Genesis and quality evaluation study of high-purity quartz deposit hosted in NYF-type granite pegmatite in the Central Asian Orogenic Belt, NW China

• An Early Carboniferous high-purity quartz deposit (impurity content = 30.36 ppm) in NYF-type granite pegmatite is discovered in the southern margin of Central Asian Orogenic Belt. • The mineralizing fluids of the Yamansu pegmatite-type quartz exhibit a spatial evolution trend from east to west. • Quartz crystallization temperature is critical for evaluating pegmatite-type quartz deposits. The recrystallization of quartz, as well as the exsolution of accessory minerals (e.g., garnet), can effectively decrease the concentrations of impurities in quartz. In this paper, the evolutionary process of ore-forming fluids and quality evaluation methods for raw quartz materials in the Yamansu quartz deposit of the Central Asian Orogenic Belt (CAOB) are explored to clarify the genetic mechanisms of granitic pegmatite-hosted high-purity quartz deposits and establishes the theoretical foundation of quality evaluation and exploration targeting for high purity quartz. The vein-type quartz from the western part of the deposit is predominantly hosted in pegmatites, whereas the quartz veins from the eastern part of the deposit are contained in volcanic rocks and granite. The ore-forming pegmatites was emplaced et 353.45 ± 0.96 Ma, and formed by magmatic activity associated with Early Carboniferous subduction. Quartz sands purified from pegmatites and associated veins yield total impurity concentrations of 31.29 ppm and 30.36 ppm, respectively, suggesting that the deposit is a high-purity quartz deposit. The quartz in the pegmatites is characterized by elevated Ti and low Al, Li and Ge contents, which are consistent with the signatures of typical NYF-type pegmatites. The hydrogen isotopic compositions of quartz (–97‰ to –51‰) and the calculated δ 18 O values for the equilibrium fluid (6.9‰ to 9.4‰) are similar to those of the magmatic hydrothermal fluid, confirming that the quartz precipitated from hydrothermal fluids exsolved from the pegmatites. Ti-in-quartz thermometry reveals a westward temperature decrease from 573°C to <500°C, accompanied by declining CO 2 content and increasing H 2 O content in fluid inclusions. The concentrations of quartz impurities are positively correlated with the temperature of the ore-forming fluid, which reaches its minimum value at approximately 500 ± 20°C. The in situ trace concentrations of impurities of the recrystallized quartz veins are nearly 100 ppm lower than those of the quartz veins without recrystallization, suggesting that recrystallization may have reduced the trace element contents in quartz. Furthermore, the quartz from garnet-poor pegmatites contains 34.1 ppm of impurities, markedly lower than in the garnet-rich pegmatites (162 ppm) in the same orebody. This finding indicates that early-stage garnet crystallization effectively sequestered abundant amounts of impurities, thus depleting the amount of impurities in the residual hydrothermal fluid and yielding quartz with significantly low contents of impurities (<100 ppm).