compositions of Fe-oxyhydroxide mineral grains separated from metalliferous sediments of the WHF.We aim to understand the probable origin, types and modes of precipitation of Fe-oxides and hydroxide deposits of the Wocan and the ridge flanks.

Materials and Methods
Metalliferous sediments were sampled by TV-grab sampler at six stations during the DY 28 th cruise in 2013 (Table 1).Rock fragments that are too large for grain mount were isolated from the samples under the binocular microscope.Representative samples from each station were washed with ultra-pure water to remove interstitial salts, dried at 60 °C and weighed.Approximately 50 g were wet-sieved and separated into >63 µm and <63 µm (silt and clay size fraction) using 240 mesh sizes.From the sieved (>63 µm) fractions, Fe-oxyhydroxide grains were examined and hand-picked under the stereographic and binocular microscope.Micromorphological observations were conducted on the separated grains with Leica M205C stereographic microscope in combination with a scanning electron microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS) analytical X-ray system.The SEM-EDS was operated at an accelerating voltage of 20 kV and a beam current of 16-22 mA at the State Key Laboratory of Submarine Geosciences (KLSG), Hangzhou, China.The isolated Fe-oxyhydroxide grains were then impregnated with epoxy resin and hardener (ratio 2:1), similar to the method of [21] and polished.Chemical compositions of Fe-oxyhydroxide grain separates were performed on a JEOL JXA-8100 Superprobe Electron Probe Microanalysis (EPMA) at 15 kV acceleration voltage with a beam current of ~2 × 10 −8 A and beam size of 5µm at KSLG.A 10-20 nm thick carbon layer was coated on the samples, before EPMA investigation.The standard used include synthetic oxide set (K2O, FeO, MnO2, TiO2, Cr2O3) for K, Fe, Mn, Ti and Cr respectively.Others include: apatite for Ca and P, barite for Ba and S; olivine for Mg and Na; SrSO4 for S. ZAF correction program was employed according to the method of [22].The results are documented with photomicrographs and micro-chemical compositions of the phases.EPMA

Materials and Methods
Metalliferous sediments were sampled by TV-grab sampler at six stations during the DY 28th cruise in 2013 (Table 1).Rock fragments that are too large for grain mount were isolated from the samples under the binocular microscope.Representative samples from each station were washed with ultra-pure water to remove interstitial salts, dried at 60 • C and weighed.Approximately 50 g were wet-sieved and separated into >63 µm and <63 µm (silt and clay size fraction) using 240 mesh sizes.From the sieved (>63 µm) fractions, Fe-oxyhydroxide grains were examined and hand-picked under the stereographic and binocular microscope.Micromorphological observations were conducted on the separated grains with Leica M205C stereographic microscope in combination with a scanning electron microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS) analytical X-ray system.The SEM-EDS was operated at an accelerating voltage of 20 kV and a beam current of 16-22 mA at the State Key Laboratory of Submarine Geosciences (KLSG), Hangzhou, China.The isolated Fe-oxyhydroxide grains were then impregnated with epoxy resin and hardener (ratio 2:1), similar to the method of [21] and polished.Chemical compositions of Fe-oxyhydroxide grain separates were performed on a JEOL JXA-8100 Superprobe Electron Probe Microanalysis (EPMA) at 15 kV acceleration voltage with a beam current of ~2 × 10 −8 A and beam size of 5µm at KSLG.A 10-20 nm thick carbon layer was coated on the samples, before EPMA investigation.The standard used include synthetic oxide set (K 2 O, FeO, MnO 2 , TiO 2 , Cr 2 O 3 ) for K, Fe, Mn, Ti and Cr respectively.Others include: apatite for Ca and P, barite for Ba and S; olivine for Mg and Na; SrSO 4 for S. ZAF correction program was employed according to the method of [22].The results are documented with photomicrographs and micro-chemical compositions of the phases.EPMA concentrations of individual analyses are given in Supplementary Materials, with a detection limit of >0.01 wt.% and analytical precision of <10%.
Furthermore, the traditional method of counting was adopted on ~250 to 260 randomly counted grains at the six stations.The grains were temporarily mounted on stud attached with carbon tapes to a set of slides using a pre-cleaned needle with methylated spirit according to [23].Fe-oxyhydroxide grains were counted in order to have an idea about its relative abundance at the six sampling stations.
Also, bulk-X-ray powder diffraction analysis (XRD) was conducted on the part of the sediment after grinding ~2 g of representative samples from each station in an agate mortar and pestle.The XRD analysis was investigated using X-Pert PRO Diffractometer, with Cu kα radiation generated at 45 kV and 40 mA and a beam diameter of 0.1 mm at KSLG.

Bulk Sediment Mineralogy
The bulk mineralogy of representative sediment fractions of Wocan-2 when compared with the standard according to [24], suggests a minor and weak goethite peak characterized by different diffraction patterns.These patterns are with d-spacing of 1  3 and  4).The characteristic diffraction patterns with d-spacings of 1.564Å (58.7 • 2θ) and 2.568 Å (34.9 • 2θ) and its comparison with appropriate standard according to [25] suggests manganosite (manganese oxide minerals, Figure 4).
The Wocan-1 samples are generally amorphous, with a non-identifiable peak of Fe-oxides and hydroxides.Other characteristic XRD patterns suggest sulfide minerals (e.g., sphalerite, chalcopyrite and pyrite) and associated sulfate mineral minerals (gypsum and barite).We have reported the detailed information on the sulfide mineral separates in Reference [26].
The bulk mineralogy of the representative sediment fractions of the ridge flank is dominated by biogenic calcite (mainly foraminifera assemblage) and traces of phyllosilicate minerals vermiculites (Figure 5).Optical microscopy and SEM-EDS investigation also revealed the presence of a minor abundance of Fe-oxyhydroxide grains (Table 2).
Minerals 2018, 8, x FOR PEER REVIEW 5 of 18 concentrations of individual analyses are given in Supplementary Materials, with a detection limit of >0.01 wt.% and analytical precision of <10%.Furthermore, the traditional method of counting was adopted on ~250 to 260 randomly counted grains at the six stations.The grains were temporarily mounted on stud attached with carbon tapes to a set of slides using a pre-cleaned needle with methylated spirit according to [23].Fe-oxyhydroxide grains were counted in order to have an idea about its relative abundance at the six sampling stations.
Also, bulk-X-ray powder diffraction analysis (XRD) was conducted on the part of the sediment after grinding ~2 g of representative samples from each station in an agate mortar and pestle.The XRD analysis was investigated using X-Pert PRO Diffractometer, with Cu kα radiation generated at 45 kV and 40 mA and a beam diameter of 0.1 mm at KSLG.
The Wocan-1 samples are generally amorphous, with a non-identifiable peak of Fe-oxides and hydroxides.Other characteristic XRD patterns suggest sulfide minerals (e.g., sphalerite, chalcopyrite and pyrite) and associated sulfate mineral minerals (gypsum and barite).We have reported the detailed information on the sulfide mineral separates in Reference [26].
The bulk mineralogy of the representative sediment fractions of the ridge flank is dominated by biogenic calcite (mainly foraminifera assemblage) and traces of phyllosilicate minerals vermiculites (Figure 5).Optical microscopy and SEM-EDS investigation also revealed the presence of a minor abundance of Fe-oxyhydroxide grains (Table 2).Fe-oxyhydroxide separates from Wocan-1 and Wocan-2 sites are poorly crystallized with a few angular to subangular texture (Figure 6A,B).Some are sub-rounded and botryoidal (Figure 6C,D).The Wocan-1 oxyhydroxides are majorly reddish-brown.While the oxyhydroxides of the Wocan-2 are reddish-brown to yellowish (Figure 7A), some primary sulfide grain separates exhibit features of partial oxidation into secondary iron-oxyhydroxides (Figure 7C).Others show inclusions of subangular to angular shards of S and Si-bearing of un-identified compounds (Figure 7B).

Morphology and Relative Abundance of Fe-Oxyhydroxide Separates
Fe-oxyhydroxide separates from Wocan-1 and Wocan-2 sites are poorly crystallized with a few angular to subangular texture (Figure 6A,B).Some are sub-rounded and botryoidal (Figure 6C,D).The Wocan-1 oxyhydroxides are majorly reddish-brown.While the oxyhydroxides of the Wocan-2 are reddish-brown to yellowish (Figure 7A), some primary sulfide grain separates exhibit features of partial oxidation into secondary iron-oxyhydroxides (Figure 7C).Others show inclusions of subangular to angular shards of S and Si-bearing of un-identified compounds (Figure 7B).
The relative abundance of Fe-oxyhydroxide grains from ~250 to 260 counts (Table 2) shows up to ~54% and ~67% abundance on the Wocan-1 and Wocan-2 samples.The oxyhydroxides of the ridge flanks are less than 12%.

Mineral Chemistry of Fe-Oxyhydroxide Grains
In-situ geochemical investigations (using Electron Probe Microanalysis (EPMA) are essential tools in geochemical studies [27,28].The improved methodology on spectrometers and efficient operations of the electron column at high probe current and acceleration voltage have increased the detection limit and precision of EPMA analysis to ppm levels [29].References [15,30] applied EPMA on iron oxyhydroxide deposits from the Valu Fa Ridge, Lau Basin and PacMamus hydrothermal field, Eastern Manu Basin.The EPMA results of this study represent the average values of several analyses.

Mineral Chemistry of Fe-Oxyhydroxide Grains
In-situ geochemical investigations (using Electron Probe Microanalysis (EPMA) are essential tools in geochemical studies [27,28].The improved methodology on spectrometers and efficient operations of the electron column at high probe current and acceleration voltage have increased the detection limit and precision of EPMA analysis to ppm levels [29].References [15,30] applied EPMA on iron oxyhydroxide deposits from the Valu Fa Ridge, Lau Basin and PacMamus hydrothermal field, Eastern Manu Basin.The EPMA results of this study represent the average values of several analyses.

Origin and Formation Process of Wocan-1 Fe-Oxyhydroxides
Previous studies reported on three processes which gave rise to the Fe-oxyhydroxide deposits (i) The Fe-Si-Mn types that are predominantly formed by diffuse hydrothermal flow [3,12].(ii) The Fe-Si-Mn types that are precipitated by seawater [31-34] and (iii) the Fe-Si-Mn types that are formed by mass wasting and corrosion of pre-existing sulfides [2,14].However, these processes rarely occur in isolation and each may play an essential role in the precipitation of Fe-Mn oxyhydroxides at different stages [35,36].The high metal content (e.g., Cu concentration >1 wt.%) collected from bottom-moored sediment traps (during the 2013 DY 28th cruise) [19] provided the direct evidence that the Fe-oxyhydroxide deposits on the Wocan-1 hydrothermal site are high-temperature hydrothermal precipitates.
Previous studies have utilized Fe/Ti and Al/(Al + Fe + Mn) ratios to estimate the hydrothermal and detrital contribution in metalliferous sediments and Fe-oxyhydroxides deposits [2,3,12,[37][38][39]. Elevated Fe/Ti and low Al/(Al + Fe + Mn) ratio is an of strong hydrothermal components and enrichment of trace metals [39].The increased Fe/Ti ratio and the low values of the Al/(Al + Fe + Mn) ratio (Figure 9) exhibited by the Wocan-1 Fe-oxyhydroxides are consistent with the enrichment of minor elements (Cu, Ba, Sr, As, Zn, Ni and Pb) in Figure 8.

Origin and Formation Process of Wocan-1 Fe-Oxyhydroxides
Previous studies reported on three processes which gave rise to the Fe-oxyhydroxide deposits (i) The Fe-Si-Mn types that are predominantly formed by diffuse hydrothermal flow [3,12].(ii) The Fe-Si-Mn types that are precipitated by seawater [31-34] and (iii) the Fe-Si-Mn types that are formed by mass wasting and corrosion of pre-existing sulfides [2,14].However, these processes rarely occur in isolation and each may play an essential role in the precipitation of Fe-Mn oxyhydroxides at different stages [35,36].The high metal content (e.g., Cu concentration >1 wt.%) collected from bottom-moored sediment traps (during the 2013 DY 28 th cruise) [19] provided the direct evidence that the Fe-oxyhydroxide deposits on the Wocan-1 hydrothermal site are high-temperature hydrothermal precipitates.
Previous studies have utilized Fe/Ti and Al/(Al + Fe + Mn) ratios to estimate the hydrothermal and detrital contribution in metalliferous sediments and Fe-oxyhydroxides deposits [2,3,12,[37][38][39]. Elevated Fe/Ti and low Al/(Al + Fe + Mn) ratio is an indication of strong hydrothermal components and enrichment of trace metals [39].The increased Fe/Ti ratio and the low values of the Al/(Al + Fe + Mn) ratio (Figure 9) exhibited by the Wocan-1 Fe-oxyhydroxides are consistent with the enrichment of minor elements (Cu, Ba, Sr, As, Zn, Ni and Pb) in Figure 8.
Moreover, the identifications of sulfide mineral assemblage with the Fe-oxyhydroxides (Figure 3) and the plot of the Fe-rich corner of the Mn-Fe-(Cu + Co + Ni) × 10 ternary diagram (Figure 10), further indicates the hydrothermal origin and secondary sulfide oxidation.Moreover, the identifications of sulfide mineral assemblage with the Fe-oxyhydroxides (Figure 3) and the plot of the Fe-rich corner of the Mn-Fe-(Cu + Co + Ni) × 10 ternary diagram (Figure 10), further indicates the hydrothermal origin and secondary sulfide oxidation.
Therefore, the high concentration of Fe (49.1-55.1 wt.%), transition metals other than Fe (0.79-1.78 wt.%) and S concentrations (mean value 0.10 wt.%) suggests that the Fe-oxyhydroxide deposits of the Wocan-1 belongs to the group-2 classifications from the detailed studies of [1,14,16].This is an indication that the Wocan-1 Fe-oxyhydroxides are precipitated by mass wasting and corrosion of pre-existing sulfides (Figure 11).
Fe-oxyhydroxide deposits from the EPR 13°N, MESO (Central Indian Ocean Ridge) and the Mothra hydrothermal field (Endeavour segment, Juan de Fuca Ridge) have been linked to secondary alteration of pre-existing sulfide deposits [2,14,40,41].

Origin and Formation Process of Wocan-2 Fe-Oxyhydroxides
Previous studies have reported the occurrence of an inactive toppled sulfide chimney in the vicinity of the Wocan-2 site, as an indication of previous widespread venting [19].The lower values of Al/(Al + Fe + Mn) concentrations (mean value ~0.0036, n = 9) (Table 3), together with the Fe-rich corner of the Mn-Fe-(Cu + Co + Ni) × 10 ternary diagram (Figure 10) also support hydrothermal origin and secondary sulfide oxidation.The enriched transition metals (e.g., Cu + Co + Ni + Zn, mean value,

Origin and Formation Process of Wocan-2 Fe-Oxyhydroxides
Previous studies have reported the occurrence of an inactive toppled sulfide chimney in the vicinity of the Wocan-2 site, as an indication of previous widespread venting [19].The lower values of Al/(Al + Fe + Mn) concentrations (mean value ~0.0036, n = 9) (Table 3), together with the Fe-rich corner of the Mn-Fe-(Cu + Co + Ni) × 10 ternary diagram (Figure 10) also support hydrothermal origin and secondary sulfide oxidation.The enriched transition metals (e.g., Cu + Co + Ni + Zn, mean value, 1.23 wt.%) further confirms these assumptions.
In comparison with the previous studies, the geochemical composition of the type-2 oxyhydroxides is similar to the group 3 classifications of [2,14].

Origin and Formation Process of Oxyhydroxide Separates of the Ridge Flanks
The low Fe/Mn ratio (0.004-0.022 mean value ~0.01) of the Si-Mn enriched grains and the depleted transition metal content suggest non-hydrothermal process on the formation of the ridge flanks oxyhydroxides.The Fe/Mn ratio is lower than the estimated value (<0.07) for hydrogenous Fe-Mn oxyhydroxide precipitates [36,41].The Zn contents show a weak correlation with Cu (R 2 = 0.124) and Pb (R 2 = 0.230) (Figure 12A,B).Also, the Pb vs.As (R 2 = 0.096) and Cu vs. Fe (R 2 = 0.053) show a similar weak relationship (Figure 12C,D).These weak correlations and strong probability values (P < 0.5) suggests a distinct geochemical process (e.g., high or low-temperature precipitates) between the two deposits.
The geochemical characteristics of the yellowish (type-2) Fe-oxyhydroxides suggest the increased influence of seawater on its precipitation.This assumption is supported by the depletion of minor element (Cu, Ba, Sr, As, Zn, Ni and Pb), and the higher Al/(Al + Fe + Mn) ratio (mean value 0.404).The Fe/Mn ratio (Tables 3-5) of the type-2 (Wocan-2) oxyhydroxides (25.87-37.66) is very low, relative to the type-1 (Wocan-2) 1062-3080 and Wocan-1 oxyhydroxides (69. .The low ratio is a further indication of seawater influence on its precipitation.Additionally, the low Fe/Ti and higher Al/(Al + Fe + Mn) values exhibited by the type-2 deposit (Figure 9) confirmed detrital dilution on the oxyhydroxides by deep sea pelagic types [37][38][39].The observed Mn oxide minerals (Manganosite, Figure 4) further suggests its precipitation in an oxygenated environment relative to Wocan-1 and type-1 (Wocan-2) deposit.The Fe-oxyhydroxide deposit of station 28II-TVG05 displays these type-2 geochemical characteristics.In comparison with the previous studies, the geochemical composition of the type-2 oxyhydroxides is similar to the group 3 classifications of [2,14].

Origin and Formation Process of Oxyhydroxide Separates of the Ridge Flanks
The low Fe/Mn ratio (0.004-0.022 mean value ~0.01) of the Si-Mn enriched grains and the depleted transition metal content suggest non-hydrothermal process on the formation of the ridge flanks oxyhydroxides.The Fe/Mn ratio is lower than the estimated value (<0.07) for hydrogenous Fe-Mn oxyhydroxide precipitates [36,41].
The sub-rounded Mn-enriched texture, with Ti, Ca and K inclusions in Figures 6D and 13A-E suggest the additional influence of pelagic oozes and terrigenous matter in the oxyhydroxides grains.These features indicate the absence of diffuse hydrothermal flow in the formation of the ridge flank oxyhydroxides [12].These features indicate the absence of diffuse hydrothermal flow in the formation of the ridge flank oxyhydroxides [12].
The observed microbial mediated features from the optical microscope (Figure 13G,H) is an indication of microbial process in the precipitation of typical oxyhydroxide of the ridge flanks.
Previous studies [42,43], have identified the existence of a spatial relationship between microorganisms and the precipitation of Fe-oxyhydroxides in the hydrothermal environments and modern marine settings.
The geochemical composition of the ridge flanks Si-Mn oxyhydroxides (Table 6) are similar to the group 4 classification of [1,14].

Conclusions
Mineralogical investigations on the Fe-oxyhydroxide deposits of the Wocan-1 hydrothermal site highlighted their amorphous and poorly crystallized character.The X-Ray diffraction patterns of the bulk Fe-Si-Mn oxyhydroxides deposits on the Wocan-2 site show minor goethite and manganosite.The observed microbial mediated features from the optical microscope (Figure 13G,H) is an indication of microbial process in the precipitation of typical oxyhydroxide of the ridge flanks.Previous studies [42,43], have identified the existence of a spatial relationship between micro-organisms and the precipitation of Fe-oxyhydroxides in the hydrothermal environments and modern marine settings.
The geochemical composition of the ridge flanks Si-Mn oxyhydroxides (Table 6) are similar to the group 4 classification of [1,14].

Conclusions
Mineralogical investigations on the Fe-oxyhydroxide deposits of the Wocan-1 hydrothermal site highlighted their amorphous and poorly crystallized character.The X-Ray diffraction patterns of the bulk Fe-Si-Mn oxyhydroxides deposits on the Wocan-2 site show minor goethite and manganosite.
The morphology and geochemical investigations revealed two types of Fe-oxyhydroxide deposits at the Wocan hydrothermal sites (type-1 and type-2).The higher concentration of transition metals and the low Al/(Al + Fe + Mn) ratio of the type-1 oxyhydroxides suggest mass-wasting, corrosion and alterations of pre-existing sulfide minerals as the probable mode of formation.The depletion in transition metals, low Fe/Mn ratio and high Al/(Al + Fe + Mn) values of the type-2 Fe-oxyhydroxides suggest an increased contribution of sea water during the process of secondary sulfide oxidation.
The presence of biogenic mediated Mn-rich oxyhydroxide precipitates, low Fe/Mn ratio and depletion in transition metals suggest mixed (diagenetic and hydrogenous) origin to the ridge flank oxyhydroxides.
Our results further indicate a marked shift from reduced to oxygenated environments in the precipitation of Fe-oxyhydroxides with distance from hydrothermal sites.
Author Contributions: X.H. conceived, designed and fund the experiments; S.O.P. wrote the first draft, prepared the samples and performed the experiments.Z.Q.analyzed the bulk samples and supplied the needed laboratory materials; Y.W. co-supervised the SEM and EPMA and proofread the manuscript; Y.Y. co-supervised the project.

Figure 1 .
Figure 1.Bathymetric map of (A) showing the locations of Wocan-1 and Wocan-2 hydrothermal sites and the sampling stations in the ridge flank; (B) The enlarged map of A showing the sampling stations in Wocan-1 and Wocan-2 hydrothermal field.

Figure 1 .
Figure 1.Bathymetric map of (A) showing the locations of Wocan-1 and Wocan-2 hydrothermal sites and the sampling stations in the ridge flank; (B) The enlarged map of A showing the sampling stations in Wocan-1 and Wocan-2 hydrothermal field.

Figure 5 .
Figure 5. XRD patterns of the bulk sediment from station 28II-TVG13 (ridge flank).Abbreviations, C: calcite; V: vermiculite.The 2θ values are enclosed in the bracket, whereas, the first values are the d values.

18 Figure 5 .
Figure 5. XRD patterns of the bulk sediment from station 28II-TVG13 (ridge flank).Abbreviations, C: calcite; V: vermiculite.The 2θ values are enclosed in the bracket, whereas, the first values are the d values.

Figure 11 .
Figure 11.Schematic diagram showing the secondary oxidation of pre-existing sulfides in the Wocan hydrothermal site, modified after [14].

Figure 13 .
Figure 13.SEM photomicrographs and EDS spectra of (A-F) typical diagenetic alterations on the oxyhydroxides of the ridge flanks (G-H).An optical photomicrograph of typical biogenic influence on Mn oxyhydroxide precipitates.The SEM photomicrograph of G-H is shown in A and C, while the EDS spectra are in B and D. The cross sign represent analyzed spot.

Figure
Figure SEM photomicrographs and EDS spectra of (A-F) typical diagenetic alterations on the oxyhydroxides of the ridge flanks (G,H).An optical photomicrograph of typical biogenic influence on Mn oxyhydroxide precipitates.The SEM photomicrograph of G-H is shown in A and C, while the EDS spectra are in B and D. The cross sign represent analyzed spot.

Table 1 .
The sampling information of studied bulk samples.

Table 2 .
Relative abundance of Fe-oxyhydroxide separates from ~250-260 grain counts at the six stations.

Table 2 .
Relative abundance of Fe-oxyhydroxide separates from ~250-260 grain counts at the six stations.

Table 6 .
Electron Probe Micro-analysis (wt.%) for typical Si-enriched grains from the ridge flanks.