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

Abstract

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.