<p>The Saxi tungsten deposit, located in the Laojunshan ore district of southeastern Yunnan Province, is a significant W-polymetallic deposit. The origins of tungsten-bearing pegmatite dikes and quartz vein mineralization in the Saxi deposit remain poorly understood. This study employs in situ U–Pb dating of apatite from the altered granite, along with trace element and S–Pb isotopic analysis of arsenopyrite, to investigate the timing, source of ore-forming fluids and the mechanisms of tungsten enrichment. The apatite in the altered granite yields a U–Pb age of 147.0±4.0&#xa0;Ma, indicating magmatic activity during the Early Cretaceous. Three generations of arsenopyrite (Apy) are identified: Apy-1 in the altered granite, Apy-2 in the pegmatite dikes and Apy-3 in the quartz veins. The S/Fe ratios for Apy-1, Apy-2 and Apy-3 range from 0.98 to 1.09, 0.89 to 0.92 and 0.86 to 1.02, respectively (average 0.97), suggesting a magmatic-hydrothermal origin. Sulfur isotope values (<i>δ</i>³⁴S = 4.29‰–8.11‰) indicate that it was likely sourced from deep magmatic-hydrothermal fluids. Lead isotopic compositions of arsenopyrite suggest that the granitic parental magma is derived from the upper crust. These findings point to a magmatic-hydrothermal origin for the vein-type tungsten mineralization, linked to a concealed magmatic-hydrothermal system in the Early Cretaceous.</p>