Geochemical study of the black rock series and kerogen elements in Wufeng-Longmaxi formations in eastern Dianqianbei Depression

Key words: 
• Element geochemistry /
• kerogen /
• Wufeng Group /
• Longmaxi Formation /
• eastern Dianqianbei Depression

Abstract: [Objective] The Dianqianbei depression is not only an important exploration area and producing area of marine shale gas in southern China, but also an important large-super large MVT type lead-zinc metallogenic area associated with key metals in China. Asphalt or ancient reservoirs with high Pb and Zn contents are constantly found in lead-zinc deposits. In this paper, the geochemical characteristics of elements in black shale kerogen are used to study the material contribution of mineralization and the sedimentary environment of black shale. [Methods] The black shale outcrops in the Dashiban, Liangfengao and Maoba areas in the eastern part of the northern Yunnan-Guizhou depression were taken as the research objects. The kerogen in the black shale was pretreated by microwave digestion, and the trace elements in the black shale and kerogen were analyzed and tested in combination with petrographic and mineralographic observations. [Results] Three redox indicators (U/Th ,Ni/Co, V/Cr) in the kerogen of the Wufeng Formation-Longmaxi Formation showed that the Wufeng Formation was oxygen-poor-anoxic, and the Longmaxi Formation was oxygen-rich-oxygen-poor environment ; the ancient water depth calculated by Co element in the study area is 6.85 ~ 54.37 m. The paleoclimate reflected by Sr / Cu is warm and humid, and the Sr / Ba value is less than 0.5, which represents the sedimentary environment of brackish water. At the same time, in terms of paleoproductivity, Wufeng Formation is greater than Longmaxi Formation. The contents of Pb, Zn, Cd, Bi, Sb, V, Cr and Ni in the whole rock are higher than those in the kerogen, and the contents of Ag, Ge, U and Th in the kerogen are higher than those in the whole rock. The light rare earth elements in the whole rock and kerogen are relatively enriched, and the heavy rare earth elements are relatively depleted. The Eu and some Ce in the whole rock show weak negative anomalies. The kerogen shows strong Ce negative anomaly, Eu weak negative anomaly, and the total rare earth content is higher than that of the whole rock. [Conclusions] The Wufeng Formation-Longmaxi Formation in the study area is generally an oxygen-rich-oxygen-poor turbulent sedimentary environment, with a warm and humid climate of brackish water shallow shelf facies sedimentary characteristics. Trace elements in black shale kerogen can provide a basis for regional prospecting.