Geochemistry of Upper Devonian Hydrothermal Mammilated Chert Guangxi,Southwest China

Geochemistry of Upper Devonian hydrothermal mammilated chert Guangxi,Southwest China

The mammilated chert (MC) studied in this paper is a kind of silica-rich concretion (SiO₂ 93%) occurring in the Upper Devonian argillaceous silicolites (SiO₂≈88%) in Mugui, Guangxi. Impurity components in the MC are relatively low and show systematic variations as compared with the country rocks. Abundances of Mg, Si, Ti, Fe, Mn, U and Th are characteristically similar to those of recent and fossil hot-water sediments. Moreover, the MC is also characterized by low REE abundance, LREE > HREE, apparent negative Ce anomaly and moderate positive Eu anomaly. Oxygen isotope data show that the formation temperature of the MC is about 20 °C, higher than that of the host rock. The results of geochemical, geological and lithological studies suggest that the MC may have been formed through metasomatism, filling and crystallization beneath the sedimentary basin during the Devonian period.