A Research On The Melt Inclusion Of Metallic Sulfide-bearing Surrounding Rock In 26°S Hydrothermal Areas Of The Mid-Atlantic Ridge

The sulphide smokers in the 26°S hydrothermal area of the mid-Atlantic ridge,which is a newly discovered hydrothermal sulfide metallogenic area in recent years,developed on basalt bedrock.The research for this hydrothermal area just started.This paper focus on research of melt inclusions of olivine and plagioclase in this hydrothermal area.Based on the host rocks,host mineral,homogenization temperature and composition analysis in-situ for inclusions,the effects of crystallization and phase separation during the evolution of basaltic magma on migration and enrichment of ore-forming metal elements are discussed and the contribution of magmatic fluid to mineralization are suggested.The thesis mainly obtains the following understanding:（1）The basalt in the study area is sodic tholeiitic basalts with depletion of light rare earth elements and incompatible elements.Belonging to N-MORB basalt,the basalt’s formation is associated with partial melting of the depleted upper mantle.The Ca O and Al₂O₃ decreased while the Si O₂,Na₂O,Fe O and Mg O increased during magma evolution process.At the same time,the magma is affected by the low Si homologous magma mixing and evolved towards the middle basic magma.（2）In the early stage of magma evolution,the plagioclase porphyry crystallized earlier than the olivine porphyry.Melt inclusions are abundant in both plagioclase and olivine porphyry,in which the inclusions in plagioclase are mainly clusters of crystalline melt inclusions and glass inclusions,while the inclusions in olivine is more isolated glass inclusions.Homogeneous temperature of inclusion is between1185-1197℃;at the same time,a large number of melt inclusions can not be homogenized,which shows that the immiscible separation between low density gas phase and high density melt phase had existed in the early stage of magma evolution.（3）Scanning Electronic Microscopy（SEM）and energy spectrum analysis show that there are pyrite,nickel-bearing pyrite and chalcopyrite on the inner wall of the bubble of the melt inclusion,which may imply the phase separation between gas phase and melt phase in the early stage of magma evolution,and the migration of metallogenic metal elements into the low density gas phase from the melt.（4）The EPMA and LA-ICP-MS composition analysis of melt inclusions,host minerals and basalt matrices indicate that metallogenic elements in basalt magma evolved from enrichment to depletion during magma evolution.From the early stage to the late stage of magma evolution,with temperature and pressure decrease,the phase separation between the gas phase and liquid phases enhance,more and more gas escape from the magma and resulted in depletion of the metallogenic metal elements in melt.During the phase separation,metallogenic metal elements such as Cu,Zn and Pb entered into the gas phase preferentially,phase separation may be responsible for metallogenic fluid.