Contrasting U-Pb geochronology and geochemistry of uraninite from the Xianshi and Xiwang uranium deposits, South China: Implications for ore genesis

Abstract

The Xiazhuang uranium ore field contains eighteen uranium deposits and is one of the most important granite-related uranium ore producers in South China. Silicified vein-type and intersection-type uranium deposits are the two main types of uranium deposits in this ore field. To constrain the mineralization ages and sources of uranium of these two types of uranium deposits, we present geochemistry and U-Pb isotopic data of uraninites from the representative Xianshi intersection-type and Xiwang silicified vein-type uranium deposits in the Xiazhuang ore field. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating on uraninite from the Xianshi deposit yielded the crystallization age of 89.9 ± 2.1 Ma, which is consistent with the emplacement age (∼90 Ma) of NNE-trending diabase porphyrite dykes in this area. Chondrite-normalized rare earth element (REE) patterns of uraninite from the Xianshi deposit are characterized by slight enrichment of light REE relative to heavy REE and weakly positive Eu anomalies. Our study suggests the involvement of mantle-derived metals in the formation of the Xianshi deposit. In situ LA-ICP-MS U-Pb dating on uraninite from the Xiwang deposit yielded a weighted mean age of 51.3 ± 0.8 Ma, which overlaps the timing of the 55–45 Ma crustal extension event in South China. Uraninite from the Xiwang deposit has much higher contents of W (5768–11305 ppm), Nb (294–1301 ppm), and Y (3013–6263 ppm) than those of uraninite from the Xianshi deposit (W = 291–371 ppm, Nb = 3–14 ppm, and Y = 1658–3149 ppm). Uraninite from the Xiwang deposit has “gull-winged” chondrite-normalized REE patterns with pronouncedly negative Eu anomalies, which resemble whole-rock REE patterns of the Maofeng and Xiazhuang granites. This study suggests that the Maofeng and Xiazhuang granites are probably the primary source of uranium for the Xiwang deposit.