SHRIMP zircon U-Pb dating of the ore-forming monzogranite from the Phalek iron deposit, Vientiane, Laos and its geological implications

The Phalek iron deposit is a large-sized skarn-type magnetite deposit in the central part of the Cu-Au-Fe polymetallic structural-magmatic metallogenic belt in the Trung Son zone at the border area of Laos and Vietnam. Judged from the geology and occurrence of ore bodies, the mineralization of the iron deposit is dominantly controlled by the magmatism of the monzogranite in the study area. The present paper focuses on the petrography and SHRIMP zircon U-Pb dating of the ore-bearing monzogranite. The integration of internal structures, cathodoluminescence images and Th/U ratios of the measured zircons shows that the zircons from the monzogranite are of magmatic origin. As indicated by the U-Th-Pb isotopic analysis, the ages of the ore-bearing monzogranite masses are dated at 280.3±2.9 Ma (MSWD=0.70). The regional tectonic evolution and dynamic mechanisms for the formation of the structural-magmatic zone have also disclosed that the Phalek iron deposit resulted from the arc magmatism during the subduction of the Early Permian oceanic plate, and are assigned to the same active epicontinental metallogenic system as the late Hercynian large-sized Cu-Au-Fe deposits in the Trung Son metallogenic belt.