Origin and evolution of uraniferous pegmatite: A case study from the Xiaohuacha granite–pegmatite system and related country rocks in the Shangdan uranium mineralization district of North Qinling Orogenic Belt, China

Abstract

ABSTRACT The Shangdan district in the North Qinling Orogenic Belt hosts the largest pegmatite-type uraniferous province in China. The Xiaohuacha U mineralization in this district occurs along the contact zone between biotite pegmatite and its host rock of biotite gneiss, adjacent to the composite Huichizi batholith, which is composed mainly of biotite monzogranite and syenogranite. Zircon U Pb dating of these granitoids reveals two-stage magmatism, namely, the early Silurian for biotite monzogranite (438.5 ± 4.4 Ma) and the early Devonian for syenogranite (417.8 ± 3.2 Ma). The biotite pegmatite shows an age of 415.8 ± 3.8 Ma for the mineralized marginal zone, and similar ages of 414.6 ± 4.3 Ma and 413.2 ± 3.5 Ma are obtained for the barren marginal and intermediate zones, respectively. The biotite monzogranite exhibits a restricted range of positive zircon eHf(t) values of +5.3 to +7.8, with TDM2 ages of 0.93–1.08 Ga, suggesting magma derivation from mainly pre-existing basaltic rocks in the orogenic root. Magmatic oxygen fugacity (fO2) was estimated by Ti-in-zircon thermometry and Ce incorporation within zircons. The biotite monzogranite was emplaced at a temperature of 736 ± 24 °C with log fO2 values of −22.28 to −14.19. In contrast, the syenogranite zircons have eHf(t) values of −1.4 to +2.4 and TDM2 ages of 1.25–1.49 Ga, indicating a derivation from the Mesoproterozoic crust with some mantle input. Pristine zircon domains from different zones of the biotite pegmatite display eHf(t) values of −2.8 to +1.8 and TDM2 ages of 1.29–1.58 Ga, similar to those of the syenogranite, indicating they share a common magma source. Combination of petrographical, geochronological, isotopic, and mineral chemical data supports a petrogenetic relationship between the high-T (842 ± 14 °C) syenogranite and biotite pegmatite (756 ± 21 °C and 709 ± 26 °C for the uncontaminated marginal and intermediate zones respectively) through magmatic differentiation from the same magma source during a period of tectonic extension. Processes of segregation and emplacement of the pegmatitic magma are consistent with open-system assimilation and fractional crystallization (AFC) leading to decreasing log fO2 values from the syenogranite (−11.23 to −7.02) to biotite pegmatite through a fractionation trajectory. However, log fO2 values increase from the uncontaminated pegmatite marginal (−15.88 to −10.9) to intermediate (−17.86 to −6.14) zones with closed-system pegmatite internal evolution. The interaction (hybridization) between magma and gneiss induced decrease of oxidation state and (K + Na)/Al, as well as modifications to zircon compositions in the mineralized pegmatite marginal zone. We propose a model with decreasing of fO2 and (K + Na)/Al during AFC being the main controls on the formation of the U deposit.