Geochemistry Characteristics Of Hydrothermal Sedimentation In The Lower Cambrian Niutitang Formation In Guizhou

Guizhou province is the main area of the hydrothermal activity in the Late Ediacaran—Early Cambrian.The black shales which is extensively distributed in the Early Cambrian Niutitang formation are not only the mineral deposits but also organic-rich source rocks.They has the highest exploration potential in the south China.Based on the research of inorganic geochemical and organic geochemical characteristics of five outcrop samples from the Guizhou province,the sedimentary environment,element anomaly features,abundance and maturity of organic matter,characteristics of saturated hydrocarbon and the relationship between organic-rich phenomenon and hydrothermal activity of the black shales in the Niutitang Formation were largely discussed.The main results are as follows:(1)The Niutitang Formation black shale in this study was mainly formed under the continental marginal sedimentary environment.While a part of the area had the characteristics of the pelagic basin sedimentary,which is caused by the upper Yangtze region seawater intrusion to the Early Cambrian.(2)It was found that trace elements such as V,As,Se,Mo,W,and U are commonly enriched in each outcrop,and Cd,Co,Tl,Sb,Ba,Cr,The Bi,Ni,Cu,Zn,Sr and Pb are noticeable enriched in some area.The enrichment characteristic of these trace elements may be related to the migration ability of the elements in the hydrothermal deposition environment.(3)Because of the invalidation of common redox proxies under hydrothermal deposition background,this paper proposed to use the Index Sc/Cr to discriminate the hydrothermal deposition.The normal seawater sedimentary process is dominant when Sc/Cr ? 0.144.When 0.120<Sc/Cr<0.144,the deposition is controlled by both normal sedimentary and hydrothermal process,when Sc/Cr ? 0.120,the hydrothermal activity dominates the sedimentary process.(4)The relationship between proxies ?Eu(PAAS normalized)/Al and ?Y(PAAS normalized)/Al is applicable for determining the type of the hydrothermal deposition that the source rock undergoes during diagenesis.When ?Eu/Al and ?Y/Al have linear positive correlation,and both less than 20,denote the transition from normal seawater sedimentation to low-temperature hydrothermal sedimentation.Whereas ?Eu/Al and?Y/Al increase at the same time and beyond 20 represents the high temperature hydrothermal deposition.(5)The Niutitang Formation black shale is enriched in organic carbon,however,the chloroform asphalt "A" content is relatively low.n-Alkanes of the Niutitang formation black shale,tested by gas chromatography-mass spectrometer,show two obviously characteristics: bimodal distribution and even-carbon preference.Since the source material plays a decisive role in the composition of the source rock,the hydrothermal impact on the deposition in this area and age is important.The above two characteristics of n-alkanes should concern to the organic matter provided by the hydrothermal organism and the excessive maturity.Moreover,they should be some characteristics of the formation of hydrothermal sediementation-controlled source rock.Combining the previous research achievements,based on the geochemical characteristics of hydrothermal sedimentation of Niutitang Formation source rocks in Guizhou,this paper put forward new element indicators for discriminating hydrothermal sedimentation,and found the distribution characteristics of n-alkanes "bimodal distribution" and "even-carbon" in hydrothermal source rocks in Guizhou,which can provide guidance for future selection and exploration of oil and gas in hydrothermal sedimentary area.