早震旦世大塘坡期锰矿成因新认识

A NEW UNDERSTANDING OF MANGANESE CARBONATE DEPOSITS IN EARLY SINIAN DATANGPO STAGE

中国南方早震旦世大塘坡期锰矿的形成一直存在热水成因、生物成因或化学成因的争议。通过对贵州松桃早震旦世大塘坡期锰矿的碳、硫同位素和藻类化石的研究 ,认为早震旦世大塘坡期锰矿是在 70 0～ 695Ma全球性Sturtian冰期后形成 ,由于大气中含有很高的CO2 与海洋中的Ca2 + 、Mn2 + 反应 ,造成大量CaCO3和MnCO3快速沉淀 ,形成“碳酸盐岩帽”(碳酸锰 )所致。对大塘坡组藻类化石研究还表明 ,在含锰矿的层位 ,藻类化石很少 ,而锰矿层上下 ,微体藻类化石都很丰富 ,这说明锰矿成矿与藻类关系并不那么密切。

There has been a long-standing controversy about the origin of manganese carbonate deposits in the Early Sinian Datangpo Stage in South China. Some researchers considered that manganese carbonate deposits were formed by biogenization, and others suggested that they were associated with hydrothermal systems. On the basis of the study of δ 13C carb of manganese carbonate deposits and δ 34S sulfate of pyrite, we found microflora from the Datangpo Formation, especially microalgal fossils, are rare in manganese carbonate deposits, but microalgal fossils are abundant in black shales or silty nudstones under and above manganese carbonate deposits. Therefore, we suggest that the origin of manganese carbonate deposits in the Early Sinian Datangpo Formation is of no relation to microflora. This mechanism of formation of manganese carbonate deposits suggested that at 700 Ma the amount of atmospheric carbon dioxide was about 350 times its modern level, and the transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium (manganese) carbonate in warm surface waters, producing the cap carbonate rocks, including CaCO 3 and MnCO 3. Additionally, manganese carbonate deposits in the Early Sinian Datangpo Stage can be correlated with the post-Sturtian cap carbonate rocks observed globally.