Alteration Mechanism of Variscan Granite in a Project Area at the Northern Foot of the Tianshan Mountains, China

Long-term erosion of granite—a type of hard rock—by hydrothermal fluids and tectonic movement can lead to a fragmentation of the internal structure of the original rock, transfer/replacement of mineral elements, and alteration of the rock’s basic properties. Such changes can be problematic for the construction of water conservancy, hydropower, and road projects. This study adopted the altered Variscan granite in a water diversion project area at the northern foot of the Tianshan Mountains (China) as the research object, and explored the alteration mechanism using thin section identification, X-ray diffraction, major element analysis, and electron probe and oxygen isotope tests. Results showed that the lithology of the granite in the study area is mainly biotite granodiorite and biotite monzonitic granite. Reductions in both the K+ content in plagioclase and the K+ and Ti4+ contents in chlorite indicate that the alteration types within the study area are mainly clayization of feldspar minerals and chloritization of biotite. Biotite granodiorite and biotite monzonitic granite both have low δ18O values. The δ18O value of biotite granodiorite decreases with increase in the Loss on Ignition. The low-δ18O-value granodiorite due to an alteration by hydrothermal fluids transformed from glacier meltwater, groundwater, atmospheric precipitation, and magmatic water; whereas the monzonitic granite might be formed by the reinvasion of low-δ18O-value granodiorite formed in the early stage, which is remelted, assimilated, and rebalanced. The research findings provide a reference for similar research on altered granite at the northern foot of the Tianshan Mountains, and also lay a foundation for subsequent research on its physical/mechanical properties and engineering characteristics.