The rare metal minerals related to pegmatite are lithium, niobium, tantalum, beryllium, cesium, tin, tungsten, yttrium, thorium, uranium and zirconium. Pegmatite is the home of rare metals and the treasure house of precious stones. It has always been the research object of mineralogy and geochemistry, and it is also an important window to explore new metallogenic theories. Pegmatite deposit, as an independent deposit type, not only plays an important role in deposit science, but also has great significance in tracing structural evolution. The study of pegmatite deposits abroad has also experienced a similar process from case study to regional study, that is, in the past, it mostly focused on pegmatite mineralogy, structural zoning, element geochemistry, isotopes, melt fluid inclusions, diagenetic and metallogenic experiments, etc. (Jollif,1986; Burnham,1986; Erci， 1992； Hansen,1992; Lenz,1992; London,1988; Thomas, 1988), established many prospecting indicators (London, 1986), which have been extended to the study of metallogenic mechanism, metallogenic model, diagenetic process (including chronology) and tectonic environment (London,1985; Cerny,1991; Swamson， 1992； Suwimonprecha， 1995； Miller,1996; Lin Ning,1998; Essaid, 2000).
Cny (1985) divided pegmatites into orogenic pegmatites and non-orogenic pegmatites. Cny (1991a) classifies pegmatites containing rare metals into three types: LCT type, ny type and NYF type. The main elements of LCT pegmatite are lithium, rubidium, cesium, beryllium, gallium, tin and niobium. ta, Y, REE, Sc, ti, Zr, Be, Th, U and F. Because mica is the main carrier of rare elements, Zou et al. (1975) classified pegmatite as biotite pegmatite (REE Different types of pegmatite may have the same genetic mechanism, while the same type of pegmatite may belong to different genetic causes.
Isotopic geochemical data prove that the isotopes between many pegmatite dikes and surrounding rocks are unbalanced, and pegmatite can be traced back to the magma source area, even highly differentiated pegmatite dikes are no exception. For example, when O 'Connor et al. (199 1) studied the lithium-rich pegmatite dikes around leinster granite in southeastern Ireland, according to the Rb-Sr isotopic age of pegmatite dikes and granite, the initial value of 87-Sr/ 86 Sr and the relationship between trace elements, they ruled out the possibility that pegmatite was a molten product of lithium-rich sedimentary rocks, and thought it was magmatic origin. Brookins( 1986), Talor and Friedrichsen (1983) ruled out the possibility that pegmatites in the United States and northern Sweden originated from surrounding rocks through Sr isotopic composition. Tomasak et al. (1998) According to the Sm-Nd isotope system, the pegmatite in the vertical mountain of Maine, USA is related to the adjacent biotite granite. There are generally three occurrences of pegmatite dikes of magmatic origin (Cě rny, 199 1b). When seepage, fluid migration and gravity convection and diffusion are the main driving forces to produce residual magma, pegmatite veins will mainly distribute in the upper part of granite body; When the cracks induced by rock mass cooling are the dominant factor of magma separation, pegmatite veins will distribute inward from the contact zone; The other is that the pegmatite melt rises under buoyancy, separates from the incompletely cooled parent magma, and forms veins in the rock mass, which is not common.
To sum up, there are three main genetic models of magmatic pegmatite deposits: pulsating model, magmatic differentiation model and liquid separation model.
1. pulse form
According to the regional zoning of pegmatite veins, солоов (1959, 1962) thinks that different pegmatite melts are precipitated in different periods of magma source, and potassium-rich pegmatite melts are precipitated first, and then Ta and Cs are enriched. However, a large number of field evidences show that most pegmatite dikes were invaded by pulses, and then contracted radially to increase the scale before crystallization began, so this genetic model was gradually abandoned.
2. Magmatic differentiation model
According to the viewpoint of crystallization differentiation, due to the incompatibility between volatile components and rare metals, with the precipitation of early crystals, they gradually enriched in the residual melt until they finally crystallized into veins. For example, Evensen and London(2002) and London and Evensen(2003) show that when the crust is remelted to form granite, refractory minerals such as cordierite will prevent Be from entering the melt, and then in the process of melt evolution, the early melt will Be slightly enriched due to the low melt/mineral distribution coefficient between Be and various minerals. When 80% of the melt crystallizes, the Be solubility at the top of the rock mass reaches (15 ~ 20) × 10-6, and the Be solubility of the differentiated pegmatite is more than 70 × 10-6 (Figure 1). Sheaer( 1992) put forward an idealized model of pegmatite field formed by continuous crystallization of mother magma (Figure 2), which holds that after partial melting of lithosphere, homogeneous magma or heterogeneous magma was formed and homogenized in magma chamber, and then mother magma was continuously crystallized to form extensive granite zoning phenomenon, with low degree of crystallization differentiation forming biotite granite and high degree of crystallization differentiation forming magma rich in rare metals. This model is different from many other models.
Fig. 1 schematic diagram of Be enrichment process in crustal melt evolution
(Based on evenson and London, 2002)
The pegmatites containing rare metals formed by magmatic crystallization differentiation can be divided into LCT type and NYF type (Cě rny, 199 1a). The composition of LCT pegmatite is peraluminum, and the parent rocks are S-type and I-type granites. Pegmatite comes from the upper part of the rock mass and is the first partial melting of rocks in the middle and upper crust (Cě rny, 65438). The parent rock of NYF-type pegmatite is A-type granite or rock mass with similar composition, and the magma and fluid produced by the secondary melting of the original rock of the lower crust in a short time participated in the formation of many NYF-type pegmatites (Cě rny, 19 1B). These two kinds of pegmatites also reflect the different crystallization processes of parent magma. For LCT type, magma crystallizes from bottom to top. For NYF type, magma crystallizes from outside to inside (London, 2005). Bea et al. (1994) studied the crystallization differentiation of Pedrobernardo in Spain, and put forward the convection and gravity differentiation models (Figure 3). The model holds that strong convection occurred in the early stage of magma emplacement due to high temperature, low viscosity and large Rayleigh coefficient; Subsequently, as the temperature decreases, the viscosity increases. When the proportion of residual melt reaches the critical fraction (30% ~ 40%), the rheological properties of melt change, which makes the high-density melt between high-density crystals unstable, sinks to the lower layer under the action of gravity, and the residual low-density melt is squeezed to the upper part, thus causing zoning of rock mass. This process includes crystallization in convection, crystallization in static melt, crystal precipitation and the rise of residual melt discharged from accumulation.
The continuous differentiation of magma forms a pegmatite field model.
(According to Sheaer, 1992)
Fig. 3 Vertical zoning of crystals caused by convection and subsequent gravity.
(According to Bea et al., 1994)
3. Liquid separation mode
The liquid separation mode of magma can be proved in a large number of lithium-rich fluorine granite. When Mapaky( 1984) describes the spherulites in acidic volcanic rocks of Amelia, it is found that spherical glass is rich in Na and Fe, while basic glass is rich in K, Mg, H2O, etc. In addition, there are concentric bands and banded structures in super-acidic fluorine-rich rhyolite, and these structural, structural and compositional characteristics are considered to be the result of magma liquid separation. At home, Wang Liankui et al. (2000) also classified Dajishan, Yichun and Jianfengling granites in Nanling area of China as the cause of liquid separation according to the abrupt changes of structure, structure and composition in different parts of the rock mass. Another form of liquid separation is gas-liquid separation of magma. Tycoн( 1977) proposed that the upper part of rare metal mineralized granite was formed by gas-liquid separation, so the upper part of the rock mass is rich in volatile magma chamber, so the upper part of the rock mass is relatively rich in rare metal elements (Nb, Ta, W, Sn, etc. ) are all rock-loving, and these elements in the lower part are relatively poor, which constitutes a dual magma chamber differentiation model. In recent years, Chen Yuchuan, Luan Shiwei and others (2003) explained the genesis of pegmatite deposits in Altai with liquid separation model, and put forward two sources of pegmatite protomagma.
In addition to pegmatites derived from magmatic differentiation, there are Lamberg (1952, 1956), сокодв (1959, 1970). Metamorphic pegmatite can be divided into metamorphic anatexis and metamorphic differentiation.
Second, the scope of application and application examples
The Greenbuss lithium, niobium, tantalum and tin polymetallic deposit (Figure 4) is located 250km south of Perth, Australia. Since the beginning of the 20th century, this mining area has become the tin sand production center in Western Australia. After 60 years, weathered pegmatite has become the main ore, and now it has been transformed into fresh and hard pegmatite to be mined as tin, tantalum and lithium ore. At present, lithium ore (containing Li2O 4.06%) is 765,438+million tons, tantalum ore (containing Ta 0.06%) is 4.7 million tons, niobium ore (containing Nb 0.42%)108,000 tons, tin ore (containing Sn 0.24%) is 4.7 million tons, and kaolin is 2.3 million tons (containing Nb 0.42%).
Fig. 4 Geological map of pegmatite in Greenbush, Australia (a) and pegmatite distribution profile (b)
(According to Fan Peifeng, 2000)
The pegmatite group in the mining area consists of a series of dikes with a length of 2 ~ 3 km and a width of 10 ~ 300 m and a few geese-shaped lenses with a diameter of several meters, which are radially distributed from the intrusion center. Late deformation and metamorphism transformed the magmatic structure and structure of pegmatite to varying degrees. Pegmatite can be divided into plum zone, potassium zone, sodium zone and marginal zone from inside to outside. The main ore minerals are lepidolite, cassiterite, tantalite, fine spar and crystalline uranium. Tin-rich tantalum ore bodies occur in albite belts. The study shows that pegmatite has three ore-forming events, the first one is related to the original crystallization of pegmatite and the metasomatism of surrounding rocks (the ore-forming time is 2527Ma), the second one is related to hydrothermal alteration of pegmatite with the same structure (2430Ma), and the last one is related to the activation and migration of ore-forming elements in the post-deformation and metamorphism stage (11000 Ma). The formation depth of Lvbushi pegmatite is greater than 1 1km, and its intrusion and crystallization are in a metamorphic environment of medium-high temperature and medium pressure. According to fabric analysis, isotope data and intrusion time, pegmatite area can be divided into three kinds of metamorphism: M 1, M2 and M3, in which pegmatite intrusion is mainly controlled by M2 metamorphism and deformation.
The main features of the deposit are as follows: ① the deposit is located in the geosyncline belt on the edge of the Archean craton in Australia; (2) The deposit is formed in a metamorphic area with medium-high temperature and medium pressure, and there is no need for obvious granite parent rock; (3) Pegmatite is zonal, which can be divided into lithium zone, potassium zone, sodium zone and marginal zone from the inside out, and tin-rich tantalum ore bodies occur in albite zone; ④ The main ore minerals are lepidolite, cassiterite, tantalite, fine spar and crystalline uranium.
Three. sources of information
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