The Major and REE Geochemistry of the Silikou Chert in Northern Guangxi Province

Key words: 
• chert /
• EdiacaranCambrian transition period /
• the Laobao Formation /
• element /
• petrogenesis

Abstract: The Silikou chert in Northern Guangxi Province was deposited in a terminal Ediacaran to early Cambrian (about 550～540 Ma) deep ocean basin. It has high SiO2 concentration (average 93.8%). Al2O3 content ranges from 0.17% to 4.92% and increases from the lower to the upper section which is more than 2%. Al/(Al+Fe+Mn) and Fe/Ti ratios for most of the chert are more than 0.42 and less than 16.3, respectively. Al2O3/( Al2O3+Fe2O3) ratios for most of the chert exceeds 0.4 and the ratios are 0.8～0.9 for chert in the upper section. Y/Ho ratios (26.4～36.9) are high (more than 32) for chert in the lower and middle section, and are similar to that of the upper crust (27) for chert in the upper section. It has no positive Eu anomalies (average 1.0) for the chert samples. REE concentrations for chert in the lower section (15.9×106～27.1×106) are low relative to PAAS, which are similar to that of the seawater but hydrothermal fluids or jasper related to seafloorhydrothermal sulfide deposits. Although REE patterns for chert in the middle section become flat since the increase of terrigenous detrital input, they have low REE concentrations (26.2×106～49.4×106) and seawater characteristics. REE concentrations for chert in the upper section range from 40.5×106 to 59×106 and they have flat REE patterns that are similar to PAAS, yet REE concentrations are just 1/4～1/3 of the PAAS. The major and rare earth elements characteristics of the Silikou chert suggest hydrothermal fluids and terrigenous detrital input were not main sources of the silica. In fact, huge amounts of dissolved silica derived from chemical weathering due to high CO2 concentration in the atmosphere and degradation of organic matter were the basic cause for the basinal chert in South China. 