北天山海西中期花岗岩蚀变普遍发育,其深部区域蚀变主要受热液交代与构造动力双重作用控制,传统量化分级方法因其局限性导致等级划分过于粗放、各等级间岩石劣化差异较大。通过薄片鉴定、XRD、XRF的全岩分析,提出了基于精细化修正CaO组分的蚀变分级指数AIG,探讨了花岗岩蚀变机理及其化学分级标准。经对比校验,AIG指数与CIA、CIW的总体相关性相较于次优指数WIG分别提升了65%和160%,蚀变量化灵敏度相对于CIA提升了50%。全岩分析表明,区内蚀变类型主要为低温热液蚀变,构造动力蚀变弱,蚀变产物主要为绿泥石与黏土矿物,分别造成K和Mg的富集与Na和Ca的流失。在此基础上构建了基于AIG指数与岩石蚀变特征的5级蚀变分级标准,从而对蚀变花岗岩进行量化评价。

[Objective] This study aims to address the limitations of traditional quantitative classification methods for altered granites, which often lead to coarse grade divisions and significant discrepancies in rock degradation between adjacent grades. Focusing on the Hercynian mid-term granites in the North Tianshan Mountains region, where deep-seated alteration is jointly controlled by hydrothermal metasomatism and tectonic dynamics, the research seeks to 1) elucidate the alteration mechanisms through mineralogical and geochemical analyses, 2) develop a refined alteration classification index (Alteration Index for Granite, AIG) by correcting CaO components, and 3) establish a five-grade alteration classification standard for engineering geological evaluations. [Methods] Twelve granite samples (including altered and unaltered rocks) from two depths (240 m and 1250 m) in the F01 area were analyzed. The methodologies included: (1) Thin-section identification to determine mineral composition, optical properties, and structural characteristics;(2) X-ray diffraction (XRD) for quantifying secondary minerals (e.g., chlorite, clay minerals) and validating CaO correction;(3) X-ray fluorescence (XRF) to measure major and trace element oxides (Al₂O₃, CaO, Fe₂O₃, MgO, K₂O, SiO₂, Na₂O) following the HJ780-2015 standard;(4) Index formulation to derive the AIG index by refining CaO contributions, excluding interference from apatite and secondary carbonates. This involved a three-step correction: XRF data acquisition, XRD-based secondary mineral quantification, and crystal stoichiometric inversion of primary silicate CaO. [Results] (1) Innovation of the AIG Index: Traditional indices (CIA, CIW, WIP, WIC, WIG) exhibit limitations, such as over-reliance on Al₂O₃ stability (CIA/CIW) or insensitivity to late-stage alteration (WIP). The AIG index addresses these by decoupling CaO contributions from secondary phases (e.g., apatite, calcite) and integrating multi-component stability (TiO₂, Al₂O₃, Fe₂O₃). Compared to the suboptimal WIG index, the AIG demonstrates 65% and 160% improvements in overall correlation with CIA and CIW, respectively. Its quantitative accuracy surpasses CIA by 50%, with reduced dispersion (<5%) in high-secondary-mineral samples.(2) Alteration Mechanisms: in terms of dominant alteration types, Low-temperature hydrothermal alteration (150-300 ℃) prevails, characterized by chloritization (K depletion, Mg enrichment) and argillization (Na/Ca depletion). Tectonic dynamic alteration is weak. In terms of mineralogical evolution,a) Chloritization originates from biotite replacement, accompanied by Fe-rich chlorite (Fe/(Mg+Fe)=0.78-0.82) and secondary epidote/sphene; b) Argillization is driven by plagioclase dissolution, forming montmorillonite-illite mixtures and kaolinite under acidic conditions. Fracture networks enhance fluid permeability, intensifying alteration.Secondary alterations: Sericitization (K⁺ metasomatism), epidotization (Ca²⁺ migration), and clinozoisitization (selective Ca-Al exchange) occur at lower intensities.(3) Five-Grade Classification Standard: Based on AIG thresholds and petrological features, the altered granites are classified into Grade I (AIG>105): Unaltered, blocky structure, negligible secondary minerals; Grade II (AIG between 100 and 105): Slight chloritization, color transition (gray-black to brown-red); Grade III (AIG between 95 and 100): Moderate alteration with illite/sericite, blurred grain boundaries; Grade IV (AIG between 90 and 95): Enhanced argillization, cataclastic structure; Grade V (AIG <90): Intense alteration with montmorillonite, complete loss of primary texture. AIG thresholds expand the CIA’s theoretical range by 1.7 times, enhancing sensitivity to multi-stage alteration processes. [Conclusion] (1) Theoretical Advancements: The AIG index innovatively resolves CaO interference through mineral-phase decoupling, enabling precise quantification of silicate-driven alteration. Its multi-component framework (Na₂O, K₂O, MgO, corrected CaO vs. Al₂O₃+Fe₂O₃+TiO₂) outperforms existing indices in sensitivity and robustness. The study confirms that North Tianshan granites undergo low-temperature hydrothermal alteration dominated by chloritization and argillization, with weak tectonic influence. Elemental migration (Na/Ca loss, K/Mg enrichment) aligns with mineralogical transformations (plagioclase depletion, secondary clay/chlorite growth).(2) Practical Implications: The five-grade AIG classification standard bridges geochemical data with macroscopic engineering properties (e.g., fracturing, water absorption), offering a reliable tool for evaluating altered granite stability in deep engineering projects. Future work should integrate mechanical testing to correlate AIG grades with rock strength and deformability, further refining the model’s predictive capability.(3) Innovative Highlights. AIG Index: a novel geochemical index incorporating XRD-XRF cross-validation to eliminate non-silicate CaO contributions, achieving 65%-160% higher correlation with established indices. Mechanistic Insights: first systematic delineation of chloritization-argillization synergy in Hercynian granites, linking element migration to multi-stage hydrothermal processes.Engineering Relevance: a robust five-grade standard enabling quantitative alteration assessment, critical for mitigating geological hazards in deep underground projects.