Various beryls in pegmatite from the Chinese Altai: implications for melt differentiation and source tracing

• Element substitutions in beryls are dominated by tetrahedral site replacements (Na + , Cs + ↔ Li + ). • Na/Cs ratio of beryl is effective to distinguishing Li- and Be-mineralization in pegmatites (threshold value of 15). • Principal Component Analysis (PCA) of beryl composition has great potential to discriminate the source and type of pegmatite. The Chinese Altai, one of the largest pegmatite provinces in the world, hosts significant pegmatite-type beryllium deposits in China. As one of the primary beryllium-bearing minerals, beryl is widespread across various pegmatites (Li-Cs-Ta and Nb-Y-F types) with different origins (anatectic and granite-related). This study examines the textures and compositions of beryls from 11 rare metal pegmatites in the Chinese Altai, aiming to decipher geochemical variations and establish new indicators for assessing melt differentiation of pegmatite-forming melts, mineralization types, and source discrimination of pegmatites. Results reveal that element substitutions in beryls are dominated by tetrahedral site replacements (Na + , Cs + ↔ Li + ), increasing in intensity with progressive melt differentiation, while octahedral substitutions are suppressed by peraluminous melt compositions. The Na/Cs ratio in beryl provides an effective indicator for assessing melt differentiation and discriminating between Li- and Be-mineralized pegmatites, while Fe content shows ancillary variation and is not a universal discriminator. The Principal Component Analysis (PCA) of beryl composition, first applied to Chinese Altai pegmatites, identifies three genetic pegmatite types: (1) NYF-affinity pegmatites, characterized by Sc- and Ga-enriched beryl and relatively depleted Hf isotopic compositions compared to LCT-types, (2) highly differentiated LCT-type pegmatites, marked by beryl enriched in Li, Na, K, Rb, Cs, and Zn, and (3) poorly differentiated granite-pegmatite system, featuring beryl rich in Fe, Mn, V, and P. This study highlights the utility of beryl composition and multivariate analysis in deciphering pegmatite genesis and guiding exploration for Li-Be deposits.