Hydrothermal chlorite geochemistry and formation mechanisms: Linking alterations to uranium mineralization and exploration in the Daliang deposit of South China

Chlorite is the dominant hydrothermal mineral in the Daliang deposit, a granite-related hydrothermal uranium deposit in South China. Despite the prevalence of chlorite, its characteristics and link to uranium mineralization have largely been neglected. Through comprehensive petrographic and geochemical analyses, we identified five distinct chlorite types (Chl-1 to Chl-5) and traced their formation mechanisms (dissolution−precipitation, layer-by-layer replacement, and dissolution−transport−precipitation) and crystallization temperatures (166−303 °C). Our analyses reveal that the chlorite compositions in the Daliang deposit effectively track the distinct evolutionary trends of the physicochemical properties of hydrothermal fluids across different paragenetic stages, including pH and oxygen fugacity. These observations suggest that the intragroup compositional variations in chlorite are primarily controlled by temperature. In contrast, the intergroup differences mainly reflect precursor mineralogy, fluid pH, and variations in oxygen fugacity. The formation of pre-ore chlorites (Chl-1 to Chl-3) provided favorable spatial conditions for uranium mineralization, whereas the evolution of mineralizing fluids from relatively oxidizing conditions (Chl-4) during the early ore stage to reducing conditions (Chl-5) during the main ore stage directly drove the pitchblende formation process. This research establishes new geochemical indicators for exploring uranium. Notably, elevated Cu/Li and Cu/Ga ratios—coupled with decreasing Li/U ratios and high U and Cu concentrations in ore-stage chlorite—emerge as effective proximal vectors for mineralization within the Daliang deposit. These results not only increase our understanding of granite-related hydrothermal uranium deposits but also provide practical tools for accelerating future exploration efforts.