## 4 Discussion  
### Genetic Mechanism of Alteration  
The K\({}^{+}\) content of some altered rock samples obtained in the study area is markedly lower than that of fresh noneroded rocks (Figure 4a,b); conversely, the MgO content of almost all of the altered rock samples is higher than that of fresh noneroded rocks. Moreover, the MgO content shows a marked trend of increase with increasing LOI (Figure 4c,d), indicating that the chloritization phenomenon is more widely distributed and is prevalent in most rock samples. In the clarification stage, K\({}^{+}\)-rich acidic hydrothermal fluids extracted Na+ and Ca\({}^{2+}\) ions from the plagioclase feldspars, which were replaced with K\({}^{+}\) ions present in the fluids to form sercite and clay minerals such as kaolinite, montmorillonite and illite [27; 28; 48; 49]. Electron microprobe data show that the K content of the plagioclase in the samples (0.00-0.34%) is generally low compared with that of typical fresh plagioclase, which is indicative of the more pronounced clay alteration of the plagioclase in the obtained samples. The low K\({}^{+}\) content in the potassium feldspar is also indicative of claying alteration. These changes in elemental content indicate that magmatic activity provided large amounts of hydrothermal fluid for alteration, leading to chloritization of smectite and clayification of feldspar minerals, which are products of medium-low-temperature hydrothermal alteration [27; 28; 48; 49]. Different from previous studies, the content of CaO was found to increase with increase in the LOI after a small initial decrease (Figure 4e,f). The main reason for this phenomenon is that calcite is common within the samples, as shown by X-ray diffraction [42]. Furthermore, with increase in the LOI, the degree of rock alteration deepens, and rock porosity increases. Carbonate is a soluble mineral, and water-encapsulated carbonate continuously invades into \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Sample** & **Rock** & **SiO\({}_{2}\)** & **TiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **TF\({}_{2}\)O\({}_{3}\)** & MgO & CaO & **Na\({}_{2}\)O** & **K\({}_{2}\)O** & **LOI** & **\(\delta^{18}\)O** \\
**Number** & **Character** & **Mass\%** & **Mass\%** & **Mass\%** & **Mass\%** & **Mass\%** & **Mass\%** & **Mass\%** & **Mass\%** & **Mass\%** & **Mass\%** & **\%** \\ \hline
14 & & 75.36 & 0.19 & 11.56 & 1.54 & 0.34 & 1.41 & 3.37 & 4.34 & 1.29 & 5.7 \\
15 & & 73.03 & 0.41 & 12.99 & 2.73 & 0.66 & 1.50 & 3.23 & 4.07 & 0.86 & 7.9 \\
16 & Bio adamellite & 73.17 & 0.41 & 13.05 & 2.66 & 0.66 & 1.69 & 3.27 & 4.19 & 0.75 & 8.6 \\
17 & & 72.83 & 0.38 & 13.09 & 2.56 & 1.34 & 1.00 & 1.78 & 4.36 & 2.52 & 8.7 \\
18 & & 69.81 & 0.55 & 13.36 & 3.46 & 1.01 & 2.07 & 3.43 & 4.09 & 1.76 & 7.7 \\
19 & Fresh-Bio & & 71.71 & 0.44 & 13.29 & 2.75 & 0.98 & 2.29 & 3.39 & 3.93 & 0.62 & 10.8 \\
20 & & 54.81 & 0.52 & 12.47 & 3.21 & 1.46 & 5.75 & 3.93 & 2.94 & 9.23 & 1.9 \\
21 & & 57.94 & 0.63 & 16.77 & 3.87 & 1.75 & 4.84 & 5.20 & 3.90 & 4.77 & 1.9 \\
22 & & 67.27 & 0.59 & 14.42 & 3.52 & 1.38 & 2.01 & 3.17 & 4.06 & 2.76 & 3.6 \\
23 & & 68.16 & 0.49 & 12.70 & 3.10 & 1.24 & 3.69 & 2.73 & 3.30 & 4.09 & 3 \\
24 & Bio & 68.45 & 0.65 & 10.64 & 4.02 & 1.29 & 4.94 & 2.70 & 2.44 & 4.11 & 8.4 \\
25 & grandodiorite & 71.29 & 0.50 & 12.90 & 3.13 & 1.30 & 2.02 & 3.31 & 3.50 & 1.82 & 6.5 \\
26 & & 59.03 & 1.67 & 13.01 & 9.98 & 2.86 & 3.51 & 2.70 & 2.39 & 3.84 & 8.3 \\
27 & & 68.74 & 0.71 & 13.86 & 5.12 & 1.45 & 2.89 & 3.00 & 3.03 & 0.53 & 13.3 \\
28 & & 71.19 & 0.51 & 13.33 & 3.36 & 1.13 & 2.50 & 3.79 & 2.51 & 1.24 & 9.6 \\
29 & & 70.52 & 0.55 & 13.00 & 3.55 & 1.29 & 2.50 & 3.33 & 3.46 & 1.19 & 10.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cont. the altered rock via cracks in surrounding rocks. As the porosity increases, more Ca2+ is invaded. Clayification is the process of Ca2+ precipitation [50], and the CaO content first decreases and then increases with increase in the LOI.