Silicalites of hydrothermal origin in the Lower Cambrian black rock series of South China

Abstract

A silicalite bed was found in the hanging wall and foot wall of the sulfide-rich bed of the Lower Cambrian black rock series in South China. Its origin was not described before. On the oxide (SiO2-Al2O3, SiO2-MgO, SiO2-K2O + Na2O) diagrams for discriminating silicalites of chemical, biological and volcanic origins (Liu Xiufeng, 1991), most of the data points of silicalites fall within the areas representing silicalites of chemical and volcanic origins. On the Al-Fe-Mn diagram for discriminating silicalites of hydrothermal and biological origins (Yamamoto, 1987), the data points fall within the areas representing silicalites of hydrothermal and hydrothermal-biological origins. On the SiO2-Al2O3 diagram for discriminating silicalites of hydrothermal and hydrogenous origins (Bonatti, 1975), the data points mostly fall within the hydrothermal area. The ratios of SiO2/Al2O3, SiO2/(K2O+ Na2O), SiO2/MgO, and K2O/ Na2O in the silicalites stand between those of volcanic sediments and of seafloor hydrothermal sediments. The total amount of rare-earth elements in the silicalites is low; the North American Shale-normalized REE patterns decline leftward with obvious negative Ce anomaly. The trace elements Mo, Zn, As, Sb, Se, U, and Ba are higher than those in non-hydrothermal sediments and U/Th ≥1. The present authors think that the silicalites are derived from seafloor hot brines which had attracted elements from igneous rocks.