Kimberlite is a serpentine phlogopite peridotite. Kimberlite is rarely distributed in nature and usually occurs in the form of small intrusions. The exposed area accounts for less than 0. 1% of the total exposed igneous rocks. This is an uncommon rock type, belonging to hypabyssal-ultrahypabyssal rocks. However, kimberlite plays an important role in petrology, especially in deep geological research and national economy. As far as academic value is concerned, kimberlite is one of the igneous rocks with the deepest origin in nature, which comes from the lower part of the mantle lithosphere at 150 ~ 200km. The initial fluid may come from the mantle transition zone, often carrying mantle peridotite and xenoliths in the lower crust, and a large number of records of deep material composition and geological process are preserved (Zheng Jianping and Lu Fengxiang, 1999), which can provide a depth of 200 km. In terms of economic value, kimberlite is closely related to diamond (diamond), an expensive gem resource, and is the main parent rock of diamond. Most precious diamonds in the world are produced from kimberlite. For example, Cullinan, the world's largest gem-grade diamond (weighing 3 106 carats), is produced in the kimberlite pipe of Premier in South Africa. The main body of diamond is not crystallized from kimberlite magma, and the age of diamond is generally older than that of kimberlite carrying it (Zheng Jianping, 199 1).
1870, the Dutoispan kimberlite cone containing primary diamond was first discovered in South Africa, and then the famous diamond-rich cones such as Kimberlite, De Beers and Balfour Tan were discovered one after another, which opened a chapter in the study of kimberlite and primary diamond deposits. By 200 1, more than 5,000 kimberlite cones have been discovered in the world, of which 100 has important economic value, accounting for 2% of the total. In China, two kimberlite areas containing diamonds were discovered at 1965 and 1970, respectively. Among them, the diamonds produced by No.50 Rock Tube in Fuxian County are of high quality and are very popular in the international market.
Most kimberlites are very strongly altered, and their primary minerals and rock structures are poorly preserved. However, a large number of studies show that the mineral composition of kimberlite is very complex, including not only minerals crystallized directly from magma, such as olivine, phlogopite, ilmenite, spinel (chromite), perovskite, apatite and zircon. There are also xenoliths (foreign minerals) after the disintegration of mantle and crustal materials carried by magma in the source area and on the way up, such as coarse olivine, garnet, chromite, diamond and zircon. In addition, because the magma is rich in volatiles, there are also hydrated carbonate and silicate minerals.
(2) Petrographic characteristics
1. Mineral composition
There are many kinds of minerals that make up kimberlite. According to the statistics of Fuxian and Mengyin rock areas in China, 86 kinds of minerals have been found. Only the main mineral types and characteristics are introduced here.
◎ Olivine: The mineral with the highest content in kimberlite can be divided into three generations. The earliest olivine crystal is round or oval, mostly 2 ~ 4 mm, reaching 65438±0cm, and its composition is forsterite; The second generation olivine phenocrysts, with good self-shape and complete hexagon, are generally less than 2mm, and are also composed of forsterite (Figure 1 1- 1). The matrix olivine is the third generation, with smaller particles and components of forsterite or forsterite. Almost all olivine in kimberlites in China has suffered from strong self-metasomatism, forming the illusion of serpentine and carbonate. Most people think that coarse olivine is not the product of direct crystallization of magma, but the xenocryst of mantle. Arndt et al. (20 10) put forward a standard to distinguish xenocrysts from porphyritic crystals by combining crystal morphology, internal deformation and composition.
Garnet is an important mineral in kimberlite, among which garnet with high chromium and low calcium has symbiotic relationship with diamond, which is of great prospecting significance. Garnet is usually produced in the form of coarse crystals and giant crystals. The coarse crystals are xenocrysts of mantle, and the giant crystals are the products of early crystallization of kimberlite magma. Coarse-grained garnet is usually round, with secondary edges, brown, dark green to black. It consists of clinopyroxene, orthopyroxene, spinel, phlogopite, serpentine and aphanitic. It is called secondary garnet, which is the result of decomposition reaction of garnet from mantle once it migrates from its stable region. The composition of garnet is mainly Mg-Al garnet-Fe-Al garnet-Ca-Al garnet series, showing a certain range of composition changes. Those with high Cr2O3 content and low CaO content are purple-blue, those with high MgO content are pink, and those with high FeO content are orange or crimson. Coarse crystals are mostly purple-blue-pink series, and giant crystals are orange series. Violet-blue garnet is closely related to diamond, with Cao < 3% and Cr2O3 > 4%.
Fig.11-/micromodel structure formed by the second generation authigenic olivine (single polarized light, Fuxian, Liaoning, 10×4) (quoted from Dr. Zheng Jianping's thesis, 1997).
◎ phlogopite: kimberlite, megacrysts, phenocrysts and matrix contain tribasic mica. Most of them are formed by magma crystallization, but the crystallization time is different. Giant crystals crystallize under high pressure, and the crystals are large, up to several centimeters, with melting erosion and edge darkening, and wavy extinction can also be found. Porphyry crystals crystallize on the way of magma rising; After the rock mass is in place, the matrix crystallizes. The phlogopite in kimberlite sometimes appears reverse absorption, that is, ng < nm < NP. The reason of back absorption is that the content of Si or Si+Al in mica is insufficient, which may be accompanied by the increase of Fe and Ti in tetrahedral position.
◎ Spinel: It is coarse-grained and matrix-like in kimberlite. Although the quantity is small, it is very common. Coarse-grained spinel comes from the mantle, which is unbalanced with the rising magma and often develops along the reaction side, and the main component of the reaction side is magnetite. Coarse-grained spinel is generally 0. 1 ~ 0.5 mm in shape, while matrix spinel is less than 0.08mm, with good self-shape. With the increase of Cr2O3 content, the color of spinel changed from transparent dark brown to opaque. Spinel (chromite) with high Cr2O3 content is an indicator mineral for finding kimberlite.
◎ Titanium-rich minerals: including ilmenite, perovskite, rutile, ilmenite, Yimeng ore (K(Cr, Ti, Fe, Mg) 12O 19), etc. The first three are magmatic crystallization, which generally occurs in the matrix of kimberlite; Magnesite is mostly coarse-grained from the mantle. Yimeng ore was first discovered by Chinese scholars in Hongqi No.27 dike in Kimberlite area of Mengyin, Shandong Province, with a size of 0.5 ~ 2mm；. Black, opaque, metallic, flaky and flaky are the products of mantle metasomatism, and together with ilmenite, they are indicator minerals for finding diamonds.
Altered minerals: refer to minerals formed by fluid replacement. The most common altered minerals in kimberlite are serpentine, carbonate and chlorite. They generally appear as the illusion of gathering and explaining. Sometimes, under the microscope, serpentine and carbonate can be seen as annular replacement olivine, indicating that the components of replacement fluid have the characteristics of interaction between H2O and CO2.
In addition to the above minerals, there are apatite, zircon, sulfide and natural elements (such as natural iron, natural silver, natural copper, natural tin and natural silicon). ) and intermetallic compounds (silicon carbide, tungsten carbide, silicon iron ore, etc. The appearance of the latter three minerals reflects the extremely reducing crystallization environment, which is consistent with the characteristics that diamonds are formed in reducing environment.
In addition, in the artificial heavy sand of kimberlite, amorphous or crystalline "melt-off pellets" with diameters mostly less than 1mm can be found, which can be divided into three types according to composition, namely, high-iron titanium pellets, sulfur-iron-nickel pellets and light-colored silicon-aluminum pellets. Molten pellets occur in the late stage of magma crystallization, and are relatively rich in CO2, SO2, FeO, MnO and TiO2 _ 2. Under the conditions of rapid rising, cooling and depressurization, various local ordered zones appear in magma (Lv Fengxiang et al., 2007).
Table 1 1- 1 genetic structure classification of kimberlite
(According to Lu Fengxiang 1996, simplified)
2. Structural structure
(1) Common structure
Kimberlite is a rock formed by the consolidation of mantle materials, magma and volatiles, which is not only manifested in mineral types, but also in structure. The genetic structure classification of kimberlite is shown in table 1 1- 1. Common structures are introduced as follows:
◎ Coarse-grained porphyritic texture: It is the most common texture type of kimberlite. This structure was formed when magma captured olivine decomposed from mantle peridotite in the source region. It is characterized by the large round olivine dispersed in the matrix, and the scale observation of the hand specimen is very clear. The content of coarse crystals in Shengli 1 tubule in Mengyin, Shandong Province is as high as 40%, and the grade of diamond is also very high, which has obvious positive correlation. Olivine is easy to serpentine. Giant crystals are sometimes difficult to distinguish from coarse crystalline phases, but giant crystals are larger, generally larger than 1cm, up to several tens of centimeters. Megacrysts are unevenly distributed in rocks, and the number is small, so they show uneven particle structure.
◎ Microscopic porphyritic structure: observed under a microscope, porphyritic crystals are uniformly dispersed in the matrix, with olivine and a small amount of phlogopite, and olivine is serpentine (figure11).
◎ Since metasomatic texture: Since metasomatic texture refers to the self-metasomatism of olivine or garnet with the participation of fluids related to kimberlite magmatism (not from surrounding rocks or atmospheric circulating water), and then the metasomatism is enhanced, thus forming a network ring structure (metasomatism along cracks), metasomatism residues (metasomatism is incomplete, fresh parts remain in minerals), metasomatism zones (more than one metasomatism product forms a ring) and metasomatism illusion (no residues are found after complete metasomatism).
(2) Common structure
Include massive structure, breccia structure and rock ball structure. The breccia composition of breccia structure comes from the surrounding rock and mantle, and they are unevenly distributed in kimberlite, thus forming this structure. Rockball structure refers to the sphere with kimberlite composition in the rock, and the size of the sphere varies from 2mm to 10 cm. The core of the sphere is mineral debris, and the periphery is fine-grained kimberlite, which is cemented by coarse-grained kimberlite.
(3) Petrochemistry
See table 1 1-2 for the chemical composition of kimberlite. It can be seen from the table that kimberlite is rich in MgO, rich in volatile matter and low in SiO2 _ 2 _ 2 and Al _ 2O _ 3.
Kimberlite belongs to SiO _ 2 unsaturated rock, similar to ordinary peridotite, and its SiO _ 2 content is low, generally less than 40%, and a few are higher than 40%. The content of compatible elements chromium, nickel and cobalt in trace elements is high. Different from peridotite, the contents of K2O, Na2O and incompatible elements Rb, Ba, Nb and LREE are high, and K2O > Na2O. In addition, kimberlite is rich in volatile H2O and CO2.
Table 1 1-2 chemical composition of some representative kimberlites and lamprophyres (WB/%)
sequential
1. Paleozoic kimberlites in Mengyin area (Lu et al.,1998); 2. Mesozoic kimberlites in Kimberly area, South Africa (Le Roex et al., 2003); 3. Paleozoic high-titanium kimberlite in kola peninsula, Russia (Beard et al.,1996); 4. Paleozoic kimberlite in kola peninsula, Russia (Beard et al.,1996); 5. Proterozoic kimberlites in Cuddapah Basin and Dharwar Craton, India (Chalapathi Rao et al., 2004); 6. Proterozoic K-Mg lamprophyres in Cuddapah Basin and Dharwar Craton, India (Chalapathi Rao et al., 2004); 7. Gausberg K-Mg lamprophyre, Antarctica (Gil, 2010); 8. K-Mg lamprophyres in Western Australia (Luo Huiwen, Yang Guangshu,1989); 9. Baifen K-Mg lamprophyre in Zhenyuan, Guizhou (Luo Huiwen, Yang Guangshu, 1989).
(4) occurrence and type
Almost all kimberlites in the world are distributed in stable platforms (cratons), such as South Africa, Siberia, South America, Canada, Australia, India and China's North China Craton. Kimberlite was mainly formed in Proterozoic (represented by Australia and India), Paleozoic (represented by Europe, Siberia and China) and Mesozoic (represented by South Africa and Canada), and a small amount was formed in Paleogene-Neogene, such as Lake Degraces in Canada (Yansai & Xie Han, 1995).
Kimberlite rocks often occur as dikes, cones or pipes, but the scale is very small, and the diameter of the pipes is only a few hundred meters, forming shallow or ultra-shallow facies; It can also overflow the surface to form crater facies.
Figure 1 1-2 Ideal model of magma emplacement of kimberlite (according to Mitchell, 1986)
According to the exposure of kimberlite in the process of South African diamond mining, Mitchell( 1986) put forward an ideal model of kimberlite magma emplacement (Figure 1 1-2), that is, the root facies (including shallow rock wall and bedrock), volcanic channel facies (volcanic neck) and crater facies were divided from bottom to top, and different facies appeared. On this basis, Field & Smith (1999) and Skinner &Marsh(2004) combined with the study of South African and Canadian kimberlite cones, divided kimberlite cones into three types: the first type of kimberlite cones consists of volcanic neck facies, transitional facies, shallow sea facies and crater facies, and the crater facies consists of spherical magma fragments and a large number of microcrystals. The second and third types of kimberlite cones are composed of shallow sea facies and crater facies, but their craters are different. Among them, the crater facies of the second kimberlite cone are mainly pyroclastic kimberlite and amoeba breccia, and the third kimberlite cone is mainly redeposited volcanic kimberlite and breccia clastic rock.
(5) Rock genesis and ore-bearing property
Petrological and geochemical studies show that kimberlite is not a product of single magma crystallization, but a porridge-like magma crystallization containing solid substances (such as xenocrysts formed by the disintegration of mantle and crustal materials) and rich volatiles. Therefore, it consists of three parts: melt, solid matter and volatile matter in mantle and crust.
It is generally believed that the magma related to kimberlite is formed by low-degree partial melting of garnet peridotite with mantle depth above 150 ~ 200 km (Eggler & Wendland T,1979; Wyllie， 1980； Canil &Scarfe， 1990； Dalton Press, 1998).Ringwood et al. (1992) believe that kimberlite is the product of low-degree partial melting of metasomatic gabbro; Chi Jishang et al. (1996) think that kimberlite and olivine-K-Mg lamprophyre are in the ternary system of mantle-magma-fluid. In a certain lithospheric dynamic environment, the mantle material triatomic, low-melting K-rich ultramafic magma and the fluid with C, H, O, N and S as the main components react and mix to form mixed dyes. According to Kamenetsky et al. (2008), the initial melt (protokimberlite magma) is a fluid rich in chloride and carbonate, and its SiO2 _ 2 content is very low. When the magma rises to the surface, it gradually becomes the composition of kimberlite magma due to its interaction with mantle rocks. The interaction between fluid and mantle includes: fluid assimilates mantle minerals such as olivine, which increases its MgO content, and finally forms the composition characteristics of low silicon and high magnesium. Kamenetsky et al. (2004, 2008) inferred from the composition of pyroxene and garnet inclusions in olivine in Udachnaya kimberlite that these inclusions were formed by crystallization in the lower mantle of lithosphere, with a pressure equivalent to 5GPa and a temperature of 900 ~ 1000℃. According to the research, the original kimberlite fluid is deep and may come from the mantle transition zone, and these fluids migrate upward due to the compaction of olivine (Grégoire et al., 2006). On the basis of these understandings, Arndt et al. (20 10) put forward a two-stage model of kimberlite formation: the first stage, in the deep mantle (mantle transition zone? ) CO2-rich fluids gather at the bottom of the lithosphere, forming fluid-rich capsules, and the fluids react with surrounding rocks, consuming pyroxene and garnet, leaving only olivine. Therefore, due to metasomatism, dunite is formed around the fluid sac, and lherzolite is formed far away from the fluid sac. In the second stage, due to the pressure in the fluid bag, the surrounding peridotite broke, and the fluid previously mixed with pyroxene and garnet entered the fracture and quickly flowed to the surface. During the ascent, dunite and other deformed peridotite will be captured one after another.
Recent studies show that diamonds with economic value are not formed by magma crystallization, but by xenoliths in the mantle. Therefore, the higher the mantle material content in kimberlite, such as coarse olivine, the better the diamond content.