## 4 Geochemical Characteristics of Overburden Soils
### Overburden Soil Major Elements  
The results of soil major elements testing in the Mahuagou mining area, along with descriptive statistics, are presented in Tables 2 and 3. Among the 18 samples, silicon (Si) exhibited a maximum value of 69.8%, a minimum value of 57.7%, and a mean value of 64.8%. Aluminum (Al) showed a maximum value of 19.2%, a minimum value of 12.9%, and a mean value of 15.7%. Both elements demonstrated a coefficient of variation (C\({}_{\text{V}}\)) below 15%, indicating relatively stable content within the samples. On the other hand, calcium (Ca) and magnesium (Mg) elements displayed variation coefficients exceeding 50%, implying significant content disparities within individual samples. Notably, samples FXS-3 and FXS-4 exhibited notably distinct calcium content from other samples, possibly attributed to mineralogical associations with abundant calcium and magnesium elements in the soil. CIA (Chemical Index of Alteration) [60] and CIW (Chemical Index of Weathering) are two commonly used weathering indices. The CIA is based on the research of weathering of magmatic rocks with relatively uniform chemical composition, and its application premise is that the chemical composition of weathered parent rocks is relatively uniform [61; 62]. Obviously, in this study, the parent rocks of the soil samples are not the same, and K\({}_{2}\)O may  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & \multicolumn{8}{c}{**Element (\%)**} \\ \cline{2-9}
**Number** & **SiO\({}_{\text{2}}\)** & **Al\({}_{\text{2}}\)O\({}_{\text{3}}\)** & **MgO** & **CaO** & **Na\({}_{\text{2}}\)O** & **K\({}_{\text{2}}\)O** & **MnO** & **TiO\({}_{\text{2}}\)** \\ \hline FXS-1 & 57.90 & 18.40 & 2.23 & 1.51 & 1.95 & 2.13 & 0.13 & 0.60 \\ FXS-2 & 66.60 & 16.30 & 0.45 & 1.60 & 3.42 & 1.94 & 0.05 & 0.24 \\ FXS-3 & 59.20 & 16.30 & 1.70 & 4.60 & 0.27 & 2.94 & 0.04 & 0.51 \\ FXS-4 & 64.20 & 17.10 & 0.67 & 3.74 & 3.13 & 2.90 & 0.03 & 0.28 \\ FXS-5 & 65.60 & 15.90 & 0.88 & 0.38 & 3.21 & 2.08 & 0.06 & 0.41 \\ FXS-6 & 67.20 & 14.70 & 0.29 & 0.11 & 2.49 & 2.59 & 0.03 & 0.26 \\ FXS-7 & 65.30 & 15.90 & 0.55 & 1.17 & 4.46 & 1.42 & 0.06 & 0.30 \\ FXS-8 & 57.70 & 19.20 & 1.03 & 0.93 & 1.81 & 1.81 & 0.07 & 0.49 \\ FXS-9 & 64.90 & 15.70 & 0.54 & 0.70 & 2.10 & 1.93 & 0.04 & 0.32 \\ FXS-10 & 67.50 & 14.30 & 0.52 & 1.14 & 2.50 & 1.94 & 0.04 & 0.33 \\ FXS-11 & 60.10 & 16.70 & 1.65 & 1.65 & 2.29 & 1.93 & 0.10 & 0.79 \\ KZ-1 & 69.80 & 14.60 & 0.77 & 0.14 & 1.42 & 3.05 & 0.06 & 0.27 \\ MHG-1 &  
be active during metamorphism; biotite alteration can be affected by K\({}_{2}\)O concentrations, which can be rich in illite and smectite clay minerals. Therefore, it is appropriate to use the CIW to evaluate its weathering. The calculation Formula (2) for CIW is provided below:  
\[\text{CIW}=100\%\times\text{Al}_{2}\text{O}_{3}/(\text{Al}_{2}\text{O}_{3}+ \text{CaO}+\text{Na}_{2}\text{O}) \tag{2}\]  
The calculated results are depicted in Figure 4. In general, CIW values between 50% and 60% indicate that the samples are fresh and have not undergone significant chemical weathering. If the CIW value of a sample exceeds 70%, it signifies intense chemical weathering [63]. The samples from the Mahuagou mining area consistently exhibit CIW values above 70%, indicating a pronounced history of substantial weathering processes. To better illustrate the compositional characteristics of major elements, the samples were categorized based on their sampling point characteristics into five sections: fault material, above the faulted structures, around the MHG ore-body, above the trench and borehole, and peripheral areas. Utilizing the (Al/Si) \(\times\) 100% ratio in conjunction with Ca (%) for plotting (refer to Figure 5), it was observed that the majority of samples exhibit a relatively stable ratio of Al to Si, forming a linear distribution parallel to the \(X\)-axis. However, some individual samples display elevated Al/Si ratios. Fault materials (fault mud and fault cataclastic rock) exhibit higher Ca content.