## 4 The Fixed-Value Method of Sn Geochemical Mapping  
Although traditional methods for geochemical maps have played an important role in geochemical exploration, traditional methods' dependence on the amount of data exposes their limitations. In theory, these limitations lead to incomparable results for areas with different amounts of data. We propose a fixed-value method to overcome the limitations of traditional Sn geochemical maps after the fixed-value method proposed by [PERSON] et al. [14] and the 19 levels illustrated in traditional geochemical maps. In the fixed-value method of Sn geochemical mapping, Sn concentrations are divided into 19 levels on 18 fixed values that range from 1 ug/g (corresponding to the detection limit) to 1000 ug/g (corresponding to the cutoff grade of Sn in hard rocks). Table 3 lists 18 fixed values and 19 colors. The first value is 1 ug/g, which represents the detection limits of the RGNR [6; 10] and NMPRGS projects [7]. However, the Sn detection limit in many laboratories is lower than 1 ug/g. The second value is 1.3 ug/g, which is the 0.5% cumulative frequency value of surface soils in China [25]. The third value was set to 1.8 ug/g after 1.7 ug/g, corresponding to the 2.5% cumulative frequency value of surface soils in China [25], and 1.96 ug/g, corresponding to the 25% cumulative frequency value of stream sediments in China [10]. The fourth value was 2.7 ug/g, which is the 25% cumulative frequency value of surface soils in China [25] and the 50% cumulative frequency value of stream sediments in China [10]. The fifth value  
Figure 5: Sn geochemical map in the Gejiu area contoured using the Avg\(\pm\)k\(\ast\)Std method. Color information is shown in Table 2. Geological legends are the same as those illustrated in Figure 1.  
was set to 3.4 \(\mu\)g/g after 3.2 \(\mu\)g/g, which is the 50% cumulative frequency value of surface soils in China [25], and 3.7 \(\mu\)g/g, which is the 75% cumulative frequency value of stream sediments in China [10]. The sixth value was set to 4.3 \(\mu\)g/g after 4.1 \(\mu\)g/g, corresponding to the 75% cumulative frequency value of surface soils in China [25], and 4.5 \(\mu\)g/g, corresponding to the 85% cumulative frequency value of stream sediments in China [10]. The eighth value is 7.9 \(\mu\)g/g, which is the 95% cumulative frequency value of stream sediments in China [10]. Based on the eighth and sixth values, the seventh value is interpolated as 6.0 \(\mu\)g/g. The 10 th value is 13 \(\mu\)g/g, which is the 97.5% cumulative frequency value of surface soils in China [25]. Based on the 10 th and 8 th values, the ninth value was interpolated to 10 \(\mu\)g/g. The 11 th value was 17 \(\mu\)g/g, which is the 98.8% cumulative frequency value of stream sediments in China [10]. The 12 th value was 28 \(\mu\)g/g, which is the 99.5% cumulative frequency value of surface soils in China [25]. The 15 th value was 200 \(\mu\)g/g, which is the cutoff grade of the Sn placer [26]. Based on the 15 th and 12 th values, the 13 th and 14 th values were interpolated to 50 \(\mu\)g/g and 100 \(\mu\)g/g, respectively. The 16 th value is 400 \(\mu\)g/g which is the minimum industrial tenor [26]. The 18 th value was 1000 \(\mu\)g/g, which corresponds to Sn's cutoff grade in hard rock [27]. Based on the 18 th and 16 th values, the 17 th value was interpolated to 600 \(\mu\)g/g. Based on the above 18 fixed values, Sn concentrations can be classified into 19 levels based on the Sn geochemical map. The first level out of 19 levels corresponds to Sn concentration, ranging from the minimum to the first value (or 1 \(\mu\)g/g). The second level out of 19 levels corresponds to Sn concentration, ranging from the first value (or 1 \(\mu\)g/g) to the second value (or 1.3 \(\mu\)g/g). The last level out of 19 levels corresponds to Sn concentration, ranging from the 18 th value (or 1000 \(\mu\)g/g) to the maximum. To better explain these levels, the 19 levels were mapped in six color tones.