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Article

The Mineral and Geochemical Features of Sulfides in the Jade Hydrothermal Field of the Okinawa Trough in Off-Shore China

by
Yujie Wang
1,2 and
Zhigang Zeng
1,2,3,*
1
Seafloor Hydrothermal Activity Laboratory, CAS Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Laboratory for Marine Mineral Resources, Laoshan Laboratory, Qingdao 266237, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(9), 1772; https://doi.org/10.3390/jmse11091772
Submission received: 17 July 2023 / Revised: 1 September 2023 / Accepted: 6 September 2023 / Published: 11 September 2023
(This article belongs to the Section Geological Oceanography)
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Abstract

:
In this study, mineralogical and elemental geochemical characteristics of massive sulfide samples collected from the Jade hydrothermal field, located in the Izena depression in the central graben of the Okinawa Trough, were analyzed by means of optical microscopy, scanning electron microscopy, and electron probe microanalysis (EPMA). The results show that the mineralization in the Jade hydrothermal field can be divided into Zn-Cu-Pb-rich massive sulfides and Zn-Fe-rich massive sulfides. The former is composed of sphalerite, galena, chalcopyrite, pyrite, and anglesite, which is the product of the low-temperature alteration of galena. The latter is mainly composed of sphalerite, pyrite, marcasite, and traces of galena. Cu and Zn in pyrite may exist in the form of microinclusions, while Ag and Pb may exist in pyrite in the form of fine galena inclusions containing Ag. Fe and Cu may enter sphalerite in the form of ion replacement. Zn may enter chalcopyrite in the form of ion replacement. Consistent with the previous understanding, the metal elements in the hydrothermal liquid system in the Jade hydrothermal field mostly migrated as sulfur complexes, and when the hydrothermal fluid mixes with seawater, the physical and chemical conditions of the fluid change, resulting in sulfide mineral precipitation. However, the chemical structure of chalcopyrite is still controversial, which restricts the understanding of the substitution mechanism of trace elements during chalcopyrite precipitation.

1. Introduction

The discovery of modern seafloor hydrothermal systems is one of the major events in the field of marine geological research in the 20th century [1]. They widely exist in extensional tectonic environments such as mid-ocean ridges and back-arc basins, as well as volcanic activity fields [2,3]. Driven by heat sources, seawater seeps down through crustal faults or cracks, transforms into hydrothermal fluids, and flows upward, forming hydrothermal products such as sulfides, hydrothermal columns, and metal-bearing sediments at the vents. Its evolution is accompanied by the interaction of seawater/fluid rock and/or sediment, injection of magmatic materials, mixing with seawater, and response and action of hydrothermal organisms, forming a large-scale massive accumulation of polymetallic sulfides. It not only has enormous economic value but also has important research significance and has become one of the major cutting-edge research fields in earth sciences today. By studying this natural laboratory, scientists can obtain information on the composition and temperature of hydrothermal fluids and improve existing theories of hydrothermal mineralization [4]. In addition, comparative research with ancient continental massive sulfide deposits can also be conducted, providing theoretical guidance for exploring seafloor massive sulfide resources and laying a foundation for further understanding the hydrothermal mineralization process during the geological history [5].
Previous systematic studies on mineralogy, geochemistry, and isotopes have been carried out in the Okinawa Trough [6,7,8,9,10,11,12,13,14]. According to the existing geological and geophysical investigations, the Okinawa Trough is a back-arc basin, which is characterized by the development of brittle normal faults and frequent intrusion of magma, providing a favorable tectonic and magmatic environment for the development of hydrothermal systems [15]. So far, over 10 hydrothermal mineralization points have been discovered in the Okinawa Trough [16]. Among them, sulfides accumulations in the Jade hydrothermal field are mainly composed of sphalerite (ZnS), wurtzite (ZnS), galena (PbS), anglesite (PbSO4), pyrite (FeS2), marcasite (FeS2), cubanite (CuFe2S3), and chalcopyrite (CuFeS2). The average element composition of the whole rock samples of seafloor polymetallic sulfides is Zn (20.2%), Pb (11.8%), Ba (7.2%), Fe (6.2%), Cu (3.3%), As (17,500 ppm), and Ag (2300 ppm) [17]. Nevertheless, further exploration is needed on important issues such as the mineral precipitation stage, chemical composition, and control factors of massive sulfides in the Jade hydrothermal field of the Okinawa Trough.
It is well known that the study of the mineral composition, fabric, mineral precipitation sequence [18,19], and geochemical characteristics [20,21] of hydrothermal sulfides can provide an important theoretical basis for further exploration of hydrothermal systems and mineralization through the indicator information of mineral genesis. Therefore, in this study, the texture, mineral paragenesis, and geochemical characteristics of sulfide samples collected from the Jade hydrothermal field in the Okinawa Trough are studied. The controlling factors of mineral paragenesis and geochemistry in hydrothermal sulfides of the Okinawa Trough and their reflection on hydrothermal activity characteristics are discussed.

2. Geologic Setting

The Okinawa Trough, located at the eastern margin of Eurasia, is a back-arc basin formed by the NW subduction of the Philippine Sea plate under the Eurasian continental plate (Figure 1). At present, it is still in the early stage of back-arc expansion [22,23], forming a complete trench-arc basin system with the Ryukyu Islands arc and the Ryukyu trench. The Okinawa Trough is 230 km wide in the north and 60–100 km wide in the south, with water depth gradually increasing from north to south [22]. There have been reports of signs of oceanic crust appearing in its southern section [24], and the trough is intersected by a series of northwest-trending strike-slip faults, often divided into the northern, middle, and southern sections by the Tukara and Gonggu fault structural belts [25]. At the same time, the volcanic rocks exposed in the Okinawa Trough are mainly medium to acidic volcanic rocks, while basic volcanic rocks are rare. The rock types are mainly andesites, rhyolites, dacites, basalts, and other calc-alkaline series and tholeiite series rocks [25,26]. In addition, due to the huge supply of terrestrial materials from the Yangtze and Yellow Rivers, the Okinawa Trough is covered by thick sediments, with the northern section covered by approximately 8 km of sediments and the southern section covered by nearly 2 km of sediments, mainly composed of terrestrial and hydrothermal origin materials [23].
There is a diversity of sulfide and sulfate mineralization that can be explained by seafloor phase separation in the hydrothermal field of the Okinawa Trough, which is consistent with the shallow water depth of 700–1600 m [16]. Among them, the Jade hydrothermal field is one of the largest mining areas in the Okinawa Trough [10], distributed on the northeast slope of the Izena depression in the central part of the trough (27°15′ N, 127°4.5′ E). The area is distributed in a southwest–northeast direction with a width of about 600 m and a length of 1800 m at a water depth of around 1200–1600 m [27]. The strong structural deformation leads to rugged terrain and the distribution of small stepped faults. The Jade hydrothermal field exhibits both high-temperature (320 °C) and low-temperature (124 °C) hydrothermal activities, with irregular distribution of sulfide-sulfate chimneys and mound-shaped accumulations. Except for some chimneys that are collapsed, most chimneys can stand upright up to 5 m. The main ore minerals in Jade hydrothermal field are sphalerite, galena, pyrite, marcasite, and chalcopyrite, as well as pyrrhotite (Fe1−xS), tetrahedrite(Cu12Sb4S13), stibnite (Sb2S3), orpiment (As2S3), and other As- and Sb-rich minerals. Bismuthinite (Bi2S3), mimetite (Pb2Pb3[AsO4]3Cl), and native metals are occasionally seen. Gangue minerals are mainly barite (BaSO4), gypsum (CaSO4·2H2O), anhydrite (CaSO4), and amorphous silica, and occasionally carbonate minerals, such as calcite (CaCO3) [10].

3. Material and Methods

The massive sulfide samples in this study were taken from the Jade hydrothermal field in the middle of the Okinawa Trough (Figure 1). They are mainly composed of sphalerite, barite, pyrite, and chalcopyrite, with minor anglesite and galena, as well as a small amount of covellite, mimetite, and crocoite (PbCrO4). The minerals are mainly fine-grained, sub-to-euhedral, and are partially characterized by abundant pore spaces.
The thin sections of the samples were examined by optical microscopy for ore mineralogy and texture, and then the mineral composition of the samples was analyzed with a TESCAN VEGA 3LMH scanning electron microscope in the Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences. The electron microscope was equipped with a backscattered electron detector (BSE), a secondary electron detector, and an Oxford INCA X-Max energy spectrometer (EDS), with an energy resolution of 124 eV (Mn Kα), Count rate > 500,000 cps, output rate > 200,000 cps. Operating conditions involved an accelerating voltage of 20 kV, an electron beam with a beam intensity of 15–798 pA, and a working distance of ~15 mm. The resolution of the system is controlled by the beam spot size (370 nm during analysis). BSE signals and EDS spectra are collected during the analysis. The following standards were used for the EDS measurements and calibrations: (1) sulfides—pyrrhotite (Fe, S), galena (Pb, S), sphalerite (Zn, S), chalcopyrite (Cu, Fe, S), greenote (Cd), stibnite (Sb), and proustite (Ag, As); (2) silicates—olivine (Ni) and basalt glass (Ti, Si); (3) carbonates—calcite (O); (4) metals—cobalt (Co); (5) oxides—Cr-spinel (Mn); and (6) system standard library (Au, Se). The EDS detection limits for Au, S, Pb, Ag, Cd, Sb, O, Se, As, Zn, Cu, Ni, Co, Fe, Mn, and Ti were 0.1 wt.% [28].
The JOEL JXA-8230 electron probe was used for major element analysis in the Analysis and Test Center of the Institute of Oceanology, Chinese Academy of Sciences. Analyses were conducted using an accelerating voltage of 15 kV and a beam current of 20 nA. The following reference materials were used for the wavelength-dispersive spectrometry measurements and calibrations: (1) metals—Co, Ag, As, Ni, and Mo; (2) natural sulfides—FeS2, CuFeS2, ZnS, PbS. Detection limits for S were ~74 ppm, for Mo ~205 ppm, for As ~137 ppm, for Fe ~218 ppm, for Cu ~238 ppm, for Co ~117 ppm, and for Zn ~223 ppm. Concentrations of Ag, Ni, and Pb are generally around or below the minimum detection limit of the instrument (~145 ppm for Ag, ~133 ppm for Ni, and ~393 ppm for Pb). After obtaining the experimental data, the Pearson correlation analysis of the elemental content was performed using SPSS (the version number of the software is 27.0).

4. Results

4.1. Mineralogical Characteristics

According to microscopic observation, the samples can be divided into Zn-Fe-rich massive sulfides (including HOK4, HOK41, and HOK42) and Zn-Cu-Pb-rich massive sulfides (including HOK3, HOK31, and HOK 32). The Zn-Fe-rich massive sulfide samples are composed of sphalerite, pyrite, and minor galena. Galena is the first mineral to crystallize. Sphalerite commonly occurs as hydrothermal aggregates that are intimately intergrown with pyrite, and it is generally dendritic (Figure 2e). Typically, the high-temperature hydrothermal environment fosters the gradual growth of pyrite, and its crystal structure tends to be more organized. The fast deposition of pyrite causes lattice defects when exposed to a low-temperature hydrothermal environment [29]. Subhedral pyrite contains galena inclusions, which are presumed to be the result of hydrothermal activity during early crystallization. Some of the crystals appear to be relatively intact, taking on the form of aggregates that resemble bundles, branches, and chimneys. The samples undergo varying degrees of oxidation in the later stages, which is a result of their exposure to the seafloor environment. It can also be seen that pyrite is intergrown with marcasite (Figure 2b), indicating that the physical and chemical conditions of the fluid have changed substantially during the later stage because marcasite is formed under pH lower than 4.5 and temperature lower than 200 °C, while the stable state of pyrite is stable at pH higher than 4.5 [18].
The massive sphalerite in the Zn-Cu-Pb-rich sample is an early-stage ore phase, so it is mostly euhedral. Coarse-grained sphalerite forms the overall framework of the ore, and part of it is cemented and interspersed along fractures by late colloform pyrite formed later. Fine-grained chalcopyrite replaces sphalerite at late stages, forming a “chalcopyrite disease” texture (Figure 2d) [17,30]. Since the contact boundary between chalcopyrite and sphalerite is smooth, no metasomatism or dissolution occurs; therefore, it is assumed that chalcopyrite spots in sphalerite may be related to exsolution processes. Anglesite is sub-to-euhedral, which encloses pyrite, sphalerite, and other minerals precipitated earlier. It may be the product of low-temperature alteration of galena, so residual galena can be observed in anglesite (Figure 2c). Pyrite occurs in various forms and can be divided into various generations according to the crystal morphology of the mineral and the symbiotic association between the ore phases. The first generation of pyrite is subhedral, surrounded by anglesite. The second generation of pyrite forms concentric ring-shaped ribbons and is a colloform due to the continuous leaching of hydrothermal fluid. It cements the sulfides generated in the early stage or existing in the particle gap of sphalerite crystals, which is the product of the late hydrothermal activity (Figure 2a). Some pyrite intergrow with marcasite (Figure 2b).

4.2. Chemical Characteristics

4.2.1. Pyrite

Pyrite is the most common mineral during hydrothermal activity. The electron probe analyses (Table 1) of sulfide minerals reveal that the three pyrite species are sulfur-deficient with mean values of S/Fe atomic ratios of 1.99, 1.97, and 1.95, respectively. In the Zn-Cu-Pb-rich samples, the w(S + Fe) of colloform pyrite is the lowest. Melekestseva et al. [31] found a decrease in trace metal concentrations in massive pyrite compared to colloidal pyrite, interpreting this as a result of the slow crystallization of late-stage pyrite and, therefore, the ease with which trace elements decompose into other sulfide phases rather than entering pyrite as solid solutions. The content and distribution of trace elements are controlled by mineral, structure, zoning, and the possible presence of microinclusions in individual minerals [32]. The vast majority of pyrite has Zn in the range of major elements (>1%), and Cu, Pb, and As are in the range of minor (<0.1%) and trace elements (<0.01%) (Table 1).

4.2.2. Sphalerite

Sphalerite is the predominant sulfide mineral in the samples. The EPMA results show that the major and trace element composition between sphalerite in Zn-Cu-Pb-rich and Zn-Fe-rich samples is significantly different (Table 1). In the Zn-Cu-Pb-rich samples, the average Zn content of sphalerite is 63.493%, the average Cu content is 1.310%, and the average Fe content is 2.312%. Compared to this, the average content of Zn in the Zn-Fe-rich samples is relatively high, about 66.129%, while the Cu and Fe contents are relatively low, at 0.162% and 0.890%, respectively. Both types show similar S (33.620% and 33.653%, respectively) and trace element content (such as As, Pb, Co, and Ni) (Table 1).

4.2.3. Chalcopyrite

Chalcopyrite is a typical high-temperature mineral [33], and its trace element composition is characterized by enrichment in Zn (0.133–7.567%, average content 1.278%) and Mo (2720–4020 ppm, average content 3470 ppm), as well as trace amounts of Co (average content 310 ppm) and Ag (average content 130 ppm), and relatively low content of other trace elements (Table 1).

5. Discussion

5.1. Seawater–Fluid Mixing and Element Enrichments

It has been shown that metallic elements in hydrothermal fluids migrate and are transported mainly in the form of complexes [34]. There are four main complex types in natural fluids: hydroxide, chloride, sulfate, and sulfide [35]. Studies on the migration mechanism of metallic elements in seafloor hydrothermal fluids have shown that chloride complexes and sulfur complexes are the two main forms of metallic elements migrating in hydrothermal fluids, and the specific forms of migration are closely related to the physicochemical properties of the fluids. Chloride complex is an important form of metal migration in high-temperature, high-chloride fluids; in relatively low-temperature and low-salinity conditions, sulfur complexes are the main form of metal migration [36]. In addition, the temperature of mineralization is often linked to the stage of mineralization, resulting in differences in the form of metal migration during different stages of hydrothermal activity [34].
Due to the lack of fluid information and the fact that the fluid inclusions in the sulfide minerals are too small to determine their compositions and temperatures, the sulfide accumulations in the Jade hydrothermal field are divided into two main stages of mineralization based on the mineral parageneses at different stages. In the first mineralization stage of Fe-rich sulfides, due to the euhedral shape of pyrite, it can be considered that its mineralization temperature is relatively high, so some metal elements will be transported in the form of chloride complexes, such as Fe, Cu, etc. In the second mineralization stage, i.e., the precipitation stage of Zn-rich sulfides and low- and medium-temperature minerals, such as colloform pyrite, most of the metal elements will be migrated in the form of sulfur complexes.
The trace element composition of pyrite can effectively reflect the composition and physicochemical properties (temperature, salinity, pH, sulfur fugacity, oxygen fugacity, etc.) of the ore-forming fluid at different stages, so pyrite can be used as an indicator mineral for the nature of mineralizing fluids [37,38,39]. The research shows that the content of Co in pyrite is positively correlated with temperature [40], and the solubility of Mo drops sharply when the temperature is lower than 350 °C [41], so the content of Mo in high-temperature sulfides is high [42]. It can be seen from the comparison that the Co (average content 470 ppm) and Mo (average content 4910 ppm) of colloform pyrite are the lowest (Table 1), so the temperature during colloform pyrite formation is the lowest when it is formed. In addition, the range of Co (430–840 ppm) and Mo content (4480–5870 ppm) of anhedral pyrite in Zn-Fe-rich samples is smaller than that of allomorphic pyrite in Zn-Cu-Pb-rich samples (Table 1). It is speculated that pyrite in Zn-Fe-rich samples was formed in a more stable hydrothermal environment. The average contents of As (6600 ppm), Ag (1080 ppm), and Pb (7230 ppm) in colloform pyrite are the highest (Table 1), and the possible existence forms of As in pyrite include substitution into pyrite lattice under low oxygen partial pressure and low-temperature conditions [43,44] and microinclusions [45], indicating that these elements tend to be enriched in hydrothermal sulfides at relatively low temperature [46]. Reith et al. [47] found that when the concentration of trace metals is only a few hundred ppm, it is likely to represent the solid solution in the pyrite lattice. In addition, the results of Vaughan and Rosso [48] show that Ni, Co, As, and other trace elements usually exist in pyrite in the form of lattice replacement.
Fe2+, Co2+, and Zn2+ have similar chemical properties and are easy to enter sphalerite lattice in the form of divalent ions instead of Zn2+ [49]. The content of Fe in sphalerite can indicate the fluid temperature when sphalerite is formed, which generally has a positive correlation relationship [50,51]. The content of Fe in sphalerite from the Zn-Fe-rich samples is low, and its variation range is small, which indicates that the temperature of hydrothermal fluid was low when it was formed, with small fluctuation.

5.2. Element Correlation Analysis

Pyrite contains a large number of trace elements, including Cu, Zn, As, Ag, Pb, Co, and Ni. Their occurrence forms in pyrite mainly include solid solution, nano invisible inclusion, and micron visible inclusion [39,41,46]. Pearson correlation analysis of element content shows that Cu, Zn, and Fe in pyrite are negatively correlated (Table 2). When the maximum value of Cu content in pyrite is 6.043%, the corresponding minimum value of Fe content is 41.448%. According to performed studies, Cu and Zn, as chalcophile elements, are difficult to substitute for Fe in a homogeneous manner and often exist in the form of microinclusions, such as very fine mesh vein-like chalcopyrite [52]. Ag and Pb in pyrite are significantly positively correlated (RAg-Pb = 0.402) (Table 2), which reflects either the similar geochemical behavior of Ag and Pb during mesothermal hydrothermal activity or the result of leaching of early sulfides by late hydrothermal fluids, e.g., elemental remobilization occurs at low temperatures [53]. Generally, Pb is enriched in low-temperature minerals [54,55], and Ag and Pb may occur in pyrite in the form of fine galena inclusions containing Ag. As is a metalloid element. Under the condition of reducing hydrothermal fluid conditions, it mainly displaces S in pyrite [41,45,56]. Therefore, As in pyrite is negatively correlated with S (RAs-S = −0.456).
The variation of trace element content in sphalerite can reach several orders of magnitude [40], which can indicate important geochemical information such as its formation environment. The content of Zn in sphalerite is negatively correlated with that of Cu and Fe (RZn-Cu = −0.923; RZn-Fe = −0.930) (Table 3; Figure 3a), which are positively correlated (RFe-Cu = 0.833) (Table 3). Fe and Cu may enter sphalerite as ion substitutions. In addition, sphalerite in Zn-Cu-Pb-rich samples shows a “chalcopyrite disease” texture, so Cu may be present mainly in the chalcopyrite inclusions [32,57]. The Fe content is an important indicator of sphalerite formation, and electron microprobe analyses show that sphalerite with “chalcopyrite disease” has a significantly higher Fe content (Figure 3b), while pure sphalerite has a lower Fe content, which may be indicative of temperature differences between the two [51].
Zn in chalcopyrite is negatively correlated with Cu and Fe (RZn-Cu = −0.778; RZn-Fe = −0.827) (Table 4). Zn enters chalcopyrite in the form of ion replacement. As exists in chalcopyrite in the form of a solid solution and may also exist in the form of inclusion. There is still controversy over the chemical structure of chalcopyrite Cu2+Fe2+ S 2 2 − [58,59], Cu+Fe3+ S 2 2 − [60], or both [61], which constrains the understanding of the trace element replacement mechanism during chalcopyrite precipitation.

5.3. Mineral Precipitation Sequence and Growth Conditions

It is known that changes in temperature and redox gradients of hydrothermal fluids can lead to changes in trace elements in different sulfides [62]. In the absence of in situ hydrothermal fluid temperature data, the fluid conditions during mineral precipitation can be inferred based on mineral combinations, mineral morphology, and elemental chemical characteristics. For example, the mineral morphology of pyrite precipitated under different fluid conditions is different, and euhedral and subhedral pyrite usually correspond to higher fluid temperature and less fluid seawater mixing. Colloform pyrite, on the other hand, shows a rapid and unbalanced crystallization process under the low-temperature condition of seawater flooding [32,63]. The precipitation temperature of sphalerite is generally lower than 250 °C [64,65]. The solubility of chalcopyrite in hydrothermal fluid below 350 °C drops sharply [66,67]. According to the temperature measurement formula of sphalerite, Fe/Znsphalerite = 0.0013 (T) −0.2953 [51], the fluid temperature of Zn-Fe-rich samples is about 229–250 °C. The content of Fe and Zn in sphalerite of Zn-Cu-Pb-rich samples varies widely. It is calculated that the hydrothermal fluid temperature during sphalerite precipitation is about 235–271 °C, reflecting the evolution process of hydrothermal activity temperature change.
It can be assumed that the mineral precipitation process follows the sequence of “high-temperature minerals precipitate first, low-temperature minerals precipitate later”. According to the mineral precipitation sequence, the formation process of Zn-Cu-Pb-rich massive sulfides can be divided into the following four stages: Euhedral and subhedral pyrite crystallized in the early high-temperature hydrothermal fluid environment. Subsequently, the temperature of hydrothermal fluid decreased. So sphalerite precipitated between 235 °C and 271 °C, accompanied by chalcopyrite dissolution. In the later stage, anglesite replaces galena, and minerals formed in the early stage are surrounded by anglesite. Finally, colloform pyrite, the product of late hydrothermal activity, was precipitated at a lower hydrothermal fluid temperature. In Zn-Fe-rich samples, galena is an early mineral. Subsequently, the increase of the temperature of the hydrothermal fluid led to the formation of euhedral and subhedral pyrite; sphalerite was crystallized at the lowest temperatures of ~228.8–250.2 °C as a result of moderate mixing of medium-temperature, acidic, reducing hydrothermal fluids with seawater. With the later turbulent hydrothermal fluid conditions, pyrite and marcasite are interwoven. In addition, the sample is devoid of gypsum due to the “quenching” of the chimney, which lowered the fluid temperature and allowed gypsum to be dissolved at low temperatures or consumed by the later sulfides [55,68].

6. Conclusions

The sulfides in the Jade hydrothermal field of the Okinawa Trough can be divided into Zn-Cu-Pb-rich massive sulfides and Zn-Fe-rich massive sulfides. In the Zn-Cu-Pb-rich massive sulfides, coarse-grained sphalerite forms the overall framework of the ore, and chalcopyrite is scattered in sphalerite in the form of tiny flecks or “milk drops”, forming “chalcopyrite disease” structure. Minerals precipitated earlier are surrounded by anglesite, which may be the product of low-temperature alteration of galena in the seawater environment. Pyrite exists throughout the mineralizing stage and occurs in a variety of forms. In the Zn-Fe-rich massive sulfides, galena is the earliest to crystallize and is surrounded by sphalerite and pyrite. Sphalerite is anhedral and coexists with pyrite, with an overall dendritic appearance. It can also be seen that pyrite and marcasite interweave and coexist.
The electron probe analyses (Table 1) of sulfide minerals in the samples show that the average S/Fe atomic ratio of the three pyrite types is 1.99, 1.97, and 1.95, respectively, which are sulfur deficient. In the Zn-Cu-Pb-rich samples, the average Zn content of sphalerite is 63.493%, the average Cu content is 1.310%, and the average Fe content is 2.312%. The average content of Zn in the Zn-Fe-rich samples is relatively high, about 66.129%, while the Cu and Fe contents are relatively low, at 0.162% and 0.890%, respectively. The trace element characteristics of chalcopyrite are characterized by enrichment of Zn (0.133–7.567%, average content 1.278%) and Mo (2720–4020 ppm, average content 3470 ppm).
Colloform pyrite has the lowest Co and Mo content and the highest As, Ag, and Pb content, indicating the lowest hydrothermal temperature at the time of colloform pyrite formation. The Zn-Fe-rich massive sulfides have a smaller range of variation in Co and Mo content in anhedral pyrite, which is speculated to form in a hydrothermal environment with a more stable temperature. The negative correlation between Cu, Zn, and Fe in pyrite suggests that these elements may be present as micro-inclusions, while the positive correlation between Ag and Pb in pyrite suggests that Ag and Pb may be present in pyrite as fine Ag-bearing galena inclusions. The negative correlation of Zn with Cu and Fe in sphalerite suggests that Fe and Cu may have entered sphalerite as ion replacement. The negative correlation between Zn with Cu and Fe in chalcopyrite may indicate that Zn enters chalcopyrite in the form of ion replacement. The chemical structure of chalcopyrite is still controversial, which limits the understanding of the mechanism of trace element replacement during chalcopyrite precipitation.
The order of formation of Zn-Cu-Pb-rich massive sulfides is euhedral–subhedral pyrite, sphalerite, chalcopyrite, anglesite, and colloform pyrite. The order of formation of Zn-Fe-rich massive sulfides is galena, euhedral pyrite, sphalerite, anhedral pyrite, and marcasite.

Author Contributions

Conceptualization, Y.W.; methodology, Y.W.; software, Y.W.; validation, Y.W.; formal analysis, Y.W.; investigation, Y.W.; resources, Z.Z.; data curation, Y.W.; writing—original draft preparation, Y.W.; writing—review and editing, Y.W. and Z.Z.; visualization, Y.W.; supervision, Z.Z.; project administration, Z.Z.; funding acquisition, Z.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant Nos. 91958213, 42221005), Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB42020402), and Special Fund for the Taishan Scholar Program of Shandong Province (Grant No. ts201511061).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data are given in the article.

Acknowledgments

We are grateful for the valuable comments and suggestions from the anonymous reviewers and editors. This work was supported by the National Natural Science Foundation of China (Grant Nos. 91958213, 42221005), Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB42020402), and Special Fund for the Taishan Scholar Program of Shandong Province (Grant No. ts201511061).

Conflicts of Interest

The authors declare no conflict of interest.

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Jmse 11 01772 g001
Figure 1. Localities of seafloor hydrothermal sulfide samples in the Jade hydrothermal field of the Okinawa Trough.
Figure 1. Localities of seafloor hydrothermal sulfide samples in the Jade hydrothermal field of the Okinawa Trough.
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Jmse 11 01772 g002
Figure 2. (a–d) are optical microscopy images; (e,f) are scanning electron microscopy images. (a) Colloform pyrite intergrown with sphalerite; (b) pyrite and marcasite are interwoven. (c) Sporadic residual galena can be observed in anglesite, which is the product of low-temperature alteration of galena; (d) fine-grained chalcopyrite replaces sphalerite at late stages, forming a “chalcopyrite disease” texture. (e) Pyrite and sphalerite appear as aggregates and overall in bundles, branches, and chimneys. (f) Pyrite with galena inclusions.
Figure 2. (a–d) are optical microscopy images; (e,f) are scanning electron microscopy images. (a) Colloform pyrite intergrown with sphalerite; (b) pyrite and marcasite are interwoven. (c) Sporadic residual galena can be observed in anglesite, which is the product of low-temperature alteration of galena; (d) fine-grained chalcopyrite replaces sphalerite at late stages, forming a “chalcopyrite disease” texture. (e) Pyrite and sphalerite appear as aggregates and overall in bundles, branches, and chimneys. (f) Pyrite with galena inclusions.
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Jmse 11 01772 g003
Figure 3. (a) Correlation of Fe-Zn content in sphalerite. (b) Correlation of Fe-Cu content in sphalerite.
Figure 3. (a) Correlation of Fe-Zn content in sphalerite. (b) Correlation of Fe-Cu content in sphalerite.
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Table 1. Results of electron probe analysis of sulfide. Major elements (S, Fe, Zn, and Cu) in wt%; trace elements (Mo, Ag, As, Ni, and C) in ppm.
Table 1. Results of electron probe analysis of sulfide. Major elements (S, Fe, Zn, and Cu) in wt%; trace elements (Mo, Ag, As, Ni, and C) in ppm.
MineralsS (%)FeZnCuMo (ppm)AgAsPbCoNi
Zn-Cu-Pb-rich massive sulfideColloform pyritemin48.38141.9960.8640.0114220160140—110—
mean50.46544.4811.4570.310491010806600723047030
max52.28245.9832.3590.7485500374020,02042,520690140
Anhedral pyritemin49.155 41.448 0.109 —4110 —— —100 —
mean52.282 46.290 0.702 0.253 5190 160 2480 440 550 40
max54.621 47.913 3.354 6.043 59204240 28,220 10,930 900 350
Sphaleri-temin31.747 0.718 49.543 —2530—— ———
mean32.882 2.312 63.493 1.310 3420—60 90 40 40
max33.620 7.828 66.736 8.502 4050 —420 1000 200 270
Chalcopy-ritemin33.21727.4210.13330.6962720———80—
mean34.404 29.908 1.278 34.052 3470 130 4040 310 40
max35.5130.777.57634.9614020580260510610160
Zn-Fe
-rich
massive sulfide		Anhedral pyritemin50.256 45.132 0.662 —4480 ———430 —
mean51.544 46.090 1.155 0.161 5120 100 2710 40 590 50
max53.178 46.814 1.572 0.648 5870 420 23,520 440 840 220
Sphaleri-temin31.625 0.139 64.102 —2450 —————
mean32.740 0.890 66.129 0.162 3470 —50 60 50 50
max33.653 1.949 67.404 1.075 4400 120 560 1350 360 370
Table 2. Correlation matrix of elements analyzed in pyrite.
Table 2. Correlation matrix of elements analyzed in pyrite.
SMoAgAsFeCuPbCoZn
S1
Mo0.1701
Ag−0.2070.0431
As−0.456 **−0.1750.2021
Fe0.2370.053−0.374−0.2091
Cu−0.220 *0.0420.0600.047−0.534 **1
Pb−0.155−0.0150.4020.017−0.435 **0.0391
Co0.1790.134−0.0580.0150.278−0.148−0.0951
Zn−0.227−0.0810.033−0.069−0.401 *−0.0550.243−0.2191
The values followed by * are correlations at 95% confidence level. The values followed by ** are correlations at 99% confidence level.
Table 3. Correlation matrix of elements analyzed in sphalerite.
Table 3. Correlation matrix of elements analyzed in sphalerite.
SMoFeCuZnAsCo
S1
Mo−0.0271
Fe0.1150.0001
Cu0.177−0.0380.833 **1
Zn−0.087−0.024−0.930 **−0.923 **1
As−0.0120.0160.030−0.0670.0051
Co0.0120.0200.0300.026−0.036−0.0621
The values followed by ** are correlations at 99% confidence level.
Table 4. Correlation matrix of elements analyzed in chalcopyrite.
Table 4. Correlation matrix of elements analyzed in chalcopyrite.
SMoAsFeCuCoZn
S1
Mo0.0161
As0.334 *−0.2711
Fe0.1900.0260.0901
Cu0.087−0.0240.1400.794 **1
Co0.0650.0920.0330.021−0.2111
Zn−0.085−0.100−0.062−0.827 **−0.778 **0.1461
The values followed by * are correlations at 95% confidence level. The values followed by ** are correlations at 99% confidence level.
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Wang, Y.; Zeng, Z. The Mineral and Geochemical Features of Sulfides in the Jade Hydrothermal Field of the Okinawa Trough in Off-Shore China. J. Mar. Sci. Eng. 2023, 11, 1772. https://doi.org/10.3390/jmse11091772

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Wang Y, Zeng Z. The Mineral and Geochemical Features of Sulfides in the Jade Hydrothermal Field of the Okinawa Trough in Off-Shore China. Journal of Marine Science and Engineering. 2023; 11(9):1772. https://doi.org/10.3390/jmse11091772

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Wang, Yujie, and Zhigang Zeng. 2023. "The Mineral and Geochemical Features of Sulfides in the Jade Hydrothermal Field of the Okinawa Trough in Off-Shore China" Journal of Marine Science and Engineering 11, no. 9: 1772. https://doi.org/10.3390/jmse11091772

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