松辽盆地大庆长垣南缘四方台组砂岩型铀矿碳氧同位素特征及铀成矿意义

In order to further discuss the diagenetic environment, uranium precipitation mechanism and metallogenic mechanism of the sandstone-type uranium deposit in the Sifangtai Formation on the southern margin of the Daqing Placanticline in the Songliao Basin, through the observation of drilling cores, rock thin sections, and the analysis of carbon and oxygen isotopes of carbonate cements in uranium-bearing sandstones, a systematic study has been carried out on the sandstone rock types, carbonate cement types, diagenetic stages, and carbon and oxygen isotope characteristics in the study area. The results show that the rock types of uranium-bearing sandstones are feldspathic litharenite and lithic arkose. The carbonate cements can be divided into three stages, with the first and second stages being dominant. The cements include calcite and ferrocalcite, and a very small amount of ankerite is observed. The δ~(13)C_(PDB) values range from -22.45‰ to -13.65‰, with an average of -18.33‰, and the δ~(18)O_(PDB) values range from -17.56‰ to -9.46‰, with an average of -13.52‰, indicating the participation of certain organic matter; the paleosalinity Z values range from 75.98 to 90.97, indicating freshwater deposition; the paleotemperature ranges from 67.31 to 94.92 °C. Combining the above rock characteristics, cement types and diagenesis, it is determined that the diagenetic environment is in the early diagenetic - middle diagenetic stage A. Comprehensive analysis suggests that the compaction and cementation of sandstones in the early diagenetic stage reduce the porosity of uranium-bearing sandstones and inhibit the migration of oxygen-rich uranium-bearing fluids. As diagenesis progresses, the burial depth increases, the maturity of organic matter rises, and thermal decarboxylation occurs. The adsorbed water generated by compaction forms an acidic solution, which corrodes feldspar, carbonate cements, etc., forming secondary pores, increasing the porosity of sandstones, promoting the downward infiltration and migration of oxygen-rich uranium-bearing fluids. The underlying oil and gas escape upward through fractures, providing reducing agents for the strata, causing uranium to precipitate and accumulate on a large scale at the bottom of the Sifangtai Formation.

为进一步探讨松辽盆地大庆长垣南缘四方台组砂岩型铀矿的成岩环境、铀沉淀机制及成矿机理,通过钻井岩心观察,岩石薄片,含铀砂岩碳酸盐胶结物碳、氧同位素分析,对研究区砂岩岩石类型、碳酸盐胶结物类型、成岩阶段以及碳、氧同位素特征进行了系统研究。结果表明:含铀砂岩岩石类型为长石岩屑砂岩、岩屑长石砂岩,碳酸盐胶结物可分为3个期次,以第一和第二期次为主,胶结物包括方解石、铁方解石,见极少量含铁白云石。δ~(13) CPDB值为-22.45‰~-13.65‰,平均值为-18.33‰,δ~(18) OPDB值为-17.56‰~-9.46‰,平均值为-13.52‰,揭示有一定有机质参与;古盐度Z值介于75.98~90.97之间,为淡水沉积;古温度介于67.31~94.92℃之间,结合以上岩石特征、胶结物类型及成岩作用判定成岩环境处于早成岩—中成岩阶段A期。综合分析认为:早成岩阶段砂岩的压实、胶结作用使含铀砂岩孔隙度减小,抑制含氧富铀流体运移;随着成岩作用进行,埋藏深度增加,有机质成熟度升高发生热脱羧基作用,与压实作用产生的吸附水形成酸性溶液对长石、碳酸盐胶结物等进行溶蚀形成次生孔隙,增大砂岩孔隙度,促进含氧富铀流体下渗运移,下伏油气由断裂向上逸散,为地层提供还原剂,使铀在四方台组底部大规模沉淀和富集。