关键词: 黑色岩系, 重晶石, 锶同位素, 冷泉, 龙马溪组, 四川盆地, 钡

Abstract: The formation tectonic background and mineralization process of the barite-witherite mineralization belt in the Silurian black rock series on the northern margin of the Yangtze plate have always been the focus of many scholars. This article selects the barite minerals in the mud shale of the Longmaxi Formation in the Lower Silurian of the Yueliangping section at the northern margin of the Sichuan basin as the research object. Starting from petrology and mineralogy, the development characteristics of barite are discussed, and the genesis of barite and its geological significance are analyzed by combining strontium isotopes. The results show that barite mainly develops into acicular shaped crystals dominated by diagenesis, larger particle size dissolution shaped crystals dominated by cold methane seeps, dendritic crystals dominated by hydrothermal processes, and small particle size ellipsoidal crystals dominated by biological processes. The barite nodules are distributed along the layers with oblate ellipsoid shape, with a concentric stratification structure, dense homogeneity structure, and turtle-crack filling structure. The genesis is mainly diagenesis, and local late hydrothermal fluid filling shrinkage cracks form turtle-cracks. The 87Sr/86Sr value of layered barite is 0.708 4, indicating that its formation process was influenced by the mixing effect of the deep mantle sources (low 87Sr/86Sr value) and the host rocks (high 87Sr/86Sr value). It is speculated that its formation was the result of the joint action of deep mantle derived hydrothermal fluids and cold methane seeps (or diagenesis). Research suggests that the sedimentary period of the Longmaxi Formation on the northern margin of the Sichuan basin was a passive continental margin with an extensional tectonic background, and synsedimentary faults developed in the North Dabashan. In the early stage of the Longmaxi Formation, the northern slope area of the Sichuan basin was subjected to weak extension, and the scale of fault development was not large, forming intermittent small-scale cold methane seepage. In the late stage of the Longmaxi Formation, the extension was strengthened, the fault deepened, and a black chimney of barite was generated. A large amount of hydrothermal fluids containing Ba2+ and H2S entered the seawater, forming barite deposits in the North Dabashan area.

Key words: black rock series, barite, strontium isotopes, cold methane seepage, Longmaxi Formation, Sichuan basin, Ba