Mineralogical and geochemical constraints on the genesis of the granite-hosted Huangao uranium deposit, SE China

Abstract

The Huangao uranium deposit, located in Jiangxi Province, SE China, is hosted mainly by the Indosinian medium- to coarse-grained porphyritic biotite granite as a batholith (Sanerer pluton) and to a lesser extent by the Yanshanian fine-grained porphyritic biotite granite as stock. The Indosinian granite has a whole-rock Rb–Sr isochron age of 215±6 Ma, and the Yanshanian granite has a whole-rock Rb–Sr isochron age of 155.7±6.4 Ma. Both granites may be formed by anatexis of uranium-rich Precambrian and Cambrian metasedimentary rocks. Deuteric alteration of primary accessory minerals (zircon, sphene, apatite, allanite, monazite and xenotime) in the granites, and erosion of the granites under a hot, dry palaeoclimate resulted in the partial liberation of uranium. These processes were critical for the formation of the mesothermal to epithermal U deposit. Uranium in the deposit occurs as pitchblende along tectonic fractures, interstitially in granular minerals, and as cements of breccias. The host-rock alteration comprises pre-ore alkali-metasomatism and ore-stage hydrothermal chloritization, pyritization, hematitization, silicification, damouritization, and carbonatization. Other minerals present in the ore include pyrite, galena, sphalerite, chalcopyrite, hematite, microcrystalline quartz, chlorite, calcite and fluorite. Fluid inclusion studies on quartz, fluorite and calcite reveal that temperatures of the mineralizing fluids range from 110° to 260°C and give salinity values ranging from 11.7 to 21.6 wt.% NaCl equivalent. The δ 18 O and δD values of the fluids range from −4.0 to 3.6 and from −81.3 to −56.6‰, respectively, indicating meteoric water. Mineralogical, fluid inclusion and isotopic data are consistent with a mesothermal to epithermal origin for the Huangao deposit. Thermal activity at the Huangao deposit was maintained from the emplacement of the Indosinian granite, at 215 Ma, until the intrusion of the lamprophyre dike, at 105 Ma. Magmatism in the district as well as radioactive decay of K, U and Th in the granite may have provided an essential or additional heat source for the formation of the hydrothermal solutions. The U-bearing hydrothermal solution was also heated and deep convective circulation was mainly driven by the regional tectonism. This is supported by the U–Pb isochron age of four pitchblende samples at 48 Ma which is in the time range of the late Yanshanian regional tectonism that affected South China.