南大西洋中脊26°S热液区成矿物质来源探讨

南大西洋中脊的26°S热液区广泛发育多金属硫化物、底泥、枕状熔岩、非活动性烟囱体和活动性烟囱体。为了有效探索硫、铜等成矿物质的来源以及成矿作用过程，分别以玄武岩、烟囱体残片及块状多金属硫化物为研究对象，开展了熔融包裹体、硫同位素和铜同位素研究。结果显示：区内玄武岩新鲜未蚀变且斑晶中产出大量熔融包裹体；熔融包裹体气泡壁附着黄铜矿、黄铁矿及磁铁矿等子矿物，说明在岩浆作用过程中可从熔浆中分离出成矿所需的金属元素和硫，这些成矿元素随着岩浆去气作用进入挥发分中，并随着脱气作用迁移出来。通过对烟囱体残片及块状多金属硫化物中黄铁矿的硫同位素组成进行比对分析，发现26°S热液区内硫化物的硫同位素与大西洋各热液区硫化物的硫同位素变化范围相一致，但δ³⁴S_V-CDT值略低（3.0‰~3.9‰）。低的δ³⁴S_V-CDT值指示硫以岩浆硫源为主，海水硫酸盐还原硫占比低。黄铜矿呈现略微富铜重同位素特征且分馏程度较低，其δ⁶⁵Cu值（0.171‰~0.477‰）趋近于大洋中脊玄武岩的铜同位素值（0）。综合硫同位素及铜同位素特征，表明热液流体经历了岩浆和海水的混合过程，成矿物质主要来自于岩浆热液，成矿作用过程中可能有少量海水混入。

Discussion on Sources of Metallogenic Materials in the 26°S Hydrothermal Field,Southern Mid-Atlantic Ridge

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.