### Regional Geological Background  
As shown in Figure 1, the South China block (SCB) was formed by the amalgamation of the Yangtze block (YZB) in the northwest and the Cathaysia block (CAB) in the southeast. The Nanling region is situated in the central part of the SCB, with the Xitian ore field located in the central Nanling area--specifically at the western outer contact zone of the dumbbell-shaped Xitian granite pluton's handle section, within the Longshang ore block.  
The study area primarily develops the Upper Devonian Xikuangshan Formation, the Shetianqiao Formation, and the Middle Devonian Qiziqiao Formation. The Xikuangshan Formation can be divided into upper and lower members: the upper member consists of grayish-white, medium-thick-bedded mica-bearing quartz sandstone intercalated with mudstone, approximately 100 m thick; the lower member comprises gray to bluish-gray banded limestone with sandy-argillaceous bands and medium-thick bedding, about 70 m thick, conformally overlying the Shetianqiao Formation. The Shetianqiao Formation has a total thickness of 200 m to 250 m, with its upper part being gray to gray-yellow, medium-thick-bedded mica-bearing quartz sandstone intercalated with sandy mudstone; the middle part consists of gray-white to gray-yellow, medium-fine-grained quartz sandstone with individual beds 40 cm to 80 cm thick and quartz content exceeding 90%, weathering to  
Figure 1: Comprehensive geological map of the Xitian mining district: (**a**) geological sketch map of China with different geological blocks and neighboring countries (modified from [49]), (**b**) schematic geological map of the Xitian W–Sn ore deposits (modified from [50]), and (**c**) detailed schematic geological map of the Xitian W-Sn ore deposit field (modified from [49]). TB: Tarm block, CAB: Cathaysian block, NCB: North China block, SCB: South China block, YZB: Yangtze block.  
a sugary texture; the lower part is gray to gray-white, medium-thick-bedded siltstone intercalated with mudstone, conformably overlying the Qiziqiao Formation. The Qiziqiao Formation is over 200 m thick, with its upper part being gray to gray-white limestone intercalated with calcareous dolomite and marl, weathering yellowish-brown and partially marmorized; the middle part shows interbedded dolomite and limestone with banded limestone and widely developed marbleization; the lower part consists of gray-white to gray thin to medium-thick bedded calcareous dolomite intercalated with mudstone, commonly containing diopside alteration, serving as the main host horizon for tungsten-tin skarn mineralization in the area, conformably overlying the Taiomajian Formation.  
The mining area generally exhibits an approximate homoclinal structure, with strata striking \(\approx\) 325 \({}^{\circ}\)NW and dipping 50 \({}^{\circ}\)SW to 70 \({}^{\circ}\)SW. The fault system develops a NE-trending fault (F1) that crosses a ravine about 20 m west of the main shaft. Another fault (F2) strikes \(\approx\) 335 \({}^{\circ}\)NW and dipping 50 \({}^{\circ}\)NE to 70 \({}^{\circ}\)NE. The fracture zone is 5 m to 20 m wide, showing silicification and griesenization within the zone, where the orebodies in the working area are hosted.  
Approximately 1700 m northeast of the mine, the Xitian pluton is exposed, which was formed during the Yanshanian period and consists of medium-fine- to medium-grained porphyritic biotite granite. The wall-rock alterations in the working area mainly include silicification, skarnization, griesenization, pyritization, and marbleization. The structural complexity of the mining area is classified as simple type. Influenced by the Indosinian-Yanshanian tectonic movements, the wall rocks exhibit well-developed joints and fractures, providing favorable ore-hosting spaces for vein-type W-Sn mineralization.