Geochronology and Geochemistry of Uraninite and Coffinite: Insights into Ore-Forming Process in the Pegmatite-Hosted Uraniferous Province, North Qinling, Central China

Abstract

The biotite pegmatites in the Shangdan domain of the North Qinling orogenic belt contain economic concentrations of U, constituting a low-grade, large-tonnage pegmatite-hosted uraniferous province. Uraninite is predominant and ubiquitous ore mineral and coffinite is common alteration mineral after initial deposit formation. A comprehensive survey of the uraninite and coffinite assemblage of the Chenjiazhuang, Xiaohuacha, and Guangshigou biotite pegmatites in this uraniferous province reveal the primary magmatic U mineralization and its response during subsequent hydrothermal events. Integrating the ID-TIMS (Isotope Dilution Thermal Ionization Mass Spectrometry) 206Pb/238U ages and U-Th-Pb chemical ages for the uraninites with those reported from previous studies suggests that the timing of U mineralization in the uraniferous province was constrained at 407–415 Ma, confirming an Early Devonian magmatic ore-forming event. Based on microtextural relationships and compositional variation, three generations of uranium minerals can be identified: uaninite-A (high Th-low U-variable Y group), uranite-B (low Th-high U, Y group), and coffinite (high Si, Ca-low U, Pb group). Petrographic and microanalytical observations support a three-stage evolution model of uranium minerals from primary to subsequent generations as follows: (1) during the Early Devonian (stage 1), U derived from the hydrous silicate melt was mainly concentrated in primary magmatic uaninite-A by high-T (450–607 °C) precipitation; (2) during the Late Devonian (stage 2), U was mobilized and dissolved from pre-existing uraninite-A by U-bearing fluids and in situ reprecipitated as uraninite-B under reduced conditions. The in situ transformation of primary uraninite-A to second uraninite-B represent a local medium-T (250–450 °C) hydrothermal U-event; and (3) during the later low-T (100–140 °C) hydrothermal alteration (stage 3), U was remobilized and derived from the dissolution of pre-existing uraninite by CO2- and SiO2-rich fluids and interacted with reducing agent (e.g., pyrite) leading to reprecipitation of coffinite. This process represents a regional and extensive low-T hydrothermal U-event. In view of this, U minerals evolved from magmatic uraninite-A though fluid-induced recrystallized uraninite-B to coffinite, revealing three episodes of U circulation in the magmatic-hydrothermal system.