BACKGROUND The eastern Liaoning Province represents the eldest hydrothermal uranium ore cluster area in China. Two ore types are classified, including independent uranium mineralization and uranium mineralization associated with Precambrian sedimentary-metamorphic iron deposits. The ore-forming age of the former type has been well constrained at ~1.85Ga, whereas the age of the latter type remains uncertain. Iron deposit-associated uranium mineralization developed in the Wengquangou B-Mg-Fe deposit, the Gaojiagou Fe deposit, and the Gongchangling Fe deposit. Geochronological studies on uraninite of the Wengquangou deposit yielded variable ages ranging from 2.0 to 1.8Ga^11-14, with some ages even older than the depositional ages of the hosting Liaohe Group (2.05 to 1.93Ga¹⁵). This inconsistency suggests that some uraninite grains used for dating are detrital in origin. The lack of reliable age constraints on the iron deposit-associated uranium mineralization has raised questions about its origin. It remains unknown whether the iron deposit-associated uranium mineralization resulted from the same hydrothermal process and formed at the same tectonic setting when compared with the independent uranium mineralization.

OBJECTIVES To determine the metallogenic age and verify the accuracy of the age in uranium.

METHODS The instrument JXA-iHP200F was used for analysis, and the age was calculated according to Ranchin’s empirical formula³⁸. The analytical conditions were 20kV acceleration voltage, 50nA beam current, and 5μm beam spot size. A further LA-ICP-MS U-Pb dating method was used to verify the EPMA chemical ages of the uraninite, using a 193nm GeoLasPro and Agilent 7900 ICP-MS with a laser spot size of 16μm.

RESULTS The calculated age of the Wengquangou deposit ranges from 1899 to 1324Ma, but it is mainly concentrated from 1899 to 1741Ma. Two peaks at 1859Ma and 1784Ma are constrained by an age frequency distribution histogram. This result is younger than the deposition age of the Liaohe Group. The age of the Gongchangling deposit ranges from 1858Ma to 1715Ma, with two peaks at 1865Ma and 1743Ma constrained by an age frequency distribution histogram. In the Wengquangou deposit, the position for uraninite LA-ICP-MS U-Pb dating corresponds to EPMA analytical points. The weighted mean ages are grouped into 1840±16Ma (MSWD=2.0) and 1787±8Ma (MSWD=0.95), which are consistent with the results obtained by EPMA. The loss of Pb can result in disruption of U-Th-Pb isotope system and thus affect the calculated ages. The loss of Pb can be probed because it is generally considered to be positively correlated with Si, Ca and Fe contents. The SiO₂+CaO+FeO contents of most measuring spots of uraninite of the two deposits is less than 1%, and show no correlation with Pb contents, indicating that the loss of Pb is negligible.

CONCLUSIONS The uranium mineralization associated with iron deposits has been constrained at ~1.85Ga and then experienced hydrothermal superposition at ~1.78Ga. The age results indicate that both the independent and iron ore-associated uranium mineralization in the eastern Liaoning Province was formed at ~1.85Ga, linking to a Proterozoic post-orogenic extensional environment in the eastern North China craton. The ore-forming fluids of different deposits of iron associated uranium deposits are all alkaline and oxidated but vary in fluid composition and temperature. This study highlights combined application of EPMA and LA-ICP-MS dating methods on uraninite, realizing the complementary advantages of spatial resolution and dating precision.