Characterization of ore-forming fluids in the Tamusu sandstone-type uranium deposit, Bayingobi Basin, China: Constraints from trace elements, fluid inclusions and C–O–S isotopes

Abstract

The Tamusu deposit is a large sandstone-type uranium deposit discovered in recent years in the Bayingobi basin, Inner Mongolia. Hydrothermal activity was recorded by veins in this deposit, but its relationship with uranium mineralization has not been clarified. In this study, petrographic observations, chemical composition of U minerals, whole rock geochemistry, fluid inclusions and C–O–S stable isotope studies of sandstone and veins were integrated to characterize the mineralizing fluids and mineralization conditions. We found uranium minerals generally co-exist with pyrite occurring in interstitial pores or on detrital grains in sandstone. The main ore mineral is pitchblende, and minor ore minerals include brannerite and coffinite. Pitchblende coexists with subhedral pyrite, whereas brannerite and coffinite coexist with cubic pyrite and calcite. Calcite also occurs in veins together with gypsum. Fluid inclusions from calcite and gypsum in the veins have homogenization temperatures ranging from 124 °C to 326 °C, with salinities of 5.7–17.2 wt% NaCl equivalent. The C–O and S isotopes of the vein minerals suggest that the hydrothermal fluids were derived from circulated formation water. Trace element analysis shows that the U-bearing sandstone is commonly enriched in V, Re and Mo, while the high-grade uranium ores are further enriched in Zn, Co, Ni and Cu. The REE patterns and δEu values in the high-grade U-bearing sandstone are similar to those in the veins, which are characterized by high LREE and low δEu, suggesting that the high-grade U-bearing sandstone is related to hydrothermal activity. The mineral assemblages, temperatures, trace elements and isotopes indicate that the Tamusu deposit was affected by two distinct types of fluids: supergene diagenetic alkaline oxidizing fluids and acidic reducing hydrothermal fluids. The oxidizing fluids started to be involved after the deposition of the ore-hosting upper Bayingobi Formation, forming a large amount of low-grade U-bearing ores. The hydrothermal fluids occurred in association with the eruption of basalt in the overlying Suhongtu Formation. The involvement of hydrothermal fluids changed the pH and Eh conditions, superimposed and reformed the low-grade uranium ores, forming high-grade uranium ores, and finally fixed the position of the uranium orebodies. Therefore, the Tamusu deposit experienced four main stages: (i) sedimentary and early diagenetic uranium concentration, (ii) oxidation and ore-forming stage, (iii) hydrothermal alteration and superimposed mineralization, and (iv) post-ore stage.