The Guangshigou uranium deposit, northern Qinling Orogen, China: A product of assimilation-fractional crystallization of pegmatitic magma

Uranium mineralization associated with biotite pegmatites occurs in the Shangdan triangular domain of the northern Qinling Orogen, constituting a significant uraniferous province. The Guangshigou deposit is the most productive uranium deposit in this area. Several Caledonian intrusions were emplaced within the ore district including the Huichizi granite (HG, 441.1 +/- 1.3 Ma), Damaogou granite (DG, 420.9 +/- 1.3 Ma), biotite pegmatite (BP, 415.6 +/- 1.5 Ma), and muscovite-biotite pegmatite (MBP, 412.1 +/- 1.4 Ma). Based on uranium concentration, each BP displays two different zones, i.e., an uranium-bearing zone (UBP hosted at the contact zone) and a non-mineralized zone (NBP hosted in the non-assimilation zone). Two magmatic pulses are distinguished. The Early Silurian HG was derived from a low degree of partial melting of thickened lower basaltic crust interfused mantle-derived materials following the subduction of the Shangdan Ocean. The Late Silurian-Early Devonian intrusions exhibited the same source region but formed during the weakening of convergent action. Fractional crystallization of high-K granitic magma (DG) produced a set of fractionated pegmatites, whereas UBP deviates from a normal magma evolution trend due to country rock contamination in the hybrid zone. Most of the U in UBP is hosted in primary uraninite. The biotite gneiss (BG) is the most favorable country rock for uranium mineralization. Bulk assimilation of country rock is driven by reactions during xenolith melting and pegmatitic magma crystallization. The mineralization occurs at the assimilation/hybrid zone, e.g., the contact zone between BP and BG, as well as the partially assimilated xenoliths. The enrichment of contact-hosted peritectic biotite and increase of FeOT, MgO, TiO2, MnO, LILE (i.e., Rb, Ba, K), HFSE (i.e., Nb, Ta) contents and initial Sr-87/Sr-86 ratios as well as decrease of epsilon(Nd)(t) values in BP arose from chemical interaction (assimilation/hybridization) with the country rock. The reactive bulk assimilation modified the normal crystallization process of pegmatitic magma. The uranium ores are the products of the assimilation fractional crystallization of the pegmatitic magma. U-F complexes were destabilized by reducing fluorine activities during the formation of peritectic biotite in the assimilation/hybrid zone, triggering decrease of U solubility and saturation.