Chenjiazhuang uraninite: mineralogy, genesis, and potential as a U-Pb dating reference material

• An integrated approach was used to study the mineralization and evolution of the Chenjiazhuang pegmatite-type uranium deposit. • The uraninite grains in the Chenjiazhuang deposit are large (100–200 μm), well-formed, and mainly composed of UO 2 (82.69–90.01 wt%), with a relatively pure chemical composition. • The pegmatite host rock formed at 425 ± 2.5 Ma, uraninite crystallized around 401 ± 5.4 Ma, and a significant tectono-thermal event occurred around 380 Ma. • The uraninite shows high dating uniformity and minimal post-crystallization alteration, making it an excellent candidate for U-Pb in situ dating reference material. The age homogeneity of GBW04420 has been questioned, and its stock is nearly exhausted, making the development of new reference materials for U‑Pb dating an urgent priority. Uraninite from pegmatite‑type uranium deposits is a promising candidate due to its high uranium content and excellent crystallinity. As a typical pegmatite‑type uranium deposit in China, the Chenjiazhuang deposit provides an ideal case to evaluate the potential of uraninite as a reference material. In this study, we systematically investigated uraninite from the Chenjiazhuang deposit using optical microscopy, electron probe microanalysis (EPMA), and laser ablation‑inductively coupled plasma‑mass spectrometry (LA‑ICP‑MS), with focus on its mineralogical, geochemical, and geochronological characteristics. Analytical results show that uraninite grains are relatively coarse (100–200 μm), dominated by UO 2 with minor ThO 2 and PbO. Their trace element compositions indicate crystallization under high‑temperature reducing conditions, associated with magma‑wall rock (biotite‑plagioclase gneiss) assimilation and fractional crystallization. Geochronological data reveal that the ore‑bearing pegmatite formed at 425 ± 2.5 Ma (zircon U‑Pb age), whereas uraninite crystallized at 401 ± 5.4 Ma, approximately 24 Ma after pegmatite emplacement. A tectono‑thermal event at 380–370 Ma, recorded by late‑stage zircon, apatite, and monazite ages (369–380 Ma), caused localized alteration but did not reset the U‑Pb system of uraninite. The alteration of uraninite is characterized by UO 2 loss, while PbO and ThO 2 remained relatively stable. EPMA dating yields a weighted mean age of 404 ± 10 Ma for uraninite, which is consistent with the LA‑ICP‑MS result (401 ± 5.4 Ma) and previous studies (∼406 Ma). The absence of significant age differences among uraninite grains of different sizes confirms a single mineralization event. Notably, both unaltered coarse‑grained and slightly altered fine‑grained uraninite show excellent compositional homogeneity: UO 2 yields RSD values of 0.79% and 2.31%, and PbO yields RSD values of 4.62% and 4.12%, respectively, which are significantly better than those of the GBW04420 reference material. These features demonstrate that Chenjiazhuang uraninite has great potential as a high‑precision reference material for in situ U‑Pb dating, as long as obviously altered domains are avoided.