## 2 Materials  
The Gejiu area in Southwest China was selected as the study area to illustrate the geochemical mapping method for Sn. The Gejiu area is located in Yunnan province, China (Figure 1a), ranging from 102\({}^{\circ}\)48\({}^{\prime}\) E to 104\({}^{\circ}\)06\({}^{\prime}\) E, and 23\({}^{\circ}\)12\({}^{\prime}\) N to 23\({}^{\circ}\)30\({}^{\prime}\) N, with an area of ca. 4224 km\({}^{2}\)[15]. The strata and magmatic intrusions of the Gejiu area are briefly described in Figure 1b after [PERSON] et al. [15]. Figure 1: Location of the Gejiu area in China (**a**) and its geological map with tin deposits (**b**) modified after [PERSON] et al. [15]. Notes for (**b**): (1) Quaternary: sand, sandy clay, clay sandwiched gravel layer; (2) Miocene: sandstone, mudstone; (3) Triassic: shale, dolomite, upper sandstone, calcareous mudstone or phyllite; (4) Upper Triassic: conglomerate, sandstone, slate, intermediate acid volcanic rocks; (5) Middle–Upper Triassic: limestone, dolomitic limestone, siltstone, sandstone, mudstone intercalated with conglomerate, coal; (6) Lower–Middle Triassic: limestone, dolomite, limestone; (7) Upper Permian:  
mudstone, sandstone, shale, limestone and basalt; (8) Middle-Upper Permian: limestone, mudstone, siliceous rock, Emeishan basalt, sandstone, shale; (9) Middle Permian: gravel limestone, micrite limestone, chert strip limestone, mudstone, siliceous rocks; (10) Carboniferous-Permanian: limestone, lower basalt, tuff; (11) Carboniferous: limestone, sandstone, dolomite; (12) Devonian-Carboniferous: mudstone, sandstone, limestone, argillaceous limestone, and middle and upper sandstone, shale; (13) Devonian: lower sandstone with shale, upper limestone, bioclastic limestone, dolomitic limestone, shale; (14) Upper Devonian: limestone, siliceous limestone, lentil-like limestone, argillaceous strip limestone, dolomite, siliceous mudstone; (15) Middle Devonian: sandstone, limestone; (16) Lower-Middle Devonian: lower mudstone, upper thin limestone, siliceous rock; (17) Lower Devonian: lower argillaceous siltstone, sandstone, mudstone, upper marl, limestone, dolomitic limestone, biogenic limestone; (18) Lower-Middle Ordovician: sandstone, limestone, siltstone; (19) Lower Ordovician: limestone, sandstone, shale; (20) Cambrian-Ordovician: dolomite, limestone, sandstone; (21) Cambrian: shale, limestone, dolomite; (22) Middle Cambrian: argillaceous strip limestone, dolomite; (23) Lower-Middle Cambrian: shale, upper limestone, dolomite; (24) Paleoproterozoic: schist, marble, granulite, marble, graphite schist; (25) Monzognarie; (26) Monzonite; (27) Svenite; (28) Gabbro; (29) lithological boundaries; (30) faults; (31) Sn deposit; (32) main residential place. The main metal resource in the Gejuu area is tin, and super-large Sn deposit clusters are mainly located in the western part. In the deposit cluster, seven Sn deposits were exploited, including Malage [16], Songshujiao [17], Gaosong [18], Laochang [19], Kafang [20], Niushipo [21], and Zhujingpo Sn-polymeretallic deposits. In addition, two super-large Sn deposits in Bainiuchang [22] in the northeast and Guanfang [23] in the southeast were also exploited. In this study, the Sn concentration data of stream sediments in the Gejuu area were retrieved from the RGNR project database, which has covered >7 million square kilometers of Chinese land surface since its implementation in 1979 [6; 10]. In the RGNR project, each composite sample representing 4 km\({}^{2}\) and corresponding to a scale of 1:20,000 was analyzed for 39 major and trace elements including Sn. The Gejuu area covers approximately 4224 km\({}^{2}\) with 132 km from west to east and 32 km from north to south. A total of 1170 analytical records were obtained from the RGNR project database to contour the geochemical Sn map. In the RGNR project, Sn was determined by the AC horizontal electrode emission spectrographic method with a detection limit of 1 ug/g.