Mineralization age and metallogenic environment of Daotuo manganese deposits in Guizhou: Evidence from Re-Os isotopes
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Résumé
The Neoproterozoic is a critical interval in Earth’s history, having witnessed the breakup of supercontinent Rodinia, global glaciations (or the snowball Earth), and the evolution of multicellular organisms. It is proposed that the breakup of Rodinia might have triggered the snowball Earth, while the termination of the Neoproterozoic global glaciations might have triggered the evolution of life in the Ediacaran Period. Two snowball Earth events, the Sturtian (717–663 Ma) and the Marinoan (654–635 Ma) glaciations, were separated by an interglacial interval of ~10 million years (663–654 Ma). In South China, the Sturtian and Marinoan glacial deposits are represented by the Jiangkou and Nantuo Formations. The interglacial Datangpo Formation is characterized by widespread manganese (Mn) ore deposits, such the Datangpo and Xixibao in Guizhou, Minle in Hunan, and Xiushan in Chongqing, representing the most important metallogenic period of Mn in China. The Daotuo Mn ore, a superlarge-scale deposit (approximately 200 million tons), was first discovered in 2010 in northeastern Guizhou. It is hosted in a fault-bound depression system between the Yangtze block and the Jiangnan orogenic belt. The Mn ores were discovered in black shale of the Member I of the Datangpo Formation, and occur as rhodochrosite (MnCO<sub>3</sub>). In order to constrain the age of Mn ores, here we report Re-Os isochron age of the ore-bearing black shale from the Member I of the Datangpo Formation. The Re-Os isochron age of the Member I black shale is 660.6±7.5 Ma, consistent with the zircon U-Pb age of the tuff layer from the base of the Datangpo Formation and other Re-Os isochron ages from other localities (such as Canada, Australia, Scotland, and Mongolia). Furthermore, the initial value of <sup>187</sup>Os/<sup>188</sup>Os from the Re-Os isochron age is 0.781, suggesting predominantly terrestrial input. Based on previous research, it is shown that during the Sturtian glaciation, under the influence of sea ice cover barrier and atmospheric material exchange, the hypoxic water led to the formation of iron and sulfide rich environments on the seafloor near hydrothermal vents, giving rise to a large number of rift basins rich in Mn<sup>2+</sup>. After the glaciation and water and atmospheric exchange of material recovery, the glacier meltwater carries the terrigenous material into the rift basin containing Mn<sup>2+</sup>, accompanied by the melting of the land glacier and the rapid increase of the atmospheric oxygen content. The surface water body of the rift basin is rapidly oxidized, and the oxidation degree is higher than the ocean area. Mn<sup>2+</sup> are also oxidized to MnO<sub>2</sub> and precipitate back into the environment, where they buried with organic matter, undergo diagenesis, and eventually form rhodochrosite.
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|---|---|---|
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