## 5 Conclusions  
In this study, the altered granite in an engineering area at the northern foot of the Tianshan Mountains (China) was taken as the research object. Following field investigation and sampling, the geochemical characteristics and alteration mechanism of the granite in this area were systematically studied via a series of laboratory tests, which included thin section analysis, X-ray diffraction, major element analysis, and electron probe and oxygen isotope tests. The main conclusions derived are as follows.  
The lithology of the granite in the study area is biotite granodiorite and biotite mon-zonitatic granite, and the K\({}^{+}\) content in the plagioclase and K-feldspar are generally low. The K\({}^{+}\) and T\({}^{4+}\) contents in the chlorite are markedly lower than those in the biotite, and the MgO content is higher than that in fresh unaltered rocks. The MgO content increases notably with increase in the LOI. These phenomena indicate that the granite in this area has been subjected to low-temperature hydrothermal alteration over a long period. The alteration types are mainly chlorite of biotite and clay of feldspar minerals. The abnormal change in CaO content with the LOI (initial slight reduction and then gradual increase) is mainly due to the widespread existence of calcite and the continuous intrusion of carbonate with groundwater into the altered rock along cracks in the surrounding rocks.  
Most of the altered granodiorites in the study area have low \(\delta^{18}\)O values. The \(\delta^{18}\)O value decreases with increase in the LOI, and the low-\(\delta^{18}\)O-value samples mostly appear in the water conveyance tunnel with large buried depth. With consideration of the geological history of the northern foot of the Tianshan Mountains, it can be inferred that the genetic mechanism was mainly water-rock reaction involving glacial meltwater, groundwater, atmospheric precipitation, and magmatic hydrothermal fluid. The irregular low-\(\delta^{18}\)O-value phenomenon in the monzonitic granite indicates that the monzonitic granite might have formed from the early low-\(\delta^{18}\)O-value granodiorite, which was remelted, assimilated, and rebalanced following reintroduction by magma.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app132212486/s1](https://www.mdpi.com/article/10.3390/app132212486/s1)]([https://www.mdpi.com/article/10.3390/app132212486/s1](https://www.mdpi.com/article/10.3390/app132212486/s1)), Table S1: The data of the electron probe micro-analyzer.  
Methodology, M.R.; Investigation, T.W.; Data curation, J.W.; Writing--original draft, S.S.; Writing--review & editing, J.D. All authors have read and agreed to the published version of the manuscript.  
This work was sponsored by the National Key Research and Development Project of China (Grant No.: 2019 YFC1509704).  
Not applicable.  
Figure 7: The depth–\(\delta^{18}\)O relationship.  
Data Availability Statement: The data presented in this study are available in Supplementary Materials.  
This work was sponsored by the National Key Research and Development Project of China (Grant No.: 2019 YFC1509704). National Natural Science Foundation of China (Grant Nos.: U1704243, 41741019, 41977249, and 42090052), and Henan Province Science and Technology Research Project (Grant No.: 192102310006).  
Author Tong Wu was employed by the company Beifang Investigation, Design & Research Co. Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.