Timing of hydrothermal activity associated with the Douzhashan uranium-bearing granite and its significance for uranium mineralization in northeastern Guangxi, China

摘要

The Miaoershan uranium(U)ore field in northeastern Guangxi is one of the important granite-related U deposits in south China and is closely related to the Douzhashan U-bearing granite.The Douzhashan granite contains primary U-rich accessory minerals,including monazite(UO2=0.98-1.75 wt%)and xenotime(UO2=1.48-6.14 wt%).Primary monazite and xenotime yield chemical ages of 231±28 Ma and 230±38 Ma by electron microprobe analysis and U-Pb isotopic ages of 220±6 Ma and 211±7 Ma by laser ablation-inductively coupled-mass spectrometry respectively.These ages demonstrate that the Douzhashan granite formed during the period of Indosinian magmatic activity.Back scattered electron imaging shows that monazite and xenotime are commonly altered to assemblages of low-U synchisite and apatite,which was associated with loss of U to hydrothermal fluids.U-Th-Pb analyses of secondary apatite yielded a chemical age of 136±17 Ma,which corresponds to the timing of Cretaceous-Tertiary crustal extension in south China.We suggest that the heat and CO2required for mineralization was the result of Yanshanian crustal extension,and that this triggered the breakdown of U-rich accessory minerals in the Douzhashan U-bearing granite.Uranium remobilization from the Douzhashan granite provided materials for mineralization within the granite and/or surrounding country rocks.Therefore,a combination of Indosinian compression and Yanshanian extensional overprint produced the hydrothermal U deposits associated with the Douzhashan granite.