Length-Slow Chalcedony in Chrysanthemum Stone of Chihsia Formation, South China and Its Geological Implications

Key words: 
• Chihsia Formation /
• cheysanthemum stone /
• length-slow chalcedony

Abstract: Length slow chalcedony (LSC) was commonly used as an indicator of evaporitional environments. LSC was also distributed widely in the chrysanthemum stone, the pseudomorph of a kind of nodular celestite, from the Permian Chihsia Formation of South China. A number of lines of evidence indicated the Chihsia Formation was accumulated in a shallow carbonate platform with normal salinity. Based on microscopic observation, a comprehensive replacement succession has been established for the celestite pseudomorph as: celestite(fine-grained cloudy calcite(LSC) (Fig.Ⅰ B, C, D)(clear granular calcite(euhedral calcite crystal (in LSC) (Fig.Ⅰ B)(clear calcite (Fig.Ⅰ C). The original celestite has also been replaced directly by LSC at places. The latest generation of coarse calcite is related to calcite vein (Fig.Ⅰ C). Electronic microprobe analysis on the replacement calcites (Table 1) shows higher magnesium content in the replacement calcites of early phase ( including fine grained cloudy calcite and clear granular calcite) than those of later phase, indicating a magnesium rich diagenetic environment. In addition, tiny celestite detected by electronic scanning microscope in the replacement calcites of early phase and strontianite in the replacement calcite of later phase indicate the geochemical compositional transition of diagenetic fluids from sulfate rich to carbonate rich. The LSC was thus formed in a fluid rich in sulfate and magnesium, agreeing well with reported cases from evaporitional and non evaporitional environments. The possibility of evaporitional sedimentary environments or saline diagenetic conditions favoring the formation of LSC, thus, was excluded for the Chihsian LSC. The LSC should not be used, by itself, as a reliable indicator of evaporitional environment of deposition.