Fluorite cement in Ordovician stromatoporoid skeletons

Fluorite crystals have been found within the skeletons of a Middle Ordovician (Llanvirn) stromatoporoid, Labechiella regularis (Yabe and Sugiyama), from peritidal carbonates of the Youngwol area, Southern Korea. Observation of sedimentary and diagenetic characteristics indicates that the carbonates were deposited and buried in an evaporitic, very shallow water environment, with some dolomite also formed. Occurrence fluorite is most common within the stromatoporoid skeletons. The fluorite occurs in pillars, galleries, and within dolomitic sediment between the latilaminae. In normal optical view, the pillars seem to exhibit a tubelike structure. However, the 'walls of the tubes' are composed of fine-grained calcite crystals representing the earliest stage of cementation, and appear to have a different refractive index to the fluorite crystals occurring in the centre of the pillars. Formation of the fluorite clearly postdates and may be related to the dolomitization, consuming the magnesium ion and releasing the fluoride to the reacting fluid. Diagenesis of the stromatoporoids, does nto resemble the diagenesis of aragonitic (molluscan) or high-Mg calcitic (echinoderm) skeletons. However, the selective occurrence in the stromatoporoid is probably related to the textual or mineralogical characteristics of the stromatoporoids.