Hydrothermal activity in the fourth member of the Triassic Leikoupo Formation in the Yuanba area, northeast Sichuan, SW China

摘要

Large quantities of saddle dolomite, dolomite, calcite, celestite, fluorite, barite, quartz, and pyrite occur as fracture fillings in the fourth member of the Middle Triassic Leikoupo Formation in the Yuanba area, northeast Sichuan. They were found either as single mineral or mineral assemblages. The properties, origin, and evolution of the fluids from which these fracture fillings were precipitated were studied using a variety of methods, including polarized light microscopy, cathodoluminescence microscopy, fluid inclusions, and stable isotope analysis. Homogenization temperatures of the liquid-gas inclusions trapped within the dolomite ranged from 112 to 138 degrees C. Based on this temperature range and the oxygen isotope data, the delta O-18 values of the fluids from which the dolomite precipitated were determined to be between -5 parts per thousand and 7 parts per thousand (Standard Mean Ocean Water). The study suggests that the fracture-filling dolomite, ubiquitous in the fourth member of the Middle Triassic Leikoupo Formation in the study area, was produced from high-temperature, high-salinity brines. These brines were interpreted to have originated from the mixtures of hot brines trapped in rock formations and hydrothermal fluids from deep sources and to have become enriched in strontium during diagenesis.