黔西织金矿区乐平世煤系菱铁矿成因探讨

Genesis of Siderite Precipitated in the Lopingian Coal Measures in Zhijin Mining Area, Western Guizhou

黔西织金矿区乐平世煤系中的菱铁矿以多种微观形态产出,基于镜下薄片观测、泥岩微量元素分析,探讨了菱铁矿成因及演化。研究表明菱铁矿可划分为第一世代的胶状集合体、第二世代圈层结构发育不良的球粒及第三世代圈层结构发育完好的鲕粒与菱面体三个成岩世代产物。菱铁矿胶状集合体是在成矿组分(Fe^(2+)与CO_3^(2-))浓度达到超饱和状态时快速结晶形成的;圈层结构发育不良的菱铁矿球粒是成矿物质围绕第一世代形成的菱铁矿胶粒聚集而成;圈层结构发育完好的菱铁矿鲕粒是大量放射状菱铁矿线晶以第二世代的球粒为核心连续共轴生长而成,晚二叠世脉动式的海侵可能是菱铁矿在结晶非平衡态与平衡态与之间往复循环驱动因素,最终形成了同心圈层结构,而同心圈层发育完好的菱面体状菱铁矿可能是成矿条件的变化促使鲕粒从放射状生长向自形菱面体生长而成。

Siderite with a wide spectrum of micro morphologies has been found in the Longtan Formation of the Lopingian coal-bearing measures in Zhijin Mining area,western Guizhou.The origin and phase evolution of siderite are discussed based on thin section observations and trace elements analysis.The study suggests that siderite with various micro morphologies could be divided into three diagenetic generation products,comprising siderite colloidal aggregates precipitated during the first generation,poorly zoned spherulites developed during the second generation,and perfectly zoned sphaerosiderites as well as rhombohedra precipitated during the third generation.Siderite colloidal aggregates were crystallized rapidly when the concentration of metallogenic components(Fe^2+and CO3^2-)reached their supersaturation stage.Poorly zoned spherulites were formed by adhering metallogenic materials to siderite colloidal particles precipitated during the first generation.Perfectly zoned sphaerosiderites were developed by the syntaxial and continuous overgrowth of numerous radiating linear crystals surrounding the spherulites developed during the second generation.Moreover,the pulse-like transgression during the Late Permian might have acted as an external driving factor which would lead to the reciprocating cycle of siderite between crystalline equilibrium state and non-equilibrium state,and finally the concentric zonation structure was formed.The formation of perfectly zoned sidetite rhombohedra might have resulted from changes of growth patterns from the radiating growth to the rhombohedral growth caused by variations of metallogenic conditions.