沉积型锰矿床的形成及其与古海洋环境的协同演化

Sedimentary manganese formation and its link with paleo-oceanic environment

海相沉积型锰矿的成矿过程受古海洋沉积环境影响,而古海洋环境又与超大陆聚合与裂解、极端地质事件、生命演化等密切相关,因此,海相富锰地层是岩石圈、水圈、大气圈和生物圈等多圈层耦合关系与物质循环相关信息的重要载体。深层海水缺氧模型、最小氧化带模型和幕式充氧模型都显示海水中氧化还原梯度的变化是导致锰矿形成的最主要原因。全球范围内海相沉积型锰矿主要形成于古元古代、新元古代和显生宙3个地质历史时期。其中,元古宙时期,地球上发育了完善的氧化还原分层的古海洋结构;古元古代早期和新元古代,超大陆裂解引起的海平面升降变化导致古海洋氧化还原结构产生动荡,并促使大规模沉积型锰成矿作用发生;地球沉寂期(1800~800 Ma)涵盖了整个中元古代,这一时期仅在华北地台发育了少量沉积型锰矿床,反映该时期古海洋中锰的迁移受到了抑制;显生宙地球再次进入活跃期,经历了数次海洋缺氧事件,冰室-温室气候交替促使海水的化学性质剧烈变化,并在局部氧化还原分层的沉积盆地中富集形成沉积型锰矿床。总之,古海洋氧化还原环境的变化是沉积型锰矿形成的必要条件,同时,区域性沉积盆地的结构、海平面的升降、火山作用导致的物缘供给等多种因素都会影响沉积型锰矿的形成。与沉积型铁矿相比,沉积型锰矿对局部海水化学性质的变化更加敏感,综合研究铁锰矿床的共生与分异过程,将有助于更加有效的识别不同尺度的沉积过程与古海洋环境变化。

The precipitation of manganese in the marine environment is intrinsically controlled by seawater redox and ocean che-mistry which are ultimately linked to many geological processes such as supercontinent reconstructions,extreme geological events,and biological evolution.Therefore,Mn-rich sediments are ideal for studying the co-evolution of the lithosphere,hydrosphere,atmosphere,and biosphere.Currently,models for manganese formation,which include euxinic basin model,oxygen minimum zone model and episodic ventilation model,all suggest that the changes of chemocline play a critical role in the formation of sedimentary manganese deposits.Three peaks of manganese formation could be identified through Earth history:Paleoproterozoic,Neoproterozoic,and Phanerozoic.A redox stratified ocean was developed in Proterozoic.During the Paleoproterozoic and Neoproterozoic,supercontinent break-up led to dramatic changes in ocean redox conditions,which promoted the forma960 tion of giant manganese deposits during these periods.However,only a few manganese deposits in North China Craton during the“boring billion”of the Earth(1800~800 Ma)were reported,probably suggesting that Mn was immobile in the stable,redox stratified ocean.Earth became more active during Phanerozoic,characterized by repeated ocean anoxic events(OAEs)in the overall oxidized Earth.Repeated greenhouse-icehouse conditions caused dramatic changes of chemocline in the Phanerozoic ocean,and enhanced manganese precipitation at the chemocline boundary.In summary,the redox stratified oceanic structure is a first-order factor to form a sedimentary manganese ore deposit;Other factors,such as ventilation to open ocean,sea-level changes,and volcanisms,may also impact the manganese mineralization.The sedimentary manganese formations,unlike iron formations that respond more to global parameters,seem to be controlled by local basin tectonics.Therefore,studies of the mechanisms of Fe and Mn partitioning will help to better understand sedimentary processes and paleo-oceanic redox environments at both global and local scales.