Genesis of the Guangshigou pegmatite-type uranium deposit in the North Qinling Orogenic Belt, China

摘要

The Guangshigou uranium deposit is an important pegmatite-type uranium deposit located in the North Qinling Orogenic Belt (NQB), China. The characteristics pertaining to its chronology, petrogenesis, and geodynamic setting remain unclear till date. In this study, we present systematic zircon U-Pb ages, Lu-Hf isotopic data, and mineral chemistry of uranium-rich biotite pegmatite dikes. In situ zircon U-Pb dating of the biotite pegmatite dikes yielded early Caledonian ages (422-417 Ma). The initial Hf-176/Hf-177 ratios of the zircons from the biotite pegmatites showed a narrow range (0.282378-0.282590), with epsilon Hf values ranging from -5.1 to 2.8 and Paleoproterozoic to Mesoproterozoic two-stage Hf model ages between 1723 and 1227 Ma. The bulk and mineral chemistries of the pegmatites also indicate that the pegmatite dikes were derived from low-degree partial melting of Qinling Group rocks with monazite as the residual mineral. Differences between the normal biotite pegmatite and the mineralized biotite pegmatite dikes with respect to mineralogy, texture, structure, and mineral chemistry indicate that the assimilation between the pegmatite dikes and the wall rock, may be the primary mechanism for the mineralization of the uranium, with the fractional crystallization process being another important mechanism. Amphibolite facies metamorphism, which was most likely caused by the exhumation of the subducted slab in the NQB, may have been a key factor in the genesis of the uranium-rich pegmatitic magma.