The genesis of granite-related hydrothermal uranium deposits in the Xiazhuang and Zhuguang ore fields, North Guangdong Province, SE China: Insights from mineralogical, trace elements and U-Pb isotopes signatures of the U mineralisation
Résumé
Granite-related hydrothermal U deposits from the Xiazhuang and Zhuguang ore fields are located within the Nanling Metallogenic Belt in South China. These deposits are hosted in Triassic and Jurassic granites that were emplaced during the Indosinian orogeny and the Yanshanian post-orogenic extension. Four main types of granitic host rocks were identified based on geochemical characteristics as the most favourable sources of U for the mineralisation in the district: (i-ii) Indosinian peraluminous S-type (Maofeng) and L-type leucogranite (Baishuizhai); (iii) Indosinian highly fractionated high-K calc-alkaline A2-type granite (Xiazhuang, Maofeng and Youdong) and (iv) early Yanshanian highly fractionated high-K, (Fe) calc-alkaline A2-type granite (Sundong, Siqian and Changjiang). They contain high U concentrations ranging from 15 to 28 ppm indicating the crystallisation of uraninite, which is a source of U easily leachable by oxidised fluids for the formation of hydrothermal mineralisation.
This study characterises the mineralogical, chemical and isotopic signatures of the U mineralisation from five representative deposits and occurrences in the Xiazhuang (Baishuizhai, Shituling, Xiwang and Xianshi) and Zhuguang (Mianhuakeng) ore fields. The 175 ± 16 Ma Baishuizhai occurrence and 162 ± 27 Ma Shituling deposit represent an early stage of U mineralisation (175–145 Ma) that occurred during the early Yanshanian. Disseminated Th-bearing uraninite at Baishuizhai and the association of U oxides with alteration minerals such as epidote and chlorite in both Baishuizhai and Shituling indicate temperatures >250 °C for the hydrothermal system. The early Yanshanian granites (190–150 Ma) provided the heat source and magmatic fluids to the hydrothermal system while U-rich Indosinian granites provided U for the Zr-Th-Ta-bearing U mineralisation. In contrast, the mineralisation from Xiwang, Mianhuakeng and Xianshi deposits (107 ± 16, 93 ± 15 and 79 ± 11 Ma, respectively) represent several pulses of the main mineralisation stage (110–50 Ma) that occurred during the late Yanshanian crustal extension. The mineralisation occurred at relatively low temperatures (<250 °C) as W-Y-Nb-Ti-rich pitchblende and uraninite veins associated with quartz, fluorite, calcite and pyrite. During Cretaceous-early Cenozoic crustal extension, oxidising meteoric and/or basinal waters percolated downward into the granitic basement through deep faults and mixed with CO2-rich magmatic fluids. These thermal solutions circulated along opening fractures and leached U from Indosinian and early Yanshanian U-rich granites and also possibly from early Yanshanian U mineralisation. After heating, the ore-forming fluids ascended along structures leading to the decrease of the hydrostatic fluid pressure, which promoted the deposition of U in veins. Therefore, granite-related hydrothermal U mineralisation in the province, formed over a long time span (>100 Ma) in two major stages related to different genetic conditions.