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

The Huangao uranium deposit, located in Jiangxi Province, SE China, is host ed mainly by the Indosinian medium- to coarse-grained porphyritic biotite g ranite as a batholith (Sanerer pluton) and to a lesser extent by the Yansha nian fine-grained porphyritic biotite granite as stock. The Indosinian gran ite has a whole-rock Ph-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 gran ites may be formed by anatexis of uranium-rich Precambrian and Cambrian met asedimentary rocks. Deuteric alteration of primary accessory minerals (zirc on, sphene, apatite, allanite, monazite and xenotime) in the granites, and erosion of the granites under a hot, dry palaeoclimate resulted in the part ial liberation of uranium. These processes were critical for the formation of the mesothermal to epithermal U deposit. Uranium in the deposit occurs a s pitchblende along tectonic fractures, interstitially in granular minerals , and as cements of breccias. The host-rock alteration comprises pre-ore al kali-metasomatism and ore-stage hydrothermal chloritization, pyritization, hematitization, silicification, damouritization, and carbonatization. Other minerals present in the ore include pyrite, galena, sphalerite, chalcopyri te, hematite, microcrystalline quartz, chlorite, calcite and fluorite. Flui d inclusion studies on quartz, fluorite and calcite reveal that temperature s of the mineralizing fluids range from 110 degrees to 260 degrees C and gi ve salinity values ranging from 11.7 to 21.6 wt.% NaCl equivalent. The delt a(18)O and SD values of the fluids range from -4.0 to 3.6 and from -81.3 to -56.6 parts per thousand, respectively, indicating meteoric water. Mineral ogical, fluid inclusion and isotopic data are consistent with a mesothermal to epithermal origin for the Huangao deposit. Thermal activity at the Huan gao deposit was maintained from the emplacement of the Indosinian granite, at 215 Ma, until the intrusion of the lamprophyre dike, at 105 Ma. Magmatis m in the district as well as radioactive decay of K, U and Tn in the granit e may have provided an essential or additional heat source for the formatio n of the hydrothermal solutions. The U-bearing hydrothermal solution was al so 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. (C) 1999 Elsevier Science B.V. All ri ghts reserved.