SIMS U-Pb Dating of Uraninite from the Guangshigou Uranium Deposit: Constraints on the Paleozoic Pegmatite-Type Uranium Mineralization in North Qinling Orogen, China

Pegmatite-type uranium mineralization occurs in the Shangdan domain of the North Qinling Orogenic Belt, representing a significant uraniferous province. The Guangshigou deposit is the largest U deposit of the district. Within the North Qinling area, a series of Caledonian granitic igneous rocks intruded the Proterozoic metamorphic rocks of the Qinling Group in two magmatic stages: (i) the Early Silurian Huichizi granite that was derived from a low degree of partial melting of thickened lower basaltic crust combined with mantle-derived materials following the subduction of the Shangdan Ocean; and (ii) the Late Silurian–Early Devonian Damaogou granite and associated pegmatites derived from the same source but emplaced in a late tectonic post-collisional extension environment. In the Guangshigou deposit, the U mineralization mainly occurs as uraninite disseminated in U-rich granitic biotite pegmatites, which formed by assimilation-fractional crystallization magmatic processes. Petrographic observations showed evidence for coeval crystallization of uraninite and other rock-forming minerals of the host pegmatite including quartz, feldspar, biotite, zircon, monazite, apatite, and xenotime. In addition, the low U/Th ratios (~19) and Th, REE, and Y enrichments characterized a magmatic origin for uraninite, which was likely derived from fractionated high-K calc-alkaline pegmatitic magma that experienced various degrees of crustal material contamination. In situ U-Pb isotopic dating performed by Secondary-Ion Mass Spectrometry (SIMS) on uraninite from the Guangshigou deposit yielded a crystallization age of 412 ± 3 Ma, which is concomitant (within errors) with the emplacement age of the host pegmatite (415 ± 2 Ma) and constrained the U ore genesis to the Early Devonian, which corresponds to the late Caledonian post-collisional extension in the North Qinling area. Uraninite then experienced various degrees of metamictization and/or post-Caledonian hydrothermal alteration characterized by an alteration rim associated with coffinite, chlorite and limonite. Finally, the characteristics of the pegmatite-related Guangshigou deposit exhibiting Th-rich uraninite which was the product of assimilation-fractional crystallization of pegmatitic magma defined a model significantly different than the one established for the world-class Rossing deposit characterized by Th-poor uraninite hosted in alaskite dykes formed by low degree of partial melting of U-rich metasediments.