Primary uranium sources for sedimentary-hosted uranium deposits in NE China: insight from basement igneous rocks of the Erlian Basin

Carboniferous–Permian, Triassic and Jurassic igneous basement rocks around the Erlian Basin in northeast China have been investigated through detailed mineralogical, whole-rock geochemistry, geochronological data and Sm–Nd isotope studies. Carboniferous–Permian biotite granites and volcanic rocks belong to a calc-alkaline association and were emplaced during the Late Carboniferous–Early Permian (313 ± 1–286 ± 2 Ma). These rocks are characterised by positive eNd(t) (3.3–5.3) and fairly young T DM model ages (485–726 Ma), suggesting a dominant derivation from partial melting of earlier emplaced juvenile source rocks. Triassic biotite granites belong to a high-K calc-alkaline association and were emplaced during the Middle Triassic (243 ± 3–233 ± 2 Ma). Their negative eNd(t) (−2 to −0.1) and higher T DM model ages (703–893 Ma) suggest a contribution from Precambrian crust during the magma generation processes, leading to a strong enrichment in K and incompatible elements such as Th and U. Highly fractionated magmas crystallised in U-rich biotite (up to 21 ppm U) and two-mica granites. In biotite granite, the major U-bearing minerals are uranothorite and allanite. They are strongly metamict and the major part of their uranium (90 %) has been released from the mineral structure and was available for leaching. Mass balance calculations show that the Triassic biotite granites may have, at least, liberated ∼14,000 t U/km3 and thus correspond to a major primary uranium source for the U deposits hosted in the Erlian Basin.