## 4 The Fixed-Value Method of Sn Geochemical Mapping
The first to fifth levels were low background areas in blue tones corresponding to Sn concentrations ranging from the minimum value of 3.4 \(\mu\)g/g. The sixth and ninth levels were high background areas in yellow tones corresponding to concentrations less than 10 \(\mu\)g/g. The 10 th to 12 th levels are low anomaly areas in pink tones, with less than 28 \(\mu\)g/g. The 13 th to 15 th levels are high anomaly areas in red tones, with less than 200 \(\mu\)g/g. The 16 th to 18 th levels were in gray tones, with less than 1000 \(\mu\)g/g corresponding to concentrations of Sn in placers (or Sn mineralization in placer types). The 19 th level was in black with Sn concentrations ranging up to the cutoff grade of Sn in hard rock (or Sn mineralization in hard rock types). The above fixed-value method for Sn geochemical mapping is mainly suggested for Sn concentrations in the RGNR and NMPRGS projects in China [6,7] and cutoff grades of Sn placers and hard rock deposits [27]. The Sn concentrations in this method range from less than detection limits to larger than the cutoff grades. Therefore, the new method is universally applicable to contouring Sn geochemical maps of geological materials. If Sn concentrations in a case study are within a narrow range, its geochemical map may illustrate \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c c} \hline \hline
**Level** & **No.** & **0** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** & **11** & **12** & **13** & **14** & **15** & **16** & **17** & **18** & **19** \\ \hline Sn & min & 1 & 1.3 & 1.8 & 2.7 & 3.4 & 4.3 & 6.0 & 7.9 & 10 & 13 & 17 & 28 & 50 & 100 & 200 & 400 & 600 & 1000 & max \\ lgSn & - & 0 & 0.114 & 0.255 & 0.431 & 0.531 & 0.633 & 0.778 & 0.898 & 1.0 & 1.114 & 1.23 & 1.447 & 1.699 & 2.0 & 2.301 & 2.602 & 2.778 & 3.0 & - \\ AlgSn & - & - & 0.114 & 0.141 & 0.176 & 0.100 & 0.102 & 0.145 & 0.119 & 0.102 & 0.114 & 0.117 & 0.217 & 0.252 & 0.301 & 0.301 & 0.301 & 0.176 & 0.222 & - \\ Color & & & & & & & & & & & & & & & & & & & & & & \\ R & 0 & 9 & 28 & 54 & 128 & 255 & 255 & 255 & 255 & 255 & 255 & 255 & 254 & 249 & 216 & 191 & 135 & 100 & 0 \\ G & 0 & 46 & 140 & 224 & 255 & 230 & 223 & 211 & 198 & 229 & 204 & 153 & 92 & 57 & 35 & 191 & 135 & 100 & 0 \\ B & 170 & 216 & 248 & 255 & 224 & 153 & 127 & 76 & 25 & 255 & 255 & 255 & 18 & 9 & 0 & 191 & 135 & 100 & 0 \\ \hline \hline \end{tabular} Note: The unit is \(\mu\)g/g of Sn concentrations.  
\end{table} 
Table 3: The information of Sn geochemical map on the fixed-value method. only some of the 19 levels, such as the Cr geochemical map presented by [PERSON] et al. [14]. In this study of the Gejiu area, Sn concentrations ranged from 0.2 ug/g to 7760 ug/g. Therefore, all 19 levels are represented on the Sn geochemical map of this area. According to the fixed-value method, Sn concentrations of 1170 stream sediments were used to contour a Sn geochemical map of the Gejiu area, as illustrated in Figure 6.  
Figure 6 shows the gray and black colors located in the Gejiu super-large Sn deposit cluster in the west. By contrast, the Bainiuchang deposit area in the northeast is in red tones and the Guanfang-Bozhushan area in the east is in pink tones. In other words, Sn deposits in the study area are located in areas with black, gray, red, or pink tones, which correspond to the Sn ore, placer, high-anomaly, and low-anomaly areas. The color tones of the fixed-level method indicate Sn deposits more clearly than traditional methods.