The Origin and Features of Late Sinian Phosphatic Stromatolites in Fuquan, Guizhou Province

AbstractThe phosphatic stromatolite discussed in this paper is a columnar type, comprised chiefly of collophane and subordinately of microcrystalline apatite. The column is composed of alternations of dark, cryptocrystalline laminae formed by the biological processes of algae and bacteria, and light, microcrystalline laminae formed by chemical precipitation. The intercolumnar filler, showing a concave anti‐stromatolitic structure, is composed of alternations of loose laminae formed by mechanical deposition, and dense, crypto‐, microcrystalline laminae formed by chemical precipitation and algal bacterial organic process.Under the SEM, the column differs clearly from the intercolumnar filler in ultramicro‐fabric. The three basic types of ultramicro‐laminae that can be distinguished in the column are: prismatic apatite laminae of chemical origin, clastic apatite laminae of mechanical origin and branched‐tubular apatite laminae of algal‐bacterial origin, of which the first and second categories constitute light laminae and the third, dark laminae. The dark and light laminae were formed during the alternation of flourishing and dormant periods of algae and bacteria.Under the SEM, the clastic texture is obvious in the loose laminae of the intercolumnar filler. And four types of ultramicro‐apatite assemblages can be distinguished in the dense laminae, i. e., the. framboidal and the tubular apatite of algal‐bacterial origin, the framboidal‐prismatic and the framboidal‐tubular‐prismatic apatite of chemical organic origin. The alternation of the loose and the dense laminae depends upon the changes of the energy in the water.In the stromatolites, no replacement phenomena have been found and there exists abundant prismatic apatite kept in the initial form of upward‐crystallization, and especially, in the forms of algae and bacteria (framboidal, tubular and branched). All these undoubtedly indicate that the stromatolites were originally formed by algae and bacteria.The ultramicro‐fabric features indicate that the collophane is not a simple mineral, but a disorderly arranged aggregate of ultramicro‐granular apatite.