南大西洋中脊26°S热液区成矿物质来源探讨

A large amount of metal sulfide debris, sedimentary mud, pillow basalt, inactive and active black smoker was found in the Southern Mid-Atlantic Ridge at 26° S segment (SMAR 26° S). The melt inclusions, sulfur isotopic data and copper isotopic data of collected samples, including basalt, chimney debris and massive sulfide, were well studied. The results show that the polymetallic sulfides, such as chalcopyrite, pyrite and magnetite, adhere to the bubble wall of melt inclusions in the phenocrysts of basalts, indicating that the ore-forming metallic elements and sulfur derived from the volatile-rich melt. These ore-forming elements may enter into the volatile phase and precipitate during the magma degassing. The role of sulfur in SMAR 26°S was examined by utilizing sulfur isotope. Isotope composition for pyrites in chimney debris and massive sulfides ranges from 3.0‰-3.9‰ in δ³⁴S_V-CDT, which points to a mixing process between sulfur from magmatic fluid and the sulfur from seawater sulfate. The sulfur isotopes in SMAR 26°S_V-CDT show the same variations as those of other hydrothermal vent systems on the Southern Mid-Atlantic Ridge, but all samples are relatively depleted in ³⁴S relative to other hydrothermal fields, reflecting the greater relative importance of the magmatic fluid. The monomineral Cu isotopic compositions of chalcopyrite are positive with the range of 0.171‰-0.477‰, indicating that the δ⁶⁵Cu values obtained for chalcopyrite are close to those for source rock (i.e. 0 for basalts), and characterized by slight ⁶⁵Cu-rich and low fractionation. The characteristics of the hydrothermal fluid and the source of metal are recorded by the combination data of the sulfur isotope with copper isotope and the melt inclusions. All the evidence shows the lower contribution of seawater sulfate and indicates that the ore-forming fluid and metallic elements derive mainly from magmatic-hydrothermal fluid.