## 4 Geochemical Characteristics of Overburden Soils  
### Overburden Soil Mineral Composition  
Utilizing X-ray Diffraction (XRD) analysis, a semi-quantitative assessment of the mineral composition of representative soil samples from the Mahuagou mining area was conducted, as illustrated in Figure 3. The X-ray diffraction peak intensities are measured using integrated intensities after background subtraction. Relevant data is obtained from the X-ray diffraction spectrum of the sample and then compared to standard X-ray diffraction data for minerals to determine the mineral species. Common mineral X-ray diffraction data can be found in SY/T 5163-2018-Appendix E. Formula (1) is used to calculate the total content of clay minerals and various non-clay minerals:  
\[X_{i}=[\frac{I_{i}}{K_{i}}/(\sum\frac{I_{i}}{K_{i}})]\times 100\%, \tag{1}\]  
\(X_{i}\) represents the percentage content of mineral \(i\), expressed as a percentage; \(I_{i}\) represents the integrated peak intensity of the selected diffraction peak of mineral \(i\); and \(K_{i}\) represents the reference intensity of mineral \(i\). Broadly, soil samples from the FXS and MHG profiles exhibit two distinct mineral assemblages. In the FXS profile, the mineral composition of the soil is primarily comprised of clay minerals, such as illite, chlorite, and kaolinite, as well as primary minerals like quartz and sodium feldspar (e.g., FXS-5 and FXS-6). Illite is an intermediate transitional mineral between montmorillonite and mica, often formed through the alteration of potassium feldspar and mica or the replacement of montmorillonite [51; 52; 53]. Kaolinite predominantly forms during the weathering of silicate minerals like feldspar and common pyroxenes [54]. Chlorite is a secondary mineral that develops during the initial stages of soil formation, arising from the alteration of ferromagnesian minerals (e.g., biotite, hornblende, pyroxenes) in granite or other igneous rocks. Field observations indicate that the rock types exposed in the FXS profile consist mainly of moyite, monzonitic granite, granite, and dioritic rocks, indicative of intermediate acidic intrusive rocks. Consequently, the formation of illite and kaolinite in the soil is connected to the weathering of minerals like mica and alkali feldspar in intermediate acidic parent rocks. At the same time, the development of chlorite often results from the weathering of dark-colored minerals in the parent rock. FXS-3 and FXS-4 exhibit a characteristic mineral composition of fault gouge. The presence of montmorillonite and a small amount of calcite in the soil leads to a notably higher Ca element content compared to other soils. The formation of this mineral assemblage is attributed to the grinding of near-surface rocks by faults under low-temperature conditions, coupled with late-stage low-temperature hydrothermal alteration and metamorphism [55; 56], resulting in the mixture of unconsolidated, powdery clay minerals (illite, chlorite, and montmorillonite) with fine particles from adjacent rocks (quartz and calcite) [57]. In the MHG profile, the clay mineral composition in the soil is relatively simple, primarily consisting of montmorillonite, with minimal presence of minerals like illite and chlorite. The content of biotite and minor plagioclase is significantly higher in the soil compared to the FXS profile. Diabase dikes (\(\beta\upmu\)) and other mafic dikes are widely exposed in MHG, which is the main factor affecting the local soil composition. Consequently, the formation of montmorillonite in the soil of MHG profile is closely related to the weathering of minerals from basic rocks in an alkaline environment [58]. The soil mineral composition adjacent to the KZY trench resembles that of the FXS profile, with illite being the primary clay mineral and a relatively lower content of quartz and sodium feldspar. Considering the mineral composition of soils in the Mahuagou mining area, it can be inferred that the soils at FXS, MHG, and KZY locations are all products of the in situ weathering of parent rock.