| Songtao manganese ore is located in Tongren City, Guizhou Province.It is the second large manganese ore in China, which is located in the binding site of the southeast edge of the Yangtze block and the Jiangnan orogenic belt. It was formed in Nanhua System, followed by Liangjiehe Formation,Tiesiao Formation,Datangpo Formation and Nantuo Formation from the bottom up. Predecessors have done more research on the genesis of "Datangpo" manganese ore. Such as bio-mineralization, turbidites mineralization, hydrothermal sedimentary mineralization, submarine volcanic eruption sedimentary mineralization and so on. The bio-mineralization think that rhodochrosite was mainly formed in a series of shallow water and low-lying areas of tidal flat area during early Datangpo. The turbidites mineralization think that the intraclast from the broken of manganese deposits in shallow water environments has been transported into the center of the basin and formed the intraclast rhodochrosite. The hydrothermal sedimentary mineralization think that the ore-forming elements and else, which is mainly for manganese, rise along fracture into the basin, then rapid accumulation of mineralization by chemically and biochemically. The submarine volcanic eruption sedimentary mineralization think that in the process of volcanic eruptions, the manganese and fine-grained volcanic material quickly become the manganese tuffaceous sediments. Then the seawater which has been heated by the volcanic gas-liquid or the gas-liquid, make manganese material enrichment and migrate into the center of the basin, and sedimentary mineralization. In recent years, Zhou Qi and Du Yuansheng research the results about manganese of the study area and describes this theory which is the metallogenic system and metallogenic model of the old gas leakage sedimentary manganese of Nanhuan "Datangpo" in Guizhou, Hunan, Chongqing and adjoining regions. They think that the "Datangpo" manganese formed in the study area is due the Nanhuan rift basins formed by the Rodinia supercontinent cracking. Then it led to a series of secondary graben basin formed in the rift basin in Guizhou, Hunan and Chongqing areas adjacent. This manganese is the deposit product of the old gas leakage events in a series of small secondary rift basin. The rift formed by Nanhuan rift provides a channel into the basin for the inorganic gas from deep with a large of manganese. The old gas eruption mouth formed in the center of the basin, leading to the formation of manganese ore. This article is mainly based on this theory as well to study the genesis of solid bitumen.There are abundant bubble solid bitumen in Datangpo Manganese Ore in Songtao County, it is very common to find them in rhodochrosite body of every mining area. Asphaltene is filled in these bubbles,and the edge of the bubbles wall was chalcedony, commonly known as "fish eyes."The bubbles are generally distributed by sparse and dense forms in the rhodochrosite body;there are three forms:circular, oblate shape and collections;the content of bubbles is generally between 5 and 30%.Since the widespread of bitumen ball in Datangpo Manganese Ore and the few previous research,the origin and genesis study of Datangpo manganese ore in Songtao is a contribution to analyzing the material source, enrichment methods and metallogenic model of manganese, and it has an important significance on summarizing the mineralization law of Songtao Manganese Ore.Based on the summary of research results at home and abroad, we use raman spectroscopy, stable carbon isotope (813C) and organic geochemical analysis techniques to analyse the geochemical characteristics of solid bitumen and carbonaceous shale in Datangpo Manganese Ore, such as physical properties, carbon isotopic composition, saturated hydrocarbon compounds composition, biomarker composition, aromatic compounds composition and maturity, then to explore the material source and sedimentary environment of bitumen, and speculate the possible genesis mode of bitumen bubbles through the parameter comparison between solid bitumen and carbonaceous shale of Datangpo Formation.The reflectance value of bitumen is between 3.245%～3.336%, showing a high degree of thermal evolution. The δ13C value of bitumen ranges from-31.49‰ to-32.52%o, with an average of-32.04%o; the normal paraffins series have a complete distribution, basically in the range of C15～C25,all samples are single peak, with peak carbon of C18 or C17, light and heavy hydrocarbon ∑C21-/∑C22+ ratio is 2.27-6.64, showing a big advantage of light hydrocarbon component, little odd even predominance is shown by OEP as 0.62～1.21; Pr/nC17 distribution in the range of 0.39-0.55, Ph/nC18 distribution in the range of 0.51 to 0.92,with an average of 0.49 and 0.72, Pr/Ph distribution in the range of 0.30 to 0.67, with an average of 0.54, indicating a reductive depositional environment; tricyclic terpane is mainly shown as normal distribution type with main peak of C21 and C23, carbon number distribution is in the range of C19-C31,the content of C30 hopane is the highest in ho pane series, while C29 hopane is the second high,there is a certain amount of gammacerane, gammacerane/C30 hopane is 0.15-0.19, Ts/(Tm+Ts) is 0.41-0.51, C3122S/22S+22R is 0.57-0.59; the distribution of C27 and C29 aaaR steranes is " V" shape;the value of DBT/P is 0.03～0.20, the value of C26- Miyoshi Sterane (20S)/C28- Miyoshi Sterane (20S) is 0.43-0.64,with an average of 0.52.The δ13C value of carbonaceous shale is -31.07‰～31.67 ‰, with an average of-31.29‰; the distribution of normal paraffins is basically in the range of C15～C25, with peak carbon of C18, light and heavy hydrocarbon ∑C21-/∑ C22+ ratio is 4.77-11.09, showing a big advantage of light hydrocarbon component, the value of Pr/Ph is in the range of 0.47～0.55, indicating a reductive depositional environment; little odd even predominance is shown by OEP as 0.62～1.21; tricyclic terpane is mainly shown as normal distribution type with main peak of C21 and C23, carbon number distribution is in the range of C19～C31, the content of C30 hopane is the highest in hopane series, while C29 hopane is the second high, there is a certain amount of gammacerane, gammacerane/C3o hopane is 0.22～0.35; the distribution of C27 and C29 aaaR steranes is" V" shape.Biomarkers parameter reflect that the original organic matter of solid bitumn is sourced from lower aquatic organisms or bacteria and algae, and formed in a reductive, brackish marine sedimentary environment. Through the comparison of saturated hydrocarbons chromatography, biomarker and organic carbon isotope, it can be concluded that carbonaceous shale in the upper and lower of rhodochrosite layer in Songtao, Guizhou and solid bitumen have similar geochemical characteristics, showing homology. It can be speculated that the formation of solid bitumen may be divided into two stages: ①In early Nanhua system, the seepage of methane gas in deep sea basin promotes the large multiplication of lower algae and bacterial microorganisms which are lived with methane, These creatures were saved in the seabed shale after dying to form a good source rocks.Before the formation of rhodochrosite, the source rocks began to generate hydrocarbons and migrated into favorable reservoir areas to forming hydrocarbon reservoirs,its shape may be dispersed or chicken coop like; ②Mn2+ in the seawater was oxidized to MnO2,then MnO2 was gradually transformed into manganese carbonate rocks during diagenesis process, while oil and gas thermal evolution constantly increased, lighter components formed earlier in oil and gas reservoirs escaped constantly, heavy components constantly concentrated to bitumen and dispersed in manganese ore which was forming to occupy some space. With the accumulation of manganese ore layer, those fully cured bitumen formed spherosomes, partly cured bitumen deformated into flat spheroid because of the pressure from the overlying manganese ore body. Thereby decentralized distribution, different sizes, different forms of solid bitumen were formed, its uneven distribution was controlled by the dotted distribution of early oil and gas reservoirs and eruption port position. |
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