Analysis of the Origin of Calcite Cements and Fluid Sources in Limestone Formations Based on Petrology and Geochemical Data

In this paper, the authors have summarized and analyzed the petrology and isotope and trace element characteristics of host rocks and calcite cements in the Yingshan and Yijianfang formations of Lower-Middle Ordovician in the Shunbei area of the Tarim Basin. The result successfully reveals four types of limestones and four types of calcite cements, such as the sparry grainstone (SG), the microcrystalline grainstone (MG), the micrite (MC), the silicified limestone (SL), the calcite coexisting with silicon in SL (C1), the calcite which develops in fractures or between the broken siliceous breccias (C2), the calcite which fills between the broken particles (C3), and the calcite filling the fractures of all the host rocks and cements (C4). The high U/Th and Sr/Ba ratios of all the calcite cements indicate that the fluids that formed them were of comparatively higher salinity in burial environments, and the differentiation of their Sr-isotopes, Fe, and Mn contents clearly shows two types of diagenetic fluids: the fluid forming C2 and C3 is the inherited seawater closely related to the host rocks, and the fluid forming C1 and C4 is the silica-rich hydrothermal fluid from the lower strata and flowed through the possible clastic strata.