In situ U–Pb dating of dolomite: Reliable ages for sandstone-hosted uranium deposits in the southern Songliao Basin, NE China

Abstract

Previously, the ages of U mineralization in sandstone-hosted uranium deposits have not been adequately constrained due to the absence of suitable minerals for precise radiometric dating. To ascertain the mineralogenetic epoch and origin of the ore-forming fluids in the sandstone-hosted uranium deposits within the southern Songliao Basin of northeastern China, we conducted U–Pb dating, major and trace element analyses, and Sr isotope analyses on carbonate minerals taken from the sandstone. In this study, we present a novel U–Pb age of 92.5 ± 6.9 Ma (MSWD = 0.56) for calcite nodules, which is interpreted as the depositional age of the target formation hosting the ore. The age of the main mineralization stage is constrained by two types of high uranium carbonate minerals: the dolomite coexists with siderite and recrystallized framboidal pyrite, whereascoarse-grained ankerite occurs with dawsonite enveloping dolomite. The dolomite vein yields an age of ∼60 Ma (62.8 ± 0.5 Ma, MSWD = 2.7; 60.0 ± 0.4 Ma, MSWD = 6.4), the coarse-grained ankerite may be contemporaneous with the widely distributed diabase with an age of ∼40 Ma, which is interpreted two U mineralization ages. The ∼92 Ma calcite nodules are characterized by low U and U/Th ratios and relatively flat REE distribution patterns, and the high ⁸⁷Sr/⁸⁶Sr ratios (0.7109–0.7327) and the dispersed trace element composition can be attributed to fluid–rock interactions. In contrast, dolomite veins with an age of approximately 60 Ma exhibit elevated MnO and Y/Ho ratios, as well as patterns enriched in heavy rare earth elements (HREEs); the ⁸⁷Sr/⁸⁶Sr ratios range from 0.7066 to 0.7092, indicating their association with hydrocarbon seep-related carbonates of microbial origin. The coarse-grained ankerite, which has high REE + Y values and hump-shaped, MREE-enriched patterns with low Y/Ho ratios and negligible to positive Eu anomalies, combined with the spatial relationship between the ankerite-bearing sandstone sample and diabase, indicates that the ankerite is the product of hydrothermal processes. This ∼60 Ma dolomite presents initial evidence indicating that the Hulihai deposit is contemporaneous with a significant regional tectonic inversion event and preceded or coincided with hydrocarbon fluid activity in the southern Songliao Basin. Based on the spatial and temporal correlations, it is plausible that both the Palaeocene (∼60 Ma) tectonic reversal event and the Eocene hydrothermal event (∼40 Ma) were causally linked to sandstone-hosted uranium deposits in the southern Songliao Basin. The present study demonstrates the robustness of in situ carbonate mineral U–Pb dating as a valuable tool for geochronological investigations pertaining to sandstone-hosted uranium deposits.