广西扶绥喀斯特型铝土矿氢氧同位素特征及地质意义

Features and significance of H-O isotopes from Fusui Karst bauxite in Guangxi Province, China

扶绥铝土矿是近年来在桂西南地区探获的一个典型喀斯特型铝土矿床。在野外地质工作的基础上,通过显微镜观察、X射线衍射、氢氧同位素测试等手段,对矿床中矿物组合及一水硬铝石、高岭石的氢氧同位素进行分析,探讨成矿环境的变化及硬水铝石的成因。结果表明:硬水铝石、高岭石是铝土矿石中的主要矿物,其次为勃姆石、赤铁矿、锐钛矿,沉积期矿石中还含有绿泥石和黄铁矿等,沉积期后矿石中含叶腊石、伊利石等,堆积矿石中均不含勃姆石。氢氧同位素测试表明:一水硬铝石的δ^(18)O值为0.95%~1.98%,δD值为-6.5%^-7.94%;高岭石δ^(18)O值为1.3%~1.89%,δD值为-7.04%^-8.97%。矿物组合指示,铝土矿沉积于缺氧环境中,沉积期后转变为富氧环境,且经历了干湿交替的气候变化。一水硬铝石氢氧同位素组成与钙红土三水铝石有较好的协调性,且兼具"风化壳"及"红土"的特征,反映其由钙红土风化壳演化而来。高岭石氢氧同位素组成推算沉积阶段温度为33℃,沉积期后为18℃,与二叠纪末期—中三叠世海水温度相似。扶绥沉积铝土矿的形成是对二叠—三叠纪全球气候变化的响应,且沉积阶段三水铝石或勃姆石向硬水铝石的转换与古特提洋俯冲作用有关的火山活动有密切关系;堆积铝土矿的形成则与印支运动及后来的表生喀斯特化作用密切相关。

Fusui bauxite deposit is a typical Karst bauxite deposit, which is found in southwest of Guangxi Province. Based on the field survey, by the means of microscope, X-ray diffraction, hydrogen and oxygen isotope testing, the mineral intergrowth and H, O sotopes date were analyzed, and the changes of metallogenic environment and the genesis of diaspore were discussed. XRD and microscope observations show that the diaspore and kaolinite are the main minerals, and boehmite, hematite and anatase are the subordinate minerals. Chlorite and pyrite form during deposition stage. Pyrophyllite and illite form during post sedimentary stage. Boehmite is not obtained in accumulative bauxite. H, O isotopes data show that δ^（18）O of diaspore is at range of 0.95%-1.98%, and δD is between-6.5% and-7.94%; kaolinite δ^（18）O is 1.3%-1.89%, and δD is-7.04%--8.97%. Mineral association indicates that the bauxite is deposited in anoxic environment, which experiences the oxidizing environment and alternation of arid and wet climates during post sedimentary stage. H-O isotopes composition of diaspore is coordinated with terra rossa, and have the characteristics of weathering crust and laterite, which indicates Fusui bauxite is transformed by terra rossa. H-O isotopes composition of kaolinite shows the temperatures of 33 ℃ in deposition and 18 ℃ in post deposition, which approximate sea water from the Late Permian to middle triassic. In conclusion, the transformation of gibbsite and boehmite to diaspore is related to the volcanic activity caused by subduction of the Tethys sea. The formation of the primary sedimentary bauxite is a response to global climate change in the Permian-Triassic. The formation of diaspore in accumulation ores are related to the indosinian movement and karstification.