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The Late Permian coal measures in eastern Yunnan, western Guizhou, and central Guangxi are significantly enriched in critical metals that could serve as important supplements to conventional critical metal deposits in China. This study collected previous geochronological and geochemical data from the Late Permian coal measures to evaluate the distribution characteristics and enrichment factors of critical metals. Moreover, metallogenic models for critical metals were also developed. The results showed that Late Permian coal measures in Yunnan, Guizhou, and Guangxi provinces exhibited abnormal enrichment in Nb, Zr, and rare earth elements (REY, or REE if Y is excluded). The Emeishan mafic rocks and intermediate-felsic volcanic ash from the Truong Son orogenic belt underwent chemical weathering, with Nb and Zr selectively preserved in situ in the form of heavy minerals (e.g., rutile, zircon, and anatase), which subsequently led to the enrichment of Nb and Zr in bauxite and Al-claystone at the bottom of the Late Permian coal measures. Intermediate-felsic volcanic ash from the Emeishan large igneous province (ELIP) and the Truong Son orogenic belt supplied Nb, Zr, and REY for the middle and upper parts of the Late Permian coal measures. The intermediate-felsic mineral material of the coal measures in the intermediate zone, outer zone, and outside zone of ELIP are derived mainly from the ELIP, the mixture from ELIP and the Truong Son orogenic belt, and the Truong Son orogenic belts, respectively. Nb, Zr, and REY were leached by acidic aqueous solutions and from the parting and roof into underlying coal seams, where they deposited as authigenic minerals or adsorbed ions on organic matter during early coalification. [ABSTRACT FROM AUTHOR]

Clay-altered volcanic ash with highly-elevated concentrations of Nb(Ta), Zr(Hf), rare earth elements (REE), and Ga, is a new type of critical metal deposit with high commercial prospects that has been discovered in Yunnan Province, southwest China. Previous studies showed that the volcanic ashes had been subjected to hydrothermal fluids, the nature of which, however, is not clear. Here we show that the volcanic ashes were originated from alkaline magmatism, followed by a continuous hydrothermal-weathering process. Heated meteoric waters, which were sourced from acidic rains and mixed with CO 2 from degassing of the Emeishan plume, have caused partial, but widespread, acidic leaching of Nb, Ta, Zr, Hf, REE, and Ga into ground water and residual enrichment of these elements, along with Al and Ti, in the deeply altered rocks. Subsequent alteration occurring under cooler, neutral or alkaline conditions, caused by water-rock interaction, resulted in precipitation of the leached critical metals in the deposit. Polymetallic mineralization of similar origin may be found in other continental regions subjected to explosive alkaline volcanism associated with deep weathering in humid conditions. [ABSTRACT FROM AUTHOR]

[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]

Volcaniclastic horizons near the Guadalupian-Lopingian Boundary (G-LB) from southwest China have attracted much attention, not only due to the End-Guadalupian mass extinction near the boundary, but also to the unique and regionally extensive Nb-Zr-REY-Ga mineralization with potential economic significance (REY = rare earth elements and Y). Previous studies have suggested that these horizons were derived from volcanism associated with the terminal phases of the Emeishan mantle plume, which however was not the only contemporaneous volcanic activity in southeast Asia at this time (~260 Ma). Here we present a systematic analysis focusing on the petrological, mineralogical, and geochemical features of the volcaniclastics from the Yiliang and Weixin boreholes from the lowest strata of the Late Permian Xuanwei and Longtan formations, respectively, eastern Yunnan, SW China. In addition, previous geochemical data obtained for the correlative layers near the investigated boreholes were used for reference. The mineralogy of the volcaniclastics is dominated by mixed-layer illite/smectite, illite, berthierine, kaolinite, and quartz, and to a lesser extent, albite, calcite, siderite, anatase, and hematite, along with traces of chamosite, rutile, and florencite. Additionally, zircon, pyrite, chalcopyrite, sphalerite, galena, rhabdophane, parisite, goyazite, and barite were identified under SEM-EDS. The values of Al 2 O 3 /TiO 2 in the volcaniclastic samples are mostly greater than eight, consistent with the characteristics of intermediate-felsic rocks. The Nb/Yb and Zr/TiO 2 vs. Al 2 O 3 /TiO 2 diagrams, and the chondrite-normalized REY distribution patterns of the samples collectively indicate that the Nb-Zr-REY-Ga mineralization was not only derived from the ELIP alkaline rocks, but also from the arc-related intermediate-felsic magmatic rocks from the Paleo-Tethys. However, the latter source for Nb-Zr-REY-Ga mineralization in SW China has not been previously reported. The two sources of clastic materials, i.e., from both within-plate/anorogenic magmatism and arc-related/orogenic magmatism, agree with previously-reported discrimination diagrams of ε Hf (t) vs. age and Hf/Th vs. Th/Nb in the zircon grains from the correlative claystones. In addition, the modes of occurrence of critical metals (Nb, Zr, REY, Ga) are identified in detail in this study. The results show that most of the HREE and almost all the Zr and Hf are in zircon; the LREE are dominantly in REE-bearing carbonates and phosphates; the Nb is largely in former Fe Ti oxide phases; and the Ga is predominantly in the clays. • Volcaniclastic horizons near G-LB are an extensive Nb-Zr-REY-Ga ore deposit. • The horizons originated from within-plate and arc-related magmatism. • The minerals in the horizons were dominantly altered from volcanic ashes. • The modes of occurrence of critical metals (Nb, Zr, REY, Ga) are revealed. [ABSTRACT FROM AUTHOR]