Geochemistry of iron oxides in the Mianhuakeng uranium deposit in South China: Implications for uranium mineralization genesis and prospecting

• Iron oxides in “reddened alteration” within uranium ores of the Mianhuakeng deposit have been identified as follows, microcrystalline hematite, phanerocrystalline hematite, magnetite, and minor goethite. • Genesis mechanism of the microcrystalline hematite, phanerocrystalline hematite and goethite and their relations with the uranium mineralization have been investigated. • The hydrothermal fluid forming the Mianhuakeng uranium deposit was oxidizing and enriched in CO 3 2– . The Mianhuakeng uranium deposit is the largest granite-related uranium deposit in South China. The uranium ores in the Mianhuakeng uranium deposit can be divided into red uranium ores and gray-black uranium ores hosted in granitic rocks, and this paper focuses on the phenomenon of “reddened alteration” in the red uranium ores. To understand the nature of this “reddened alteration” within uranium ores and its indicative significance for uranium mineralization, we made a detailed study on mineralogy, petrography, and in situ element geochemistry of samples with “reddened alteration” in ores, by using optical microscopy, scanning electron microscopy (SEM), laser Raman spectroscopy, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The iron oxides in “reddened alteration ” within uranium ores include microcrystalline hematite, phanerocrystalline hematite, magnetite, and minor goethite. The enrichment of U, Mo, and W in goethite, microcrystalline hematite, and phanerocrystalline hematite is consistent with the geochemical signature of local granitic rocks, pointing to a significant genetic contribution from these nearby granites to the uranium mineralization. These iron oxides show pronounced enrichments in heavy rare earth elements (HREE) (ranging from 36.5 to 838 ppm) and low Y/Ho ratios (15–27, lower than the standard value of 28). The REE patterns of the phanerocrystalline hematite, which exhibits “W”-shaped tetrad effect with TE1,3 values ranging from 0.17 to 0.88, and the Ce anomaly values (δCe = 0.05–0.60), suggest that the ore-forming fluids were oxidized and enriched in CO 3 2– . Most of the goethite and phanerocrystalline hematite are in pyrite pseudomorph. During ore-forming fluid and rock interaction, the goethite and phanerocrystalline hematite were formed through oxidization of pyrite formed in an earlier fluid process and through partial oxidization of magnetite grains in the altered host granitic rocks. Furthermore, during the fluid-rock interaction, the microcrystalline hematite was formed through oxidation of Fe 2+ releasable in the host rocks. In the goethite and microcrystalline hematite, uranium is in adsorption forms, whereas in the phanerocrystalline hematite, U presents as pitchblende-, coffinite- and brannerite-microinclusions. The mineral assemblage of uranium minerals and phanerocrystalline hematite with high U contents could be used an important signal prospecting for granite-related uranium deposits.