桂西南早-中三叠世东平-足荣大型锰矿床地球化学、年代学及成因研究

广西东平-足荣大型锰矿床位于桂西南锰矿带上,是广西重要的锰矿富集区。其含矿地层为三叠系北泗组,是一套以硅-泥-灰为主浅海台盆相含锰碳酸盐岩沉积,含锰矿物主要为钙菱锰矿、锰方解石和锰白云石。本文对矿区锰矿层顶部的沉凝灰岩及含锰岩系开展了详细的岩石学、岩石地球化学和锆石U-Pb年代学研究,进而对锰矿成矿时代、成因、沉积环境、沉凝灰岩源区和大地构造背景进行了探讨。锆石SHRIMP U-Pb定年结果显示,3件沉凝灰岩样品的锆石呈自形-半自形板柱状,发育良好的岩浆锆石振荡环带结构。~(206) Pb/~(238)U年龄加权平均值分别为250.8±2.1Ma(MSWD=0.98)、250.6±2.2Ma(MSWD=0.49)和243.6±2.3Ma(MSWD=0.44)。进一步限定了"东平"式锰矿的成矿时代为早-中三叠世(不晚于241～246Ma的范围内)。沉凝灰岩的构造环境判别图解及微量、稀土元素特征显示其岩浆可能形成于与俯冲消减作用相关的碰撞-弧相关或活动大陆边缘相关背景,推测东平-足荣锰矿的形成可能受到广西凭祥-东兴火山作用的影响。含锰岩系的U、V和Mo元素含量及U/Th、V/Cr、V/(V+Ni)、Ni/Co 比值显示东平-足荣锰矿形成于氧化-次氧化的沉积环境。logU-logTh、Fe/Ti-Al/(Al+Fe+Mn)图解及微量元素Ba含量、Ba/Sr值均指示成矿作用受到热水作用的影响。综合矿床地质研究,东平-足荣锰矿形成于弧后盆地拉张构造背景中,锰成矿受同期火山及海底热液作用影响,水体沉积环境为氧化-次氧化条件。

Geochronology, geochemistry and genesis of the Early-Middle Triassic Dongping-Zurong large manganese ore deposit, southwestern Guangxi

The Dongping-Zurong large manganese ore deposit is located in the southwest of Guangxi manganese ore belt which is an important enrichment region of manganese ore in Guangxi Province. The Beisi Formation of Lower Triassic which is deposited in a trench is the mineral hosted strata, and it is mainly composed of a set of siliceous and argillaceous rocks, together with limestones. Calcium rhodochrosite, calcimangite and mangandolomite are the main ore minerals. In this paper, the manganese mineralization age, genesis, sedimentary environment, tuffite source area and tectonic backgrounds of the deposit are studied through analyses of petrology, zircon U-Pb geochronology and geochemistry of the manganese ore and tuffites at the top of the formation. Zircon SHRIMP U-Pb dating results show that the zircons of three tuffites are in the shape of an autochthonous to semi-autochthonous plate-column, with well-developed magmatic zircon oscillating belt structure. Three weighted mean ²⁰⁶Pb/²³⁸U ages of 250.8±2.1Ma(MSWD=0.98), 250.6±2.2Ma(MSWD=0.49)and 243.6±2.3Ma(MSWD=0.44) have been obtained for the tuffites on top of manganese ore, which indicates that the "Dongping" type manganese ore was formed in Early to Middle Triassic (no later than 241~246Ma). The characteristics of trace and rare earth elements and multiple diagrams for discrimination of structural environment on tuffites display geochemical characteristics of typical subduction-related arc volcanic rocks, which indicated that mineralization may be affected by volcanic action from Pingxiang-Dongxing in Guangxi. Such ratios as U/Th, V/Cr, V/(V+Ni) and Ni/Co of redox sensitive trace elements from manganese ores indicate that the manganese ore deposits were formed in sub-oxidation to oxidation water conditions in Dongping-Zurong manganese ore deposit, southwestern Guangxi. The diagrams of logU-logTh, Fe/Ti-Al/(Al+Fe+Mn), the trace elements content of Ba, the ratios of Ba/Sr also show hydrothermal sedimentary characteristics, implying that hydrothermal activities played a vital role in the enrichment of manganese and the formation of the deposit. By comprehensive analyses on the ore deposit geology research results, Dongping-Zurong manganese ore is formed in the back-arc basin, and the hydrothermal activities played a vital role in the enrichment of manganese and the formation of the deposit. In addition, the manganese ore deposits were formed in sub-oxidation to oxidation water conditions.