Qiao Xiufu Gao Linzhi Peng Yu Xu

The original text was published in Journal of Geology 1999, Volume 7 1 No.3; The English version was published in Journal of Geology, 1997, Volume 7 14.

There are many explanations for the genesis of the Bayan Obo superlarge deposit. In this paper, it is proposed for the first time that its host dolomite is a huge deep-water microcrystal mound. Later, in Zhang et al.' s research, it was considered that the material of deep-water microcracks originated from submarine hydrothermal solution and CO2 overflow, and the organisms in them were accompanied by deep-sea hydrothermal activity (New understanding of the genesis of microcracks, 2005, Progress in Earth Science, Vol.20, No.20). The significance of reprinting is as follows: ① The dolomite of Shalinhudong Group and the dolomite of Bayanobo super-large niobium-bearing rare earth iron ore are related to deep material activities, and the earthquake events are located directly below and inside the microcrystal mound, which also reflect the results of deep material activities; Earthquake events are related to endogenetic metal mineralization. In the study of mineral deposits, we should pay full attention to the study of stratigraphic seismic records. ② The seismic records of Group 4 of Hudong Group in Jilin Province are typical "molar structure" and the age is Ordovician. Recently, Gao Linzhi and others dated the zircon SHRIMP U-Pb of the porphyry in the east group of Shalin Lake as Ordovician, which is completely consistent with the paleontological data, thus indicating that the bright crystal veins of liquefied mud (called molars abroad) are not limited to Mesoproterozoic and Neoproterozoic, as some scholars know.

According to the study of sequence stratigraphy, event stratigraphy, biostratigraphy and lithostratigraphy, Shalinhudong Group is a part of Baiyunebo Group. Trilobite fragments were first found in the lower rock slices of Shalinhudong Group, and the Ordovician suspected provenance and chitin fossils were first separated. For the first time, carbonate seismite formation and giant micrite limestone mound at the top were found in the upper horizon. The ore-bearing dolomite of the super-large Nb-RE iron deposit in Bayan Obo is neither igneous carbonate rock nor general layered sedimentary rock, but a huge microcrystal mound, which is consistent with the macroscopic characteristics of the microcrystal mound dolomite at the top of Shalinhudong Group and may belong to the same horizon. According to the existing fossil data, the Shalinhudong Group and Baiyunebo Group belong to Paleozoic rather than Mesoproterozoic. The new discoveries and new understandings in this paper will provide new ideas for the genetic interpretation of the Bayan Obo super-large deposit. It is possible to find new deposits of the same type in its south.

Xilin Lake East Group and Baiyunebo Group are located in Baiyunebo-Bailingmiao grassland in the northwest of Hohhot, Inner Mongolia Autonomous Region, and are distributed in the east-west direction between 4 1 38 ′ and 4155 ′ north latitude (Figure1). The research on Baiyunebo Group and Baiyunebo deposit has a long history and high precision, but it lacks typical sedimentary rock structure and layered rock structure and weathered terrain. Some people think it is igneous carbonate rock. Although supported by petrochemical and geochemical data, it is impossible to deny the biological fossils and algae laminae. Some people think that it is related to volcanism, and the genesis of ore-bearing dolomite is directly related to the genesis of the deposit. In recent years, Zhang Pengyuan and Pan Qiyu should be mentioned in the study of basic geology and genesis of Baiyun Obo Group. The drainage vein is composed of bright calcite, and horizontal stripes are pierced at the upper and lower ends of the vein, which makes the stripes stretch and bend (plate Ⅰ-1). Besides drainage veins, liquefaction and curling deformation (unit B) and interlayer faults (unit C) are widely developed in striated limestone. There are 7 seismites in DS5 and 3 seismites in DS7 microcrystal mound, representing 10 seismic episodes. Obviously, DS5—DS7 is the most intense period of tectonic activity (fault activity, volcanism and igneous intrusion) from the east of Xilin Lake to the Aola trough, and it may also be the metallogenic period of the corresponding Bayanobo deposit. The seismite is the product of a strong earthquake (M > 6) caused by tectonic activity at that time.

3 strata

Seismic records of plate ⅰ of Hudong Group in Jilin Province and microcrystalline hills

Seismic records in strata

Shalin Hudong Group was originally named Shalin Hudong Group, and the Geological Survey of Inner Mongolia was established by the first regional geological survey team of Inner Mongolia Bureau of Geology and Mineral Resources in 197 1 year. Regional Geological Survey Report of Dahan Maoming United Banner (K-49X X) 1: 200000, 1976544.

, divided into a rock section and two rock sections. As mentioned earlier, the thickness of "Shalinhu East Formation" is 900 meters, and there are many sedimentary discontinuities. Therefore, according to the definition of lithostratigraphic group, "Shalin Lake Cave Group" should be called "Shalin Lake Cave Group", which can be divided into five rock groups (Figure 2). DS 1 and DS2 are a group, that is, clastic rock and carbonate mixed rock group, which is equivalent to the original rock profile. The second, third and fourth strata are carbonate rocks, the top and bottom of which are defined by clear continental weathering crust, which is easy to identify in the field. Among them, the third rock group is layered limestone rock group, which should be transformed horizontally with limestone; DS5 alone constitutes the fourth group, which is a strong earthquake event rock group. DS6 and DS7 (Group 5) are micrite dolomite.

Biostratigraphy

4. Trilobite remains were first discovered in1year.

More than a dozen tiny biological fragments (plates II-3, 4, 5) were found in the upper layer of 10 in the east section of Xilin Lake, that is, the microcrystalline limestone slice with quartz sand at the top of DS 1. The particles are arc-shaped, some are wavy, and the shell of the fragments has dark powdery iron edges. When the surrounding gypsum crystallizes into fine crystals, it still maintains the original bioglass fiber structure and is in an orthogonal polarization state. A large number of deformed S-shaped chronotropic crystals can be observed in the thin slices running through the rock, and tiny cracks filled with fine iron powder can be seen cutting trilobite fragments.

4.2 The Ordovician suspected source rocks were first discovered.

Micropalaeophyte samples were collected from DS 1, DS2 and DS3 (Figure 2), 2 1 copy. The analysis results are as follows:

①DS 1 in the second partition rock: basidium. Dryopteris simplicifolia, Dryopteris simplicifolia. , Chlorella. Zoospphaerium sp. Taenia solium.

② In the black structural limestone in the 22nd layer (CS section) of ②DS2: micrhystidium sp. 1, microsporidia. 2, small ring rot pathogen. , Microcystis. Gonio-spherical. ，Goniosphaeridia sp .，Baltisphaeridium solidium(Sin， 1962)Fu，Ancyrochiti-nasp。 ，Rbabdochitina sp .，Cyathochitinasp。 .

③ In the black structural limestone of CS section in the 35th layer of ③DS3:? Rbabdochitinasp。 ，Goniospheridiasp。 ，Leiopso-phosphaera sp .。

Due to the slight metamorphism of the Hudong Group in Jilin Province, only horny and spiny fossils remain in the spiny suspected provenance. The fossils in DS 1 belong to CAMBRIAN molecules. On the other hand, the spiny suspects and chitin insects of DS2 and DS3 belong to Ordovician molecules (plate ⅱ-6 ~ 17), especially the CS section of DS2, which reflects that the CS section has been deposited for a long time and can retain more genera and species. Coccidium solidum And chitin were first discovered in the Lower Paleozoic of Russian platform and Yangtze platform. The suspected provenance age of DS2 and DS3 in Shalinhu Cave Group should be Early Ordovician.

1. The seismite (rock slice, scale length 0.5cm) in the fourth group gill (DS5) limestone, and the liquefied vein pierces the horizontally textured limestone and bends at both ends of the vein. It can be clearly seen from the thin slice that the vertical veins are formed by liquefaction and drainage of countless horizontal micrite layers under strong earthquake vibration (unit A of seismite sequence). Vertical bright veins can be seen at the top, middle and bottom of the figure, indicating that the liquefied bright veins originated from horizontal striated limestone. 2. Macroscopic distribution characteristics of ore-bearing microcrystal mounds in Bayan Obo Group. 3. Macro-morphology of weathered microcrystalline mound dolomite in Shalin Lake Cave Group. 4. Blue-gray microcrystalline limestone rich in organic matter and yellow algae clusters are alternately arranged on the upper part of the microcrystalline mound of Shalin Lake Cave Group. 5. The contact relationship between microcrystalline calcite and bright calcite in the yellow algae crown in the microcrystalline mound at the top of Shalin Lake Cave Group. 6. Dark algae mud and a large number of holes (as shown by arrows) in the lower part of microcrystalline mound of Hudong Group, Jilin Province, with the scale length of 0.56 mm. 7. The mineral-bearing microcrystalline mound of Bayanobo has turned into fine-grained dolomite, in which reconstructed timely debris can be seen, with the scale length of 0.5 mm.. The samples were taken from the surface in the east of Bayan obodong mine.

Plate Ⅱ: Paleontological Data of Hudong Group in Jilin Province

Seismic records in strata

No.5 microcrystal mound-host rock of Bayanobo super-large deposit

Microcrystalline mounds are generally discus-shaped bodies with flat bottom and convex top, and the thickness is generally from several meters to several tens of meters. They appear in strips in the gentle slope area of deep water, parallel to the ancient coastline. Microcrystalline mound is composed of microcrystal limestone, biological components, stromatolites, bright crystals and terrigenous sediments. Qian Xianhe systematically studied and summarized the microcrystalline mound. He believes that during the formation of microcrystalline mound, microorganisms, such as fungi, cyanobacteria, etc., deposited a lot of microcrystalline plaster in the process of metabolism, and at the same time captured and precipitated part of the plaster, resulting in a lot of microcrystalline limestone. When studying the CAMBRIAN-Ordovician sequence stratigraphy of North China platform, the author identified the pure limestone at the bottom of the Yeli Formation in Xishan, Beijing and Hunyuan, Shanxi as microcrystalline hills, and made a preliminary study on its macro and micro characteristics. The new fossil materials discovered by the author in the East Group of Gilles Forest Lake support the work of Zhang Pengyuan and others. According to the available data, only the Ordovician-Triassic microcrystal mounds have been found in the world, which also proves that the Baiyunebo Group and the Hudong Group in Jilin are more likely to be the Lower Paleozoic. According to the new geological records, the author revised the view of the original Mesoproterozoic, and thought that the Shalinhudong Group-Baiyunebo Group was a Caledonian aulacogen deposit. According to seismic strata, rifting started in the fourth gill stage, that is, DS5 stage. As mentioned above, this period may be the main metallogenic period, and the fossil data of Lower Paleozoic in Baiyun Obo Group are consistent with a large number of Caledonian metallogenic age data. As for the age data of 1400 ~ 1200ma, its geological significance needs further study.

6.2 Ore deposits controlled by hills

Based on the new understanding that ore-bearing dolomite is microcrystal mound, the formation of Bayanobo deposit is not only related to mantle-derived materials and volcanism, but also related to biomineralization and ore storage of microcrystal mound.

7 conclusion

The study of Shalinhudong Group and Baiyunebo Group involves the understanding of the tectonic evolution in the northern margin of North China Platform and the genesis of Baiyunebo deposit. The author hopes that the new materials and knowledge provided in this paper will arouse geologists' interest in stratigraphy, sedimentology, paleontology, structural geology, isotope chronology and deposit science, and further study them from a new angle and idea.

How to explain the contradiction between some isotopic age data and paleontological data at present? How to compare the seismite of Shalin Lake Cave Group with the seismite of Baiyun Obo Group in isochronous time? What is the tectonic background of strong earthquakes? Is it possible that the microcrystal mound of Shalin Lake Cave Group is higher than the ore-bearing microcrystal mound of Baiyun Obo, but a diachronic microcrystal mound? What are the biomineralization and ore storage mechanisms of microcrystal mounds? These problems need further study.

The proposal of hill-controlled deposits makes us have to focus on the microcrystalline hill in Shalinhu Cave parallel to the ore-bearing microcrystalline hill in Baiyun Obo, and whether there is similar iron, rare earth or (and) niobium mineralization in this microcrystalline hill. This is the expectation of this paper.

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