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1、Vol. 82 No. 4 pp. 745-749 ACTA GEOLOGICA SINICA Aug. 2008 维普资讯 http:/ Origin and Superposition Metallogenic Model of the Sandstone-type Uranium Deposit in the Northeastern Ordos Basin, China LI Ziying1 *, CHEN Anping2, FANG Xiheng1, OU Guangxi1, XIA Yuliang1 and SUN Ye1 1 Beijing Research Institute
2、of Uranium Geology, China National Nuclear Corporation, Beijing 100029, China 2 Geological Exploration Team No. 208 Baotou, Inner Mongolia 014010, China Abstract: This paper deals with the metallogenic model of the sandstone type uranium deposit in the northeastern Ordos Basin from aspects of uraniu
3、m source, migration and deposition. A superposition metallogenic model has been established due to complex uranium mineralization processes with superposition of oil-gas reduction and thermal reformation. Key words: Ordos Basin, sandstone type uranium deposit, superposition metallogenic model 1 Intr
4、oduction The sandstone-type uranium deposit in the northeastern Ordos Basin is a relatively large one in scale discovered by recent uranium exploration, which occurs in the Zhiluo Formation (Li et al., 2002, 2005). Its discovery makes the Ordos Basin a more important energy resources basin except oi
5、l-gas and coal deposits. Uranium mineralization occurs in the transitional zone between gray-green and gray sandstones in the Zhiluo Formation, which is middle Jurassic in age. So, both kinds of sandstones show reduced color, which is different from that of ordinary sandstone- type uranium deposits
6、(Li et al., 2002, 2005, 2007). Therefore, figuring out uranium mineralization mechanism and establishing a exploration model for the deposit is of not only practical significance in widening prosepecting idea and guiding exploration, but also important theoretic contribution to uranium metallogeny.
7、2 Geologic Setting The study area is located at the southern margin of Yimeng Uplift in the northeast Ordos Basin, and its adjacent Hetao graben at the northern margin. Mesozoic sedimentary strata are exposed mainly in the study area (Li et al., 1992) (Fig.l). The Upper-Triassic Yanchang Formation i
8、s composed mainly of gravel-bearing sandstone interbeds with siltstone and mudstones, which hosts oil- and coal-deposits; the Lower-Middle Jurassic Yanan Formation is composed mainly of coal-productive arkose, mudstone and siltstone. The Middle Jurassic * Corresponding author. E-mail: Zhiluo Format
9、ion is the uranium-bearing ore bed, composed of gray, gray-green sandstone and mottled siltstone and mudstone, which is parallelly and locally, angularly unconformably underlain by the Yanan Formation. The Anding Formation consists of gray-green argillaceous sandstone, purple-red fine-grained sandst
10、one, mudstone interbeds with white fine-grained calcareous sandstone, which is parallely unconformably underlain by the Zhiluo Formation. The Upper Jurassic Fenfanghe Formation is poorly developed in the study area. The Lower Cretaceous strata are mainly composed of purple, gray-green sandy conglome
11、rate, sandstone and purple-red, brown-red silt mudstone interbeds with thin beds of sandstone and conglomerate, which has angular unconformities with overlying and underlying horizons. The Tertiary strata are absent, the Quaternary sands and soils range from several to tens meters in thickness. Sedi
12、mentary strata show that the study area underwent multiple tectonic events, which were closely related to uranium mineralization (Li et al” 2005; He，2004; Zhang, 1989). Uranium mineralization occurs in the transitional zones between gray-green and gray sandstones of the Zhiluo Formation (Li et al.,
13、2005; Jiao et a., 2005), The greenish color of gray-green sandstones is originated from secondary oil-gas reduction process after paleo-oxidation, being due to acicular-leaf chlorite covering surfaces of the sandstone grains (Li et al., 1992). 3 Uranium Source Uranium of the sandstone-type uranium d
14、eposits in the northeastern Ordos Basin should come from Mesozoic ore 746 Metallogenic Model of the Sandstone-type Uranium Deposit 维普资讯 http:/ Li etal. Fig. 1. Simplified geological map of the Dongsheng area in the Ordos Basin. 1-Lower Cretaceous series, 2 Jurassic system, 3Triassic system, 4 - Pale
15、ozoic erathem, 5 - Road, 6- Study area (Li et al., 2007). strata and oil-gas fluids, characterized by multiple sources. 3.1 Uranium from provenance area Sedimentary materials in the northeastern Ordos Basin mainly come from northwestern and northern areas around the basin. The uranium contents are u
16、sually high in the Archean and early Proterozoic crystalline and granitoid rocks in different ages, which provide uranium not only for primary uranium enrichment in the Jurassic Zhiluo Formation and the Yanan Formation, but also make uranium contribution to late uranium mineralization. The U content
17、s of the rocks in middle Variscan ages are relatively low, generally 3x10-6 to 4x10-6; the U contents of the rocks in middle Variscan-early Yanshan ages are relatively high, usually 4x10-6 to 8x10_6， and specially Indosinian granites contain uranium up to 12.0x10-6. Provenance rocks of the northeast
18、ern Ordos Basin are rich in uranium, and they should provide abundant uranium resources for the sandstone uranium deposit in the study area. 3.2 Uranium source from Mesozoic ore strata Yanan Formation, Zhiluo Formation and Huaci- Huanhe Formation formed in warm and humid climatic conditions contain
19、lots of humus, carbon and coal seam, and they have strong ability of adsorbing and reducing uranium. Uranium could be enriched in the sedimentary depositional procedures. The rocks of Yan5an Formation, Zhiluo Formation and Huaci-Huanhe Formation are rich in uranium, forming preliminary uranium enric
20、hment in those strata (Fig. 2). Based on radiometric data, it can be found that there exist high uranium fields in the northeast Ordos basin (Li et al., 2002), where the high contents of uranium are correspondent with those in the outcrops of Triassic and Jurassic Systems. Many areo-radiometric abno
21、rmal occurrences with uranium contents up to llxlO-6 compared to normal 2xl0_6-3xl0-6 have been found in the Yanan Formation outcrops of Jurassic System. All of these show that the Triassic and Jurassic System are rich in uranium, which as target horizons for uranium ore-formation may provide abunda
22、nt uranium for the deposit. The uranium contents in the western part of the study area are relatively low, which are probably caused by the thick sediments of Cretaceous system. The data of radioactive elements U, Th and their often associated elements Mo, Se, V, Pb, Y, Sn, Sc, Zr, Nb in the sandsto
23、ne-type uranium mineralization are shown in Table 1 for both gray-green and gray sandstones of the ore bed. The data show that gray-green sandstones with average value 3.38x10-6 of uranium contain lower uranium than gray sandstone with average value 14.33x10_6 of uranium, but no big difference in th
24、eir thorium contents, which shows that uranium in gray-green sandstones moved out and provided uranium for mineralization during paleo- oxidation process. This process is further confirmed by the lower contents of Mo and V in the graygreen sandstones as well (Table 1). Based on the U-Pb isotopic stu
25、dies on the ore horizon of Zhiluo Formation sandstones, the initial uranium contents of the sandstones have been calculated, and then gain and lost of uranium ( A U ) also can be furthermore calculated compared with their present contents. As shown in Table 1, the present average uranium content of
26、greenish sandstones in Zhiluo Formation is 3.38x10_6， however, their average initial uranium content (U ) is up to 21.95X10-6. This shows the uranium immigrated out during paleo oxidization processes again, and contributed to the deposit, being important source for uranium mineralization. At the sam
27、e time, the data also indicate that the Zhiluo Formation has preliminary uranium enrichment, but uranium distribution being inhomogeneous. 3.3 Uranium source from oil-gas fluids The analysis of data shows that the uranium contents in oil and water samples from oil beds are a few times to tens Table
28、1 Trace element compositions of both kinds of sandstones (xlO-6) Element Th U Mo Se Pb Y Sn V Sc Zr Nb Th/U (1 Wan values (10) 6.02 3.38 3.43 1.63 18.39 21.41 1.92 | 58.78 8.19 193.65 13.35 1.78 (2)mean values (5) 4.46 14.33 25.21 0.89 18.38 7.40 2.26 71.56 6.12 151.60 9.62 0.31 Note: Analyzed in Be
29、ijing Research Institute of Uranium Geology through plasma spectra analysis, relative error is less than 5%； (1) gray-green medium-coarse grained rock fragments bearing arkose, (2) gray medium-coarse grained debrisbearing arkose (data in the brackets are statistic numbers of samples). 维普资讯 http:/ Vo
30、l. 82 No.4 ACTA GEOLOGICA SINICA Aug. 2008 747 Fig. 2. Metallogenic model of Dongsheng sandstone-type uranium deposit in the northeastern Ordos Basin. A - Preliminary enrichment stage; B - Paleo-phreatic oxidation stage; C - Paleo-phreatic+paleo- interlayer oxidation stages; D - Oil reduction+therma
31、l modification stages. of times more than those in the surface water (Qu et al., 2004). The higher concentrations of uranium and other metal elements are attributed to acid dissolution, extraction of organic acid and metal-organic complexation from U- rich sandstones, uranium-bearing volcanic rocks,
32、 volcanic clastic rocks, basement rocks and preliminary uranium enrichment rocks producing oil. The oil and gas reformation process in the northeast Ordos Basin is a common phenomenon. Uranium mineralization zones have more oil and gas inclusions, and more uranium in them than primary zones and anci
33、ent oxidation zones. All of these show that oil-gas liquids from the depth should contribute to uranium source for mineralization. 4 Uranium Migration Uranium usually migrates as uranyl carbonate during typical sandstone-type uranium mineralization; it is clear and will not be discussed here. This p
34、aper shall focus on the influence of the organic acid on uranium migration. 4.1 Formation of organic acid Kerogen and coal can produce abundant organic acid by pyrolysis. The oxygen-containing groups of kerogen can also produce organic acid in the oxidation under mineral oxidant (such as Fe3+ in cla
35、y mineral etc.). Organic acid has a great ability to take uranium away, so it can be explained why that organic matter also decrease in oxidation zones and enriched in transition zones together with uranium. Moreover, organic acid can be also formed in the deeply buried diagenetic process Ro 1.3%) b
36、y the thermo-chemical redox reaction between hydrocarbons and sulfate minerals. At the different stages forming organic acid, organic origin C02 can be also produced, and the produced C02 is favorable to uranium migration as uranyl carbonate, leading to carbonation at the same time. So the organic m
37、atters in ore beds are not only useful for uranium mineralization as reducing agents, but also helpful to accelerate uranium migration. 4.2 Rock solution of organic acid Experimental study in recent years shows that organic acid in oil-gas and reservoir water, especially bifunctional carboxylic acid
38、 can enhance activities of aluminum ions in aluminum silicate minerals greatly， which can lead to solution of aluminum silicate minerals like K-feldspar. Under organic acid condition, aluminum silicate minerals 748 Metallogenic Model of the Sandstone-type Uranium Deposit 维普资讯 http:/ Li et al. will b
39、e unstable. Aluminum complex compounds are easy to migrate, and then secondary dissolved pores will be produced in those minerals. However, stability of the complex compounds is related to pH value. If the pH value changes, the complex compounds will become unstable, then minerals like kaolinite wil
40、l deposit and fill in pores, leading to kaolinite alteration. Kerogen can be changed into phenols acid organic compounds under the temperature greater than 80C, and those compounds can complex with metals, dissolving mineral and producing secondary pores. Organic acid can complex with silicon dioxid
41、e. Organic acid and its counterparts by biodegradation from original oil can complex with dissolved silicon dioxide, which will accelerate dissolving of quartz and aluminum silicate minerals in the condition of medium pH value. The experiment shows that the oxalic acid can complex with Si2, and in n
42、atural and polluted groundwater silicic acid is easy to complex with phthalic acid, dibasic acid, humic acid and griseofulvin acid. This process is considered as the reason of increasing silicon in the mineralization fluid under thermal geotectonic conditions. Above all, organic acid is widely distr
43、ibuted; the oil and gas reformation process is very intensive and common in the northern Ordos Basin. Because of strong complex process of organic acid and dissolution ability of oil and related fluids in oil beds, the main rock-forming minerals, such as feldspar, quartz and so on, will be dissolved
44、 to different degrees and altered to produce abundant secondary pores. At the same time, it can accelerate uranium migration from the rocks as uranium organic complex compounds. 5 Uranium Mineralization When uranium bearing fluids flow to the places where physiochemical conditions change, both stabl
45、e compounds and complex compounds become unstable, uranium stops migrating to deposit, resulting in the uranium mineralization. Organic matter is the important medium which changes physiochemical conditions of uranium bearing fluids, and plays a reduction and absorption role in uranium depositional
46、processes. 5.1 Absorption of organic matter In the reduction condition, organic matter has strong ability of absorption, and is specially related with humic acid and fulvic acid. The molecule texture of humic acid has bridge bond, carbon net character which comprise loosen sponginess structure, and
47、abundant molecule can distribute in the pores. The absorption amount of U 2+ is determined by both enrichment of U 2+ ion and the agglomeration degree of uranium organic complex compounds. In the acidic condition of pH = 3.4, the adsorption degree of uranium can reach to the highest. The absorbing a
48、bility of turf and brown coal is high. With their metamorphic degree increase, the ability absorbing uranium decreases. 5.2 Reduction of organic matter Nearly all solid bitumen and many kinds of coal (mainly humic acid) have ability reducing uranium. Most materials which can reduce uranium are plant
49、 debris formed during sedimentation, and bitumen and oil from outside ore beds postdates digenesis. The rate of reducing process mainly depends on organic matter characters and reaction temperature. 5.3 Strong reduction through acidification The study area experienced special tecto-thermal event after uranium mineralization. Because of the event, the thermal fluids had strong migration ability and were rich in U, Mo, Re, V, Se, Si, Ti, P, REE and so on. Those fluids were characterized