Abstract: 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 (Fe2 +
and CO32-) 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.

Key words: colloidal aggregates, spherulite, sphaerosiderite, rhombohedron, diagenetic generation, genesis