Mineralogy and geochemistry of two stages of uranium mineralization in the Jiling albitite-hosted uranium deposit, northwest China

Abstract

The Jiling Na-metasomatism uranium deposit in northwest China was initially regarded as a low-temperature mineralized system. Recent exploration has identified high-temperature mineralization at depth. However, the genesis of high-temperature U mineralization at Jiling remains unclear, resulting in the absence of a comprehensive genetic model for the Jiling deposit. This study integrates whole-rock geochemistry, mineral paragenesis and chemistry, and stable and radioactive isotope compositions from unaltered, altered, and ore-bearing samples to characterize five stages of magmatic and hydrothermal evolution at Jiling: (1) the first stage involved the emplacement of the Jiling high-K calc-alkaline granitoids, representing the primary uranium source for mineralization; (2) Na-metasomatism stage is characterized by the albitization accompanied by ductile deformation. The alteration assemblage includes albite, hematite, and chlorite. U–Pb dating of apatite from ore-barren albitite yielded an age of 433 ± 14 Ma. Stable isotopes of albite and chemical compositions of apatite indicate that the metasomatic fluids were high-temperature, Na-enriched, and alkaline, likely exsolved from the mafic dykes in the Jiling area; (3) Na-Ca-metasomatism followed immediately after Na metasomatism. This stage, characterized by weak brittle deformation of the albitite, is associated with high-temperature uranium mineralization. Alteration assemblage includes albite and calcite, with uraninite as the primary uranium mineral. Uranium was transported by a high-temperature, alkali-rich, alkaline, and reducing fluid, primarily as hydroxide complexes. The removal of Na and Ca from the fluids destabilized uranium complexes, leading to uraninite precipitation; (4) Ca-Fe-Mg-metasomatism, occurring during the Middle Devonian, is associated with the low-temperature uranium mineralization. Alteration minerals include calcite, chlorite, hematite, and barite. Pitchblende is the dominant uranium mineral. Metasomatic fluids were primarily meteoric water with a minor magmatic component. Uranium was likely transported as uranyl sulfide complexes; (5) the post-ore alteration stage contains calcite, chlorite, illite, and quartz. During this stage, uraninite and pitchblende were partially or totally altered into coffinite. By delineating these five stages, this study provides a comprehensive genetic model for the Jiling deposit, elucidating the processes and conditions leading to high- and low-temperature uranium mineralization. These findings highlight the crucial role of initial high-temperature, alkali-rich magmatic fluids in high-temperature uranium mineralization and subsequent meteoric water in low-temperature uranium mineralization of the Jiling deposit. This understanding will facilitate the development of precise ore-forming models for future exploration at the Jiling deposit.