贵州省松桃大塘坡组含锰岩系的形成环境分析

Formation environment analysis of Mn-bearing rocks in Datangpo Formation,Songtao,Guizhou Province

贵州黔东断裂坳陷带是我国重要的锰矿成矿区,发育了新元古代大塘坡早期富锰黑色页岩。一般认为,锰矿是氧化环境下沉积的MnO2在早成岩过程中与有机质之间发生氧化—还原反应形成的;但最近研究显示,也有可能直接从缺氧海水中沉淀而成。本文对贵州省松桃县西溪堡锰矿区大塘坡组一段含锰岩系和大塘坡锰矿区铁矿坪锰矿床的大塘坡组一段含锰岩系中的样品,采用总有机碳、总硫、显微镜薄片鉴定、扫描电镜、电感耦合等离子体质谱等研究方法,探讨了大塘坡组一段的沉积环境,为锰矿成因提供新的线索。本研究分别提取和测试了碳酸盐矿物和残余物(主要为粘土矿物)微量元素、稀土元素的含量后发现,去除碳酸盐矿物后余样Sr/Ba比值为0.06~0.19、B/Ga比值为1.44~5.01,碳酸盐矿物成分中Y/Ho比值为16.18~29.78,均指示锰矿层沉积于Sturtian冰期后冰川消融而成的淡水—半咸水的环境。锰矿的稀土总量高,类似于大洋铁锰氧化壳的“帽型”稀土元素配分模式,具有Ce/Ce^(*)正异常;氧化还原敏感元素双原子比U/Th比值为0.16~0.35、V/Cr比值主要为0.35~5.84、Ni/Co比值介于0.64~12.45。这些值中绝大部分都低于氧化—还原临界值,表明西溪堡和铁矿坪含锰岩系都沉积于氧化环境。于是,锰以氧化态形式(锰氧化物)从水沉淀下来,后期在成岩过程中被还原而形成碳酸锰矿床。

The East Guizhou fault-depression belt is an important manganese deposits-forming area in China with the occurrence of manganese deposits mainly in the Neoproterozoic Datangpo Formation Member 1 black shales.It is generally believed that the formation was caused by redox reactions between MnO2 and organic matter during early diagenesis,but recent studies have shown that it may also be directly precipitated from anoxic seawater.By means of microscope,scanning electron microscope and inductively coupled plasma mass spectrometry,the sedimentary environment of the Member 1 of Datangpo Formation from Xixibao and Tiekuangping manganese deposits,Songtao,Guizhou was studied,to provide new clues for the genesis of manganese deposits.Trace elements and rare earth elements from clay minerals and carbonate minerals were extracted separately to reflect freshwater/marine environment and redox conditions.The results show that the Sr/Ba values from clay minerals after removing carbonate minerals range from 0.06~0.19,B/Ga ratios from 1.44~5.01,and Y/Ho ratios from carbonate minerals are from 16.18~29.78.All these values are less than that of modern seawater,respectively,indicating a fresh-brackish water environment.Redox-sensitive elements U/Th ratios from non-carbonate fraction is from 0.16~0.35,V/Cr ratios from 0.35~5.84,and Ni/Co ratios from 0.64~12.45,most of the values are less than the thresholds of oxic/anoxic boundary,indicating that the manganese ore beds were originally deposited under a suboxic environments.This proposal is further supported by REEs distribution from the carbonate fractions which are characterized by high total rare earth elements,positive Ce anomaly and higher middle REEs than light and heavy REEs,which is similar to the"cap type"rare earth element partitioning pattern of modern seabed manganese nodules(shells).The positive Ce anomaly of the carbonates is typically inherited from Mn oxides as the result of strong absorption of Ce(Ⅳ)oxides on the surfaces,thus strong pointing to an origin of Mn carbonates from reduction of Mn oxides.