Characteristics of rock fissure fillings and their relationship with the accumulation of uranium and associated elements in the Kailu Sag of the southern Songliao Basin, Northeast China

Abstract

The relationship between deep fluids and the enrichment of uranium and associated elements in sedimentary basins remains unclear. Sandstone-type uranium deposits in the southern Songliao Basin exhibit abundant fault systems, and evidence of hot fluid activity is present in the ore-bearing strata, including the presence of hydrothermal minerals such as galena and sphalerite. To elucidate this relationship, we conducted detailed thin-section observations and micro-X-ray fluorescence spectroscopic analysis on the fissure fillings of mafic rocks, granite, sandstone, and mudstone in or near the ore-bearing strata. Our findings indicate that rock fissures are filled with minerals such as pyrite, quartz, ankerite, and calcite, suggesting that deep fluids have multiple sources and stages. Specifically, ankerite, rich in Fe and Mn, is closely associated with uranium mineralization, and is observed in both the fissure fillings and the ore-bearing strata. Fluids enriched in elements such as Tb and Dy likely originate from or are related to the mantle, while fluids with few other components may come from deep strata. However, U and Re contents are low in the fissure fillings, being insufficient for their mineralization. Furthermore, given the high content of carbonate minerals in the ore-bearing strata and their close association with uranium mineralization, we analyzed fluid inclusions and carbon–oxygen isotope composition of the carbonate minerals present in the rock fissures. Fluid inclusions in carbonate minerals indicate moderate to low temperatures (ranging from 145.5 to 298.5 ℃, with an average of 215.3 ℃) and low salinities (ranging from 0.18 to 7.99 wt%, with an average of 2.22 wt%). δ13CV-PDB values of carbonate minerals ranges from − 15.1 ‰ to 0.4 ‰, and δ18OV-SMOW values range from 0.5 ‰ to 14.4 ‰, suggesting a composite genesis of carbonate fluids involving magma and atmospheric precipitation. These test results on fissure fillings match roughly with those of the ore-bearing strata, indicating that deep fluids have infiltrated into the ore-bearing strata and contributed to uranium mineralization. Therefore, we propose that the U and Re enrichment in the ore-bearing strata originates from supergene fluids, whereas deep fluids may provide ore-forming materials such as ankerite, reducing fluids, and heat sources for uranium mineralization.