Thallium (Tl), a strategically critical metal, holds significant application value in high-tech industries. The Nanhua As-Tl deposit in Yunnan Province represents one of the few independent thallium deposits worldwide, with ore bodies occurring as stratiform layers within thin-bedded dolomite of the Upper Jurassic Series. This study systematically investigates the deposit's mineral assemblages, textural characteristics, thallium occurrence modes, and enrichment mechanisms through comprehensive field surveys, petrographic observations, and analytical techni- ques including scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results demonstrate that realgar and orpiment dominate the ore minerals, with hydrothermal mineralization divisible into two stages: Stage Ⅰ comprises realgar-orpiment-pyrrhotite-pyrite-(thallium minerals-chalcopyrite-galena) assemblages, while Stage Ⅱ is characterized by realgar-orpiment-thallium mineral assemblages. Stage Ⅰ realgar (Rlg1) appears as orange-yellow fine veins, contrasting with Stage Ⅱ realgar (Rlg2) exhibiting coarser orange-red veins. EPMA analyses reveal thallium primarily exists in two forms: discrete mineral phases and isomorphism. Beyond discrete thallium minerals, realgar, orpiment, pyrite, and pyrrhotite display anomalously high Tl concentrations, serving as principal carrier minerals. Pyrrhotite contains the highest Tl content (0.47%～1.89%, average 1.06%), showing strong positive correlation with As and negative correlation with Fe, suggesting coupled substitution of Fe by As³⁺+Tl⁺+Tl³⁺?2 Fe²⁺+Fe³⁺. Pyrite exhibits Tl concentrations of 0.04%～0.18% (average 0.10%) with similar elemental correlations, indicating direct Tl⁺ substitution for Fe²⁺ via 2 Tl⁺?□+Fe²⁺ (□ representing vacancy). Realgar contains slightly higher Tl (≤0.59%, average 0.13%) than orpiment (≤0.17%, average 0.07%), both likely incorporating Tl through isomorphism. Integrated studies suggest differential Tl enrichment correlates strongly with Tl-rich ore-forming fluids under low sulfur fugacity and reduced low-temperature conditions, identifying pyrrhotite as a key prospecting indicator for Tl-rich resources.