Mineralogical characteristics of polymetallic sulfides from the Deyin-1 hydrothermal field near 15°S, southern Mid-Atlantic Ridge

A seafloor hydrothermal field, named Deyin-1 later, near 15°S southern Mid-Atlantic Ridge（SMAR） was newly found during the 22 nd cruise carried out by the China Ocean Mineral Resources Research ＆ Development Association（COMRA）. Sulfide samples were collected at three stations from the hydrothermal field during the26 th cruise in 2012. In this paper, mineralogical characteristics of the sulfides were analyzed with optical microscope, X-ray diffractometer, scanning electron microscope and electron microprobe to study the crystallization sequence of minerals and the process of hydrothermal mineralization. According to the difference of the ore-forming metal elements, the sulfide samples can be divided into three types：（1） the Ferich sulfide, which contains mainly pyrite and chalcopyrite;（2） the Fe-Cu-rich sulfide consisting predominantly of pyrite, chalcopyrite and isocubanite, with lesser amount of sphalerite, marmatite and pyrrhotine; and（3） the Fe-Zn-rich sulfide dominated by pyrite, sphalerite and marmatite, with variable amounts of chalcopyrite, isocubanite, pyrrhotine, marcasite, galena and gratonite. Mineral precipitations in these sulfides are in the sequence of chalcopyrite（isocubanite and possible coarse pyrite）, fine pyrite,sphalerite（marmatite）, galena, gratonite and then the minerals out of the dissolution. Two morphologically distinct generations（Py-I and Py-II） of pyrite are identified in each of the samples; inclusions of marmatite tend to exist in the coarse pyrite crystals（Py-I）. Sphalerite in the Fe-Zn-rich sulfide is characterized by a＂chalcopyrite disease＂ phenomenon. Mineral paragenetic relationships and a wide range of chemical compositions suggest that the environment of hydrothermal mineralization was largely changing. By comparison, the Fe-rich sulfide was formed in a relatively stable environment with a high temperature, but the conditions for the formation of the Fe-Cu-rich sulfide were variable. The Fe-Zn-rich sulfide was precipitated during the hydrothermal venting at relatively low temperature.

Ga isotopic fractionation in sulfides from the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge

This study examines the Ga isotopic compositions of sulfides in the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge,mid-ocean ridge basalts(MORB),and calcareous sediments around the hydrothermal fields.Theδ^(71/69)Ga_(NIST-994) values of theMORB samples vary little(+1.20‰to+1.23‰,with an average of+1.22‰)and are consistent with the δ^(71/69)Ga_(NIST-994) values of two standard basalt samples(BCR-2 and BHVO-2),indicating that Ga isotopes may either not fractionate or fractionate only slightly under high-temperature geological processes;therefore,the δ^(71/69)Ga_(NIST-994) value of oceanic crust may be+1.22‰.The sediments(+1.28‰to+1.47‰,with an average of+1.38‰)are rich in heavier Ga isotopes than the basalts,and the Ga present in the sediments may have originated from soluble Ga present in the seawater that was adsorbed by(Mn,Fe)oxides/hydroxides.The Ga contribution of basaltic debris to the sedimentswas almost negligible.Thus,wespeculate that theδ^(71/69)Ga_(NIST-994) value of seawater in the study area fell within a range from+1.92‰to+2.36‰.Theδ^(71/69)Ga_(NIST-994) values of the sulfides in the Yuhuang hydrothermal field range from+0.99‰to+1.57‰,with an average of+1.25‰,and theδ^(71/69)Ga_(NIST-994) values of the sulfides in the Duanqiao hydrothermal field range from+0.93‰to+1.55‰,with an average of+1.19‰.Theδ^(71/69)Ga_(NIST-994) ranges of the sulfides in the Yuhuang and Duanqiao hydrothermal fields are similar,with the Ga isotopic fractionation reaching 0.58‰and 0.62‰,respectively.The averageδ^(71/69)Ga_(NIST-994) values in the sulfides are close to those in the MORBs.This suggests that Ga within the sulfides in the Yuhuang and Duanqiao hydrothermal fields mainly originated from MORBs,with seawater and sediments making only small contributions.The Ga isotopic fractionation in the sulfides may be related to processes associated with the formation of sulfides,such as rapid precipitation or the admixture of different stages of sulfide.This study is of great significance for understanding the global distribution of Ga isotopes and the Ga cycle in submarine hydrothermal systems.

Mineralogy,geochemistry,and sulfur isotope characteristics of sediment-hosted hydrothermal sulfide minerals from the southern Okinawa Trough

Studies of the mineralogy and sulfur isotope composition of sediment-hosted hydrothermal sulfide minerals in cores are important for understanding the seafloor mineralization environment and material source and reconstructing the hydrothermal history.However,the source of ore-forming materials and the history of hydrothermal activity in the southern Okinawa Trough(SOT)remain unclear.Here,the mineralogy and sulfur isotope characteristics of sulfides from gravity core HOBAB4-S2,collected between the Yonaguni Knoll IV hydrothermal field(HF)and the Tangyin HF,was investigated.Enrichments in Zn(up to 321×10^(-6)),Cu(up to73.7×10^(-6)),and Pb(up to 160×10^(-6))and the presence of pyrite,galena,pyrrhotite and minor sphalerite and chalcopyrite provide evidence for the input of hydrothermal materials into the sediments.The pyrite morphologies include disseminated cubic,pentagonal dodecahedron,and framboidal forms.Except for minor framboidal pyrite,euhedral pyrite from core HOBAB4-S2 is mainly of hydrothermal origin with Co/Ni ratios>1 and S/Fe atomic ratios<2 in the Cu-Zn-Pb-rich layers.The occurrences of hexagonal pyrrhotite,high-Co(up to0.17%)pyrite and high-Fe sphalerite indicate that the hydrothermal precipitates formed at medium-high temperatures and low-sulfur fugacity(f S_(2))environments.The δ^(34) S values of sulfides(0.21‰–3.45‰)with low-f S_(2) mineral assemblages(e.g.,pyrrhotite±high-Fe sphalerite)in the core are significantly lower than those of magmatic rocks and seawater,indicating possible incorporation of previously formed biogenic sulfur in the sediment.Combined with the age model of the core,it is suggested that hydrothermal activity likely began in the Tangyin HF before AD 1445–1483 and that at least three active episodes may have occurred since then.

The occurrence of gold in hydrothermal sulfide at Southwest Indian Ridge 49.6°E

Massive sulfide precipitates found in the Southwest Indian Ridge （SWIR） 49.6°E hydrothermal field are enriched in gold. Here, the mineralogy and mineral chemistry of these massive sulfides to constrain the process of gold precipitation were studied. Sulfide samples in this field include lower- temperature Zn-rich sulfides and relative higher-temperature Fe-rich sulfides. Zn-rich sulfides are dominated by sphalerite-pyrite-chalcopyrite assemblages, with concentrations of gold ranging from 9.08 to 17.0 μg/g. Fe-rich sulfides consist mainly of pyrite-marcasite-isocubanite assemblages, with gold concentrations from 2.17 to 3.79 μg/g. The significant enrichment in gold within the lower- temperature Zn-rich sulfides and the effective separation of Zn and Fe in hydrothermal precipitates at the surface of this field are here interpreted to reflect the strong temperature dependence of gold transportation and deposition within the sulfides. In Zn-rich samples, large amounts of isolated native gold grains were identified. They were found mainly as inclusions up to 8 μm in diameter, occupying porous cavities in sphalerite or in the elevated iron content rim of sphalerite. The fineness of the gold ranged from 810 to 830. Unlike previously published results on other hydrothermal fields, these data show a low gold fineness values in SWIR 49.6°E. The FeS content of sphalerite associated with gold grains ranged from 3.2 mole % to 18.9 mole %. This was higher than in other fields, indicating that the sulfur activity is relatively low during the gold precipitation process and that sulfur activity may be one of the main factors affecting gold fineness in the SWIR 49.6°E hydrothermal field. Evidence regarding gold fineness and sulfur activity suggests that gold was quite likely transported as AuHS° rather than as a Au（HS）2- complex.

Numerical simulation for fluids mixing within seafloor hydrothermal sulfide deposit:taking the Trans-Atlantic Geo-Traverse (TAG) field for example

The formation mechanism of the large hydrothermal sulfide deposit is a complex geological process involving many controlling factors. Mixing between hydrothermal fluid and seawater plays a key role in this process. The results of the Ocean Drilling Program （ODP） indicate that mixing of the evolved seawater and hydrothermal fluid, which is wildly developed within the Trans-Atlantic Geo-Traverse （TAG） hydrothermal deposit, governs the internal structure and chemical compositions of the deposit to great extent. Taking the TAG field for example, the mixing processes of hydrothermal fluid with the seawater heated to different extent are calculated, so as to discuss the impact of hydrothermal fluid/seawater mixing on the formation process of the sulfide deposit. The results indicate that： （1） mixing between the heated seawater and hydrothermal fluid derived from the deep deposit is largely responsible for the wild precipitation of anhydrite within the TAG hydrothermal deposit; （2） 330-310℃ is a special temperature range in the mixing process; （3） the mixing and hydrothermal processes in different zones of the TAG hydrothermal deposit （TAG-1, TAG-2 and TAG-5, etc.） have been discussed based on the simulated results.