In-situ hydrothermal zircon U & ndash;Pb and phlogopite Ar-40 & ndash;Ar-39 geochronology of uranium mineralisation in Luzong ore district scientific drilling (LTZK01), Anhui Province, SE China: Constraints on the mineralisation process

摘要

The economically important Luzong ore district, located at the north-eastern margin of the South China Block, developed numerous Fe-Cu-Pb-Zn-U deposits during the Early Cretaceous. The identified uranium deposits are primarily granite-related vein-type deposits distributed in the exposed granites and adjacent Jurassic sediments along the SE edge of the Luzong volcanic basin. Recently, new uranium mineralisation has been discovered by scientific drilling in concealed syenite in the centre of the Luzong basin. In this study, we conducted detailed studies of geological characteristics and determined the geochronology of the uranium mineralisation to constrain the ore-forming process. Luzong scientific drilling (LTZK01) penetrated Quaternary loose surface deposits (0-2 m), Cretaceous volcanic rocks (2-1488.84 m), concealed syenite and monzonite (1488.84-2012.35 m), and uranium mineralisation (1506-1742 m). Uranium mineralisation predominantly occurs as uranothorite in hydrothermal veins along the intragranitic fractures and breccias, which are associated with a pyrite-quartzalbite-tourmaline-phlogopite gangue assemblage. LA-ICPMS U-Pb dating of zircons from syenite and monzonite yields crystallisation ages of 131.4 +/- 1.6 (MSWD = 0.45) and 131.7 +/- 1.0 Ma (MSWD = 0.87), respectively. Based on the close textural relationship with other hydrothermal minerals, morphology, and trace element features, the zircons from the uraniferous veins are of a hydrothermal origin. In-situ LA-ICPMS U-Pb dating of hydrothermal zircons yielded a weighted mean age of 131.1 +/- 1.6 Ma (MSWD = 3.6), which is consistent with the hydrothermal phlogopite 40Ar-39Ar plateau age of 129.25 +/- 1.51 Ma (MSWD = 7.70). These ages indicate that uranium mineralisation in LTZK01 is marginally younger than the crystallisation of the host igneous rocks and might be associated with the post-magmatic hydrothermal process. Based on the magmatic and mineralisation ages obtained in this study and previous researches, we propose that the uranium was probably fractionated from the felsic melt into F-rich magmatic fluids to form uraniferous veins in LTZK01 as the pressure and/or temperature decreased.