Modes of occurrence of critical metals (Nb-Ta-Zr-Hf-REY-Ga) in altered volcanic ashes in the Xuanwei Formation, eastern Yunnan Province, SW China: A quantitative evaluation based on sequential chemical extraction

•Most components of polymetallic deposit occur in the HF-soluble silicate assemblages.•Approximately 89% of Zr is hosted in zircons of varying grain size.•The SCEP results strongly do not support the previous hypothesis that incompatible elements are adsorbed by clay minerals.•The incompatible elements can replace K, and/or Si, and/or Al in the clay crystalline structure.