黔南—桂西软玉中锰质“草花”的矿物学特征、成因机理及成矿启示

Mineralogical Characteristics, Formation Mechanism， and Significance of Manganese Dendrites in South Guizhou—West Guangxi Nephrite

摘要:

选取黔南—桂西软玉矿床中的“草花”为研究对象，采用电子探针、激光拉曼光谱、电子顺磁共振谱和扫描电镜及能谱就其物质组成及显微结构进行测试分析，并对成因机理进行探讨。结果表明，“草花”呈（准）二维平面状或三维立体状分布于软玉、透闪石化大理岩、滑石岩、透闪石透辉石岩等多种岩石内，系非平衡、非线性条件下快速生长而成的多枝状聚集分形结构。“草花”主要由钙锰矿和少量水钠锰矿、钡硬锰矿、锰钡矿等锰氧化物组成，并显示由大小均一鳞片?叶片状晶体无序卷曲成层或立体团聚成似“沙漠玫瑰”球体的显微结构。“草花”中的锰主要由滇黔桂盆地内的晚古生代—早中生代含锰岩系提供，软玉的原生孔隙和低围压、应力作用下产生的脆性裂隙为锰质运移、沉淀提供了通道和空间，裂隙两侧矿体可在压应力或热液充填作用下愈合。

Abstract:

The compositions and microstructures of dendrites in south Guizhou-west Guangxi nephrite deposits were investigated by electron microprobe （EPMA）， laser Raman spectroscopy， electron paramagnetic resonance spectroscopy （EPR）， and scanning electron microscope （SEM） equipped with energy disperse spectroscopy （EDS）， and the formation mechanism was discussed as well. The result indicates that the dendrites are （quasi） two-dimensionally or three-dimensionally distributed in various rocks such as nephrite， tremolite marble， talc rock as well as tremolite diopside rock， and have essentially multi-branched fractal structures or patterns formed by rapid growth under non-equilibrium and non-linear conditions. They consist of mainly todorokite and minor birnessite， romanechite， hollandite， meanwhile displaying a microstructure of uniformly scaly to leaf-like crystals disorderly curled into layers or three-dimensional aggregated into balls reminiscent of the desert rose stones. Manganese for the dendrite growth were mainly provided by the Late Paleozoic to Early Mesozoic manganese-bearing sedimentations in the Yunnan-Guizhou-Guangxi Basin. The primary porosity and the induced brittle cracks at a low confining pressure and tectonic stress of nephrite could provide channels and spaces for manganese migration and precipitation. The rocks on both sides of the fracture would heal when pressed or hydrothermal filled.