The metallogenic mechanism of a granite-type uranium deposit in south China in special reference to its U-Pb isotopic systematics

The uranium deposit studied in this paper occurs in a Precambrian (Xuefengian) granite batholith. The emplacement age of the granite is 760 m.y. The average content of uranium in the granite is 7 ppm, but the yield of leached U is higher with an average of about 33.8%. The granitic rocks contain some uraninite. Uranium mineralization mainly takes the form of siliceous veins with an age of 47 m.y. K-feldspar present in the granite and pyrite associated with pitchblende were mass-spectrometrically analyzed for Pb isotopic compositions, characterized by anomalous lead. A reasonably linear array can be seen on the 207Pb/204Pb-206Pb/204Pb plot, implying a genetic connection between the rock and the ore, and a derivation of rock-forming materials from a uranium-rich source. According to the continuous-growth model for anomalous lead evolution, two isochrons give the ages of about 764 m.y. and 708 m.y. for ore-forming materials, close to the emplacement age of the granite batholith. This indicates that the ore-forming materials came from the granite batholith. Eight granite samples collected from the mining area were analyzed, but no uranium-lead ages could be worked out because of the progressive destruction of the closed U-Pb isotopic system since granite emplacement, which resulted in the loss of uranium (78%). It can be imagined that in the period of continental weathering the surface water would infiltrate downwards, leaching out large amounts of uranium from the granitic rocks to form infiltration solution. The solution was heated at great depth and then found its way upwards, on which it dissolved constantly U from the rocks to form the uranium deposit