The Phapon orogenic gold deposit, located in the Luang Prabang (Laos)–Loei (Thailand) metallogenetic belt along the northwestern margin of the Indochina Block, is a unique vein-type deposit with calcite as the main gold-bearing mineral. Primary sulfides in gold-bearing veins in this deposit are strongly oxidized and cannot be dated. Thus, the precise age of gold mineralization has been a matter of debate and its tectonic setting remains unresolved. Previous studies confirmed that the ore-forming fluids belong to a NaCl-H₂O-CO₂ system, but their evolution process is unclear. Based on detailed field investigation, and microscopic and CL studies, three ore-forming stages were recognized as the pre-ore calcite(Cal-1) ± quartz ± pyrite veinlets, main-ore calcite(Cal-2)-siderite-hematite-realgar ± orpiment-gold vein or veinlets, and post-ore calcite(Cal-3) vein or veinlets. Three calcite generations show comparable HREE flat patterns and Eu and Ce negative anomalies with the carbonate wallrock, suggesting the potential inheritance of some ore-forming materials. The Cal-2 has the highest REE, Mn, and Fe concentrations and the most obvious LREE-enrichment patterns than Cal-1 and Cal-3, indicating that the ore-forming fluids are more acidic and have higher temperature and more intense fluid-rock interaction at the deposit trap. The Cal-2 is intergrown with gold and was dated by LA-SF-ICP-MS and yielded a lower intercept U-Pb age of 221.6 ± 7.6 Ma, which is interpreted as the Au mineralization age for the Phapon deposit. This new age indicates that the orogenic gold mineralization event continued in the Late Triassic, after the late Permian–middle Triassic epithermal and porphyry-skarn Au mineralization events along the northwestern margin of the Indochina Block, and corresponds to the collision between the Sibumasu Terrane and the Indochina Block. A Miocene hydrothermal activity was constrained at 11.8 ± 2.4 Ma by Cal-3.