Characteristics of Carbonate Alteration and Its Organic Genetic Mechanism in Uranium‐Bearing Red Bed Luohe Formation, <scp>SW</scp> Ordos Basin

ABSTRACT Organic fluid activity and carbonate alteration are developed in the Lower Cretaceous uranium‐bearing red beds in the southwestern Ordos Basin. To elucidate the origin of organic‐related carbonate alteration, this study investigated the mineralisation through core alteration observations, spectral core scanning, scanning electron microscopy, and electron probe microanalysis. In addition, fluid inclusions were characterised using microscopy and microthermometry. The results indicate that: (1) Secondary carbonate alteration commonly occurs within bleached zones in mineralised sandstone intervals and surrounding rocks of the Luohe Formation, accompanied by evident hydrocarbon shows. Carbonate minerals exhibit paragenetic and associative relationships with uranium minerals and pyrite. (2) The homogenisation temperatures of organic inclusions in secondary calcite cements of mineralised sandstone range from 60°C to 165°C, with salinities between 2% and 23.18% NaCl eq. The distribution of secondary carbonate alteration is spatially correlated with organic fluid activity, and the extent of alteration shows a positive correlation with variations in the temperature–salinity conditions of the organic inclusions. It is concluded that the association of secondary carbonate alteration with organic fluids, bleaching, pyritisation, and uranium mineralisation may be attributed to the dual alteration of sandstone by moderately acidic and reducing organic fluids at low to medium temperatures.