There are two main types of the oldest crust distributed in all continents. One is the advanced metamorphic area, which is often composed of rocks that have reached the deep metamorphism of granulite facies; The other is granite greenstone belt, which is mainly composed of greenschist facies low metamorphic mafic extrusive rocks, in which there are often granite intrusions of different ages, and the diagenetic age in this area extends from Archean to Proterozoic. There are few minerals with important industrial significance in high-grade metamorphic areas, but there are many very important ore-forming areas (belts) in greenstone belts.
1. Iron, copper, zinc and gold mineralization areas in greenstone belt
Mafic volcanic magma in greenstone belt comes from mantle and is the product of continuous frequent eruption and intrusion into active environment. The iron deposits in the greenstone belt are Algoma-type iron deposits, represented by the iron deposits in Ontario, Canada, and also distributed in central Australia, Swaziland in Africa and northern Norway. Iron formations occur in tholeiite-calc-alkaline volcanic rocks and pyroclastic rocks. Layered ore bodies can be iron oxide, carbonate, sulfide and silicate, which are formed in volcanic-sedimentary environment with low Eh. Massive sulfide deposits usually occur near this iron ore layer. Archean massive sulfide deposits are characterized by rich copper and zinc, accompanied by gold and silver; The main rocks of the deposit are basaltic-rhyolitic volcanic rocks, which are often related to volcanic domes and volcanic breccia tubes. This kind of deposit is the most typical in the greenstone belt such as Abitibi in Canada, belonging to Archean and Early Proterozoic. The greenstone belt-type gold deposits are widely distributed and large in scale, and occupy an important position in the world's gold resources. The main producing areas include Canada, the United States, southern Africa, India, Brazil and Western Australia. The host rocks of the gold deposit are tholeiite and felsic volcanic rocks with complex conglomerate and turbidite. Gold deposits mainly occur at the edge of greenstone belt not far from granite, and some gold deposits often occur in the same area as massive sulfide deposits. Nickel ore in greenstone belt is mainly produced in Komatite ultrabasic rocks, with typical producing areas such as South Africa and Western Australia.
2. Gold, uranium and iron (manganese) in the huge basins on the ancient continent were built into mining areas.
The Witwatersrand Basin in South Africa is one of the largest gold mines in the world. Gold deposits occur in thick metamorphic paleovolcanic rocks and sedimentary rock series, and gold bodies occur in chronological conglomerate layer, which was formed with the appearance of the earliest sedimentary basin, and gold came from the nearby greenstone belt. The existence of pyrite and pitchblende fragments in the gold-bearing conglomerate layer indicates that the reducing atmosphere at that time was a necessary condition for the preservation of these substances. The ore-bearing rock series in South Africa were formed 2.5 billion to 2.9 billion years ago. In addition to South Africa, Brazil and Ghana have similar mineral deposits. The iron (manganese) formation mentioned here is called dominant type, which is distributed in all continents and is a large iron deposit in the Great Lakes region of North America, Brazil, Australia, Europe, Ukraine and other places. This kind of iron-bearing formation is formed in and around stable continental basins, mainly composed of shallow-sea clastic rock series and carbonate rocks, and the ore-bearing strata are mainly composed of iron oxide and silicon oxide minerals. Around the world, the formation of this kind of iron is concentrated in a period before 2000 ~ 1800 Ma. At the same time and later in some similar environments, important manganese carbonate deposits were formed, which are found in South Africa, Brazil and India.
3. Metallogenic areas of Cr, Cu, Ni, PGE, V, Ti and Fe in large mafic and ultramafic magmatic rocks in the ancient continental rift system.
Typical ore-forming areas are Bushveld rock mass in South Africa and large rock wall in Zimbabwe. There are large chromite deposits, copper-nickel sulfide deposits and platinum group elements deposits in Bushveld rock mass. The Great Wall of Zimbabwe produces chromite. Rock masses are huge in scale, some of which can span continents. The emplacement of mantle-derived magma along the fault zone that cuts the continental crust is the basic metallogenic background. Rock intrusion into Archean shield or greenstone belt. Canada, the United States and Greenland have similar rock masses, which were formed 2 billion to 2.5 billion years ago.
The earliest Precambrian crust in China is concentrated in the north of North China and the southeast of Northeast China. Liaoji, Jidong and Jiaodong have famous gold deposits and iron-floor deposits. Because the metamorphic rocks in these areas are generally above amphibolite facies and the characteristics of the deposits are different, until recent years, these deposits can still be compared with the greenstone belt deposits in the world in essence. In addition, there are no large-scale gold-bearing conglomerates and layered ultramafic complexes in China, which is considered to be related to the strong structural changes in the late paleocrust of China.
2. Metallogenic areas related to lithospheric plate tectonic movement.
Plate tectonics is a global tectonic movement based on the study of oceanic crust. Modern plate tectonic movement refers to the pan-continental division and convergence movement since 200 million years ago. People think that the movement mechanism of lithosphere in the form of "plate" is actually much earlier. Many people think that the movement mechanism of plate form may appear in 2 billion to 654.38+08 billion years. First, the mainland broke after 654.38+08 billion years, and then merged into a global continent again before 654.38+00 billion years. That is, the supercontinent (Rodinia) once again experienced a rupture-convergence cycle between about 900 million and 800 million years, and it was not until 300 million to 200 million years that it gradually merged into Pangea. The specific situation of convergence of each continent is different, but its kinematics and dynamics mechanism and characteristics are similar. The evolution of plate movement includes rifting, ocean floor spreading, subduction, collision and other stages. In the development process of each stage, corresponding distinctive metallogenic belts are formed under different tectonic backgrounds (Figure 1 1-2).
1. Tin, uranium, apatite, niobium, rare earth, diamond, chromium, copper, nickel, platinum and ilmenite mineralization areas in continental hot-spot rift and rift.
The upwelling of mantle asthenosphere leads to the stretching and thinning of continental crust, which shows the evolution from hot spot to rift valley. The hot spot of continental crust arch is the product of continental plate cracking mechanism, and the rift is characterized by intense magmatic activity, the formation of rift basin and the continuous supply of deep matter and energy. Rift is a part of the rift zone. When it evolves into an intracontinental rift, it stops moving forward, such as a branch of the original trigeminal rift. Rift basin not only has deposits related to magmatism and hydrothermal process, but also has favorable areas for the formation of deposits related to sedimentation.
Continental hotspots are characterized by the development of over-alkaline and alkaline granite strains and ring-shaped rock masses, resulting in magmatic hydrothermal tin ore, some of which are accompanied by niobium. Jos Plateau in Nigeria is a typical example, and its age is Jurassic. There are similar deposits in the Late Proterozoic in southeastern North America.
The characteristic deposits in rift and rift environment are niobium, rare earth, apatite and vermiculite deposits related to alkaline complexes and carbonate rocks. Diamonds in kimberlites represented by the East African Rift Valley and South Africa were mainly formed in Mesozoic and Late Proterozoic. Controlled by the intracontinental rift system, the deposits related to large-scale basic ultrabasic rocks include chromium, copper-nickel, platinum group and vanadium-titanium iron ore deposits, and the largest deposits in the world are Bushveld in South Africa and Ocean in Zimbabwe. Time is early Proterozoic. It is difficult to see such a large-scale rock mass in the relatively new rift valley area. Iron and copper-nickel deposits related to Triassic shallow rocks and extrusive rocks in Siberia are another example with different characteristics.
There are sediments with a thickness of more than 10,000 meters in the rift valley area, and there are important sedimentary and stratabound deposits in several different sedimentary structures. There are layered copper deposits in clastic rock-dolomite formation and asphaltene shale formation, and Cretaceous stratabound deposits in the Atlantic margin of Africa are its representative, which can be compared with Permian copper-bearing shale in Europe and late Proterozoic copper deposits in Central Africa. Another type of deposit is hydrothermal sedimentary Pb-Zn-Ag deposit, including Canadian Sullivan deposit, Australian Queensland Monte Elsa deposit and German Devonian Maigen deposit. The similar environment of modern plate is called Benue vein lead-zinc mine. In addition, there are evaporites in magnesium-bearing carbonate rock series, such as Permian salt and potash deposits in Chustan, Europe, and Quaternary salt, potash and magnesite deposits in East Africa Rift Valley.
Figure 1 1-2 Schematic diagram of plate movement evolution stage and mineralization distribution (quoted from Zhu Shangqing et al., 1997)
2. Passive continental margin metallogenic belt and marine environment.
Passive continental margin evolved from intracontinental rift system mainly due to submarine expansion. The initial rifting margin becomes an ocean basin with passive margin due to the formation of shelf and slope by subsidence, and its further development is related to the tectonic action of the active expansion center and its two wings. The main geological process in the formation of passive continental margin is sedimentation accompanied by subsidence. The related mineralization types include evaporite deposits in magnesium carbonate rocks in the transgression sequence, such as the South Atlantic and the Red Sea, which are Miocene; Phosphate deposits in black shale-flint-dolomite assemblage in transgression sequence, such as Miocene phosphate deposits in Florida, Peru and Holocene phosphate deposits in West Africa; There are also black shale rich in metals in some areas, such as Mesozoic in southern Europe and Cambrian in Sweden. Oolitic limonite in Jurassic clastic rock series in shallow sea of western Europe is mostly formed in regressive sequence. There are also important lead-zinc deposits in some deep-buried continental shelf carbonate rocks in the relevant environment, which are considered to be caused by atmospheric precipitation or syngenetic hot liquid. Typical ore-forming areas, such as the Mississippi valley in the United States, include CAMBRIAN and Carboniferous, Carboniferous in Ireland and Triassic in the southern Alps.
In marine environment, unique deposit types can be formed in mid-ocean ridges and ocean basins. Firstly, massive sulfide (VMS) copper, iron and zinc deposits in mid-ocean ridges, such as many places in the East Pacific uplift, were formed in Holocene. Most researchers think that they are similar to the geological background of Cretaceous massive sulfide deposits in Cyprus. Secondly, the nodules and crusts containing oxides and hydroxides of manganese, nickel, cobalt and copper in the ocean basin are distributed in the Atlantic Ocean, the Pacific Ocean and the Indian Ocean in Holocene. As for whether there is an earlier equivalent deposit, it is still inconclusive.
3. Metallogenic areas in tectonic environment related to plate subduction.
The subduction of the oceanic crust along the Beowulf Belt to the continental margin or under the oceanic island arc belt has formed related tectonic environments, including magmatic arc, extra-arc and extra-arc trough, back-arc magmatic belt and back-arc basin. These environments often extend to a large space, and related magmatic and hydrothermal processes have formed many distinctive metallogenic belts.
Magmatic arc includes continental margin magmatic arc and island arc in the ocean, which is characterized by the development of calc-alkaline, acidic and neutral magmatic rocks, mainly andesite or basalt, dacite and plutonic rocks emplaced in volcanic activity. The best representatives of magmatic arcs related to modern plates are the boundary between the Pacific plate and the American plate and the boundary between the Pacific plate and the Asian plate. The main types of deposits in the magmatic arc zone are: ① Porphyry and breccia tube-shaped copper and molybdenum deposits and copper and gold deposits related to I-type granite. Tertiary deposits in Andes of South America and Cenozoic and Mesozoic porphyry copper belts in Cordillera of North America are the best examples. The deposits in the island arc belt include tertiary copper and gold deposits in the Philippines and Papua New Guinea belt. In addition, there are Iranian, Cenozoic-Mesozoic copper belts in China and Tibet, and there were deposits in Apara and Kazakstan in Paleozoic. ② Tellurium-gold-silver deposits and gold-bearing quartz veins in andesite craters, such as tertiary deposits in Colorado and Fiji. (3) Sn-W deposits in S-type peraluminous granite, such as the tin ore belt in Southeast Asia. ④ Antimony, tungsten and mercury deposits in basic volcanic rocks or andesitic volcanic rocks, such as the Eastern Alps. ⑤ Magnetite-hematite-apatite deposits in basic volcanic rocks, such as Laco, Chile, are considered to be comparable to Kiru sodalite in Sweden.
Trenches and outer arcs are places where the seabed shrinks. The outer arc is the highland where the volcanic arc rises on the sea side and trench slope. The outer arc is sometimes separated from the magmatic arc by the outer arc trough, and its descent and deposition are influenced by the magmatic arc and the outer arc, which is indirectly related to subduction. Trench deposit, outer arc flysch belt contains ophiolite with tectonic emplacement and ophiolite blocks in slump layer, and may contain chromite or massive sulfide deposits, such as podiform chromite in Indo-Myanmar mountains. It has also been explained that massive sulfide deposits in Cyprus proliferated to the outer arc. Some ore-bearing granitoids are thought to be emplaced in the outer arc, such as tin-bearing granite in southwestern Japan. The older ones are uranium mineralization and copper mineralization in Devonian granite in Scottish Highlands. In addition, there are antimony, mercury and gold deposits in some flysch, such as California coastal mountains.
The mineralization and mineralization of the back-arc environment are also very important, and the back-arc magmatic belt can be hundreds of kilometers away from the contemporary magmatic arc. Back-arc thrust fault zone and back-arc compression basin are often developed near continental back-arc magmatic belt, while back-arc extensional basin is developed in island back-arc magmatic belt. The typical back-arc magma belt is the Tertiary volcanic arc in Bolivia, the Cretaceous-Eocene granite belt in Myanmar and Thailand, and the Cretaceous-Eogene granite belt in Cordillera Mountains in North America. The intrusive rocks in the back-arc magmatic belt are generally rich in silicon and peraluminum, and siliceous volcanic rocks are also preserved on the intrusive bodies. The related deposit types are hydrothermal tin-tungsten deposits occurring in S-type granites and volcanic rocks, such as the western belt of tin ore belt in Southeast Asia, with the age from Cretaceous to Eocene. The tin mining area in Bolivia is Miocene. The rock mass is a large batholith, such as Idaho batholith in the United States, and the related molybdenum-tungsten deposits are in the Early Tertiary. Some porphyry copper (gold, silver) deposits are also considered to have been formed in this environment. The granite belt in the southeast of China is related to the late Mesozoic intrusive rocks in the southwest, which is generally considered as the product of the back-arc magmatic belt. Hydrothermal uranium deposits exist in some back-arc thrust faults. The rock series dominated by clastic deposits in the back-arc compressional basin is the zone where potassium salts are formed, such as Cretaceous deposits in the Nakhon Ratchasima Plateau in Thailand. The deposits in the back-arc extensional basin include epithermal gold and silver veins in volcanic rocks, such as Miocene deposits in the basin-ridge area of the United States.
4. Metallogenic areas in tectonic environment related to collision zone
Collision is caused by the decrease of lithosphere. The lithosphere is narrowed by folds and compressions, forming a linear active zone, that is, an orogenic belt, which is often accompanied by a series of magmatic activities, medium-deep to deep metamorphism, thrust faults and the emplacement of ancient oceanic crust. Related mineralization is also varied, which can occur in foreland basin, foreland thrust fault zone, back continental margin of suture zone and intermountain basin.
Hydrothermal tungsten-tin deposits related to S-type peraluminous granite occur in foreland thrust belt, and the typical areas are tertiary deposits in Himalayan mountains, such as early Triassic deposits in main mountains of Malaysia and early Permian deposits in southern England. Hydrothermal uranium deposits related to S-type light-colored granite, such as Devonian deposits in central France and Late Proterozoic deposits in Namibia.
The deposits in the suture zone include podiform chromite deposits in ophiolite napped by thrust (such as Cretaceous deposits in Oman) and massive sulfide copper-iron deposits (such as Ordovician deposits in Newfoundland). There are jadeite, nephrite and gem deposits in metamorphic rocks in suture zone and post-continental margin, such as some places in Myanmar. In addition, there are more important uranium, vanadium and copper deposits in foreland basins, such as tertiary deposits in India and Pakistan.