Trace Metal Distribution in Sulfide Minerals from Ultramafic-Hosted Hydrothermal Systems : Examples from the Kairei Vent Field , Central Indian Ridge

The ultramafic-hosted Kairei vent field is located at 25◦19′ S, 70◦02′ E, towards the Northern end of segment 1 of the Central Indian Ridge (CIR-S1) at a water depth of ~2450 m. This study aims to investigate the distribution of trace elements among sulfide minerals of differing textures and to examine the possible factors controlling the trace element distribution in those minerals using LA-ICP-MS spot and line scan analyses. Our results show that there are distinct systematic differences in trace element distributions throughout the different minerals, as follows: (1) pyrite is divided into three types at Kairei, including early-stage euhedral pyrite (py-I), sub-euhedral pyrite (py-II), and colloform pyrite (py-III). Pyrite is generally enriched with Mo, Au, As, Tl, Mn, and U. Pyrite-I has high contents of Se, Te, Bi, and Ni when compared to the other types; py-II is enriched in Au relative to py-I and py-III, but poor in Ni; py-III is enriched in Mo, Pb, and U but is poor in Se, Te, Bi, and Au relative to py-I and py-II. Variations in the concentrations of Se, Te, and Bi in pyrite are most likely governed by the strong temperature gradient. There is generally a lower concentration of nickel than Co in pyrite, indicating that our samples precipitated at high temperatures, whereas the extreme Co enrichment is likely from a magmatic heat source combined with an influence of serpentinization reactions. (2) Chalcopyrite is characterized by high concentrations of Co, Se, and Te. The abundance of Se and Te in chalcopyrite over the other minerals is interpreted to have been caused by the high solubilities of Se and Te in the chalcopyrite lattice at high temperatures. The concentrations of Sb, As, and Au are relatively low in chalcopyrite from the Kairei vent field. (3) Sphalerite from Zn-rich chimneys is characterized by high concentrations of Sn, Co, Ga, Ge, Ag, Pb, Sb, As, and Cd, but is depleted in Se, Te, Bi, Mo, Au, Ni, Tl, Mn, Ba, V, and U in comparison with the other minerals. The high concentrations of Cd and Co are likely caused by the substitution of Cd2+ and Co2+ for Zn2+ in sphalerite. A high concentration of Pb accompanied by a high Ag concentration in sphalerite indicates that Ag occurs as Pb–Ag sulfosalts. Gold is generally low in sphalerite and strongly correlates with Pb, suggesting its presence in microinclusions of galena. The strong correlation of As with Ge in sphalerite from Kairei suggests that they might precipitate at medium temperatures and under moderately reduced conditions. (4) Bornite–digenite has very low concentrations of most trace elements, except for Co, Se, and Bi. Serpentinization in ultramafic-hosted hydrothermal systems might play an important role in Au enrichment in pyrite with low As contents. Compared to felsic-hosted seafloor massive sulfide deposits, sulfide minerals from ultramafic-hosted deposits show higher concentrations of Se and Te, but lower As, Sb, and Au concentrations, the latter often attributed to the contribution of magmatic volatiles. As with typical ultramafic-hosted seafloor Minerals 2018, 8, 526; doi:10.3390/min8110526 www.mdpi.com/journal/minerals Minerals 2018, 8, 526 2 of 21 massive sulfide deposits, Se enrichment in chalcopyrite from Kairei indicates that the primary factor that controls the Se enrichment is temperature-controlled mobility in vent fluids.


Introduction
Seafloor massive sulfide (SMS) deposits occur at mid-ocean ridges and also in submarine volcanic arcs and related back-arc basins associated with host rocks of highly variable composition [1].The geochemical composition of SMS is also highly variable.For example, sulfide minerals from basalt-hosted mid-ocean ridges appear to be depleted in Au, Ag, As, Sb, and Pb compared to sulfide minerals from hydrothermal systems in back-arc basins (e.g., [1,2]).Additionally, sulfide minerals from occurrences associated with ultramafic host rocks at mid-ocean ridges have significantly higher average Au and Cu contents, when compared to the basalt-hosted deposits (e.g., [3][4][5]).Hence, the geodynamic settings of seafloor hydrothermal systems with their variable host rock compositions represent a first-order control on the geochemical composition of SMS deposits [6].
Sulfide precipitation from hydrothermal fluids that transport metals leached from the host rock, is governed by physico-chemical parameters, such as temperature, salinity, pH, and f O 2 -f S 2 .Processes such as cooling, boiling, mixing, and fluid-rock interactions are key for sulfide precipitation in hydrothermal systems, as they can influence the main parameters above [7][8][9].Previous studies suggest that the recognition of distinct trace element signatures of individual sulfide grains from seafloor hydrothermal systems can assist with tracking sequences of crystallization and stages of mineralization, sources of metals, and evolution of ore-forming fluids over time in SMS deposits (e.g., [10][11][12]).Recent work by Wohlgemuth-Ueberwasser et al. (2015) [13], Keith et al. (2016) [8], Melekestseva et al. (2017) [14], and Wang et al. (2017) [15] investigated trace element distributions using laser ablation ICP-MS analysis in sulfide minerals from SMS deposits in different geodynamic settings including ultramafic-hosted and basalt-hosted fields as well as a number of fields that are associated with submarine volcanic arcs and back-arc basins.Those studies demonstrated that all of the parameters mentioned above influence geochemistry and mineralogy and may account for the variable metal enrichment of SMS.There is comparably little data in hydrothermal fields from the Indian Ocean.A recent publication [4] focused on the mineralogy and bulk geochemistry of massive sulfides from the Kairei vent field on the Central Indian Ridge and suggested that the enrichment of Cu, Zn, and trace elements, such as Au, Co, and Sn, is related to the involvement of ultramafic rocks in the subseafloor, thereby strengthening earlier observations from fluid chemistry (hydrogen and methane enrichment) and petrology (sampling of ultramafic rocks to the east of the active vent field).High-resolution geochemical analyses on sulfide minerals have only been conducted for a few ultramafic-hosted SMS deposits at mid-ocean ridges.For the Kairei vent field, LA-ICP-MS analyses have been performed on pyrite [8], but there is still little known about the incorporation mechanism of trace elements that are normally enriched in the ultramafic-hosted SMS deposits (e.g., Co, Ni, Sn) in sphalerite and Cu-(Fe)-sulfides.
In the present work, we therefore analyze the major sulfide minerals from the Kairei vent field that come from a wide range of sample types (chimney, massive sulfides, and breccias) for trace elements using LA-ICP-MS spot analyses and line scans.This study aims to investigate the distribution of trace elements among sulfide minerals of differing textures, such as pyrite, chalcopyrite, sphalerite, bornite, and covellite, and examine the possible contributing factors (e.g., formation temperature, seawater-fluid mixture, redox) controlling the trace element distribution in these minerals.

Geological Background
The Kairei vent field is located at 25 • 19 S, 70 • 02 E, towards the Northern end of segment 1 of the Central Indian Ridge (CIR-S1), at a water depth of ~2450 m (Figure 1; [16]).Kairei occurs ~1800 m above the rift valley floor on its Eastern wall and is situated over 7 km away from the axial volcanic ridge.The rift valley wall has a stair-step morphology in this region and the active vent field occurs on the Western flank of Hakuho Knoll within a larger area of massive sulfides (Figure 1b; [17,18]).The discharging fluids at the vent field reach temperatures of 365 • C and are characterized by high concentrations of H 2 and CH 4 [17,19], which is comparable to ultramafic-hosted hydrothermal fields along the Mid-Atlantic Ridge (e.g., [20][21][22][23][24]).The outcrops in the vicinity of the vent field are entirely made up of pillow basalt and sheet flows, and no exposures of ultramafic rocks were observed [17][18][19].However, two oceanic core complexes (Uraniwa Hills and Yokoniwa Rise; Figure 1b) consisting of peridotitic and gabbroic rocks have been identified within ~15 km of Kairei [25,26].Another large core complex has been documented on the Western slopes of the CIR in this area [27].It was suggested that the combination of the troctolite alteration at the recharge zone near the oceanic core complex and subsequent basalt alteration at the discharge zone under Kairei is causing the distinct chemistry of the vent fluids [18,25].Recent investigations have shown the presence of other ultramafic-hosted massive sulfide deposits (e.g., the Yokoniwa vent field) in the area [26].
Minerals 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/minerals The Kairei vent field is located at 25°19′ S, 70°02′ E, towards the Northern end of segment 1 of the Central Indian Ridge (CIR-S1), at a water depth of ~2450 m (Figure 1; [16]).Kairei occurs ~1800 m above the rift valley floor on its Eastern wall and is situated over 7 km away from the axial volcanic ridge.The rift valley wall has a stair-step morphology in this region and the active vent field occurs on the Western flank of Hakuho Knoll within a larger area of massive sulfides (Figure 1b; [17,18]).The discharging fluids at the vent field reach temperatures of 365 °C and are characterized by high concentrations of H2 and CH4 [17,19], which is comparable to ultramafic-hosted hydrothermal fields along the Mid-Atlantic Ridge (e.g., [20][21][22][23][24]).The outcrops in the vicinity of the vent field are entirely made up of pillow basalt and sheet flows, and no exposures of ultramafic rocks were observed [17][18][19].However, two oceanic core complexes (Uraniwa Hills and Yokoniwa Rise; Figure 1b) consisting of peridotitic and gabbroic rocks have been identified within ~15 km of Kairei [25,26].Another large core complex has been documented on the Western slopes of the CIR in this area [27].It was suggested that the combination of the troctolite alteration at the recharge zone near the oceanic core complex and subsequent basalt alteration at the discharge zone under Kairei is causing the distinct chemistry of the vent fluids [18,25].Recent investigations have shown the presence of other ultramafic-hosted massive sulfide deposits (e.g., the Yokoniwa vent field) in the area [26].The main mineralogical characteristics (Figure 2) of Zn-rich chimney and massive sulfide samples from the Kairei vent field were described in Wang et al. (2012Wang et al. ( , 2014) ) [4,30].Three mineralization stages were identified at this site: (1) a high temperature stage consisting largely of chalcopyrite, isocubanite, and pyrite; (2) a medium to low temperature stage characterized by the mineral assemblages of sphalerite and pyrite; and (3) a late stage of waning hydrothermal activity and weathering characterized by secondary Cu-(Fe)-sulfides (bornite, digenite, covellite, and idaite), Fe-oxyhydroxides, opal-A, and Cu-chloride (paratacamite and atacamite) were related to the increasing influence of oxygenated seawater.The observed Cu-(Fe)-sulfide sequence of isocubanite → chalcopyrite → bornite (including bornite-digenite intergrowths) → minerals of the Cu-S system (covellite-idaite solid solutions) exists along a path of increasing oxidation and sulfidation state and is common at Kairei.Dating ( 230 Th/ 234 U) of massive sulfides collected from Kairei shows ages ranging from 8.4 to 96.3 ka [30], indicating a prolonged hydrothermal history at this site.

Samples and Methods
The samples from Kairei are Zn-rich chimneys, Cu-rich massive sulfides, and sulfide-bearing breccias that were recovered using a TV-grab during R/V Dayangyihao DY17A and DY19III cruises in 2005 and 2007 (Table 1).In advance of the laser-ablation analyses, eight thin, polished sections were examined microscopically in order to identify the different mineral phases (Figure 2) and to avoid measuring grains with visible mineral inclusions.A total of 99 representative analyses were performed by electron microprobe for major element composition [4], and on these samples, 64 points were analyzed by laser ablation ICP-MS.
Minerals 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/mineralsanalytical precision, and minimum detection limits for each element.The average concentrations of trace elements were calculated for the different sulfide minerals and morphologies (Table 2).
Representative time-resolved LA-ICP-MS depth profiles are shown in Figure 3 for pyrite, chalcopyrite, sphalerite, bornite-digenite, and covellite-digenite.Figure 4 shows the trace element distribution in different sulfide mineral phases.In addition, line scan analysis of the pyrite in the Znrich chimney sample (17A-TVG9) was carried out, and the resulting mass spectrum is shown in Figure 5.

Results
Full details of the LA-ICP-MS dataset are given in Table S2, including the concentration, analytical precision, and minimum detection limits for each element.The average concentrations of trace elements were calculated for the different sulfide minerals and morphologies (Table 2).Representative time-resolved LA-ICP-MS depth profiles are shown in Figure 3 for pyrite, chalcopyrite, sphalerite, bornite-digenite, and covellite-digenite.Figure 4 shows the trace element distribution in different sulfide mineral phases.In addition, line scan analysis of the pyrite in the Zn-rich chimney sample (17A-TVG9) was carried out, and the resulting mass spectrum is shown in Figure 5.A line scan analysis in chimney sample 17A-TVG9 conducted across a boundary between colloform py-III and subhedral pyrite II (Figure 5) exemplified the differences.The ablation of colloform py-III was marked by high trace element concentrations (Mn, Pb, Cu, Zn, Co, and Mo) that generally decreased towards the more homogeneous subhedral py-II.

Pyrite
A total of nineteen spot analyses was completed on pyrites selected from different samples of the Kairei vent field, including eight euhedral pyrites (py-I), five subhedral pyrites (py-II), and six colloform pyrites (py-III).Pyrite is generally enriched in Mo, As, Tl, Mn, and U compared to the other investigated sulfide minerals (Figure 4, Table 2).Pyrite with different textures showed distinct variations in the trace element contents (Figures 5 and 6, Table 2).Compared to the other textural types, euhedral py-I tended to have higher contents of Se (up to 168 ppm; average: 65.3 ppm), Ni (up to 363 ppm; average: 51.4 ppm), Bi (up to 123 ppm; average: 30.4 ppm), and Te (up to 46.0 ppm; average: 14.3 ppm), when compared to py-II and py-III.Colloform py-III had distinctly higher concentrations of Mo (46-307 ppm; average:136 ppm), Pb (14-295 ppm; average: 105 ppm), and U (up to 2.73 ppm; average: 0.90 ppm) but was poor in Se, Te, Bi, and Au.Subhedral py-II was poor in most trace elements, but one single spot showed abnormally high Au concentrations (28.4 ppm; Table S2).However, enrichments of As (up to 2041 ppm) and Co (up to 1453 ppm) were common in all generations of pyrites.Py-I contained 395 ppm Co and 135 ppm As, on average; py-II had average As and Co concentrations of 254 ppm and 230 ppm, respectively; while py-III was characterized by concentrations of 614 ppm As and 466 ppm Co, on average.Minerals 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/mineralsbelow).The Co-enrichment in all sulfide minerals at Kairei may therefore also point to an ultramafic influence at this site.Higher contents of Au (up to 28.4 ppm) are most likely related to inclusions of native gold which were observed in a single grain (Figure 3A), which is similar to observations by Keith et al. ( 2016) [8].Uranium, which is normally derived from seawater, is also enriched in colloform pyrite compared to the other pyrite textures.This U enrichment is commonly observed in SMS deposits (e.g., [10,15,42]).A line scan analysis in chimney sample 17A-TVG9 conducted across a boundary between colloform py-III and subhedral pyrite II (Figure 5) exemplified the differences.The ablation of colloform py-III was marked by high trace element concentrations (Mn, Pb, Cu, Zn, Co, and Mo) that generally decreased towards the more homogeneous subhedral py-II.

Chalcopyrite
Sixteen chalcopyrite grains were analyzed in samples from the Kairei vent field.The chalcopyrite samples were characterized by high concentrations of Co (32-1781 ppm; average: 445 ppm), Se (62-1283 ppm; average: 363 ppm), and Te (up to 110 ppm; average: 29.7 ppm).The mean concentrations of Mo, Ag, Pb, Sb, As, Cd, and Mn were generally lower than in the other sulfide minerals; however, single measurements with enrichments in Zn (1740 ppm), Ge (243 ppm), and Au (3.5 ppm) did occur (Figure 4, Table S2).

Bornite-Digenite Assemblage
Ten measurements were performed on bornite-digenite intergrowths in the Cu-rich sulfide samples.The results given in Table 2 indicate that bornite-digenite intergrowths were depleted in most trace elements when compared to the other sulfide minerals, except for Se (70.3-2272 ppm), Co (0.02-1669 ppm), and Bi (<0.001 ppm-191 ppm).Interestingly, the highest concentrations of Se (up to 2272 ppm) and Bi (up to 191 ppm) in all the sulfide minerals were observed in a bornite-digenite grain.

Controls on Trace Elements in Pyrite
Pyrite is the most common sulfide mineral in hydrothermal ores and large mineral chemical data sets exist for SMS deposits over a wide range of geodynamic settings, including mid-ocean ridges, submarine volcanic arcs, and related back-arc basins (e.g., [6,8,13,15,[36][37][38]).Previous studies have shown that most trace elements in pyrite likely reside in microinclusions of different grains, except for Cu, Co, Ni, Se, and As, which commonly occur in solid solution and result from Fe or S substitution [8,39].
Colloform pyrite (py-III) in the Zn-rich chimneys was consistently enriched with Pb, Mo, As, and U (Figure 6) relative to the other pyrite varieties.Euhedral pyrite (py-I), which is a main constituent of both sulfide-bearing breccia and Zn-rich chimneys, had high concentrations of Se, Te, Ni, Bi, and Au when compared to the colloform py-III but was depleted in Mo and U. Enrichments of As (mean 376 ppm) were common in all generations of pyrites at the Kairei vent field, and a positive correlation was observed between As and Ag (R 2 = 0.74).This could indicate co-incorporation of the two elements resulting from As 3+ and Ag + coupled substitution for 2Fe 2+ [40,41].Variations in the concentrations of Se (<0.04 to 168 ppm), Te (<0.05 to 46.0 ppm), and Bi (<0.02 to 123 ppm) in pyrite are likely governed by the strong temperature gradient [41].Nickel concentrations (<0.04 to 363 ppm) were generally lower than those for Co in pyrite (Co/Ni = 3-172 ppm), indicating that the investigated samples precipitated under high temperature conditions [e.g., [42,43]].Similar enrichments of Co in pyrite have been described from sulfides sampled at the Mid-Atlantic Ridge 5 • S vent field with an average of 602 ppm (Table 3; [8]).Previous studies have shown that the Co is concentrated in high temperature fluids and is highly sensitive to changes in fluid temperature [44,45].The high exit temperature for black smoker fluids at the Kairei vent field (up to 365 • C; [19]) indicates little or no interaction with seawater during upwelling.In addition, the results of Gallant and Von Damm (2006) [19] indicate that diking may be a mechanism for the heat source of the Kairei vent field.Hence, the Co enrichment in pyrite at Kairei might be related to a phase of recent magmatic activity [8].However, besides pyrite, chalcopyrite and sphalerite at Kairei also have higher Co contents compared to minerals from the mafic-hosted Wocan vent field at the Carlsberg Ridge (Table 3; see discussion below).The Co-enrichment in all sulfide minerals at Kairei may therefore also point to an ultramafic influence at this site.Higher contents of Au (up to 28.4 ppm) are most likely related to inclusions of native gold which were observed in a single grain (Figure 3A), which is similar to observations by Keith et al. (2016) [8].Uranium, which is normally derived from seawater, is also enriched in colloform pyrite compared to the other pyrite textures.This U enrichment is commonly observed in SMS deposits (e.g., [10,15,42]).[48] and references therein.
Our analyses showed that Se (61.5-1283 ppm) and Te (2.6-110 ppm) were enriched in chalcopyrite from the Kairei vent field, with a strong positive correlation between these two elements (R 2 = 0.96).Both elements are likely incorporated into the chalcopyrite lattice at high temperatures, with high solubilities of Se and Te in reduced hydrothermal fluids of ~350 • C [10,[49][50][51].Similar enrichments of Te in chalcopyrite were described by Logatchev and Semyenov-2, with Te concentrations reaching 39.6 and 42.0 ppm, respectively (Table 3; [13,14]).However, the Te content of chalcopyrite in most other SMS deposits is typically low.LA-ICP-MS analyses of chalcopyrite from Wocan, Turtle Pits, and PacManus yielded an average Te concentration of around 1 ppm (Table 3; [13,15]).Compared to the basaltic-hosted Wocan field, chalcopyrite from Kairei was also shown to be highly enriched in Co, with a wide range of concentrations from 32.1 to 1781 ppm (Table 3; [15]), which could be derived from ultramafic rocks in the reaction zone.Maslennikov et al. (2009) [42] suggested that the Co enrichment in chalcopyrite from fossil vent chimneys in a volcanogenic-hosted massive sulfide deposit in the Southern Urals is related to Co-bearing mineral inclusions, such as cobaltite, arsenides, sulfoarsenides, and telluride.However, no positive correlation was observed between the concentrations of Co and Te/As in chalcopyrite from the Kairei vent field, and the pattern of the Co signal intensities correlated with Fe (Figure 3B, C), indicating that Co most likely substitutes for Fe.Antimony (<2.3 ppm) and As concentrations (<50.6 ppm) were relatively low in chalcopyrite from Kairei.These concentrations are similar to those described from other SMS deposits from mid-ocean ridge systems worldwide (Table 3).For example, a similar low concentration of Sb has been observed in chalcopyrite from Turtle Pits, Logatchev, Wocan, and Semyenov-2 (average usually <30 ppm).In contrast, Sb-rich chalcopyrite is common in back-arc basins and the enrichment there is thought to be mainly controlled by the occurrence of Sb-bearing mineral inclusions related to an increased flux of elements such as Sb due to the input of magmatic volatiles [13].Most chalcopyrite from Kairei contains <0.2 ppm Au; however, single measurements with enrichments in Au (3.5 ppm, 2.3 ppm; Table S2) do occur and might indicate the occurrence of native gold inclusions (Figure 2F).

Controls on Trace Elements in Sphalerite
As one of the major sulfide minerals in the SMS deposits, sphalerite precipitates under a range of pressure, temperature, and f O 2 -f S 2 conditions [52].At Kairei, sphalerite from the Zn-rich chimneys was shown to be highly enriched in Cd, Pb, Co, As, Sn, Ga, Ge, Ag, and Sb compared to the other minerals.Cadmium, Co, Ga, and Sn showed clear positive correlations with an increasing temperature (Figure 7) calculated following the Fe/Zn sphalerite equation of Keith et al. (2014) [53].In contrast, Ag, As, Ge, and Pb were characterized by negative correlations with temperature and precipitate as a consequence of an increasing f O 2 of the hydrothermal fluids due to significant mixing with seawater (Figure 7).Trace element concentrations in sphalerite showed strong positive correlations (Figure 8) for Pb-Ag (R 2 = 0.98), Pb-Au (R 2 = 0.83), Co-Cd (R 2 = 0.67), Cu-Sn (R 2 = 0.91), Ga-Sn (R 2 = 0.90), As-Ge (R 2 = 0.95), and Sb-Cd (R 2 = 0.79).The high concentrations of Cd (609-1792 ppm) and Co (196-993 ppm) were likely caused by the substitution of Zn 2+ by Cd 2+ and Co 2+ (e.g., [52,54]).A high concentration of Pb (420-6396 ppm) accompanied by similarly high Ag concentrations (44-882 ppm) in sphalerite most likely indicates that the Ag occurs as Pb-Ag sulfosalts rather than as native silver associated with galena.The concentration of Au was generally low in sphalerite (<0.5 ppm); however, Au is related to Pb (R 2  Ag-Au = 0.68), suggesting co-precipitation with Au as electrum inclusions in galena.The positive correlation between Sn and Cu, as well as between Sn and Ga, may be attributed to the substitution of Zn 2+ by Ga 2+ or Ga 3+ [12], or 3Zn 2+ by Sn 4+ + Cu 2+ [52].This substitution would also explain the Sn enrichment in bulk samples from Zn-rich chimneys (101-510 ppm; [4]).Sphalerite is also enriched in Ge and As with concentrations ranging from 72 ppm to 336 ppm and from 61 ppm to 502 ppm, respectively, which are similar to sphalerite from the Wocan hydrothermal field at the Carlsberg Ridge [15].Although Ge 4+ represents the more common oxidation state, Cook et al. (2009) [52] proposed a direct Ge 2+ ↔ Zn 2+ substitution in sphalerite.The strong correlation of As with Ge (R 2 = 0.95) in sphalerite from Kairei suggests that both elements are enriched in sphalerite precipitating under medium temperature and moderately reduced conditions [52].[52].This substitution would also explain the Sn enrichment in bulk samples from Zn-rich chimneys (101-510 ppm; [4]).Sphalerite is also enriched in Ge and As with concentrations ranging from 72 ppm to 336 ppm and from 61 ppm to 502 ppm, respectively, which are similar to sphalerite from the Wocan hydrothermal field at the Carlsberg Ridge [15].Although Ge 4+ represents the more common oxidation state, Cook et al. (2009) [52] proposed a direct Ge 2+ ↔ Zn 2+ substitution in sphalerite.The strong correlation of As with Ge (R 2 = 0.95) in sphalerite from Kairei suggests that both elements are enriched in sphalerite precipitating under medium temperature and moderately reduced conditions [52].

Controls on Trace Elements in Bornite, Digenite, and Covellite
Bornite, digenite, and covellite are common Cu-(Fe)-sulfide minerals in SMS deposits which often precipitate under more oxidizing conditions caused by the ingress of seawater during the waning stages of hydrothermal activity (e.g., [3,4,51,55]).Due to the interaction of cooling hydrothermal fluids with cold, oxygenated seawater, U, V, and sometimes Ag may reach high concentrations in low temperature bornite and digenite formed during the latest hydrothermal phase or during secondary oxidation of primary sulfides (e.g., [14,15,47]).In contrast, bornite precipitated at higher temperatures has relatively high concentrations of Sn and In [15].Additionally, Cook et al. (2011) [56] stated that high temperature primary bornite is an excellent host for Bi and Se.In our dataset, the highest Bi (up to 191 ppm) and Se (up to 2272 ppm) concentrations were found in bornite and may be explained by the incorporation of Bi into the bornite crystal lattice [56].Cobalt concentrations in bornite-digenite were typically close to their respective minimum detection limits.However, in some spots, high Co concentrations (>1000 ppm; Figure 4) were observed, and based on

Controls on Trace Elements in Bornite, Digenite, and Covellite
Bornite, digenite, and covellite are common Cu-(Fe)-sulfide minerals in SMS deposits which often precipitate under more oxidizing conditions caused by the ingress of seawater during the waning stages of hydrothermal activity (e.g., [3,4,51,55]).Due to the interaction of cooling hydrothermal fluids with cold, oxygenated seawater, U, V, and sometimes Ag may reach high concentrations in low temperature bornite and digenite formed during the latest hydrothermal phase or during secondary oxidation of primary sulfides (e.g., [14,15,47]).In contrast, bornite precipitated at higher temperatures has relatively high concentrations of Sn and In [15].Additionally, Cook et al. (2011) [56] stated that high temperature primary bornite is an excellent host for Bi and Se.In our dataset, the highest Bi (up to 191 ppm) and Se (up to 2272 ppm) concentrations were found in bornite and may be explained by the incorporation of Bi into the bornite crystal lattice [56].Cobalt concentrations in bornite-digenite were typically close to their respective minimum detection limits.However, in some spots, high Co concentrations (>1000 ppm; Figure 4) were observed, and based on the flat signal intensities of Co, we suppose this to represent local substitution of Co into the Fe site within bornite (Figure 3E).

Comparison with Mafic-and Ultramafic-Hosted SMS Deposits
The concentrations of trace elements in the ores from the SMSs, especially those that are mobilized at relatively low temperatures, are remarkably sensitive to the source rock concentrations in different tectonic settings (e.g., [3,7,57]).For example, mafic-and ultramafic-hosted SMSs at mid-ocean ridges are often enriched in Ni and Co [3,4,58], whereas felsic-hosted deposits in volcanic arc settings are enriched in Pb, As, Ag, Sb, and Sn [7].Previous studies concluded that the presence of elevated Ni and Co concentrations in the sulfide minerals from ultramafic SMS deposits originates from the ultramafic substrate (1960 ppm Ni and 106 ppm Co in depleted mantle versus 104 ppm Ni and 44 ppm Co in MORB [59,60]).However, depositional conditions may also play significant roles in determining the final metal concentrations in the sulfide minerals [58].
Pyrite from ultramafic-hosted Kairei, Logatchev, and Semyenov-1 showed higher concentrations of Au (>1 ppm on average) than pyrite from the mafic-hosted Wocan, MESO, TAG, Turtle Pits, and MAR 5 • S (Au <1 ppm) along the mid-ocean ridge (Table 3; Figure 9).Keith et al. ( 2016) [8] proposed that serpentinization in ultramafic-hosted vent systems plays an important role on Au enrichment in pyrite with low As contents.However, compared with felsic-hosted SMS deposits, sulfide minerals from the ultramafic-hosted deposits presented higher concentrations of Se and Te, but lower As, Sb, and Au concentrations (Table 3), which is most likely attributed to more reducing conditions at mid-ocean ridges and the contribution of magmatic volatiles in arc-related occurrences [13].Significant variations of Ni/As versus Au/As ratios in pyrite were observed between ultramafic-and mafic-hosted SMS occurrences (Figure 9).This implies that there is an ultramafic influence on Au and Ni enrichment in pyrite from Kairei.However, the ultramafic-hosted Semyenov-1 showed lower Ni concentrations that were associated with higher Au and As concentrations (averages of 33.1 ppm Au and 510 ppm As; Table 3; Figure 9), Melekestseva et al. (2014) [46] suggested that a mafic substrate might contribute to those elements in the Semyenov-1.Clearly, investigations on the trace element behavior in other ultramafic-hosted sites, such as Rainbow, Ashadze and others, are needed to verify this observation.3E).

Comparison with Mafic-and Ultramafic-Hosted SMS Deposits
The concentrations of trace elements in the ores from the SMSs, especially those that are mobilized at relatively low temperatures, are remarkably sensitive to the source rock concentrations in different tectonic settings (e.g., [3,7,57]).For example, mafic-and ultramafic-hosted SMSs at midocean ridges are often enriched in Ni and Co [3,4,58], whereas felsic-hosted deposits in volcanic arc settings are enriched in Pb, As, Ag, Sb, and Sn [7].Previous studies concluded that the presence of elevated Ni and Co concentrations in the sulfide minerals from ultramafic SMS deposits originates from the ultramafic substrate (1960 ppm Ni and 106 ppm Co in depleted mantle versus 104 ppm Ni and 44 ppm Co in MORB [59,60]).However, depositional conditions may also play significant roles in determining the final metal concentrations in the sulfide minerals [58].
Pyrite from ultramafic-hosted Kairei, Logatchev, and Semyenov-1 showed higher concentrations of Au (>1 ppm on average) than pyrite from the mafic-hosted Wocan, MESO, TAG, Turtle Pits, and MAR 5°S (Au <1 ppm) along the mid-ocean ridge (Table 3; Figure 9).Keith et al. (2016) [8] proposed that serpentinization in ultramafic-hosted vent systems plays an important role on Au enrichment in pyrite with low As contents.However, compared with felsic-hosted SMS deposits, sulfide minerals from the ultramafic-hosted deposits presented higher concentrations of Se and Te, but lower As, Sb, and Au concentrations (Table 3), which is most likely attributed to more reducing conditions at mid-ocean ridges and the contribution of magmatic volatiles in arc-related occurrences [13].Significant variations of Ni/As versus Au/As ratios in pyrite were observed between ultramafic-and mafic-hosted SMS occurrences (Figure 9).This implies that there is an ultramafic influence on Au and Ni enrichment in pyrite from Kairei.However, the ultramafic-hosted Semyenov-1 showed lower Ni concentrations that were associated with higher Au and As concentrations (averages of 33.1 ppm Au and 510 ppm As; Table 3; Figure 9), Melekestseva et al. (2014) [46] suggested that a mafic substrate might contribute to those elements in the Semyenov-1.Clearly, investigations on the trace element behavior in other ultramafic-hosted sites, such as Rainbow, Ashadze and others, are needed to verify this observation.3.
To be able to compare the differences in trace element concentrations of chalcopyrite between ultramafic/mafic-and felsic-hosted deposits, we normalized the trace elements to global MORB [48] Figure 9. Variations of Ni/As vs. Au/As in pyrite from ultramafic-and mafic-hosted hydrothermal systems at mid-ocean ridges and felsic-hosted sites occurred in the back-arc basin/arc volcanoes.See the data source in Table 3.
To be able to compare the differences in trace element concentrations of chalcopyrite between ultramafic/mafic-and felsic-hosted deposits, we normalized the trace elements to global MORB [48] (Figure 10).Chalcopyrite from Kairei showed a higher concentration of Se relative to Wocan, Semyenov-2, and Logatchev, but lower concentrations of As and Sb (Table 3; Figure 10).The general enrichment of Se in chalcopyrite is a factor of fluid temperature, with high solubility of Se in fluids of ~350 °C [10,49,61].A host rock influence (mafic versus ultramafic) can probably be neglected since significant Se enrichment in chalcopyrite has been found in both, ultramafic-and mafic-hosted SMSs (e.g., average: ~2000 ppm, Broken Spur [62]).Supplementary Materials: The following are available online at www.mdpi.com/link,Table S1: LA-ICP-MS reference materials, Table S2: Trace and main element concentrations (ppm) of the analyzed sulfides at the Kairei vent field obtained from LA-ICP-MS and EPMA.

Figure 1 .Figure 1 .
Figure 1.(a) Geological map of the Central Indian Ridge (CIR) and (b) location of the Kairei vent field.Other known hydrothermal fields are indicated by red stars.Note, segment numbering is from Okino Figure 1.(a) Geological map of the Central Indian Ridge (CIR) and (b) location of the Kairei vent field.Other known hydrothermal fields are indicated by red stars.Note, segment numbering is from Okino et al. (2015) [28].SWIR = Southwest Indian Ridge; SEIR = Southeast Indian Ridge.Bathymetry data source: Global Multi-Resolution Topography (GMRT) synthesis [29].

Figure 3 .
Figure 3. Representative time-resolved LA-ICP-MS depth profiles of selected elements in sulfide minerals from the Kairei vent field.(A) subhedral pyrite in Zn-rich chimney sulfides with inclusions of native gold; (B) chalcopyrite in the Cu-rich massive sulfide samples shows a homogeneous distribution of Ni, As, and Tl; (C) chalcopyrite showing a homogeneous distribution of Co, As, Mo, and Pb; (D) sphalerite with co-varying amounts of Fe and Co, Ag, Ga, Sn, and Mn; (E) early bornite with homogeneous Bi, Co, Mo, Ag, and Te distribution indicated by the flat signal intensities; (F) early bornite with elevated Zn, Bi, and Se.

Figure 3 .
Figure 3. Representative time-resolved LA-ICP-MS depth profiles of selected elements in sulfide minerals from the Kairei vent field.(A) subhedral pyrite in Zn-rich chimney sulfides with inclusions of native gold; (B) chalcopyrite in the Cu-rich massive sulfide samples shows a homogeneous distribution of Ni, As, and Tl; (C) chalcopyrite showing a homogeneous distribution of Co, As, Mo, and Pb; (D) sphalerite with co-varying amounts of Fe and Co, Ag, Ga, Sn, and Mn; (E) early bornite with homogeneous Bi, Co, Mo, Ag, and Te distribution indicated by the flat signal intensities; (F) early bornite with elevated Zn, Bi, and Se.

Figure 5 .
Figure 5. Signal sequence of a line scan analysis of colloform pyrite III and subhedral pyrite II in sample 17A-TVG9.The mass spectrum of colloform pyrite III is marked by high trace element concentrations (Mn, Pb, Cu, Zn, Co, and Mo) that generally decrease towards the more homogeneous subhedral pyrite II.

Figure 5 .
Figure 5. Signal sequence of a line scan analysis of colloform pyrite III and subhedral pyrite II in sample 17A-TVG9.The mass spectrum of colloform pyrite III is marked by high trace element concentrations (Mn, Pb, Cu, Zn, Co, and Mo) that generally decrease towards the more homogeneous subhedral pyrite II.

Figure 6 .
Figure 6.Average trace element concentrations in different pyrite types of the Kairei vent field.Figure 6.Average trace element concentrations in different pyrite types of the Kairei vent field.

Figure 6 .
Figure 6.Average trace element concentrations in different pyrite types of the Kairei vent field.Figure 6.Average trace element concentrations in different pyrite types of the Kairei vent field.

Figure 7 .
Figure 7. Correlation between the concentrations of trace elements in sphalerite and precipitation temperatures calculated from Keith et al. (2014) [53].

Figure 7 .
Figure 7. Correlation between the concentrations of trace elements in sphalerite and precipitation temperatures calculated from Keith et al. (2014) [53].

Figure 9 .
Figure 9. Variations of Ni/As vs. Au/As in pyrite from ultramafic-and mafic-hosted hydrothermal systems at mid-ocean ridges and felsic-hosted sites occurred in the back-arc basin/arc volcanoes.See the data source in Table3.

Table 1 .
Coordinates of sampling stations and mineral abundances of the sulfides from the Kairei vent field.

Table 2 .
Average trace element concentrations from LA-ICP-MS for the sulfides of the Kairei vent field.
Note: Avg.= average content of trace element; SD = standard deviation of the average (1δ); n = number of analyses; b.d.l = below the minimum detection limit.

Table 3 .
Average contents (ppm) of selected trace elements in the sulfide minerals in the seafloor hydrothermal systems.
flat signal intensities of Co, we suppose this to represent local substitution of Co into the Fe site within bornite (Figure Minerals 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/minerals the