| Citation: | Jianbin Zheng, Zhimin Cao, Wei An. Mineral Components, Texture, and Forming Conditions of Hydrothermal Chimney on the East Pacific Rise at 9°–10°N. Journal of Earth Science, 2007, 18(2): 128-134.
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To characterize the hydrothermal processes of East Pacific rise at 9°–10°N, sulfide mineral compositions, textural, and geochemical features of chimney ores were studied using ore microscope, scanning electron microscope, X-ray diffraction analysis, and electron microprobe techniques. Results show that there are three mineral assemblages for the hydrothermal chimney ores, namely: (ⅰ) anhydrite + marcasite + pyrite, (ⅱ) pyrite + sphalerite + chalcopyrite, and (ⅲ) chalcopyrite + bornite + digenite + covellite. Mineral assemblages, zonational features, and geochemical characteristics of the ore minerals indicate that ore fluid temperature changed from low to high then to low with a maximum temperature up to 400 ℃. The chimney is a typical black smoker. The initial structure of the chimney was formed by the precipitation of anhydrites, and later the sulfides began to precipitate in the inner wall.
The distribution of submarine hydrothermal locations and sulfide spots of east and southeast Pacific are controlled by the Pacific rise, structural axis, and rift valley. East Pacific rise (EPR) is a mid- to fast-spreading ridge with many types of hydrothermal fluids eruptions (Wu et al., 2000). Inside this ridge, EPR 9°–10°N is a fast-spreading ridge with a maximum rate of 11 cm/a. Since the discovery of hydrothermal fluids eruptions and vent biocommunity (Haymon et al., 1993), this region has been regarded as a typical place to study the hydrothermal systems at fast-spreading ridges. International organizations, such as Interridge and Ridge 2000, had included this place for extensive study in the coming years. Previous studies had emphasized on the volcanic structure, the distribution of hydrothermal plume, the variations of fluid temperature and chemical compositions, and the vent biocommunity (Heather et al., 2004; Kurras et al., 2000; Shank et al., 1998; Christeson et al., 1996, 1992; Cochran et al., 1996; Oosting and Von Damm, 1996; Feely et al., 1994; Carbotte and Macdonald, 1992; Haymon et al., 1991; Lupton et al., 1991; Shanks et al., 1991), while neglecting the hydrothermal sulfide mineralogical studies. Mineral paragenetic sequences, textures, and typomorphic characteristics (e.g., mineral size, crystal habit, and mineral inclusions) can provide important information about the variations of hydrothermal fluids, ore-forming periods, chimney growth patterns, and geochemical environments (e.g., Münch et al., 1999; Chu and Chen, 1995; Tivey et al., 1995; Janecky and Seyfried, 1984). This article presents the results of sulfide mineral paragenetic association, textures, and mineral compositions, in the hope of providing novel evidence to the study of hydrothermal processes of this area.
Sulfide ores were sampled at Venture hydrothermal vents, EPR (9°46′N–9°51′N) (Fig. 1), by the submersible Alvin in 2001. The EPR at 9°46′N–9°51′N has an axial summit caldera (ASC), consisting of a 50–80 m wide, 5–8 m deep, volcanic collapse zone within which most primary volcanic and hydrothermal processes occur. Seismic reflection studies indicated that a narrow, partially molten magma region exists at shallow depths (~1.5–1.6 km) beneath the ASC here (Gregg et al., 1996). Chimney stacks and sulfide mounds discontinuously distribute along the spreading ridge axis, mainly localize at the ASCs, which are the shallower parts of the ridge. Biologic communities exist around the hydrothermal vents (Feely et al., 1994; Haymon et al., 1993; Shanks et al., 1991).
Thin sections and polished sections were made for the sulfide ores, and mineral assemblages, fabrics and mineral-forming order were determined under ore microscope and scanning electron microscope. Parts of the samples were analyzed using X-ray diffraction analysis, and mineral chemical compositions were determined using electron microprobe.
Sampled massive sulfide chimney from the EPR at 9°–10°N shows deep-gray with a diameter of 3–5 cm. Mineral zonation (anhydrite-pyrite zone, polymetal sulfide zone, and chalcopyrite zone) clearly lay out on the cross section (Fig. 2). Ores show granule texture with geode structure.
Anhydrites as early formed minerals occur as slaty, nemaline, and radiated aggregates, distributing around the chimney's outer crust. Pyrites show hypidiomorphic granular texture, associating with anhydrites (Fig. 3a).
This zone is mainly composed of pyrites, sphalerites, and few chalcopyrites, distributing at the chimney's inner crust. Pyrites are cubic euhedral crystals, distributing along the copper-sulfides' outer margin. Some pyrites have been replaced by chalcopyrites, showing relic texture. Few pyrites show tubeworms' shape (Fig. 3b), indicating the mineralization of living body. Compared with the sulfide ores from other mid-ocean ridges, this sulfide chimney lacks of pyrrhotite. One explanation is the replacement of pyrrhotite by pyrite. There are two types of sphalerites, one occurs as solid solution with chalcopyrites (Fig. 3c), which represents high-temperature hydrothermal products; the other type is allotriomorphic granular to idiomorphic crystal, which is in intergrowth with chalcopyrites and pyrites.
Distributing mainly in the inner wall, chalcopyrites compose the main sulfides, occurring as disseminated to massive, idiomorphic to hypidiomorphic crystal aggregates. The gradual enrichment of divalent copper due to hydrothermal alterations resulted in a disseminated contact relationship among chalcopyrites, bornites, digenites, and covellites (Fig. 3d). On the chimney's outer crust, some deuterogenic copper sulfides precipitated from the hydrothermal fluids are associated with anhydrites (Fig. 3a). This may indicate the drop of the temperature and the increase of oxygen and sulfur fugacity during the later stage of hydrothermal vents.
X-ray diffraction analyzed by Prof. Wang Wenzheng, Test Center of Ocean University of China with D/max-rB shows that, hydrothermal sulfides are dominated with chalcopyrites (Diffraction peaks: 0.305 18 nm, 0.265 28 nm, 0.185 96 nm, 0.187 33 nm, 0.159 49 nm, 0.157 74 nm, 0.132 39 nm, and 0.120 64 nm), followed by anhydrites (0.350 92 nm, 0.285 54 nm, 0.233 17 nm, 0.221 29 nm, 0.191 79 nm, and 0.163 48 nm), pyrites (0.221 29 nm, 0.163 48 nm, 0.150 39 nm, 0.144 83 nm, and 0.124 39 nm), and sphalerites (0.313 36 nm, 0.271 37 nm, 0.191 79 nm, and 0.124 39 nm). X-ray diffraction analysis also revealed the existence of marcasite (Diffraction peaks: 0.345 56 nm, 0.271 37 nm, and 0.175 03 nm), which has not been observed under ore-microscope.
To summarize, (1) sulfide minerals are mainly composed of chalcopyrite, followed by pyrite, marcasite, sphalerite, anhydrite, and few bornite, digenite, covellite, exhibiting the existence of typical sulfide minerals of mid-ocean ridges; (2) hydrothermal processes of the sampled region can be split into three stages, i.e., anhydrite-pyrite, polymetal sulfide, and chalcopyrite stages (Fig. 4).
Results of electron microprobe analysis for the chimney sulfides are listed in Table 1. Sulfide minerals precipitated from different stages exhibit variations in chemical composition.
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The difference of pyrites formed from stages Ⅰ and Ⅱ mainly finds expression in the contents of copper and zinc. Cu contents of stages Ⅰ and Ⅱ pyrites are 0.054%–0.145% and 0.417%–1.37%, and Zn contents of stages Ⅰ and Ⅱ pyrites are 0.128%–0.256% and 0.039%–0.063%, respectively (Table 1). The variation of Cu and Zn in pyrites of stages Ⅰ and Ⅱ indicates that, the temperatures of stage Ⅱ hydrothermal fluids were higher than these of the stage Ⅰ.
Sphalerites were mainly precipitated from stage Ⅱ with a prominent feature of high iron contents, indicating a high precipitation temperature. Chemical composition indicates three generations of sphalerites. Generation ii sphalerites have higher Fe contents than those of generation i, indicating the temperature increase of the hydrothermal fluids. Compared with generations i and ii, the sphalerites of generation iii with solid solution texture have higher Cu contents with Cu: Fe approximating to 1:1 (Table 1).
As the region's main hydrothermal sulfide mineral, most of the chalcopyrites are precipitated at stage Ⅲ. Compared with stage Ⅱ chalcopyrites, stage Ⅲ chalcopyrites have lower zinc contents, indicating the decrease of the fluids' temperature. Generations i and ii of the stage Ⅱ chalcopyrites also have varied zinc contents, chalcopyrites with solid solution have higher zinc contents.
Most of the deuterogenic copper minerals, including bornite, digenite, and covellite, are hydrothermal alteration products of chalcopyrites. They mainly belong to the generation i, stage Ⅲ. Some deuterogenic copper minerals, which are distributing around the chimney's outer crust, and have relatively higher silver contents, belong to the generation ii of the stage Ⅲ.
The cause of formation of "chalcopyrite disease" texture of intimate intergrowths of chalcopyrite and sphalerite is still under debate, mainly between views of solid solution and replacement (e.g., Hu et al., 2000; Bortnikov et al., 1991; Li and Wang, 1990). In this study, chalcopyrite zone shows smooth contact with sphalerite zone (Fig. 3c), rather than sawtoothed fringe resulted from replacement. This smooth zonal texture indicates the solid solution of chalcopyrite and sphalerite under relatively high temperature (decomposition temperature of chalcopyrite and sphalerite in laboratory experiments may reach 400 ℃). This is in agreement with the results of in situ measurements of hydrothermal vents of this region (Haymon et al., 1993). Experiments of Cu-Fe-S system verified the existence of intermediate solid solution (ISS) at high temperature, which has sphalerite-type lattice cell. At lower temperature, sulfide minerals may separate from ISS as the range of ISS reduced (Yund and Kullerud, 1966). Experiments of Cu-Fe-Zn-S system also showed that, exsolved sphalerites have varied Fe and Cu contents and sulfide mineral assemblages at different temperature stages. The contents of Cu in exsolved sphalerites depend on phase species, temperature, and FeS and sulfur fugacity (Lusk and Calder, 2004; Kojima and Sugaki, 1985; Wiggins and Craig, 1980). Sphalerite exsolved from high temperature has high Cu contents. At given temperature, the content of FeS and the ratio of Fe/Cu in sphalerites will increase as sulfur fugacity decreases (Lusk and Calder, 2004). High contents of Cu and Fe, and the ratio of Cu/Fe (~1) in the studied sphalerites from EPR at 9°–10°N may indicate that, sphalerites formed at high temperature, and Fe and Cu in sphalerites came from same Cu-Fe sulfide particulates, such as ISS.
Mineral texture and chemical composition can reflect the property of ore-forming hydrothermal fluids. Hydrothermal chimney stacks from EPR at 9°–10°N are mainly composed of chalcopyrites, sphalerites, and pyrites. This indicates that, hydrothermal fluids are rich in Cu, Fe, and S, acidic with relatively high temperature. Coexistence of marcasites and pyrites indicates a wide range of geochemical environment of the hydrothermal fluids. In the aspect of mineral assemblages, stage Ⅰ had low temperature assemblage (anhydrite + pyrite); stage Ⅱ experienced the precipitations of pyrite (with high zinc contents), Fe-rich sphalerites, and chalcopyrite-sphalerite solid solution, indicating the increase of temperature of the hydrothermal fluids; the main mineral of stage Ⅲ is chalcopyrite with lower zinc content, indicating the decrease of temperature. The occurrence of bornite at the final stage also indicated the decrease of temperature and the increase of oxygen and sulfur fugacity. In a word, temperature of the hydrothermal fluids had varied from low to high, and back to low. This variation trend revealed by mineral features is similar to that of other hydrothermal vents (Münch et al., 1999; Langmuir et al., 1997; Fouquet et al., 1993; Herzig et al., 1993).
Composed mainly of sulfide minerals, the chimney stack from EPR at 9°–10°N is typical of "black smoker" formed at relatively high temperature. The outer wall of the chimney is anhydrite layer, which played an important role in the growth of the smoker. Many ancient volcanic-hosted massive sulfide (VHMS) deposits also had anhydrite layer, such as Gacun VHMS deposit, Sichuan Province (Hou et al., 2001). Because of the anhydrite's retrograde solubility, they would precipitate to form the chimney's frame in the early stage of hydrothermal processes (Haymon, 1983), within which sulfide precipitated during later stages. After hydrothermal processes, the anhydrite frame would be dissolved resulting in the collapse of the chimney and the stack up of sulfide mound. So, the formation of "black smoker" began with the precipitation of anhydrites, whose porous outer crust could reduce the mixing rate of seawater and hydrothermal fluids, and capture the suspended sulfide particles. Inside the chimney, pyrites, marcasites, sphalerites, chalcopyrites, and deuterogenic copper minerals precipitated. Besides, because of advection and diffusional effect, hydrothermal fluids and seawater may mix in the porous chimney wall, precipitating some pyrites, sphalerites, and bornites in the chimney's fissures surface.
(1) Hydrothermal chimney stacks from EPR at 9°–10°N are mainly composed of chalcopyrites, pyrites, marcasites, sphalerites, and anhydrites, together with few bornites, digenites, covellites, displaying the typical mid-ocean ridge sulfide assemblage.
(2) The hydrothermal chimney of the sampled region is a typical "black smoker". Anhydrites precipitated to form the chimney's frame in the early stage, and later sulfides precipitated in its inner wall. Ore-forming processes can be divided into three stages, i.e., anhydrite-pyrite stage Ⅰ, polymetal sulfide stage Ⅱ, and chalcopyrite stage Ⅲ.
(3) Hydrothermal fluids are rich in Cu, Fe, and S with varied geochemical conditions. Temperature of the hydrothermal fluids had varied from low to high, and back to low, with a high temperature up to 400 ℃.
ACKNOWLEDGMENTS: This study was supported by the National Natural Science Foundation of China (No. 40273025), Key Laboratory of Marine Sedimentology and Environmental Geology, State Oceanic Administration, and National High Technology Research and Development Program of China (No. 2006AA09Z219). The authors also wish to thank Prof. Kang Ding, University of Minnesota, for providing sulfide samples for this work.| Bortnikov, N. S., Genkin, A. D., Dobrovol, M. G., et al., 1991. The Nature of Chalcopyrite Inclusions in Sphalerite: Exsolution, Coprecipitation, or "Disease". Econ. Geol., 86: 1070–1082 doi: 10.2113/gsecongeo.86.5.1070 |
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Jianbin Zheng, Zhimin Cao, Wei An. Mineral Components, Texture, and Forming Conditions of Hydrothermal Chimney on the East Pacific Rise at 9°–10°N. Journal of Earth Science, 2007, 18(2): 128-134.
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