The Nam Xan Gold Deposit, Laos: Evidence for a Distal Intrusion-Related Gold System in the Truong Son Fold Belt

The Nam Xan gold deposit is located in the central Truong Son Fold Belt of Laos. It is a newly identified distal intrusion-related gold system (IRGS) in a continental arc setting. This study uses whole-rock geochemistry, Pb and S isotope systematics, and mineral-scale analyses to trace magmatic evolution and ore-forming processes. Whole-rock data indicate that the associated intrusive suite is a calc-alkaline volcanic-arc granite (VAG) series, derived from a subduction-modified mantle source with notable crustal contributions. Pb isotopes reveal mixing arrays rather than true isochrons. Monte Carlo modeling shows binary mantle–crust mixing for igneous rocks and ternary mixing with an additional radiogenic component in ore samples, indicating enhanced fluid–rock interaction during mineralization. Sulfur isotope data show a shift from magmatic sulfur (δ34S ≈ −5‰) in early skarn-stage pyrite to heavier values (δ34S ≈ +6‰) in gold-bearing stages, reflecting fluid evolution driven by cooling and redox changes. Mineral chemistry data demonstrate that gold is present both as invisible gold within arsenian pyrite and as free gold in late-stage fractures. Strong correlations between Au and As, along with elevated Co/Ni ratios and enrichments in Bi, W, and F, collectively support a magmatic-hydrothermal origin. These findings define a three-stage mineralization process: an initial phase involving high-temperature magmatic fluids, a main stage characterized by sulfidation and gold deposition, and a final stage marked by polymetallic overprinting. The Nam Xan deposit is therefore interpreted as the distal manifestation of a Permian arc-related magmatic system in which magmatic fluids migrated along structural conduits and precipitated gold through interaction with carbonate host rocks. The identification of these intrusions in the distal IRGS at Nam Xan informs regional exploration models in the Truong Son Fold Belt, demonstrating the potential of carbonate platforms near Permian intrusions for future mineral exploration.