| Polymetallic sulfide deposits, being found on the mid-ocean ridges, backarc spreading centers and volcanic arcs, have caused a great interest for the geologists. Because of the high economic and scientific research potentials, a number of countries are actively exploring the resources worldwide. Last year, China and Russia have signed contracts with the International Seabed Authority for prospecting and exploration for polymetallic sulfides located in the Southwest Indian Ridge and Mid-Atlantic Ridge (MAR), respectively. In the past over30years, the investigations for polymetallic sulfides are mainly focused on the fast-spreading East Pacific Rise (EPR) and the slow-spreading MAR, very few study conducted on the intermediate-spreading Central Indian Ridge (CIR). The ultramafic-hosted Kairei field (25°19.23'S,70°02.42'E) and mafic-hosted Edmond field (23°52.68'S,69°35.80'E) are the first two active hydrothermal fields found on the CIR. In this thesis, systematic researches have been carried out to understand the characteristics of mineralization, timing of mineralization, sources of precipitating, and the control factors of ore formation and metallogenetic models of the sulfide deposits.Through this study, the following achievements have been obtained.1,Mineralization at Kairei mound consists of massive sulfides, sulfide chimneys, and rock breccias. Geochemically, the sulfide is characterized by high concentrations of base metals (up to29.3%combined Cu+Zn) and Au (ca.5.28ppm), and high Cu/Zn ratios (ca.3.5), which is comparable to those of sulfides from ultramafic-hosted high-temperature hydrothermal systems, namely those occurring along the MAR. Three mineralization stages were subdivided at the Kairei mount.(a) chalcopyrite, isocubanite, and pyrite were largely formed in a high-temperature stage;(b) sphalerite and pyrite in a medium-to low-temperature stage; and (c) secondary Cu-sulfides (bornite, digenite, covellite and idaite), Fe-oxides phases, amorphous silica (or opal-A), and Cu-chloride (paratacamite and atacamite) in a late stage of waning temperatures and weathering stage. Chalcopyrite altered to secondary Cu-sulfides and secondary copper chloride minerals indicates that the chemo-physical condition of mineral precipitating, such as the H2S activity, oxygen and sulfur fugacity, have been changed. These phenomena also indicate a high maturity of the deposit at Kairei.Mineralization at the Edmond mound also consists of massive sulfides, sulfide chimneys, and rock breccias, but the mineral abundances are distinct compared to those in the Kairei field. Similarly three mineralization stages were also divided at the Edmond hydrothermal area.(a) A pyrite+marcasite+chalcopyrite-anhydrite assemblage precipitated in the early high-temperature stage:(b) pyrite and sphalerite-marmatite in a medium-to low-temperature stage; and (c) amorphous silica (opal-A) +Fe-oxides+barite-covellite-gypsum and framboidal pyrite assemblage in a low-temperature stage with a seafloor weathering. The sulfides are characterized by high concentrations of Zn (ca.22.4%), Cu (ca.6.8%), Co (ca.0.1%), Ag (ca.206ppm) and Au (ca.3.34ppm), but low Cu/Zn ratios (ca.0.6), which is comparable to those of sulfides from mafic-hosted hydrothermal systems, such as along the MAR, EPR and CIR.2,Sulfur isotope values of sulfides at the Kairei and Edmond fields range from5.30to6.71‰and5.67to7.18‰, respectively, which are significantly higher than those reported from other hydrothermal fields along MAR and EPR. A two end-members mixing about70/30between basalt-derived sulfur and seawater sulfate reduction is suggested for the sulfur source. While a long-life cycles of the deposit, so-called "time effect", and a shallow hydrothermal circulation system under the sulfide mound, so-called "space effect" may be as the main mechanism for the heavy sulfur isotope signature at the Kairei and Edmond fields, respectively.Massive sulfides, rock breccias, and metalliferous sediments at the Kairei field, exhibit a range of Pb isotopes (206Pb/204Pb=17.345-17.502,207Pb/204Pb=15.438-15.489,208Pb/204Pb=37.264-37.561), while massive sulfides at the Edmond field exhibit a narrow range but higher Pb isotope ratios (206Pb/204Pb=17.879-17.970,207Pb/204Pb=15.433-15.550,208Pb/204Pb=37.743-38.130). Reported in Pb-Pb diagrams, the data from the Kairei and Edmond sulfides are concentrated in a field with low values compared to those of upper oceanic crust (MORB) in a field with high values, indicating that the main lead sources for the Kairei and Edmond mineralizations are the components of the upper oceanic crust and minor from the components of the seawater at the Edmond field.3,Elemental abundances and patterns of Ne, Kr and Xe in fluid inclusions in all samples of sulfides from both hydrothermal sites indicate that the noble gases are derived from mixing between ambient seawater and MORB, while Kr and Xe may be partly derived from low mantle. The mean3He/4He ratios in fluid inclusions from the Kairei and Edmond fields are6.08and5.80Ra, respectively, indicating that helium is predominantly derived from a magmatic source beneath the hydrothermal systems. However, there are wide ranges of3He/4He ratios in different mineral-hosted fluid inclusions in both sites, indicating that main-stage minerals hosted fluid inclusions have higher ratios than those in later-stage minerals, which means much more seawater coming into the late-stage minerals. The40Ar/36Ar ratios in fluid inclusions from the Kairei and Edmond fields are ca.283.8and295.1, respectively, and are comparable to the seawater, and different to the MORB and Reunion hot spot. Besides, the Ar concentrations in the fluid inclusions are close to the values in seawater, but much higher than the values in MORB and Reunion hot spot. These indicate that Ar is predominantly derived from seawater, not MORB or hot spot mafic rocks. 4,Massive sulfide, sulfide chimney and rock breccia samples at Kairei and Edmond fields were age-dated using the230Th/234U and210Pb/Pb methods. Four episodes of hydrothermal activity were determined at Kairei:94.5to96.3ka,56.6to61.2ka,8.4to10.6ka, and<180a to present. Hydrothermal activity first initiated in the southern part of the mound with low-temperature hydrothermal mineralization. Then it followed by two major hydrothermal events occurred at central and east part of the mound, characterized by Cu-rich high-temperature mineralization. The latest hydrothermal event started at least180a ago, and is now still ongoing indicated by active black smokers in the central part of the mound. The ages of sulfide at the Edmond field have much younger than those at Kairei field, which the former range from906±55a,108±4a,130±3a,146±5a and172±6a.5,The ultramafic-hosted Kairei field shows a strong tectonic control of mineralization, by ridge-transform interaction or nontransform offsets, and deep rift valley faults near the end of the asymmetrical ridge segment. Diving force of the Kairei hydrothermal system are enhanced by a combination of two factors including the heats of mafic intrusion and exothermic serpentinization reactions between mantle rocks and seawater. The metallogenetic model of the sulfide deposit at the Kairei field is similar to those along the slow-spreading MAR. In contrast with ultramafic environment, the geological control of mineralization at the mafic-hosted Edmond field is not so strong, only by the deep valley faults. The high-temperature Edmond hydrothermal system is enhanced by a deep hot magmatic intrusion. The metallogenetic model of the sulfide deposit at the Edmond field is also proposed which is similar to those mafic-hosted sulfide deposits along the MAR and CIR.
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