The Characteristics And Formation Mechanism Of The Middle Jurassic Batongian Iron Oolitic In The Nyalam Area, Southern Tibet

Oolitic ironstones(or ooidal ironstones)are clastic rocks with ooid content of >5 vol.% and Fe >15% by weight(=21.4% by weight of Fe2O3),mainly marine sediments.The mineral types in the Paleozoic iron shale are mainly hematite,while the Mesozoic are mainly goethite,hematite,and chlorite.Oolitic ironstones are important deposits of iron-rich rocks in the geological history,especially in the Ordovician and Jurassic.It not only records the changes in the physical and chemical conditions and redox states of seawater during its formation,but also reflects the changes in sea level up and down,tectonic activities and interactions with organisms.The set of iron-bearing rocks exposed in the Middle Jurassic Dingjie Formation in the Nyalam area of southern Tibet.Some predecessors have already had some basic stratigraphic and petrological work,but the study of its internal microstructures,formation processes and mechanisms are relatively few.This paper analyzes this by using rock flake observation,cathodoluminescence observation,scanning electron microscopy(SEM),X-ray diffraction(XRD),and energy dispersive spectroscopy(EDS).The microstructure,mineral composition and mineral distribution pattern of the ticcites reveal the formation process and mechanism of the Middle Jurassic Batonian Oolitic ironstones Formation in the Nyalam area of southern Tibet,and explore its formation and iron composition at that time.The material source,the relationship between seawater redox conditions and microbial activity.The main understandings are as follows: 1.According to the results of paleontology research of predecessors,it is determined that the formation period is the Middle Jurassic Batonian period.2.The mineral composition of the whole rock is mainly calcite,a small amount of quartz,chlorite and goethite by XRD analysis.the cement is mainly calcium cement,and a small part of the area is impregnated with iron components which are precipitated later.3.The granules are mainly composed of chlorite and carbonate fluorapatite(CFA)interbeds.The results show that there is a significant negative correlation between Fe and P,and there is a positive correlation between Ca and P,which confirms the above observation of cathodoluminescence,and the iron-rich layer is chamosite,the layer rich in Ca and P is carbonate fluoroapatite.This interbedded structure indicates that the iron ooids are formed during in water rather than early diagenesis.According to the above results,it is concluded that the formation of chlorite is formed by the combination of ferrous ions and clay minerals in the water under suboxidation conditions.At this time,under the synergistic action of acid-reducing bacteria and sulfide oxidizing bacteria,The sulphide oxidizing bacteria decomposes the organic matter in the first sediment,releasing the polyphosphate,which is stored as an energy source by the oxidized oxidizing bacteria.When the environment becomes anoxic,it oxidizes the polyphosphate.Phosphate is released into the body of water while it combines with carbonate and calcium ions in the water to form CFA.