The uranium and gold mines in the Witwatersrand are located in a large area outside the south and southwest of Johannesburg. Latitude and longitude coordinate positions are E26 00' ~ E27 00', S26 00' ~ S27 20'.

The geotectonic position of the deposit is mostly listed as the post-craton depression or original activation area in the middle of Kapwal Craton in the southeast of African Shield. According to the diwa theory, it belongs to the diwa area of South Africa. Capval-Roger Zick diwa is the capval diwa in Waters Levit. Diwa area was formed in Paleoproterozoic, which belongs to the diwa area activated by intrusive structure.

This deposit is the earliest and largest uranium-gold super-large deposit discovered in the world, and has a long-standing reputation. The average uranium grade of the deposit is 0.024%, the uranium grade of rich ore is 0. 1%, the total uranium reserves exceed 400,000 tons, and the average gold grade is 5 ~10g/t. The total gold reserves figures have not been published. It is estimated that the annual gold output of this deposit is about 1000t, accounting for 2/3 of the world's total output for a long time.

The deposit was discovered as early as 1887. At that time, only gold mineralization was discovered, and gold mining has a history of 100 years. This is an old mine with a depth of 2500 meters. By 1975, the annual gold output was as high as 700 tons and the total output was 35,000 tons, making it the largest gold-uranium deposit in the world. However, uranium mineralization was discovered only in 1923 from the mineral processing workshop. In 1945, a large amount of uranium was found in the waste residue of 29 gold mines after research by C.F.Davison of the American Atomic Energy Agency, so the United States and Britain successively established 17 smelters to treat the waste residue and built seven new mines in South Africa. After 1952, the output of uranium increased continuously. Gold, uranium and manganese oxide are extracted from the ore, and pyrite is also extracted to produce sulfuric acid, which is used to process uranium ore.

Page (abbreviation of page) Ramdohr, C.F. Davidson, A.E. McLeavy, J.M. Nelson, etc. The uranium-gold deposit in Wit sea area is studied in detail, and it is considered that the deposit is of sedimentary metamorphic type. Belevtsev thinks it belongs to a subclass of metamorphic deposits. However, some people think that it is hydrothermal origin, ancient placer deposit and residual deposit. B.и。 Vilic Gold believes that the deposit belongs to composite genesis, that is, uranium and gold ancient sand were mineralized and enriched during primary deposition, and the current deposit was formed by hydrothermal transformation and mineralization superposition during Proterozoic intrusion. According to Tan Keren's report (1995), pyrite pellets in uranium-bearing gold conglomerate are pseudogravels of late hydrothermal origin, which have nothing to do with ancient alluvial placers. There are five types of false gravel in mining area: ① pyrite accounts for schist gravel and other debris false gravel; ② Chrono-pyrite pseudogravel containing breccia or volcanic eruption; ③ pyrite oolite or nodular pyrite pseudogravel; ④ Timely pseudogravel, which is gelled by mineral liquid, with fine grain structure in the center and recrystallization or secondary enlargement at the edge; ⑤ Timely false gravel, colloidal structure, and fine striped lace. It shows that the deposit is a complex process of multi-stage, multi-genesis, multi-source and multi-genesis.

There are many literatures about the research results of mineral deposits, but some problems need further study. The genesis and mechanism of Paleoproterozoic uranium-gold deposits have always been controversial. Most scholars agree with P.Ramdohr that uranium, as a heavy mineral, migrates in the atmosphere with no or very low free oxygen and is deposited at the same time with surrounding rocks. However, after 1980, different views were put forward one after another. The first is the rounding of uranium and gold mineral particles. Cousins and others believe that uranium mineralization is not a clastic cause, because crystalline uranium particles are too small to be rounded due to mechanical wear. It is pointed out that platinum group mineral particles in uranium-gold deposits in South Africa are rounded by chemical action during weathering and transportation. However, after microscopic analysis, T. Utter still thinks that relatively small mineral particles can also form circles, which is caused by mechanical wear during water treatment. Secondly, as to whether the metallogenic environment of uranium-bearing gold conglomerate is anoxic or not, for a long time, most scholars believe that the formation of Paleoproterozoic uranium placer gold was deposited under the reducing condition of atmospheric anoxia, when many metals were inert. However, according to E.Dimroth's research, the existence of Archean red beds indicates that there was a lot of oxygen in Proterozoic. The comparison between paleoproterozoic land weathering and today shows that the iron-poor soil of Proterozoic Huron system in Canada is similar to that of Quaternary soil. Archaean submarine basalt and rhyolite are all submarine alteration, that is, the edge of pillow basalt is rich in iron, manganese and potassium, which is caused by the precipitation of clay minerals such as FeO, Fe2O3 and K in oxygen-enriched environment. This shows that the Witwatersrand deposit is not a simple metamorphic placer. Therefore, whether Paleoproterozoic was oxygen-deficient or oxygen-rich needs further demonstration. In addition, in the early Proterozoic (2.5 ~ 2.3 billion years), most scholars thought it was a platform or a sub-platform. However, there are many volcanic rocks in the caprock, indicating that the crust was not as stable as the platform at that time. According to the analysis of diwa theory, some of them may be classified as diwa area deposits, that is, activated platform deposits, probably because the traditional trough platform theory failed to separate diwa area from platform area. Besides, B.I. каанский thinks that the mining area has undergone two metamorphisms, one of which was 65.438 billion years ago. According to the regional data, the metamorphism was more intense 2 billion years ago, but whether this metamorphism should be distinguished from the regional metamorphism in the return period of geosyncline needs further study.

2. Geological characteristics of the deposit and its multi-genetic basis.

1) mining strata and ore-bearing surrounding rocks

The oldest strata exposed in the mining area are Archean crystalline schist, gneiss and granite. They constitute the Archean crystalline basement of the mining area. There are extremely thick Proterozoic shallow metamorphic rocks on it, including widely developed gold-bearing uranium conglomerate layers.

Proterozoic strata in the 900×300km2 Witwatersland basin can be divided into: ① Dominant Group, which is dominated by acidic and basic volcanic lava with less uranium-bearing gold mineralization; (2) The Rand Group in Weite sea area is mainly composed of quartzite, conglomerate and slate, which is the main ore-bearing rock series in the mining area. There are traces of oblique bedding and wave beating in quartzite, which indicates that there was scouring in the ore-bearing rock series. The total thickness of the rock formation is 7800 meters, which can be divided into two series, the lower series is slate, iron-bearing quartzite, iron-bearing slate, volcanic rock and chronological conglomerate, and the upper series is slate and volcanic rock mixed with quartzite, fine conglomerate and chronological conglomerate. The Upper Series is the most important ore-bearing layer in the mining area (Figure 5-46), and the four layers of Main River, Bier Driver and Kimberly River conglomerate are the most uranium-rich layers, which are the main targets of mining. The average uranium content of this formation is above 30g/t, but the uranium content of pebble conglomerate layer is relatively high, and the cumulative thickness of conglomerate can reach 212m. . (3) Wintersdorp Group, unconformity covering Witwatersgroup or Archean rocks, is mainly composed of basic volcanic rocks and tuffs, with quartzite and conglomerate at the bottom, gold-bearing conglomerate and low grade of uranium and gold; ④ The Transvaal Group consists of slate, quartzite and conglomerate, and the bottom conglomerate contains low-grade gold and uranium mineralization. The total thickness of uranium-bearing and gold-bearing geological profiles is about 12000m, but the ore-bearing strata are eroded and missing in many places. Paleozoic continental deposits and volcanic rocks, as well as Mesozoic coal-bearing continental deposits of Kalu Group are unconformity covered on Yuanguyu, mostly distributed in the south, and almost horizontally produced.

Figure 5-46 Profile Comparison of Paleoproterozoic Uranium-bearing Gold Conglomerate Rock Series

(according to пи. Sharon)

1. Complex ore conglomerate; 2. Timely and less ore conglomerate; 3. Granite and complex sandstone; 4. Timely and less ore sandstone; 5. Siltstone; 6. Clay shale and phyllite; 7. Dolomite and dolomitic limestone; 8. Basic volcanic rocks; 9. Acid volcanic rocks; 10. Red iron ore in carbonate rocks; 1 1. iron quartzite; 12. Layered limestone; 13. Basement stratum; 14. Formation unconformity; 15. Angle unconformity; 16. Uranium and gold mineralization (large, medium and small scale). The thickness of rock stratum is the average thickness.

Gold-uranium mineralization is obviously controlled by horizon and lithology. The ore-bearing surrounding rock is Proterozoic chronological conglomerate. Most of the timely pebbles have good roundness and a few have poor roundness, which were formed during the transgression of Archean crystalline basement. Uranium and gold are distributed in gravel cements or gravel fractures. Sometimes gravel is broken by tectonic activities. The timely gravel size is 3 ~ 6 cm. The cement content accounts for about 2% ~ 16% of the conglomerate, and the main minerals are sericite, chlorite, muscovite, pyrophyllite, carbonate, carbonaceous matter, fine-grained quartz and trace metal sulfide minerals mainly pyrite and pyrrhotite. The content of pyrite is 2% ~ 16%. According to the composition of conglomerate cement, the metamorphic degree of ore-bearing surrounding rock has reached greenschist facies. In addition, a small amount of uranium gold mineralization is distributed in pyritized quartzite and quartzite cement.

Gold-uranium mineralization is concentrated in the north, northwest and southwest of the Witwatersrand Basin, with 7 main ore sections (Figure 5-47). Each ore block is a large alluvial fan distribution area, controlled by granite dome, or distributed near granite dome, or distributed in the depression between granite dome and anticline fold structure (Figure 5-48). The most favorable structure and formation mode of ore-forming alluvial fan are shown in Figure 5-49. The position of mineralization on the profile is often above the unconformity surface or sedimentary discontinuity, and it is related to a certain sedimentary rhythmic cycle. It is often at the bottom of each rhythmic cycle that uranium and gold mineralization is better. Planarly, uranium gold mineralization is concentrated in the head or tail of alluvial fan. Uranium-gold deposit is a metallogenic mechanism reformed on the basis of sedimentary ancient placer.

Figure 5-47 Cross-sectional view of Witwaterslander deposits passing through the central lander.

(According to L.J. Meroll)

1. basement paleogranite; 2. slate; 3. Quartzite; 4. Conglomerate; 5. Extrusion layer; 6. Gold-bearing uranium conglomerate layer and its quantity

2) Tectonic morphology and metallogenic structure

The overall structural form of the mining area is a large-scale compound syncline formed in the Paleoproterozoic depression basin. Surrounded by Archean crystalline schist, gneiss and granite, the basin is deposited with Proterozoic sandstone, conglomerate, shale and acidic and basic volcanic lava. Influenced by the late tectonic movement, these strata formed a large northeast composite syncline, in which secondary folds and fault structures were widely developed. It can be seen from the stratigraphic profile of the Witwaterst continental rock series passing through the central land block that the occurrence of ore-bearing strata is steep, inclining from south to east, and the dip angle is greater than 60, while the dip angle of Li Leishi-Gold mining area is gentle, which is 20, and both are monoclinic structures (Figure 5-50). There are a series of Archean granite domes in the basin and its edge, and their appearance plays an important role in restricting the paleoproterozoic sedimentary facies and the structural occurrence of ore-bearing rock series.

Ore-controlling structures are mainly Proterozoic granite domes distributed in the north wing and northwest wing of syncline. Each ore-bearing alluvial fan is usually controlled by a granite dome or a depression controlled by a NW-trending fault structure between two domes (Figure 5-5 1). The spatial distribution of each coal seam is related to the sedimentary discontinuous structure, which is caused by local small angle unconformity and parallel unconformity (Figure 5-52).

There are also NW-trending, NE-trending, EW-trending and N-S-trending fault structures in the mining area, mainly normal faults. Many of them cut Archean crystalline basement and some filled diabase veins. Faults at the edge of the basin have long controlled the formation and evolution of the basin syncline structure, and may also control volcanic eruption, intrusive rock intrusion and other magmatic activities, so that deep uranium sources enter the uranium-bearing gold conglomerate layer for superimposed mineralization.

Figure 5-48 Distribution map of each ore section of Witwatersrand deposit.

(According to ви Vilic Kim)

1. Rock series (non-layered) covering Rand Rock Series in Vite waters; 2. Upper Witwatersrand Rock Series; 3. Lower Witwatersrand Rock Series; 4. Rock series of Weng River in Pogliani; 5. Granite and basement crystalline schist; 6. The boundary of the Witwatersrand Basin (under the young caprock); 7. failure; The numbers in the circle represent each ore section: ① central rand; (2) the east rand, Yi Deer than; 3 Rogers; ④ Western Rand; (5) Rand in the far west; ⑥Craxsdorp； ⑦ Odendahl siles

3) Magmatic rocks in the mining area

The magmatic rocks in the mining area are mainly Archean granite, which are dome-shaped and distributed in the middle, northeast and northwest of the Witwatersrand Basin. Archean granite directly controls the temporal and spatial distribution of uranium gold mineralization and provides the source of ore-forming materials for uranium gold mineralization. The isotopic age of Archean granite measured by Rb-Sr method is 3.2 ~ 2.9 billion years. There are 2.82 billion years of potash granites in the granite gneiss in the basement, among which pegmatites rich in rare earth minerals and uranium minerals are produced.

In addition, during the Paleoproterozoic sedimentation of the basin, acidic, neutral and basic volcanic lava erupted to form Paleozoic volcanic rocks. On the northern periphery of the mining area, there is a Bushville complex intrusion with an age of 2.05 ~ 65.438+0.95 billion years. The intrusive body extends in the east-west direction, cutting the Transvaal strata and being covered by the Paleozoic strata. The center of the rock mass is red granite, surrounded by basic-ultrabasic rocks (dunite, pyroxenite and syenite). Basic-ultrabasic rocks appear in the form of interlayer rocks, which formed earlier and invaded red granite later. After the main body of the complex was formed, there was a small-scale supplementary intrusion of nepheline syenite.

4) Ore body shape and surrounding rock alteration near the mine.

The uranium-gold ore bodies are layered, layered and lenticular. The strike of the seam extends for tens of kilometers, and the thickness is mostly 1 ~ 2m. The average thickness of the seam is 1.5 ~ 1.6m, and there are 20 uranium-bearing gold conglomerate beds, usually consisting of 2 ~ 4 ore-bearing beds. In addition, some gold-bearing conglomerate layers do not contain uranium mineralization, so there is no linear relationship between uranium and gold grade in the ore, or only a partial linear relationship.

Fig. 5-49 Relationship between uranium mineralization and alluvial fan and fold axis in Witwatersrand deposit.

(According to district attorney Polo Torres)

1. Main mine scope; 2. There are many rivers rich in Au-U in the east; 3. Inference range of alluvial fan; 4. Exploded outcrops and potential outcrops in main rivers and main rivers; 5. Basement granite

Due to the characteristics of multi-stage and multi-genesis, the existing ore beds have been transformed or superimposed, forming a more complex structure and shape. For example, the Main-Reeve uranium-gold deposit is a rich comprehensive industrial deposit formed on a new horizon on the basis of the original poor ore bed after strong erosion and transformation. Sometimes the uranium grade reaches 0. 1%, but generally the uranium grade is 0.034% ~ 0.042%. In the Main-Reeve-Riter conglomerate, the richest uranium-gold ore body is controlled by a northwest narrow trough and inserted into the underlying stratum. The coal seam is 3 meters thick, and it extends to 65 kilometers in East Witwatersrand and Middle Witwatersrand.

The alteration of surrounding rocks near uranium-gold ore bodies is not developed.

Figure 5-50 Cross section of Leischrigold mine

(According to Фиииииииииииии)

Karoo system: 1. Green rock wall; 2. Coal seam; 3. Green rocks; Winter stope series: 4. Almond lava; 5. Porphyrite and tuff; 6. Diabase (footwall deposition); Kimberly-ellsberg Construction Company: 7. Intermittent reef zone; 8. Quartzite belt; 9. Kimberly Reef; 10. Kimberlite shale; Menin-Bader group: 1 1. Almond lava; 12. Quartzite; 13. Blue fine conglomerate; 14. Fault structure

Fig. 5-5 1 unconformity production diagram of uranium-bearing gold conglomerate layer

(According to Jiutuo. com)

1. Pusta Weng schist; 2. Rock wall conglomerate layer; 3. basta Del layer; 4. Quartz schist; 5. Main laminae schist; 6. Main Reeve-Ritter conglomerate; 7. denudation surface

5) Ore structure and material composition

The most common ore structure is conglomerate, and gold-uranium mineralization is mostly distributed in conglomerate cement, accounting for 30% ~ 40% by volume, and gravel generally does not contain ore. The binder is mainly timely, accounting for 70% ~ 80% of the binder, and the remaining 20% ~ 30% is chlorite, sericite and some metal minerals.

The main uranium minerals in the ore are crystalline uranium, pitchblende, thorium pitchblende and ilmenite, and the uranium-bearing minerals are zircon, monazite, sphene and titanium dioxide. Other metallic minerals include iridium, chromite, cassiterite, pyrite and natural gold. Crystalline uranium deposits are oval, round and a small amount of angular particles, which are often evenly distributed in conglomerate cements or piled up into grapes. After long-distance transportation, most of the crystalline uranium deposits are equigranular, distributed in the conglomerate of the terrestrial series of Witwaters, and the average diameter of crystalline uranium deposits is 75μ. Crystalline uranium ore is usually produced with monazite, iridium osmite, chromite, cassiterite, zircon, natural gold and pyrite. Crystalline uranium ore contains thorium as high as 1.63% ~ 2.7%, sometimes as high as 6.52%, and rare earth elements as high as 1.0% ~ 2.65438.

Figure 5-52 Geological and Ore-bearing Sketch of Capvalli Dome Series

(adapted from the data of T.0.Relmer)

1. Paleozoic and its post-deposition; 2. Transvaal Group; 3. Winterthorpe Group; 4. Loki Angkor Group; 5. Witwaterslander Group; 6, 7. Green rock belt; 8. Granite gneiss; 9.Vaild red granite; 10. Basic-ultrabasic rocks in effective rock mass; 1 1. 12. Witwatersrand uranium and gold deposit; 13. metasomatic gold deposits; 14. Iron ore deposit; 15. Chromite deposit; 16. Cu-Ni-Pt deposit; 17. ilmenite deposit; 18. Vein gold deposits; 19. cassiterite deposit; 20. Diamond cone; ⅰ. Limpopo active zone; Two. Mozambique activity area; Ⅲ. Namakua-Natal active zone

Thorium pitchblende is a heterogeneous mixture of solid hydrocarbons and crystalline uranium, which occurs as nodules and is distributed in the lower layer of conglomerate and the contact zone between conglomerate and quartzite. Hydrocarbons partially or completely account for crystalline uranium deposits. Crystalline uranium in thorium pitchblende is formed simultaneously with a single crystalline uranium particle under the unified action.

Pitch uranium is widely distributed in various conglomerate layers of ore-bearing rock series, and is often filled in the cracks of crystalline uranium ore, or forms the edge of crystalline uranium ore particles, and is filled between cement fragments or in their cracks. Pitchblende is usually distributed along thorium pitchblende. When pitchblende is veined, it often coexists with pyrite, pyrrhotite, nickel pyrite and chalcopyrite, and sometimes these sulfide minerals account for pitchblende. The distribution of ilmenite is relatively limited. It is distributed along anatase, crystalline uranium and thorium pitchblende and produced as an aggregate of fine crystals.

Gold is in the form of extremely fine angular particles or irregular flakes, which are mainly distributed in conglomerate cements. In addition, there are vein gold and gold filled in the gap between two mineral particles or produced in the form of inclusions. Gold and uraninite rarely occur in gravel, and if they exist, they only occur in veinlets. Gold can also reprecipitate and enrich skutterudite, cobalt sulfide, galena, chalcopyrite and pyrrhotite.

Pyrite is also one of the important industrial minerals in the deposit. Because of its wide distribution, its content accounts for about 5% ~ 10% of the whole conglomerate, and it is in the form of equigranular grinding circle with a diameter of 0.5 ~ 3.0 mm, and its crystals and inclusions are irregular. Early pyrite was replaced by sphalerite, chalcopyrite, galena, pyrrhotite, arsenopyrite and gold. Sometimes pyrite occurs in the form of round gravel, but the formation of pyrite gravel may be later than gold mineralization. Late pyrite was formed at the same time with other metal sulfide minerals, such as shell pyrite and skeleton pyrite. Some pyrite filled the cracks of minerals in gravel or cement, or occurred in rocks in the form of veinlets. Crustal pyrite can even be formed under modern conditions. Pyrrhotite occurs at the bottom of conglomerate layer, which is a typomorphic mineral and evolved from pyrite at high temperature. In addition, there are two generations of time, the early time is clastic time, and the late time is fine time, in which mica minerals account for it.

The uranium grade of uranium gold ore is low, generally 0.034% ~ 0.042%, and the uranium grade of the ore currently mined is about 0. 1%. However, due to the gold grade of 5 ~ 10g/t, the comprehensive development and utilization of gold and uranium can still ensure the economic benefits of enterprises.

6) Isotope Geology

The isotopic ages of Archaean granites in An Duomin-Reeve area measured by Rb-Sr method are 3.2 billion years, 600 million years (5 samples) and 2.9 billion years (65,438+0 samples) respectively. The isotopic ages of granites and pegmatites invading the Archean top are 3.05 ~ 3.2 billion years. The uranium-bearing gold conglomerate in the Witwatersrand system was formed in 2.48 ~ 2.37 billion years, and the gold mineralization age in the conglomerate was 365.438+0 ~ 2.7 billion years, including 3.04 billion 65.438+0 billion years of crystalline uranium, which obviously shows that the gold-uranium mineralization age is much older than the horizon age of the ore-bearing conglomerate.

According to the ratio of 207Pb/206Pb of most pitchblende, the isotopic age of most pitchblende is 20 ~10.90 billion years, which is very close to the isotopic age of the Bushwell complex running through the Transvaal system of 205 ~10.90 billion years. Therefore, it is considered that 2 ~ 6,543.8+0.9 billion years is the main age for uranium ore activation and transformation. In addition, the age of pitchblende is 654.38+0 billion years, which indicates that the deposit underwent remoulding and mineralization in late Proterozoic.

There are northeast and nearly north-south greenstone belts in the Neoarchean crystalline basement in this area, which are over 3.2 billion years old. They are separated by widely developed granite gneiss. Potash granites formed in 2.82 billion years and 55 million years can also be separated from granite gneiss. Pegmatite rich in rare earth minerals and uranium minerals is related to this kind of potash granite.

3. Formation conditions of ore deposits

There are three main sources of uranium and gold mineralization, namely granite, gneiss and greenschist from Neoarchean crystalline basement. The source beds of uranium and gold deposits in Proterozoic basin and the deep uranium and gold resources brought by Proterozoic magmatic intrusions. During the sedimentary period, uranium enrichment came from the crystalline basement around the basin, especially in the north and northwest, eroded Archean granite and gneiss in the source area, and gold also came from the greenschist belt. The Neoarchean granite uplift at the edge of the basin provides rich uranium and gold resources for the deposition and enrichment of uranium and gold in the basin. Some gold may also come from the greenstone belt far away from the basin, because there are 3500 gold deposits in the greenstone belt. However, the contents of gold and uranium in Neoarchean granite, gneiss and greenschist have not been published, and it is only speculated that there are more natural gold, crystalline uranium and uranium-bearing clastic particles in sediments, especially in chronological conglomerate.

The uranium and gold resources transformed and mineralized after the sedimentation of the Paleoproterozoic basin come not only from the uranium and gold deposits deposited in the basin, but also from the deep uranium and gold resources brought about by the intrusion period represented by Bushville rock mass. According to statistics, the average uranium content of Lande Group in the Paleoproterozoic Vite waters is 0.003%, while the uranium content of pebble conglomerate layer is higher, reaching 0.024%, and the cumulative thickness of conglomerate layer is 2 12m, which can be inferred to provide extremely rich and sufficient uranium sources for mineralization. The gold content in the conglomerate layer is 5 ~10g/t, which reaches the grade of industrial utilization and provides richer gold resources for mineralization and transformation.

The basic mineralization of the deposit is the enrichment of uranium and placer gold, which is carried out under the delta facies condition of shallow water basin with near-horizontal atmospheric hypoxia, and belongs to the physical and chemical conditions of atmospheric normal temperature and pressure, gravity and biochemical action. Only under the condition of atmospheric hypoxia can crystalline uranium and ilmenite form ancient placer in the form of clastic minerals. In addition, the conglomerate containing uranium and gold is multi-layered, generally as many as 20 layers, which shows that the basin needs a long-term stable tectonic environment with alternating subsidence and relative uplift, so that uranium and placer gold enriched in shallow water can be kept near the water level for many times, and at the same time, the erosion source area can continuously provide erosion source materials to enter the basin for deposition.

The metallogenic space of uranium placer gold is closely related to the Archean granite dome at the edge of the basin and its interior, or to the unconformity controlled by the granite dome, accompanied by alluvial fans. This alluvial fan may be limited by a granite dome, or it may be a depression between two granite domes, which is usually limited by NW-trending faults.

From the point of view of reforming mineralization superposition, it is found that there are fault structures developed in and near the alluvial fan, or there is intrusive contact relationship of intrusive rocks, which is beneficial for ore-forming hydrothermal solution to enter the pre-existing ore body along the fault structures and intrusive contact zones and superimpose and enrich the ore. There are pitchblende veins and gold-bearing sulfide veins in the ore, which are filled in the cracks of pebbles and gravel cements at the right time, which is good evidence.

The dynamic conditions of mineralization are mainly manifested in the hydrodynamic changes during the formation of Paleoproterozoic uranium placer gold. When the river enters the delta facies at the edge of the lake basin or shallow sea, the water velocity suddenly decreases, and the hydrodynamic force correspondingly decreases, and the uranium and gold mineral debris transported by the water body is deposited and enriched. In addition, the development of faults at the edge of the basin makes the basin continue to subside and maintain hydrodynamic momentum.

From the analysis of regional or geotectonic geological dynamic conditions, it is found that the area is in a relatively stable tectonic environment, such as platform stage, and the alluvial fan formed is large in scale and stable in thickness. In the early Proterozoic, the mining area was in the platform stage, forming a stable uranium-bearing gold conglomerate layer. However, there is a small-scale oscillation movement in the platform stage, which provides a multi-stage accumulation of uranium-bearing gold-bearing pebble conglomerate for delta deposition and forms a multi-layer production condition of uranium-bearing gold conglomerate. After the formation of uranium placer gold mine, the ore was enriched into industrial tonnage deposit through tectonic-magmatic activation mineralization in diwa period.

4. Evolution of uranium-gold mineralization

To sum up, it can be seen that the crust of the mining area and its area has undergone three structural stages: geosyncline, platform and diwa/the evolution of main geological structures and metallogenic events in 10/0. 3.8 ~ 3.2 billion years ago, it may have experienced the core stage of the mainland. The specific events in each major tectonic stage are as follows:

Ⅰ. Geosynclinal stage

Ⅰ- 1. Formation of the Neoarchean greenstone belt (> 3.2 billion years)

I-2。 Neoarchean granite (potash granite) and pegmatite intruded into the Neoarchean top stratum (3.2 ~ 2.9 billion years).

I-3。 Formation of Neoarchean gold mineralization (365,438+0 ~ 2.7 billion years)

I-4。 Formation of Neoarchean Crystalline Uranium Deposits (3 billion years)

Ⅱ. Platform stage

Ⅱ-5. Uranium-bearing gold conglomerate deposits in the Paleoproterozoic African platform (2.5-2.3 billion years)

Ⅲ. Diwa stage

Ⅲ-6. Proterozoic Weil complex and other intrusive rocks (20.5 ~10.95 billion years).

Ⅲ-7. Formation of pitchblende in uranium-bearing gold conglomerate series (20 ~ 65.438+900 million years)

Ⅲ-8. Formation of vein gold sulfide and pitchblende in uranium-bearing gold conglomerate system (654.38+0 billion years)

Ⅲ-9. Continental Deposition and Volcanic Eruption of Paleozoic Wattberger Rock Series

Fig. 5-53 Metallogenic evolution of the Waite River Rand uranium-gold deposit.

1. schist; 2. Granite gneiss; 3. Granite; 4. pegmatite; 5. Timely pebble conglomerate; 6. Sandstone; 7. Uranium mineralization; 8. Gold mineralization; 9. Mineralization of sulfide veins of uranium and gold; 10. Fracture. Ⅰ. Metallogenic period of dispersed disseminated uranium and gold in geosyncline stage; Ⅱ. The metallogenic period of uranium placer gold in platform stage; Ⅲ. Transformation and mineralization period of pitchblende in diwa stage; Ⅳ. Sulfide mineralization period of veinlet uranium and gold in diwa stage

Ⅲ- 10. Mesozoic coal-bearing continental deposits in the Kalu Group

From the above geological structure and metallogenic events, it can be seen that uranium-gold mineralization has also experienced three structural stages and four metallogenic periods (Figure 5-53).

Ⅰ. Ore-forming period of dispersed disseminated uranium and gold in geosyncline stage (31~ 2.7 billion years). Uranium-gold mineralization is in the regional fold structure and metamorphism of geosyncline stage. With the intrusion of granite and the formation of pegmatite, dispersed disseminated uranium-gold enrichment is formed in greenschist belt and granite. The metallogenic age of clastic gold measured in the Paleoproterozoic conglomerate in the mining area is 36,543.8+0 ~ 2.7 billion years, and the age of clastic crystalline uranium mine is 3.04 billion years, 65.44 billion years and 38.8+0 billion years, which is confirmed. In addition, the age of pegmatite containing rare earth minerals and uranium minerals and its potash granite surrounding rock and potash granite is 2.82 billion years, which can also be used as evidence.

Ⅱ. The metallogenic period of uranium placer gold in platform stage (2.5-2.3 billion years). In this platform stage, the crust of the mining area was generally in a state of slow subsidence, forming a rhythmic deposit of multi-layer uranium-bearing gold chronological conglomerate represented by the Witwatersrand Group, with a total thickness of 7800m and a cumulative thickness of 212 m. The lowest uranium content in conglomerate layer is 50g/t, generally between 190 ~ 380g/t, and the average uranium content is 0.024%. The gold content is 5 ~ 10g/t, and the average uranium content of the whole Witwatersrand group is as high as 0.003%. Therefore, the gold mineralization in the timely conglomerate layer has reached the industrial grade, while the uranium mineralization grade is low. However, as a comprehensive uranium-gold ore, uranium can be extracted in this way, which has comprehensive development and utilization value. In the past, this deposit was classified as an ancient placer uranium-gold deposit. In fact, uranium did not reach industrial grade during placer mineralization, so it cannot be included in the main mineralization.

Ⅲ. Transformation and mineralization period of pitchblende in diwa stage (2-65.438+0.9 billion years). Due to the tectonic-magmatic activation in the subsidence stage of the mining area and its region, the intrusive body represented by Bushville complex (205 ~10.95 billion years) has transformed the pre-existing depleted uranium ore body to some extent, forming the enrichment of pitchblende (20 ~10.90 billion years). Pitch uranium ore is mostly filled in the cracks of crystalline uranium ore, or distributed around crystalline uranium particles in an edge shape, or filled in the cracks of conglomerate cement. During this period, some uranium sources rich in pitchblende may come from plutonic uranium sources related to granite. What grade of uranium is enriched in this metallogenic period remains to be studied.

Ⅳ. The metallogenic period of vein uranium-gold sulfide in diwa stage (654.38+0 billion years). Due to the reactivation of the crust in the mining area (mainly tectonic activation), uranium and gold are redistributed, and pitchblende veinlets with isotopic age of 1 100 million years are produced with natural gold and gold-bearing sulfide veinlets. In the process of vein mineralization, some gold ore-forming materials of deep uranium mineralization may participate in mineralization, and finally form an ore grade with uranium content of 0. 1%.