Studies On Seafloor Hydrothermal Sulfides And Its Ore-forming Processes

Hydrothermal activity and its ore-forming process is one of the most important internal ore-forming processes. Its driving power comes from earth ' s inner, volcanic activities-tectonic changes-hydrothermal vents provide necessary conditions for hydrothermal deposits. It has important meaning to study componential characters of hydrothermal sulfides and its forming processes for exploring seafloor hydrothermal deposits further.The samples of hydrothermal sulfides studied in this paper come from Venture hydrothermal spouts in the East Pacific Ridge (9?46' N-9?51' N) , where modern seafloor hydrothermal investigations performed by Woods Hole Oceanographic Institute using "Alvin" in 2001. We process the samples and polish them for mineralogical and element geochemical researches, then we use microphotographics to observe the annular structure in hydrothermal sulfides. Such minerals as chalcopyrite. bornite. digenite. chalcocite which distribute near the inner wall of "Black Smokers" , and sphalerite scatters in middle circle presenting annular structures together with chalcopyrite, at last, pyrite, gypsum and barite precipitate at the outer of black smoker chimneys. In addition, electron probe results show that the main ore-forming elements in EPR9?-10?N are cuprunK ironx zincunu briunu calciunu plumbum, trace elements are seleniunu arsenics telluriunu bismuth aurunu argentine and so on. The hydrothermal fluids have higher sulfur ions. vitriol radix ions and lower carbonic acid radix ions. chlorine ions.Through thermodynamic calculate combining elements ' geochemistry behaviors, we conclude: in early stage of hydrothermal activity, temperature is from 200 to 400 , forming chalcopyrite and sphaleriat mineral intergrowth; in middle stage, ore-forming temperature increases rapidly from 300癈 to 550癈, subsequently, pyritex chalcopyrite. bornite.digenitev chalcocite etc deposit: in later stage, with the input of cold seawater along structural cracks, temperature of hydrothermal fluids decreases to less than 250癈, in this progress, the reactions happened between hydrothermal fluids and rocks, thus sphalerite and pyrihotite precipitate in lower temperature. The ore-forming process present the multi-phases of hydrothermal activity.Sulfur concentration and oxygen concentration in hydrothermal system have directive effects on sulfides deposition. In basic solution with lower sulfur concentration, sulphur ions reactwith metal ions and form metal sulfides; but in acidic fluids metal sulfides precipitate easily in the conditions with higher temperature and higher oxygen concentration. From mineral intergrowth characters, we can see that EPR9?-10?N hydrothermal system has middle or high temperature and hydrothermal fluids is acidic, so these conditions induce to massive sulfides deposition. In hydrothermal system with multi-content, metal elements combine with such ions which can reduce active abilities of compounds. After a certain mineral deposits, transformations of elements don ' t stop quickly, minerals and solutions still keep a dynamic balance. Hydrothermal sulfides deposit in term of such alternant sequence, which reflects the relationship between elements transformations and ore-forming processes.