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[Display omitted] • 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. Altered volcanic ash with highly-elevated concentrations of Nb(Ta), Zr(Hf), Ga, and rare earth elements and yttrium (REY) is an untraditional type of critical metal deposit that has been discovered in Yunnan Province, southwest China. Previous studies have briefly described the modes of occurrence of Nb(Ta)-Zr(Hf)-REY-Ga in the ore deposit, showing that Zr and Hf are mostly hosted in zircon; light rare earth elements (LREE) mainly occur in REY-bearing carbonates and phosphates; Nb and Ta are primarily hosted in the former Fe-Ti oxide phases and zircon; and Ga is predominantly associated with clay minerals. However, previous studies of the modes of occurrence of critical metal elements (Nb-Ta-Zr-Hf-REY-Ga) have been limited to qualitative or semi-quantitative studies of individual mineral grains, and quantitative research of the modes of occurrence of these elements in whole rocks is absent. This paper quantifies the modes of occurrence of critical elements (Nb-Ta-Zr-Hf-REY-Ga) using a sequential chemical extraction method. The results show that about 65.3–86.7% Nb is associated with hydrofluoric acid (HF)-soluble aluminosilicates (clays) and 13.3–34.7% in the insoluble phases such as Nb-rutile. Approximately 87.6–90.2% Zr is the primary hosted in zircon and 9.5–12.0% Zr appears to be associated with HF-soluble aluminosilicates in the altered volcanic ash. In felsic altered volcanic ashes (Al 2 O 3 /TiO 2 ratio between 21 and 70), the REY are predominantly in the authigenic minerals such as REY-bearing aluminum phosphates and sulfates of the alunite supergroup (APS minerals), carbonates or fluorocarbonates. About 74.0–89.4% LREE, 26.5–34.2% middle rare earth elements (MREY), and 22.1–24.8% heavy rare earth elements (HREE) occur in HF-soluble aluminosilicates. In intermediate altered volcanic ash samples (Al 2 O 3 /TiO 2 ratio between 8 and 21), 93.0% LREE are associated with insoluble phases such as monazite and zircon. Gallium is predominantly associated with HF-soluble aluminosilicates (73.3–80.5%) in all altered volcanic ash samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Songshan tin deposit occurred at the end of Late Cretaceous, not in Triassic as previous suggested. • The end of Late Cretaceous is another important tin metallogenic period in the Lancangjiang tin belt. • The tin mineralization not only can occur in the magmatic arc, but also in the hinterland behind the magmatic arc. The Songshan tin deposit is located in the northern Lincang granitic batholith along the Lancangjiang tin belt, Western Yunnan, southwestern China. The ore bodies mainly occur as tourmaline-quartz veins with NNE- and/or NW-trending in the Triassic Pinghejie granitic pluton, and the schist of the Proterozoic Songshan Formation. The previous study suggested the tin mineralization in the Songshan deposit should occur in the Triassic, but no directed age data support it. This hinders the understanding of the genesis of the Songshan tin deposit, especially the tin mineralization in the Lancangjiang tin belt. Here, we report the zircons U-Pb ages of 222.7 ± 1.6 and 222.0 ± 2.4 Ma for the granodiorite, 215.1 ± 1.5 and 223.0 ± 2.0 Ma for the biotite monzogranite, and 222.3 ± 2.3 Ma for the alter granite from the Pinghejie granitic pluton. The monazite from the monzogranite yields a U-Pb age of 212.8 ± 1.9 Ma. We also obtain two garnet U-Pb ages of 215.1 ± 8.7 Ma and 213.5 ± 7.2 Ma from skarn, and two cassiterite U-Pb ages of 76.6 ± 1.5 Ma and 79.6 ± 3.6 Ma from tin ores. These data suggest that tin mineralization in the Songshan deposit occurred at the end of Late Cretaceous, which is quite different from the ages of the Pinghejie granitic pluton emplaced and the formation of skarn. The zircon Hf isotopes (εHf(t) = –22 ∼ -12) and monazite Nd isotopes (εNd(t) = -13.04 ∼ -11.80) indicate that the Pinghejie granitic pluton was derived from the remelting of Paleoproterozoic crust. The paper concluded that the tin mineralization in the Songshan deposit occurred at the end of Late Cretaceous. It is another important metallogenic period in the Lancangjiang tin belt, which may be related to the subduction of the NeoTethys oceanic crust. The tin mineralization can not only occur in the magmatic arc, but also in the hinterland behind the magmatic arc. [ABSTRACT FROM AUTHOR]