Genesis Of The Early Mesoproterozoic Yunmengshan Iron Formation In The Southern Margin Of The North China Craton And Its Paleoenvironmental Implications

Iron formations(IFs)are iron-and silica-rich chemical sedimentary rocks deposited throughout much of the Precambrian that bear information of the composition and evolution of the ancient seawater,hydrosphere,atmosphere,and biosphere through much of the Precambrian eons.The depositional mechanism and abundance of the IFs evolved along with the protracted,oxygenation,tectonic activity,and volcanic activity of the early Earth in the first two billion years of life's history.After the Great Oxidation Event(GOE),the weight and quantity of marine IFs decreased rapidly,finally resulting in the disappearance between ca.1.80Ga?0.75Ga.During the transition between the late Paleoproterozoic and Mesoproterozoic(1.8?1.6Ga),a new type of iron formation(IF)deposited in the restricted spring and coastal zone.In the North China Craton,it was named as Daimeishan type iron ore deposit(or Yunmengshan IF)and Xuanlong type iron ore deposit(or Chuanlinggou ironstone).Petrologically,the new-type IF is characterized by granular mineral aggregates,such as oncoid and ooid,and commonly contain sedimentary features,such as cross-bedding and local stromatolites.Mineralogically,the IFs mainly consist of hematite but lack magnetite,iron?carbonates,and sulfide.The appearance of the new-type iron formation,together with the disappearance of IF,indicates an abrupt change of the redox condition and chemistry of Earth's surface between 1800 and 750 million years ago.However,The cause of the abrupt change and the redox condition of Earth's surface between ca.1.80 Ga and 0.75 Ga are still a matter of significant debate.The Yunmengshan IF is located in the upper part of the Paleoproterozoic Yunmengshan Formation in the Ruyang Group of the Mianchi-Queshan stratigraphic provinces,which belong to the southern margin of North China Craton.The study on the source of Fe and the mechanism of Fe(?)-oxidation of the Yunmengshan IF will help to understand the redox condition of Earth's surface and atmospheric oxygen concentration at the transition between Paleo-and Mesoproterozoic eras.To reveal the genesis of the Yunmengshan IF,the redox condition of Earth's surface,and atmospheric oxygen concentration at the transition between Paleoproterozoic and Mesoproterozoic eras,we conducted integrative studies of sedimentology,petrography,mineralogy,and geochemistry on the shallow-water iron formation from the Daimeishan section of the Paleoproterozoic Yunmengshan Formation(ca.1.75Ga)in the southern margin of North China Craton,using microscopy,SEM,TEM,XRD,M?ssbauer spectroscopy,EPMA,and MC-ICP-MS techniques.Based on detailed analyses of sedimentology,petrography,mineralogy,and geochemistry,the main contributions of this thesis are listed as followed:1)Sedimentological study on the Yunmengshan IF of the Daimeishan section shows that the Yunmengshan IF was deposited primarily in the shallow-water environment of the intertidal to upper subtidal zone,which is similar to the habitats of modern terrestrial Fe OB.2)The content of the Si element is much lower than that of the typical BIFs deposited in the marine.Meanwhile,all of the analyzed samples lack a significant Eu anomaly,with Eu/Eu*[=Eu _SN/(0.66×Sm _SN+0.33×Tb _SN)]ranging from 0.94 to 1.11 and an average bulk-rock Eu anomaly of 1.06.The?⁵⁶Fe values vary from-0.09‰to-0.46‰with an average?⁵⁶Fe of-0.28‰,exhibiting an obvious negative anomaly.All those suggest that the Fe was likely leached from the terrestrial rock exposed to the coastal environments,and precipitated primarily as amorphous Fe-oxyhydroxide in the coastal depressions that finally lithified to irregular ore depositions as observed via dehydration during diagenesis.3)Minerals of the Yunmengshan IF are almost exclusively consisted of hematite and lacks quartz and magnetite.Morphology and mineral assemblages,together with the geochemistry of the Yunmengshan IF imply that the Yunmengshan IF is the transitional type between the earlier deposited Precambrian BIF/GIF and the latter Phanerozoic ironstone.4)The Yunmengshan IF yielded samples with markedly positive?⁵³Cr_auth values(?⁵³Cr_auth=1.03‰?2.29‰,average at 1.42‰),which is higher than the?⁵³Cr records before(<0.5‰)and can be compatible with the highly positively fractionated?⁵³Cr record related to the Neoproterozoic Oxygenation Event(NOE,?0.85 to?0.54 Ga),whose estimated atmospheric O₂ content during was over 10%PAL.The positive?⁵³Cr_auth values indicate that there was a pulsed oxygenation event at the end of Paleoproterozoic,which leads to the rising of atmospheric oxygen,reaching 1%-10%PAL.The enhanced oxidative weathering increased the input of Fe(?)into the terrestrial environments,providing abundant in-situ Fe(?)for the metabolism of Fe OB but at a further reduced scale.5)The REE+Y,redox-sensitive elements,Fe isotope,and Cr isotope of the Yunmengshan IF indicate that the concentration of the atmospheric oxygen at the end of Paleoproterozoic is still far lower than that of the modern atmosphere.The oxygen concentration in the aqueous environments where the Yunmengshan IF deposited was extremely low,probably 2.8?28.2?M,which would inhibit the chemical oxidation of Fe(?)by free O₂.However,the circumneutral microaerophilic Fe OB,such as Gallionella sp,thrive and might become the predominated oxidation mechanism of Fe,playing an important role in the new-type IFs,which obtain energy by oxidizing ferrous iron to ferric iron.Therefore,Fe OB in this age is globally distributed in the terrestrial environments(including the subaerial environments of the coast)and is a short but glorious age of FeOB.