Although Yao De et al. (1998) suggested that the formation of Bayanobo deposit was related to meteorites, they did not provide new evidence that dolomite was formed by the interaction between meteorites and seawater, that is, the whole ore body was not an exotic block. Moreover, in the sandstone underlying the ore body, obvious carbonate veins (walls) can be seen cutting through the stratum, and there is obvious aegiring on both sides of the carbonate veins (walls), which has similar geochemical characteristics to the main ore body (Zhang et al.,1998b; ; Yang Xiaoyong et al., 2000; Ni Pei et al., 2003; Yang et al., 2004; Fan et al., 2006), indicating that the main ore body and carbonate vein (wall) are homologous products, and there is no possibility of foreign rocks.
For a long time, geologists and paleontologists have shown that the Baiyun Obo Group was not earlier than Sinian, but probably from Sinian to Ordovician (Sun Shufen et al.,1992; Zhang Pengyuan et al.,1993; Qiao Xiufu et al.,1997; Tan Li can wait, 2000). However, the isotopic ages obtained by geochemists and isotopic chronologists range from Proterozoic to Ordovician, and most of them are concentrated in Mesoproterozoic.
On the other hand, the mineralization of Bayan Obo is extremely special, with obvious and unique geochemical characteristics, such as extremely high Nb, REE, F- and CO2 fluids (hydrothermal or magmatic). Because the geological environment and tectonic attributes change with time, it is hard to imagine that the same mineralization with extremely special characteristics can occur in the same place in different geological ages. Obviously, the viewpoint of multi-stage mineralization of Bayan Obo deposit is not credible.
This chapter will use dialectical and systematic geological research ideas (Hu, 1983,1992; Hu et al., 1992, 2006) and logical common sense (Zhang, 2006), to discuss the significance of isotopic age and the most basic problem of metallogenic age of Bayan Obo deposit.
First, the era of Baiyun Obo Group
Baiyun Obo Group is mainly distributed in the east-west line of Baiyun Obo iron mine area, east to Huade County in the south of Xilin Gol League, called Huade Group in Hebei Province, and west to Xiongbaozi in Darhan Maoming Union Banner. From bottom to top, it can be divided into six strata, with a total of 15 layers, namely H 1 ~ H 15, which are Dulahala Formation (H 1 ~ H3), Jianshan Formation (H4 ~ H5), Hala Hoggatt Formation (H6 ~ H8) and Bibi Formation. In addition, there are Ayaden Formation, Alehuduge Formation, Hu and Ailigeng Formation in the upper part, which also belong to Baiyun Obo Group. The regional geology of Inner Mongolia Autonomous Region (Bureau of Geology and Mineral Resources of Inner Mongolia Autonomous Region, 199 1) divides the uppermost Alehuduge Formation and Huhe Aligeng Formation into the Middle and Upper Ordovician Bao Erhan Figure Formation. Cancel the seventh rock formation, Ayaden Rock Formation, and return to the upper part of the sixth rock formation, namely the Hugirtu Formation.
There are only six lower strata in Baiyun Obo area, which are divided into 15 layers, that is, H 1 ~ H 15. Many documents think that ore-bearing dolomite is equivalent to H8, but Zhang Pengyuan and others (1993) think it is equivalent after detailed study.
The geological age of Baiyun Obo Group has been controversial for a long time and has been changed many times: ① When Li Yuying and others founded the group in 1957, they classified it as Proterozoic Hutuo System; (2)1964, when the first investigation team of Inner Mongolia surveyed the area of1:200,000, it was divided into CAMBRIAN-Early Silurian according to the Favositidae fossils (disputed honeycomb corals) in the stratum; ③ In 0966, cephalopods, brachiopods and gastropods were found in the Ayaden Formation (above the Hujirtu Formation) of Baiyun Obo Group in Shangdu, with a regional scale of1:200,000, and changed to Cambrian-Ordovician; ④ The Bureau of Geology and Mineral Resources of Inner Mongolia Autonomous Region (199 1) classified the top two groups of Baiyunebo Group as Middle Ordovician, and all other rock groups were classified as Mesoproterozoic Great Wall System, and the seventh rock group Ayadeng Rock Group was abolished and included in the upper part of the sixth rock group, namely the Hujirtu Formation; ⑤ When the strata were cleared in 0994, 1 ~ 6 strata were classified as Mesoproterozoic-Qingbaikou Great Wall System; ⑥ The first and second teams of Inner Mongolia Regional Research Institute restored Ayaden Formation, and classified Dulahala Formation and Jianshan Formation into Mesoproterozoic Great Wall System; The Hala Hoggatt Formation and Birut Formation in the central part belong to the Mesoproterozoic Jixian System. The Upper Baiyinbaolage Formation, the Hujirtu Formation and the Ayaden Formation belong to the Neoproterozoic Qingbaikou System.
Sun Shufen (1992) and Zhang Pengyuan (1993) have found 33 species of microfossils 10 genus in Jianshan Formation (H4-H5) of Baiyun Obo Group, and their assemblage features are mainly spines and spherical molecules. Baltisphaeridium and Micrhystridium are dominant in Echinococcus group. Micro-palaeophytes are generally10 ~ 30μ m. Except for 3 new species and 4 undetermined species, 9 species were found in the Lower Cambrian of Zhumiaoguanshan section in Kunming, Yunnan, and 6 species were found in the Lower Cambrian of Scotland, Greenland, Norway, Canada and Russia. Six species were found in Qingbaikou in China, and five species were originally found in Ordovician in Russia. Therefore, Sun Shufen (1992) thinks that "the microfossils of Jianshan Formation in Baiyun Obo Group are distributed in the Lower Cambrian, and the inherited molecules can reach about 8 1%, with only a few molecules at a higher level; Therefore, the Jianshan Formation of Baiyun Obo Group can be compared with the Lower Cambrian in Kunming, Yunnan, China and the Lower Cambrian in parts of Europe and North America in terms of microfossil assemblage. " Zhang Pengyuan et al. (1993) put Baiyun Obo Group in Cambrian-Ordovician.
Tan Like et al. (2000) discovered 8 genera and 8 species (including 1 similar species and 2 undetermined species) of Meishucun fauna in the 4th floor of the lower part of Ayadeng Formation in the west Wudaowan section of Agutu Station, Shangdu: soft-tongued snail: Conothecasubcurvata；; Soft-tongued snail; Sword-shaped protohutt vertebrate; ; Taurelle's bacteria. (undetermined); Prismatic shell: prismatic shell (undetermined). , broadband shallow shell Lopochites latazonalis；; Toothed crustaceans: Cambrian crustaceans (similar species), Ganloudina longispina；; Carmen crustacean: Tannuolinamultifora, with a porous Tangnuoula shell and a large number of sponge spicules (3 genera): protosponges. Hunan sponge (undetermined species). And glass sponge (unidentified species). (All the above fossils were identified by Professor Jiang from Yunnan Provincial Department of Geology and Mineral Resources). Therefore, there is no doubt that the middle and lower part of Ayaden Formation belongs to Cambrian, the bottom of Ayaden Formation can be regarded as the Sinian-Cambrian boundary, and the lower Hugletu Formation is in integrated contact with Ayaden Formation, which should be regarded as Sinian stratum. The upper 6 ~ 7 layers (No.8 ~ 9 in Beishan section) belong to Ordovician, among which 6 layers have been found with early Ordovician cephalopods, brachiopods and gastropods.
The research group including the author (Qiao Xiufu et al., 1997) thinks that the original Shalinhudong Formation, more than 20 kilometers southeast of Baiyun Obo, is equivalent to the lower part of Baiyun Obo Group, and changed it into Shalinhudong Group. Microcrystalline mounds were found at the top of the Xilin Hudong Group, and their characteristics are very similar to those of the ore-bearing dolomite in Bayan Obo, so they may be isochronous. That is to say, Hudong Group in Lin Xia is equivalent to H 1 ~ H5 of Baiyun Obo Group. At the same time, trilobite fragments, microfossils and chitin insects were found in caves in Jilin forest. ① In the upper part of 10 layer in the east section of Xilin Lake, that is, in the microcrystalline limestone slice containing quartz sand at the top of DSl, there are more than a dozen tiny biological debris particles, which are arc-shaped and partially wavy, and there is a dark powdery iron edge on one side of the debris shell. When the surrounding gypsum has crystallized into fine crystals, it still maintains the original bioglass fiber structure and shows tracer extinction under orthogonal polarization. This structural structure is a typical trilobite debris (Figure 5-65433 ② Microfossils and chitin fossils are found in the slate interlayer near the bottom: Lophophorium sp., LeiopsophosphaeraSimplesin, Leiopsophosphaerasp. Microsporum ... Lepidoptera simplex. Molecules belonging to CAMBRIAN. ③ In the lower part of the limestone with black structure, there are: Dianthus microphylla. 1, microsporidia. 2. Scutellaria, phosphorus-phosphorus bacteria. , slightly concentrated sp. Goniosphaeridiasp。 ，Goniosphaeridiasp。 ，Baltisphaeridium solidium(Sin 1962)Fu，Ancyrochitinasp。 ，Rbabdochitinasp。 ，Cyathochitinasp。 . 4 there are: (? )Rbabdochitinasp。 ，Goniospheridiasp。 ，Leiopsophosphaerasp。 . The middle and lower spiny suspected species and chitin insects should belong to early Ordovician molecules.
Fig. 5- 1 trilobite debris found in the east group of branchial forest lake (quoted from Qiao Xiufu et al., 1997)
Left: tiny trilobite fragments in rock slices (arrows). It can be seen in the figure that the fine cracks cut off the trilobite fragments, and the scale length is 0.15 mm; ; Right: Trilobite detritus with glass fiber structure, and the dark part is mixed mud and iron impurities. Single polarization, the proportional rod is 0.52 mm long.
To sum up, the only basis for the classification of Baiyunebo Group in Mesoproterozoic is the isotopic age of mineralization. However, it is hard to avoid these paleontological evidences.
Two. Mineralized isotope age
Zircon SHRIMP and D-TMSU-Pb Method
Fan et al. (2002) measured the U-Pb isochron age of five zircons in the carbonate vein on the south side of Boluotou Mountain in Bayan Obo mining area, which should represent the crystallization age of zircons, that is, the emplacement age of carbonate walls. Fan et al. (2006) used isotope dilution mass spectrometry (D-TMS) to determine the U-Pb isochron age of three zircons in Wudi carbonate rock wall as 14 16 77ma, and the age of the other zircon as 1925 8Ma. The author thinks that the older age of1925 8ma should represent the age of zircon capture by surrounding rocks, and the age of 2070±33Ma obtained by Fan et al. (2002) is also the age of zircon capture by surrounding rocks, while the emplacement age of carbonate rock walls may be about 1400Ma.
Liu Yulong et al. (2006) determined the ages of four carbonate dikes (called carbonate dikes by the author): ① The age of the lower intersection of zircon SHRIMP U-Pb is 1984 180ma, and the age of the upper intersection of zircon SHRIMP is 2085±330ma；; ② The age of the upper intersection of zircon Shrimp is 2035 565438. ③ The age of the upper intersection of zircon D-TMS is1934 64ma; ④ The Pb-Pb isochron age of the whole carbonate vein is 65438±0236±300Ma. It is considered that the age of about 2.0Ga represents the formation of carbonate veins, while1236 300 Ma represents the activation of rifts.
(2) Sm-Nd era
It is understood that this document is mainly written by or in cooperation with researcher Zhang. But the age gap is still quite large (Table 5- 1). Although it is mainly concentrated in 1.2 ~ 1.6 Ga, there are also many values exceeding 400 million years, 800 million years and 1 100 million years.
Cao Ronglong et al. (1994) gave three isochrones, two of which are parallel and very close. The other is just two points outside these two lines (very close) plus two points on the line, and the author did not give any reason for choosing two points on the line, so in fact, it can be said that this line is unique and should be cancelled.
Zhang et al. (1994) think that the source of rare earth is the fluid rich in CO2, F, alkali metals and rare earth elements separated from metamorphic (metasomatic) depleted mantle, and it rises to mineralization around 1298Ma.
(3) Re-Os era
Liu Lansheng et al. (1996) measured the Re-Os model age of molybdenite as 439±8Ma, which the author thinks represents the late metallogenic age according to the popular view.
Liu Yulong et al. (2005) determined that the Re-Os isochron age of pyrite is also 439±86Ma, which is also interpreted as the late age.
(4) Other ages
Liu Yulong et al. (200 1) dated the H8 limestone in the north of Kuangou. The results show that due to the obvious surplus of uranium in the system, the U-Pb system can not give the isochron age, while the Pb-Pb isochron age has a large error, and two ages can be obtained by different data selection:1500 400 Ma and 820.
Liu Yulong et al. (2005) attempted the U-Th-Pb-Sm-Nd isotopic joint dating of 12 monazite in dolomite rare earth ore of Bayan Obo deposit. The uranium content is too low for U-Pb system to give isochron. The isochron ages of eight monazite thorium-lead systems are 123 1 200ma.
Table 5- 1 test results of mineralization age of baiyunebo deposit by Sm-Nd method
① Yellow River mine, albite, albite, albite. ② There are 65,438+00 monazite, bastnaesite, Yellow River ore, soluble stone, amphibole, sodium amphibole, aegirine and apatite, and the data of the other four samples are from Nakai et al., 65,438+0989. The author gives three isochrones, two of which are parallel and very close; The other is two points outside these two lines (very close) plus two points on the line. Actually, it's just two points. Two points outside the line should be cancelled as abnormal points, so this line is cancelled, so only two lines are listed. ③ Whole rock, dolomite, fluorite, apatite and monazite.
Zhao Jingde et al. (199 1) measured that the K-Ar or Ar-Ar age of 10 alkaline amphibole is about 820 ~ 396±4ma；; Th-Pb model age, mineral isochron or internal isochron age of monazite 10 rare earth minerals are 596 3 ~ 40712ma.
Three. discuss
(1) Biostratigraphy
According to stratigraphic paleontology, the age of Baiyun Obo Group should be Sinian to Ordovician.
(2) age of uranium and lead
It can be seen from the above literature that zircons used for zircon U-Pb dating are all from carbonate veins, and the carbonate veins where Fan et al. (2002) samples are located are all in metamorphic rocks or H 1 ~ H4 sandstone underlying the Bayan Obo Group. Although they may have the characteristics of magmatic zircon, it is difficult to rule out that they come from source sandstone or metamorphic rocks. As we all know, zircon is rarely seen in the ore body of Baiyunebo deposit, and it is rich in thorium and depleted in uranium. Carbonate veins have the same geochemical characteristics as ore bodies, so it is difficult to produce zircon in large quantities. The author thinks that ore-bearing dolomite was formed by hot water deposition, and carbonate veins were formed by homologous hydrothermal metasomatic metamorphic rocks or sandstone (Zhang et al., 2005). Zircon in carbonate veins is zircon in metamorphic rocks or sandstone, which may be reformed by hot water, and it is normal that its age is older than that of ore-bearing dolomite. Whether the age measured now can represent the transformation time of zircon by ore-forming hydrothermal process, I am afraid we have to find circumstantial evidence to support it.
(3) Sm-Nd age
The reported age of Sm-Nd mode is concentrated, with tDM greater than 1.6 GA and Tchur greater than1.2 ga; . However, the ages of isochrones vary greatly, from 1.7Ga to 0.4Ga, and the data of the same author, Mr. Zhang, are also between 1.6Ga and 0.8Ga, and the ages of many isochrones themselves have great errors (Table 5- 1).
Liu Yulong et al. (2005) pointed out that the Bayan Obo deposit is characterized by high light rare earth content and low Sm/Nd value, and most samples are lower than 0.05, which is the reason why most dating results have large errors and poor isochron linearity. Looking up these original Sm-nd dating data (Table 5-2), it can be found that the Nd values of most samples are less than 8%, and among 70 samples from ore-bearing dolomite or ore, only 7 samples have Nd values of 147Sm/ 144Nd greater than 8% (3. Many are below 5% (Figure 5-2b). In addition, the half-life of 147Sm is very long-106.0ga, so 143Nd decaying from 147Sm is compared with non-radioactive 143Nd. The share is small. It is known that λ (147 sm-143 nd) = 6.54×10-12a/, N (143Nd *) is the present 143 Nd 12. 18% due to the n (147sm)/n (/. Therefore, even if it is assumed that Bayan Obo was mineralized before1500,000 years ago, the radioactive origin of 143Nd* only accounts for all 143Nd in most samples dating now.
. Therefore, a big error is inevitable.
Based on the knowledge that Bayan Obo is a single mineralization, it can be assumed that samarium and neodymium in Bayan Obo deposit have the same source and time, so we tried to collect 98 original data, of which 70 came from ore-bearing dolomite or ore body, 25 from overlying slate and 3 from underlying carbonate veins (Table 5-2). They are mixed together and calculated by Isoplot program. There is a clerical error (from Zhang et al., 1997, a dark slate sample, 147Sm/ 143Nd is 0.0134 in the data table, but it is located at 0.65438 in the isochronous diagram. Analysis shows that the latter is reasonable), and the other two blocks (both from Cao Ronglong and others, respectively, 1994) are abandoned. The 96 Sm-Nd data can be fitted into a straight line with R=0.96325 and slope = 0.00739, and the age t =125.8 32.5 Ma (λ = 6.54×165438+5-2a-.
If only 70 pieces of ore-bearing dolomite or whole rock or monazite, bastnaesite, Yellow River ore and other rare earth-bearing minerals are used, two samples are far from the isochron, and the remaining 68 pieces should be combined into a straight line, with R=0.8863 1, slope = 0.00700, age t =1066.68.8ma..
If only 25 slate (whole rock) samples are used, the results are R=0.96642, slope = 0.00809, age t =1232.0 68.5 Ma (λ = 6.54×10-12a-65438+.
As can be seen from Figure 5-2a, all data points are close to a straight line (except two abnormal samples).
However, if the data are divided into two groups: ore-bearing dolomite (Figure 5-2b) and overlying slate (Figure 5-2c), the overlying slate is earlier than the underlying dolomite or ore body, which is obviously inconsistent with the geological facts: if the dolomite and ore body are deposited by hot water or submarine volcano, then the slate must be in the dolomite and ore body tomorrow night; If dolomite and ore bodies are magmatic intrusion or hydrothermal metasomatism, then the rare earth in overlying slate should be homologous with them. Therefore, this may be caused by a large error. At the same time, we noticed that Zhang et al. (1997) divided their 25 data into three groups according to potassium-rich slate, dark slate and metamorphic volcanic rocks, and the three age values obtained were different from each other, and their own errors were also large.
Fig. 5-2 is an isochronal diagram based on the published original data of Sm-Nd age in Bayan Obo.
A- all 98 data; B-70 whole rock, ore or single mineral data of ore-bearing dolomite or ore body; C-25 whole rock data from overlying slate. The data in ▲ and ▲ in Figure A are abnormal data and do not participate in the isochron calculation.
Table 5-2 publishes the original data table of Sm-Nd isotope test of metallogenic age of Bayan Obo deposit.
① This data is 0.0 1 134 in the original data table, which is far from the isochron in all data projections made in this paper; But it is located near 0. 1 134 on the original isochronous diagram; It is inferred that the value in the table is a clerical error of the author, and now it is corrected to 0. 1 134.
But in any case, this multi-sample mixed calculation reveals the fact that the Sm-Nd isotope clock has only been started once and has not been reset since. The start-up time is about 1 1 100 million years.
Therefore, it can be considered that the Sm-Nd isotope clock starts at t =1125.8 32.5 Ma, and subsequent geological processes, whether magmatism or hydrothermal (water) processes, will not disturb the Sm-Nd isotope clock. That is to say,1125.8 32.5 ma is only the time of source separation of Sm-Nd and has nothing to do with the subsequent mineralization process. But it is difficult to explain how strong this geological process is. Zhang et al. (1994), a famous Sm-Nd isotope expert, once "tended to think that rare earth may be a fluid rich in CO2, F, alkali metals and rare earth elements separated from metamorphic (metasomatic) depleted mantle around 1670Ma, and it seems that it can be modified and transplanted into this book." However, the εNd values of our three algorithms are all negative, indicating that the original source area of rare earth may be the crust, which needs expert explanation.
(4) Re-Os era
The Re-Os ages of the two documents, one is the model age of molybdenite and the other is the isochron age of pyrite, but they are both abnormally coincident, both of which are 439Ma, but the errors are different. This age is consistent with the evidence of paleontological fossils. Although the authors all think it is a late metallogenic epoch, it is probably a real metallogenic epoch.
Liu Lansheng et al. (1996) described that "molybdenite is mined from the east stope of Baiyunebo mining area, and the ore minerals containing molybdenite include dolomite, fluorite, albite, decomposed ore, zircon, molybdenite, pyrite and chalcopyrite. This kind of ore seems to belong to dolomite type niobium rare earth ore type. Microscopically, molybdenite ... appears in the thin slices around fluorite and dolomite minerals and in their cracks. It can be seen that the mineral has no late characteristics and is a metallogenic mineral coexisting with fluorite and dolomite. In fact, there are quite a few sulfides coexisting with rare earth, niobium and iron minerals, including molybdenite.
Liu Yulong et al. (2005) pointed out in the sample description: "Three kinds of pyrite are observed in the Bayanobo deposit: ① pyrite is embedded in various ores; ② Pyrite coexists with pyrrhotite; (3) massive pyrite coexisting with barite. The author collected pyrite associated with barite, which is massive and closely contacted with pyrite. There are aegirine and magnetite, bastnaesite, monazite, magnetite and fluorite in the veins, and pyrite also forms veinlets. Rare earth minerals and magnetite also occur in pyrite-bearing veins, and the mineralization cannot be multi-stage (see the first section of this chapter). Moreover, the mineral assemblage of Bayanobo deposit has an important feature, that is, the lower part is composed of hematite-barite and other high oxidation conditions, and the upper part is composed of magnetite-sulfide high reduction conditions, and the two combinations are continuous transition. In the author's sample, barite coexists with magnetite and sulfide, which is still a typical combination of Bayan Obo deposit. The author interprets this age as a thermal disturbance event caused by the crust after the main mineralization period, which does not conform to the general law. In fact, this era is still the age of mineralization.
Four. conclusion
(1) collected and recalculated the published original data of Sm-Nd isotopic age. The formation or mineralization time of ore-bearing dolomite is1066.6 68.8ma (r = 0.8861). The formation time of overlying slate of 25 samples is1232.0 68.5 ma (r = 0.96642). However, this is contradictory to the data itself, which cannot be supported by other evidence and should be abandoned.
(2) To sum up, the metallogenic age of Bayan Obo deposit should be later than or equal to1125.8 32.5 Ma (starting from the Sm-Nd isotope clock) and earlier than or equal to 439±86Ma (sulfide formation in the ore body). If the Sm-Nd isotopic clock is assumed to be "very firm" and difficult to reset based on the paleontological evidence, it can be considered that the metallogenic age is 439 86 Ma (or 439±8Ma optimistically). The existing problems are what geological significance1125.8 32.5ma stands for and what geological process causes the Sm-Nd isotope clock to start at this moment.