| A series of important global geological events took place between 1000~541Ma in Neoproterozoic, the whole Earth system were greatly changed. Especially the global glacial deposits related to the "snowball earth event" in between 800 to 600 Ma ago not only changed the climate of the Earth’s surface, the atmosphere, oceans, continents of environment but also had associated large-scale or oversized- scale mineral deposits. At least there are two global major ice ages existed in the Neoproterozoic, which were Marinoan ice age(650~635Ma) and Sturtian ice age(710~720Ma). There are sedimentary records corresponding to the Marinoan ice age and Sturtian ice age in Eastern Guizhou, the famous "Datangpo Style" manganese ore occurred within the Datangpo formation which were formed between the two interglacial periods in the region.Since the Datangpo manganese deposit in Songtao county had been discovered in 1950 s by 103 Geological Party of Guizhou Bureau of Geology and Mineral Resources, the "Datangpo Style" manganese deposits has become the most important manganese deposit type with the greatest resources potential. The "Datangpo type" manganese ore bodies all occur within the black carbonaceous shale of member 1 of the Datangpo formation, the surrounding rock is called "black manganese-bearing rock series", the distribution of which varies widely along the graben basin center controlled by early Nanhua Period regional tectonic. The manganese ore body mainly occur in the graben basin center. The thickness of manganese ore bodies, thickness of the "manganese-bearing rock series" and complexity of the rock combination appear significant positive correlation.The predecessors have carried out extensive research on the ore-forming sedimentary environments, the metallogenic conditions and so on of the "Datangpo style" manganese deposits, but its genetic mechanisms have been disputed. The main viewpoints include biogenic origin, biochemical deposition origin, hot water origin, volcanic eruption- sedimentary origin and ancient natural gas leakage origin.This article focuses on study of the geochemistry of the manganese ore and carbonaceous shale in the Menmber 1 of the Datangpo Formation formed in the interglacial period. The works include main element, trace element analysis, strontium whole rock isotope, carbon and oxygen isotope, sulfur isotopes, and pyrite rhenium and osmium isotopes of the manganese ore assemblage, to constrain the manganese ore-forming paleoclimate conditions in the Neoproterozoic interglacial period, to study the manganese ore material sources and genesis, and study the effects of the paleoclimate changes on the manganese ore deposition in interglacial period.The samples of manganese oxide(MnO) content greater than 10% are all occurred in Menber 1 of the Datangpo Formation, the maxmium manganese oxide(MnO) content up to 35.21%. Overall, the MnO contents of the massive rhodochrosite ore are higher than those of the banded rhodochrosite ore, and the higher MnO content, the lower the SiO2 content. In other words, the higher sandy content, the lower the manganese content. Generally speaking the Mn O content gteater than 20% are related to the massive rhodochrosite, and the MnO content of 7%-20% are associated to the banded rhodochrosite ore. The SiO2 contents of the Tiesiao formation siltstone underlieing the Datangpo formation are significantly higher than SiO2 contents of the manganese ore in the Member 1 of the Datangpo Formation, but no significant differences from the black carbonaceous shale of the Datangpo Formation. The SiO2 contents significantly increase from Member 1 to Member 2 of the the Datangpo Formation. The obvious variasion of SiO2 content from the Tiesiao Formation to the Member 1 of the Datangpo Formation, and from the Member 1 to Member 2 of the Datangpo Formation, may reflect changes of the substance source, and changes of sea level.Magnesium and aluminum ratio(m=100×MgO/Al2O3) can be used as one of the signs identifying sedimentary environment. Generally m? 1 indicates freshwater environment, m betwen 1 and 10 indicates land and sea environment transition, m from 10 to 500 indicates sea sedimentary environment, and m>500 indicates epicontinental environment. 42 samples from the Member 1 of the Datangpo Formation were analyzed, the m values vary widely from 5.07 to 303.6, with average of 59.2, which indicate a relatively deep water sedimentary environment.Sr/Ba value is usually used as an important indicator to distinguish between marine and continental facies. The Sr/Ba values of marine deposits are generally greater than 1, and continental sedimentary environment is generally less than 1.42 samples from the Member 1 of Datangpo Formation in which the manganese ore occur were analized, there are 10 samples with Sr/Ba>1, and 32 samples with Sr/Ba<1, with an average of 0.91, which may indicate that the deposition environment should be mainly transitional environment of land and sea, but the continental features are more obvious.The total rare earth elements of the Member 1 of Datangpo formation are between 122×10-6~428×10-6, with an average of 256×10-6. REE curve of the all samples are basically similar of the flat type, which are distributed between the pattern curves of coastal estuarine sediment and deep clay, the tcurves are similar to that of nearshore estuarine sediment. These features suggest that the deposited material should come more from the weathering crust, and part from the ocean substance.Rb/Sr ratio values are typically used to reflect climate change, and the degree of chemical weathering. Generally chemical weathering in hot and humid conditions is strong, there would be more of Sr entering the sedimentary environment, Rb/Sr value will be smaller, on the contrary chemical weathering in cold and dry environment Rb/Sr values go up. The Rb/Sr value of the samples from the Member 1 of Datangpo formation fluctuate between 0.01 to 2.95, with an average of 0.46, whihc reflect relatively strong chemical weathering in hot and humid environment, while the larger Rb/Sr values fluctuate also implies that climate fluctuations. High MgO/CaO values react hot and dry climate, low MgO/CaO values indicate humid climate. MgO/CaO values of the most samples form the Member 1 of Datangpo formation are less than 1, mostly in between 0.3 to 0.7, with an average of 0.58, which reflect the climate was relatively dry and hot.V/Cr ratio, Ni/Co ratio, and U/Th ratios are sued to determine the redox environment. The manganese ore and shale samples from the Member 1 of Datangpo formation have V/Cr of 0.8~3.2 with an average of 1.6, Ni/Co of 0.29~3.95, with average 0.87, and U/Th of 0.15~0.84, with average 0.28, which are respectively low and show an oxidizing depostion environment.The carbon isotope for samples of manganese ore and carbonaceous shale from the Member 1 of Datangpo formation have obvious negative offset section, the δ13CV-PDB is less than-5.5‰, with minimum of-9.4‰, and an average of-7.2‰, which are significantly lower than those from the Doushantuo formation(0.5‰ and 1.4‰). The δ13C values are of no significant difference between manganese ore and carbonaceous shale of he Member 1 of Datangpo formation, in overall δ13C values are lower than the normal deposition of carbonate(δ13C>0.0‰). and higher than that of biological algal(-12‰~-23‰), and also significantly higher than mud organic matter(-20‰), but basically close to the modern ocean-floor hot springs( δ13C-5‰~-8‰), which may indicate that Datangpo sedimentary might have genetic relation to hot springs on the ocean floor.The pyritic sulfur isotope composition for samples from the Datangpo Formation of the interglacial period, with little various range of δ34SV-CDT value, are distinct from those from the Nantuo and Doushantuo formations, except slightly smaller value of 19.4‰ for the sample of ZK0608-26 a, the value of the other samples are higher than 35‰, with an average of 42‰. Moreover, pyrite sulfur isotope δ34SV-CDT for Nantuo and Doushantuo formation are significantly quickly decreased to negative values, the lowest of-18.5‰, with a large variatrion range. The current studys show that pyrite, and rhodochrosite-bearing black shale of the interglacial period Datangpo Formation commonly have unusually high positive sulfur isotope compositions, which are quite different from those of the Doushantuo Formation, but are very close to the authigenic pyrite isotope δ34SV-CDT value(47.8‰~67.1‰) from the South China Sea Ocean Drilling, and are higher than the methane seep genetic barite δ34SV-CDT value(21.0‰~38.6‰) of modern Okhotsk sea floor. These characteristics may indicate that the genesis of the manganese might have little to do with cold seeps, but are more likely related to deep sea hot water.The 87Sr/86 Sr initial ratios of the sample vary between 0.7036~0.7135, which are higher than hydrothermal value(0.7031). Among 6 samples from the Member1 of the Datangpo Formation, the initial values of 3 samples are higher than seawater average value(0.709), and the other 3 ones less than this value. Lower 87Sr/86 Sr initial ratio reflects a greater amount of material origined from submarine hydrothermal deposition, which might provides hydrothermal manganese source material. The high 87Sr/86 Sr initial ratios might reflect the weathering crust were the main source of sediments, and less the ocean and seafloor hydrothermal material participation, the material source region should be high Rb/Sr ratios.The Re content of pyrite from the Member 1 of Datangpo Formation ranges 0.440~3.008ng/g, with mean 1.477ng/g, which are slightly lower than the continental crustal Re abundance of 2ng/g. The Os content are 0.0045~0.0063ng/g, with mean 0.0052ng/g, which are almost in line with that of continental crustal abundance 0.005ng/g. The 187Re/188 Os ratio are higher, with minimum value of 334 and highest value of 2706, which are higher than average 227 for the world’s major rivers, but less than the average of 4270.8 of sea water. The 187Os/188 Os values of the 8 samples are higher, with the minimum of 3.69 and the highest up 21.69. The 187Os/188 Os initial value of 1.02±0.41 is significantly larger than the mantle values(0.12 to 0.13), indicating radiogenic features. The low Os content and high Re/Os ratios as well as high 187Os/188 Os isotope ratios for the all 8 samples show that the samples have characteristics of higher radiogenic Os, and are of significant crustal genetic features, which indicate the materials of the Member 1 of Datangpo Formation were from the sedimentary basin itself and the sedimentary basins was in a mixed environment of seawater and river water.To sum up, the study of the author suggests that the "Datangpo style" manganese deposits of the Neoproterozoic interglacial period were formed in relatively warm climates and in an oxidizing environment as a whole. The manganese sedimentary environment was in a land and sea transition zone, and the sedimentary basin has some characteristics similar to the modern submarine. The hydrothermal hot spring of the sedimentary basin provided the material of the manganese ore, when the sedimentary basins had less terrigenous injection, the manganese deposits were formed, when the sedimentary basins had more terrestrial weathering sources, the sandy shale were formed. |
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