Enrichment mechanism of REE in rhyolite and its relationship with uranium mineralization in the Dazhou uranium ore-field, Southeast China

Abstract

The Dazhou rhyolites in the Gan-Hang Belt host uranium deposits and show high rare earth element (REE) concentrations. However, the occurrence, source, and enrichment process of REE and its association with uranium mineralization in rhyolite remain unclear. In this study, fresh and altered rhyolites were selected, and the petrography, elemental and isotope geochemistry were investigated to explore the genetic mechanism of REE-rich rhyolites and its relationship with uranium mineralization. Based on the alteration and mineral paragenesis, the Dazhou rhyolites can be classified into four types: fresh rhyolite (Fr-rhyolite), hematitic rhyolite (Hem-rhyolite), clayified rhyolite (Cla-rhyolite), and hydromica rhyolite (Hyd-rhyolite). The altered rhyolites share similar average εNd(t) values with the fresh rhyolites (Fr-rhyolites = −5.8, Hem-rhyolites = −6.1, Hyd-rhyolites = −6.2), indicating their same REE sources, that means there are no external REE input for the REE-enriched altered rhyolites. In the Fr-rhyolites, the REE contents (average 341 ppm) and high field strength element (HFSE) contents increase with a decrease in the Al2O3/TiO2 ratio, indicating that magmatic crystallization differentiation leads to the initial enrichment of both REE and HFSE. In the altered rhyolites, the REE concentrations (average 655 ppm, 1045 ppm, 999 ppm for Hem-rhyolites, Cla-rhyolites, Hyd-rhyolites, respectively) are elevated compared to those in the Fr-rhyolites, indicating a significant hydrothermal enrichment for the REE. Mineralogical and petrographic studies have shown that REE are mainly hosted in parisite and monazite; therefore, the ore-forming fluid should be enriched in CO32– and F−. During the hydrothermal alteration, hematitization occurred early, followed by clayization and hydromicazation in the late stage, recording a fluid property change from an early higher-temperature, oxidizing fluid to a late lower-temperature, more reducing and acidic fluid. The early-stage fluid was responsible for the REE migration, while the decrease of temperature led to a decrease in the stability of the REE-complex that promote the precipitation of REE minerals. Combined with previous studies, REE mineralization has similar mineralization age and alteration characteristics to uranium mineralization in the Dazhou uranium ore-field, indicating the two mineralization types have derived from the same source and activated, migrated, and precipitated under the same mineralization event. The factors that controlled the precipitation of REE and uranium ore minerals are different, with the former mainly controlled by temperature, whereas the latter controlled by redox conditions.