| Argillized alkaline volcanic ash-altered tuff layers have been found in the late Permian coal-bearing strata from eastern Yunnan Province. Those argillized tuff layers are highly abundant in rare metals such as Nb, Ta, Zr, Hf, REY(lanthanides and yttrium), Th and U, and characterized by the high positive anomalous in natural gamma log curves. The arigillized tuff layers are thick(up to 10 meters) and are widely developed within Upper Permian, SW China. Notably, the contents of oxides of rare metals are highly enriched, especially the(Nb, Ta)2O5 which has reached the required industrial grade for a weathered crust niobium deposit. However, from a genetic viewpoint, this Nb(Ta)-Zr(Hf)-REE-Ga polymetallic mineralization does not belong to any previously reported niobium deposit before. Since the rare metal-enriched argillized tuff generally occurs in coal-bearing strata, and in some cases within coal measures directly, such Nb-predominated polymetallic mineralization was defined as coal-hosted niobium deposit. Until now, although several samples were collected from typical drillholes, the studies focus on mineralogy and geochemistry of the samples. The enrichment mechanism of rare metals in the coal-hosted Nb deposit has been unknown. Based on the anomalous in natural gamma log curves, in this study, detailed sampling was executed from Nos. 301, 802, 1001 and 1201 drillholes for examining detailed petrographic, mineralogical and geochemical characteristics. Combined with the latest research progress of the Emeishan mantle plume, we propose an ore-forming model for this coal-hosted Nb deposit and discuss the relationship between the studied tuff and the Emeishan large igneous province(ELIP). At last, this study will provide material-based evidence for the felsic volcanism which could be a possible cause of end-Guadalupian mass extinct. For convenience, we define the samples with Nb≥150 μg/g as “ore samples” while those with Nb<150 μg/g as “non-ore samples”.Based on the current study, most of the samples have been argillized, mostly pale grey-green mudstones with argillized textures and blocklike structures. Some of the samples are identified as siltstones and sandstones. There are no obvious macro differences between the studied samples and normal sedimentary rocks; however, typical volcanic textures have been observed in the studied samples including concentrically-zoned amygdales, radial spherulites, angular volcanic glass shards and high temperature minerals. High-T minerals such as beta-quartz, angular quartz shards, plagioclase shards, and euhedral zircon, apatite and albite etc. As for the mineral composition identified by X-ray diffraction(XRD), it seems that the ore samples have the similar mineral compositions to the non-ore samples. Mineral phases identified in ore samples comprises clay minerals(illite, illite-smectite mixed layer, kaolinite, berthierine and chamosite), quartz, anatase, calcite, hematite, siderite, albite, florencite and pyrite. While using scanning electronic microscope(SEM), trace minerals such as ilmenite, barite, zircon, sphalerite, galena, chalcopyrite, and REE-bearing phosphate(florencite and rhabdophane) and carbonate(parisite). Ore samples usually have high contents of K2 O and SiO2, corresponding to the high contents of illite-smectite mixed layers(I/S) and quartz therein; nor samples have high contents of TiO2 and Fe2O3, corresponding to the high contents of anatase and hemetite therein. All samples have more SiO2, Al2O3, and Fe2O3 and low Na2 O, MgO, P2O5, K2 O and MnO. Generally, the ore samples possess high concentrations of rare metals such as Nb, Ta, Zr, Hf, REY, Th, U and Ga, while the non-ore samples have high concentrations of Sc, V, Cr, Co, Ni and Cu. In some cases, the enrichments of CaO, P2O5 and Na2 O may be induced by the enrichments of calcite, apatite and albite.According to mineral occurrence modes, mineral paragenesis and the geochemistry, minerals and elements in the studied samples were derived from alkaline volcanic ashes and multi-stage hydrothermal fluids.(1) alkaline rhyolitic volcanic ashes: The direct evidence for the volcanic origin of the studied samples is volcanic texture found under both microscope and SEM. Although most of the samples have been argillized, typical volcanic texture, euhedral mineral phases(zircon, beta-quartz and apatite), and shards-like materials(angular quartz, volcanic glasses and plagioclase) can be observed. Those high-T minerals with high-T crack and embayments, and angular volcanic glasses could be originated from rapid-eruption of magmas. Such widespread tuffaceous layers with uniform mineralogy and geochemistry, have lateral continuous across SW China indicating the ore samples were derived from same phase of extensive volcanic eruption.(2) multi-stage hydrothermal fluids: Those colloidal minerals fill in plant cells and high-T mineral phases indicating that they are probably derived from re-deposition of elements in hydrothermal fluids. The colloidal I/S, kaolinite, berthierine within angular berthierine and anatase suggest that these clay minerals were precipitated from hydrothermal fluids through reassembly of Al3+, Si4+, Fe2+/Fe3+, Na+, K+, and Ca2+ in hydrothermal environment. Other minerals like daisy-like rhabdophane, florencite aggregates and parisite veins may also form in hydrothermal alteration process.Paragenetic sequence of minerals implies that the authigenic hydrothermal minerals formed after volcanic minerals i.e. the hydrothermal fluids invaded after volcanic-ash fall. Additionally, epigenetic calcite and pyrite veins indicate the presence of epigenetic hydrothermal activity.Integrated with all the evidence, we suggest that the ore samples within the lowest Xuanwei Formation are derived from the coeval ELIP Nb-Ta-mineralized syenitic magmas at waning stage. The ELIP Nb-Ta-mineralzied syenitic magmas erupted on the surface and subsequently subjected hydrothermal alteration at early diagenesis. We suggest that the ore-forming process can be divided into two parts as following.1) Nb-Ta-mineralized syenites evolved to high evolved rhyolitic magmas prior to final eruption. As is clear in the chondite-normalized REE patterns, the Eu/Eu* values for ore samples are lower than those of ELIP Nb-Ta-mineralized syenites indicating that the primary source magmas of the ore samples are more evolved than the ELIP Nb-Ta-mineralized syenites. It has been demonstrated that ELIP magmas could evolve during ascent due to sustainable fractional crystallization. Due to the presence of strong negative Eu/Eu*, euhedral albite, zircon and quartz, we believe that the Nb-Ta-mineralized syenitic magmas could evolve to trachytic or rhyolitic magmas.2) Critical elements assembly during hydrothermal stage. Rare metals in ELIP Nb-Ta-mineralized rhyolitic magmas either occur within high-T mineral phases or within volcanic glasses due to the rapid-cooling during eruption. The syngenetic hydrothermal fluids dissolved the Nb-Ta-enriched volcanic ashes and released at least a part of rare metals into fluids so that the rare metals could reassemble to new phases such as colloidal REE-bearing minerals. Note that ore samples contain thousands-μg/g Zr and hundreds-μg/g Nb, both zircon and Nb-bearing Ti-oxides are rarely to observe. Most of Zr and Nb may occur as absorbed ions in clay minerals.Previous study suggested that in the lowest Xuanwei Formation, rocks with Al2O3/TiO2>7 are derived from weathering of ELIP uppermost silicic rocks; however, according this study, samples with Al2O3/TiO2>7 are mainly the ore samples. Thus the nature of the lowest Xuanwei Formation is that the alkaline rhyolitic volcanic ashes altered tuffaceous materials with Al2O3/TiO2>7 interbeded with the ELIP high-Ti basalt-derived non-ore samples with Al2O3/TiO2<7. Since the ore samples(including alkali tonsteins) are originated from ELIP highly evolved silicic magmatism, the ore samples-bearing sequences may represent the ELIP associated felsic tuff. The extensive felsic volcanism that induced the formation of Nb-Zr-REE-Ga-mineralized tuffaceous layers may caused the end-Guadalupian mass extinction. |
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