Sandstone-type uranium deposits refer to uranium deposits occurring in sandstone, feldspathic sandstone, conglomerate and clastic rocks, generally referring to in-situ leachable sandstone-type uranium deposits, but excluding timely conglomerate-type uranium deposits. Sandstone-type uranium deposits have the advantages of large reserves, low mining cost and environmental protection, and have become one of the main types of uranium prospecting in the world. By 2002, the total proven uranium resources in the world were 4.486 million tons, and sandstone-type uranium deposits were second only to unconformity uranium deposits (Wang Zhengbang, 2002).
I. Classification of sandstone-type uranium deposits
Sandstone type uranium deposits play an important role in uranium deposits. At present, great progress has been made in the theoretical study of prospecting and mineralization of sandstone-type uranium deposits, but there is no unified standard for the classification of sandstone-type uranium deposits. Dalkamp( 1993) divides sandstone-type uranium deposits into three categories (plate/quasi-conformity type, coiled type and structural-lithologic type) and eight subcategories according to the structure, ore body shape, tectonic environment and deposit genesis. However, the classification standards of different types in the same scheme are inconsistent, and the classification is often used alternately according to the genesis of the deposit and the shape of the ore body, and the classification is not clear. Li et al. (200 1) put forward four different classification schemes according to the ore-bearing sedimentary formation, the sedimentary environment of ore-bearing surrounding rocks, the shape of ore bodies and the genesis of ore deposits, but each classification scheme is not unified.
In order to be unified and accurate, this book divides sandstone-type uranium deposits into four categories according to the genesis of the deposits (Wang Zhengbang, 2002): The first category is superimposed sandstone-type uranium deposits infiltrated by late diagenesis and supergene, such as most deposits in the Glanc uranium belt in the United States; The second type is sandstone-type uranium deposits infiltrated by supergene, such as most uranium deposits in the United States and Central Asia; The third type is supergene exudation-infiltration sandstone-type uranium deposits, such as Sa Belsa Yi uranium deposit and Texas uranium deposit. The fourth category is epigenetic hydrothermal superimposed sandstone-type uranium deposits, such as Niger uranium deposit in Africa and Rabe uranium deposit in Europe. The second category can be divided into three subcategories, namely, sandstone-type uranium deposits in phreatic oxidation zone, sandstone-type uranium deposits in interlayer water oxidation zone and sandstone-type uranium deposits in phreatic-interlayer water oxidation zone. The former, such as Gorjat and Lower Ili uranium deposits in Yili Basin of Central Asia; Sandstone-type uranium deposits in interlayer water oxidation zone, such as some uranium deposits in Chuzaresu-Sirdarin basin and uranium deposits in Wyoming basin, USA; The latter are Halat uranium deposit in Mongolia and Yi Musi uranium deposit in Russia.
Practice shows that among the above types of sandstone-type uranium deposits, the second type of supergene permeable sandstone-type uranium deposits is the most important, especially the sandstone-type uranium deposits in interlayer oxidation zone (Wang Zhengbang, 2002). This kind of deposit is shallow buried, suitable for in-situ leaching mining, widely distributed and large in scale, and has important industrial value. Although the first type of deposit is large in scale, it is deeply buried and contains high concentration of ore-accumulating agent, which is not conducive to in-situ leaching mining; At present, the third and fourth types of deposits are few and small, which are not suitable for large-scale in-situ leaching and have low industrial value.
Second, the temporal and spatial distribution characteristics
Sandstone-type uranium deposits are widely distributed in the world, and their temporal and spatial distribution has the following main characteristics and laws (Wang Zhengbang, 2002):① Most sandstone-type uranium deposits discovered at present are relatively new, mainly concentrated in Cenozoic, especially from Oligocene to Pleistocene; ② Most sandstone uranium deposits occur in the caprock of Mesozoic basin, and the ore-bearing strata are mainly Jurassic, Cretaceous and Paleogene, followed by Carboniferous, Triassic, Neogene and Quaternary; ③ Sandstone-type uranium deposits are mainly distributed in the Meso-Cenozoic basins in the subtropical high zone and its trade winds and westerlies in the mid-latitude (20 ~ 50) of the northern and southern hemispheres, or in the arid and hot Gobi desert grasslands in the inland and western parts of the mainland, mainly distributed in North America, Central Asia and some Asian countries, Africa, Australia, South America and Europe. However, the sandstone-type uranium deposits in the United States and Central Asia are the most typical (Figure 65438 ④). Most of them are concentrated in the shallow-buried gently inclined slope belt in the sedimentary basin at the inner and outer edges of the stable land mass, and the basin basement and erosion source area often undergo multiple tectonic-magmatic activations, which leads to the extensive development of uranium-rich formations. For example, uranium deposits in Meso-Cenozoic basins activated by Cenozoic Laramie movement in North America and sandstone uranium deposits in sub-orogenic belts formed by Himalayan movement in Central Asia.
Figure 1 1- 1 global distribution map of sandstone uranium deposits
Three. metallogenic theory
Sandstone-type uranium deposits are not syngenetic deposits, and there is a lack of integration between ore bodies and sandstone surrounding rocks, and uranium-bearing substances are filled in the gaps between cuttings. After sandstone deposition, uranium is usually brought into the surrounding rock by aqueous solution.
Crawlry et al. (1983) put forward four genetic types on the genetic theory of sandstone-type uranium deposits in the United States:
1) Late diagenesis-supergene infiltration superimposed to form a kind of sandstone-type uranium deposits, also known as humic acid-uranium deposits (Turner et al., 1986), represented by the deposits in the Glanc ore belt in the southwest edge of the San Juan basin in the Colorado Plateau. The deposit has multi-stage mineralization. In the late diagenetic stage, the uranium-bearing humic acid tabular sandstone-type uranium deposit was formed, and its age was similar to that of the host rock. In the epigenetic transformation period, based on the plate-shaped ore bodies, the rolled uranium deposits and the accumulated uranium deposits in the interlayer oxidation zone controlled by the fault structural oxidation zone were formed. The age of the latter two uranium deposits is relatively new, which is consistent with the age of regional interlayer oxidation zone type uranium deposits.
2) Supergene epigenetic infiltration into uranium-vanadium-copper sandstone-type uranium deposits. Take the deposits in the Yulafan sandstone-type uranium belt in the Colorado Plateau as an example (Thamm et al., 198 1). This type of tabular sandstone-type uranium deposit is transformed from potassium-rich permeable oxygen-bearing uranium-bearing groundwater, and its mining age is relatively new, which is consistent with the age of sandstone-type uranium deposits in regional interlayer oxidation zone.
3) Bacterial rolling sandstone-type uranium deposits, represented by deposits in Wyoming Basin (Harshman et al., 198 1). This type belongs to the typical genesis of supergene epigenetic infiltration interlayer oxidation zone. The ore body is controlled by the geochemical barrier of redox interface in interlayer oxidation zone. Under the action of bacterial nutrients, the groundwater rich in uranium reduces SO2-4 in the basin brine, which provides important reduction conditions for precipitation of uranium.
4) Non-bacterial sandstone-type uranium deposits, represented by sandstone-type uranium deposits in the coastal plain of Texas, are characterized by lack of organic matter and low reduction capacity. However, due to the transformation of the reducing solution oozing from the underlying strata before mining, it is rich in reducing agents such as sulfides, and the reduction rate is high (Goldhaber et al., 1978). Then mineralization is formed by epigenetic oxidation transformation of supergene interlayer permeable oxygen-containing uranium-containing water.
Four. Metallogenic model
Wu Bolin (2006) established the evolution model of main uranium-producing basins of sandstone-type uranium deposits (figure 1 1-2) according to the deposit characteristics and metallogenic regularity of different sandstone-type uranium deposits abroad, taking the redox environment of mineralization as the main line and combining the tectonic evolution background (orogenic belt, sub-orogenic belt and weak neotectonic movement active area). Among them, the orogenic belt indicates that the amplitude of vertical fault block movement during orogeny is > 2000 m; The secondary orogenic belt is a small orogeny of 500 ~ 1500m or 2000m; The weak active area of neotectonics is between 200 and 500 meters.
Fig. 1 1-2 schematic diagram of dynamic (redox environment) evolution model of sandstone-type uranium basin.
The model diagram shows two different metallogenic endmembers, namely oxidation endmembers and reduction endmembers. Reducing endmember means that mineralization is in a strong reducing environment, in which reducing agent is very rich, and there are generally large-scale natural gas escape and charging events, and the reduction is sufficient. Most of the ore-controlling alteration zones are formed in strong reduction environment, and the deposits formed in this environment are generally large and super-large. Typical areas and deposits, such as Grant ore belt in the American Colorado Plateau, ancient river-type uranium deposits in Russia and Mongolia, Dongsheng deposit in Ordos Basin, China, etc. Oxidation endmembers show that mineralization is long-term. The interlayer oxidation zone has a huge mineralization scale, sufficient groundwater recharge, arid climate, stable mineralization period, large and stable sand body scale, and the ore-forming reducing agents are mainly solid organic matter and solid inorganic matter in the stratum, and the reducing substances are widely distributed and the reducing environment is stable. Typical areas are some uranium deposits in Chuzaresu-Sirdarin basin in the second category, others are uranium deposits in Wyoming basin in the United States and Halat uranium deposits in Mongolia, the third category is epigenetic seepage-seepage sandstone-type uranium deposits in central Texas, and the fourth category is epigenetic hydrothermal superimposed sandstone-type uranium deposits.
The schematic diagram shows that the two end-member uranium-producing basin models are all produced in the weak activity area of neotectonics, and the basin model can produce large and super-large deposits; Intermediate type is produced in secondary orogeny and orogeny. In contrast, the scale of uranium-producing basin model deposits in secondary mountainous areas is much smaller than that in weak neotectonic movement areas. These facts show that the oxidation or reduction metallogenic environment in orogenic and secondary orogenic areas is in turbulent change, while the stable tectonic environment is conducive to the continuous and full implementation of oxidation or reduction mineralization, and the corresponding deposit scale should be large. It can also be seen from Figure 1 1-2 that the stable craton area without tectonic activation is not the prospective area of sandstone-type uranium deposits, but the weak neotectonic movement area and secondary orogenic mountain area are the most favorable areas for sandstone-type uranium deposits.