Study Of Mineralization At The Longqi And Duanqiao Hydrothermal Fields,Southwest Indian Ridge

The Longqi?49.6°E?and Duanqiao?50.5°E?hydrothermal fields are modern seafloor polymetallic sulfide prospective mineralization areas located at the Southwest Indian ridge,with great economic and scientific significance.Detailed studies for volcanic rock and sulfide samples from the two fields are conducted by petrology,mineralogy,bulk geochemistry,EPMA and LA-ICPMS analyzation.This paper systematically discussed the mineralization characteristics,element enrichment characteristics,mineralization conditions and important ore-forming processes of the two fields.The basement rocks of the two study areas are mafic basalts,which are prominently enriched in Pb.The mineralization processes in the Longqi field are divided into three main stages:?1?colloform pyrite?Py 1?+marcasite?euhedral pyrite?Py 2?,?2?isocubanite?±exsolved chalcopyrite?+pyrrhotite?coarse-grained chalcopyrite?Ccp 1?,and?3?sphalerite+fine-grained chalcopyrite inclusions?Ccp 2??aggregates of anhedral pyrite?Py 3?±marcasite.The mineralization processes in the Duanqiao field are divided into two main stages:?1?subhedral and euhedral pyrite?Py 1'??coarse-grained chalcopyrite?Ccp 1'?and?2?sphalerite?fine-grained chalcopyrite?Ccp 2'?+chalcopyrite inclusions?Ccp 2''??silica-cemented pyrite?Py2??+marcasite.We suggest that the fine-grained chalcopyrite inclusions in sphalerite from Longqi and Duanqiao were formed by coprecipitation and replacement mechanisms,respectively.All the sulfides in this study are Fe-sulfides.On the whole,sulfides from the Longqi hydrothermal field are enriched in Zn,Au,Ag,Cd and Sn,while sulfides from the Duanqiao hydrothermal field are enriched in Cu,Pb,Si and Ca relatively.EPMA and LA-ICPMS analyzations reveal that pyrites from the Longqi and Duanqiao fields are enriched in Co,Ag,and Cu,Zn respectively.In both areas,euhedral pyrites are enriched in Se,Co,while the colloform or anhedral pyrites are enriched in Tl,V,Mn and As.Sphalerites from the Longqi field are enriched in Mn,Co,Se,Cd,Ga,In and Sn,and the ones from the Duanqiao field are enriched in Cu,Ge,As,Ag and Sb.Zn,Se and Ag are the dominant trace elements in chalcopyrites from both areas.This study reveals that trace elements such as Co,Ni,Sn,Ga,Ag,Sb,As and Ge,distributed in pyrite and sphalerite,are greatly controlled by geological settings and mineralization types.An integrated study of the mineralogy and mineralogical chemistry suggests that fluids from the Longqi field are likely conditioned with higher temperatures and lower?O₂ and?S₂ than that of the Duanqiao field.The distribution trends of Zn and Pb in sulfides from the two fields may be affected by zone-refining and the process of leaching from surrounding rock.