Abstract:

Calcite is an important cement type of Zhiluo Formation uranium-bearing sandstones in Dongsheng area, and the carbonation
is closely related to the uranium mineralization. Based on petrographic，mineralogical, carbon and oxygen isotope analysis of calcite cements, the composition, precipitation mechanism, and uranium mineralization significance of carbon and oxygen isotope of calcite were researched in the study area. The results show that sandstones include feldspathic lithic sandstone, lithic sandstone and quartz sandstone, and clay minerals are mainly composed of smectite, illite, kaolinite and chlorite and iron-containing calcite, followed by iron calcite, as the main types of carbonate cements. The δ13CV-PDB is -15.7‰~-1.6‰with an average of -9.08‰. The δ18OV-PDB is -15.6‰ ~-10‰ with an average of -12.4‰ . The results indicate that the formation of calcite is related to the organic acid decarboxylation and the carbon source is organic carbon. Analyses of carbon and oxygen isotopic results reveal that the composition of isotope composition has characteristics of sealed atmospheric precipitation and the heavier isotope composition reflects the participation of deep low-temperature thermal fluid with oil and rich-gas during the process of diagenetic mineralization. The comprehensive analysis indicated that calcite in Zhiluo Formation sandstones of Dongsheng area was a result of interaction of surface water and deep oil and gas. Early organic prompted feldspathic matrix particle dissolution and the quartz grains overgrowth edge was formed, along with kaolinite precipitation. Later, with a large number of hydrocarbons injected into sandstones, diagenetic metallogenic
environment changed from acidic to basic and the reducibility is enhanced. The combination of CO2, Ca2+, and Fe2+ in medium water formed the iron calcite and precipitated in all kinds of primary and secondary pores. The uranium migration and precipitation during the whole process, which indicate the formation of uranium deposits, was a result of interaction of surface water and deep oil and gas.

Key words: calcite cements；carbon and oxygen isotope；diagenetic metallogenic environment；paleofluid；Dongsheng area