Mineralogy, Geochemistry, and geochronology of the Paleoproterozoic pegmatite in the northern margin of the North China Craton, China: implications for U-Th-Pb-Nb-Ta-REE polymetallic mineralization

Abstract

Rare metals represent strategically critical mineral resources, with rare-metal pegmatite deposits constituting one of their most significant sources. These deposits have become a focal point in contemporary geoscientific research and mineral exploration. This contribution provides the first documentation of extensively outcropped Paleoproterozoic rare-metal mineralized pegmatites along the northern margin of the North China Craton. A representative pegmatite dike from the Daqingshan area was selected for systematic investigation, including field geology, petrology, geochemistry, LA-ICP-MS zircon U-Pb geochronology, and EPMA U-Th-Pb_total chemical dating of uraninite. Results reveal significant enrichment of U, Th, Pb, Nb, Ta, and rare earth elements (REE) during the evolution from non-mineralized graphic pegmatite to mineralized varieties. The mineralized pegmatite contains maximum concentrations of 47,300 ppm U, 1,490 ppm Th, 44,800 ppm Pb, 1,940 ppm Nb, 22,600 ppm Ta, and 69,636.3 ppm ΣREEs. These elements primarily occur in polycrase, fergusonite, allanite, uraninite, thorite, zircon, monazite, Y-bearing minerals, and xenotime. Zircon from non-mineralized graphic pegmatite yields a crystallization age of 1876 ± 14 Ma, whereas uraninite from mineralized pegmatite records a U-Th-Pb_total chemical age of ∼ 1854 Ma. These zircons exhibit ε_Hf(t) values ranging from −10.8 to −2.5, corresponding to two-stage Hf model ages (t_DM²) of 2569–3018 Ma (mean = 2769 Ma). Integrated with field evidence and existing data, we propose that these pegmatites formed via melt migration following Paleoproterozoic high- to ultrahigh-temperature metamorphism and anatexis of ancient North China Craton crust. Their emplacement likely correlates with the late-stage assembly of the Columbia supercontinent during the transition from compressional to extensional tectonics. The extreme rare-metal enrichment predominantly resulted from magmatic fractional crystallization, with subordinate contributions from late-stage hydrothermal processes. This study advances metallogenic theories of Paleoproterozoic rare-metal pegmatites in China and provides critical insights for exploration targeting ancient rare-metal deposits.