Timing of ore mineralization using ore mineralogy and U-Pb dating, Iron Oxide Copper Gold Sin Quyen deposit, North Vietnam

Magnetite, pyrite, pyrrhotite, chalcopyrite and sphalerite are the major minerals identified in the deposit, while ilmenite, marcasite, tennantite, cubanite, arsenopyrite, galena, allanite, chevkinite, apatite, Bi-native, bismuthinite, electrum, native gold, and tellurides are the minor ones in the Iron Oxide Copper Gold (IOCG) Sin Quyen deposit. The REEs are hosted mostly by allanite, and the minor minerals by chevkinite, monazite, apatite and uraninite. Based on chemical analyses and Raman spectroscopy, two varieties of allanite have been documented: (1) with lower total REE contents of 13-19 wt.%, and (2) with higher contents of 20-23 wt.%. Uraninite from copper-iron massive ores is inhomogeneous in both optical properties and chemical composition. The concentrations of uranium and total rare earth element oxides (REOs) in the paragenetically earlier uraninite are 84.55-85.96% and 1.9-8.0% on average, respectively, whereas in paragenetically later uraninite, the U and ΣREE2O3 concentrations are 96.2-96.7% and 1.3-2.7% on average respectively. The thorium concentration in both the early and late uraninites is very low (0.21-0.22% and 0.2 % on average). These are the highest REE concentrations as compared with the known concentrations of these elements in uraninite. Electron microprobe “chemical” dating of the uraninite yielded an age of 500 ±33 Ma (n = 35) for the paragenetically early uraninite, and an age of 73 ±15 Ma (n = 6) for the paragenetically later ones. The minerals of the older age, which is interpreted as the primary ore mineralization stage, correspond in age with a range of deposits along the East Gondwana margin. The measured δ34S of sulphide minerals from -2.78 to +8.65‰ suggests hydrothermal origin of fluid that was responsible for transportation and crystallization.