Study Of Hydrothermal Mineralization Of Duanqiao Hydrothermal Field In Southwest Indian Ridge

The ultraslow spreading ridges represent an important part of the global mid-ocean ridge system and its hydrothermal activity has been increasingly concerned since last century in 1990s.So far,the hydrothermal activity has been found in the ultraslow spreading southwest Indian Ridge.In this study,detailed mineralogical,geochemical analyses have been carried out on basalts,the sulfide chimney and massive sulfide samples from Duanqiao hydrothermal field.Combined with the underwater camera towed data,the metallogenic setting,the characteristics of mineralization,the material source,the formation conditions and hydrothermal mineralization model have been discussed.1.The basalts from Duanqiao hydrothermal field of SWIR belong to low-potassium tholeiite series,and the phenocrysts are mianly plagioclase and olivine.The compositions of elements and Sr,Nd,Pb isotope characteristics suggest that its original mantle material is mainly derived from the depleted mantle,contaminated with continental crust and pelagic sediments.2.The mineral assemblages of the all the samples consist mainly of chalcopyrite,sphalerite,pyrite and marc as ite.Five distinct mineralogical associations were recognized:fine grained pyrite(Pyl)+chalcopyrite+granular sphalerite(Spl)+Intermediate solid solution(Iss),dendritic clusters of pyrite(Py2)+sphalerite(granular,Spl)+Isocubanite(Iso),colloform pyrite(Py3)+marcasite +colloform sphalerite(Sp2),pyrite(mainly)+ colloform sphalerite(Sp2),amorphous silica+nantokite.Amorphous silica and nantokite are the later minerals to precipitate under low temperature.The sulfide geochemistry is characterized high concentrations ofPb(?7500 ppm),As(?830 ppm),Ag(15.6-?100 ppm),Sb(?98.9 ppm),Cd(?920 ppm).The average contents ofCu,Fe and Zn in all the samples are 2.5 wt%,27.6 wt%and 3.1 wt%,respectively.The unusual high concentrations of Pb,Cd,As,Sb and Ag in modern seafloor system can be explained by leaching of some felsic differentiates underneath the basaltic cover and subsequent zone refining processes.3.Sulfides from Duanqiao show a range of Pb isotopes(206Pb/204Pb=18.237-18.396,207Pb/204Pb=15.512-15.721,208Pb/204Pb=38.101-38.792).Those data indicate that the main lead sources for Duanqiao are basalts,the contribution of seawater is very low.22 sub-sulfides separate from hydrothermal chimneys show narrow variable ?34S values,ranging from+4.426 to +4.715‰ which are significantly lower than those of massive sulfides(+4.945 to +5.621‰).The ?34S values of sulfides indicate that two end-member mixing between seawater reduction and basalt-derived sulfur is suggested for source in SWIR hydrothermal system.20%to 27%of the sulfur was derived from ambient seawater for Duanqiao hydrothermal field.Sr isotope data show that hydrothermal fluid is from the evolution of the seawater.4.The 231Th/238U dating results show that hydrothermal activity of Duanqiao probably started about 84.3(±0.5)kyrs ago from the northern part of the field.And the hydrothermal activity ceased about 0.737(±0.023)kyrs ago from the central part of the field.Four main episodes of hydrothermal activity were determined according to the restricted results:84.3-68.9,48.4-43.9,34.8-25.3,and 17.3-0.7 kyrs.5.The heat for the Duanqiao hydrothermal field may be supplied due to the intrusion of gabbroic material in the footwall of the hydrothermal field or an axial magma chamber(AMC)may develop beneath the Duanqiao hydrothermal field which can provide a source of magma.A series of normal faults serve as the pathway for circulation and could transport the hydrothermal fluids away from the heat source and focusing them on vent site.