Carbonatite

Classification：Igneous Rock

Igneous rocks composed of carbonate minerals are related to minerals such as phosphorus, niobium and rare earth elements.

Carbonatite, the term carbonatite, which is mainly composed of carbonate minerals, related to alkali ultrabasic complex in space and in space and origin, was formally introduced by Norway geologist and mineralogologist W.C. Bragg in 1921.
The rocks are pale gray to gray, granular structure, fine to coarse grain, and sometimes Mega crystalline structure, often massive, sometimes with primary strips, pellets and spheroid structures. The chemical composition is special and is rich in CaO and CO2, poor SiO2 and Al2O3 compared with ordinary silicate magma. Compared with deposition, it is rich in SiO2 and Fe, Mg, Al, Ti, P and other oxides, while CaO and CO2 are lower.
Mainly composed of minerals and iron dolomite, occasionally seen. In addition, there are many (180 or so) secondary minerals and secondary minerals, such as pyroxene,,, cerium, rare earth fluoro carbonate minerals, niobium and Tantalum Minerals, uranium and thorium minerals, and carbon silicalite. It is generally divided into calcite Carbonatite, dolomite Carbonatite, iron dolomite Carbonatite and siderite Carbonatite according to the carbonate minerals.
Carbonatite is mainly distributed in the central intrusive complex. There are central rock bodies, circular, tapered and radial dykes, bedrock, rock flow and rock cover. It is known that the central strain is 10 thousand meters from the top to the bottom. Carbonatite often has strong separation crystallization, liquation and alkali metasomatism. The distribution of Carbonatite is related to deep faults. It mainly occurs in the ancient platform margin fault system and the intermediate block fault zone in the fold belt. In space, it is often associated with alkaline rock ultrabasic complex. Except for Antarctica, all continents have Carbonatite distribution.

Reservoir characteristics:
Carbonate rocks are widely distributed, and mainly consist of marine carbonate rocks. Lacustrine carbonate rocks are distributed only in Meso Cenozoic era. The sedimentary facies of carbonate reservoir are composed of beach and dam facies, shallow water platform facies and reef facies, Lake shallow shoal facies and semi deep lake facies, as well as pure fissure type and paleo weathered crust type reservoir. The carbonate reservoir has four characteristics as follows:
(1) under the relatively stable tectonic environment, the reservoir is widely distributed. This means that the sedimentary environment and the diagenetic environment are relatively stable, that is, the large area distribution of the land surface sea environment and the whole vertical ascending and ascending atmospheric fresh water dissolved in the diagenetic environment is an important reason for the large area distribution of the reservoir.
(2) there are many types of reservoir space, mainly fracture one pore type or fracture and hole type, and the heterogeneity of reservoir is strong, but there are open cracks and caves outside the pores in carbonate reservoir besides pore and throat. Open cracks and caves have improved the physical characteristics of carbonate reservoirs with low porosity, low permeability and small larynx, so that they can produce industrial oil and gas or become high oil and gas reservoirs with high yield.
(3) the heterogeneity of carbonate gas reservoirs is strong, which is due to the strong heterogeneity of structural opening and slit distribution.
(4) carbonate reservoir has the ability of generating oil and gas, and it can form self generating or self storing or multiple oil and gas source gas reservoirs.

New type of rock:
In the field, "marble" is dyke, rock branch and veinlet invaded quartz diorite. According to the data of the trend surface analysis of Shi Zhan Li, the contact zone of marble and quartz diorite in this mining area is always characterized by the diorite in the quartz diorite. Marble size is observed near the contact zone between marble and limestone. The size of marble is finer due to the cooling of temperature, and there are residual rocks in marble. "Marble" itself calcite granularity coarse fine on both sides of the central. Marble crystal grains are coarser than those of marble formed by contact heat metamorphism in the middle and lower reaches of Yangtze River. The pale marble of marble is white and often has a directional structure of fine stripes (which may be a linear structure caused by a flow structure or tectonic event) and a sharp change in local yield.
The "banded pyroxene one garnet marble", which is produced in the "marble" section of the Jianshan mine, mainly contains pyroxene and garnet strip, which is inclined to the extension direction of the "marble" rock mass, and is related to a group of fracture structures in the area. Microscopic observation showed that melt inclusions or fluid inclusions and fluid inclusions were observed in the slices of about 65 inclusions. The experimental results show that the temperature of the molten inclusions in the garnet and calcite of the marble rock and the calcite of the "marbles" is 880~1055 degrees centigrade, and the homogenization temperature of the fluid inclusions is 645~740 C.
The C and O isotopic compositions of the "marble" and "banded pyroxene garnet marble" are roughly the same as the limestone in the middle and lower reaches of the Yangtze River, and the projection points in the delta ^13C- Delta ^18O correlation diagram fall in the sedimentary carbonate range. The electron probe analysis shows that a molten inclusion in the calcite of the "zonal pyroxene garnet marble" is a mixture containing Si, Ca, Mg, Al and K (that is, glass). The composition of the round solid inclusion in marble is calcite examined by electron probe. The results of energy spectrum analysis indicate that the solid inclusions in dolomite of dolomite marble are similar to dolomite, but are slightly different from host minerals. On the basis of these preliminary studies, it is believed that the "marble" and "banded pyroxene garnet marble" reported in this paper are not the result of the recrystallization of calcite caused by contact metamorphism, but may be a new type of Carbonatite - shell source Carbonatite.

Conditional simulation technology:
Conditional simulation is a geostatistical technique that gets the characteristic distribution. The foundation of the simulation model is the basis of the reservoir simulation model, which characterizing the physical parameters of the reservoir in the three-dimensional space. The usual model is to grill the reservoir and give each grid its own parameter values to reflect the three-dimensional spatial variation of reservoir parameters. The smaller the mesh size is, the smaller the model is. The smaller the error between the grid parameter and the real value, the higher the accuracy of the model.
There are many reservoir parameters that affect the flow of fluid in the reservoir, such as permeability, porosity, saturation, capillary pressure, wettability, and the distribution of thin interlayer impermeable layer. The scale of the reservoir can be divided into four stages, that is, microscopes (a few pore ranges), macro range (experimental measurement of rock plug and flow characteristics), large scope (large scale block range of oil field), and huge range (reservoir range). The reservoir description has the characteristics of probability, because the useful information is incomplete, the space configuration of the reservoir building block is complex and the geological features have internal changes, which hinders the deterministic mapping of the properties between the measured points.
The step of the conditional simulation is to remove one of the many random fields from the geological statistics and let it pass through the measured data and maintain the overall contrast structure of the random field. With variable range simulation, we can use surface Kriging to divide variables into large and small ones. Kriging has all the large range variables that can be observed at the sampling point, but smoothes a small range of variables. The results of Kriging are only a small range of variables and the exact number of small range variables, but these must be added to Kriging in order to reach the uniform distribution of the rules.
The flow parameters are proportional to average flow parameters, including absolute permeability, dispersion rate and relative permeability. When the simulation ratio is larger than the macro scale, the effective flow parameters and effective fluid components generally need to be determined in the simulation model. The main factor affecting effective flow parameters is heterogeneity of permeability distribution.
The uncertainty of reservoir characterization is not unique due to the realization of spatial random function by conditional simulation. Therefore, we need to evaluate the uncertainty in reservoir description. This can be accomplished by simulating the process characteristics of multiple permeability fields. As the time required to simulate complex multiphase flow processes in detail is too long and cost too high; therefore, the possible ethnic spectrum should be retrieved to verify which implementations are conditional to simulate the most appropriate, unsuitable and most likely results.
Conditional simulation is a geo statistical technique that gets the characteristic distribution. It can predict the physical parameters of Carbonatite reservoir. If the information of rock structure is integrated into the permeability conversion equation, the permeability calculated by electrical logging can be greatly improved. There is almost no porosity in the Donne oilfield, and three pore systems are determined according to the size and separation of the particles. These three pore systems occupy a specific position in the map of the porosity of the acoustic logging, which has the relationship between the specific porosity and permeability, and can be accurately calculated by the resistivity and acoustic logging. Permeability.

Petrological characteristics:
The Carbonatite rock wall is located in the northwest foothills of dulhara mountain, about 3 km north east of the East Mine of Baiyunebo deposit, and the bottom of the Baiyunebo group, H1 coarse grain quartz sandstone and conglomerate and H2 fine quartzite. The trend of the dyke is about 40 degrees, which tends to the North West, the dip angle is 85~89 degrees, and the outcrop of the dyke surface is about 60 m, and the width is 1.1 ~ 1.5 m. The boundary between the rock wall and the surrounding rock of H1 and H2 is distinct, and the contact zones on both sides of the rock form a neon belt with a wide about 10~20 cm, characterized by natriuretic magnesium sodium amphibole, albite, and gold mica. These minerals, which are characterized by alkali, distribute along cracks and joints in the surrounding rock and reach 20~30 m away from the contact zone.
Although the lithology of Carbonatite is relatively homogeneous, there are common xenoliths and xenoliths with surrounding rocks, such as quartz sandstone, quartzite and neigenite, and sodium iron amphibole. The rocks are fine-grained structure, sometimes porphyritic structure. Due to the influence of late structure, the rock has been strongly deformed, and most of the minerals are arranged in directional arrangement. The cleavage joints of the large calcite speckles bend and appear wavy extinction.
The main mineral of the rock is calcite, which is self shaped and semi self shaped, with a particle size of 0.2 to 0.4 mm and a large speckite up to 5~7 mm, and a typical fine grained Carbonatite structure with a fine grain of fine grained calcite Carbonatite. Calcite is characterized by Sr and Mn rich, but its REE content is mostly below the detection limit of electron probe.
The secondary minerals are BIC and CFC, most of which are self shaped and semi self shaped, with a particle size of 0.01 to 0.07 mm, but the large ones can reach 0.3 to 0.5 mm, and some fine particles are composed of granular aggregates, which are in contact with calcite, indicating that they are primary minerals directly crystallized from Carbonatite pulp. In addition, the rocks also contain BCE, sodium ferric amphibole, magnesium sodium amphibole, apatite, magnetite, monazite, limonite, quartz, fluorite, dolomite and barite.

Rock geochemical characteristics:
Analysis method: each sample carefully selected 500 g small pieces, cleaned with distilled water, and dried by oven (100 degrees). The samples were crushed into agate ball and crushed to 200 mesh. The total rock analysis was measured by the Philips PW 1400 X fluorescence spectrometer at the Geological Department of Leicester University in England. The main elements were measured by the fusion sheet method (rhodium tube). The trace elements were measured by the press method. The Ni, Zn, Rb, Sr, Y, Zr, Nb and Th were measured by the rhodium tube. In order to make trace elements within the scope of instrumental analysis, the samples were diluted with the spectral pure SiO2 at a ratio of 1 to 1. The analysis of the rare earth elements in the sample was also measured by a plasma spectrometer at the University of Leicester. The original ICP data were corrected by superposition of Ba, Sr, Ca, Fe and Zr lines.
Rare earth elements: the REE content of Carbonatite whole rock samples is high and the range of change is large. It is between 1.45% and 19.92%, with an average of 8.36% (mass fraction), which has already formed a REE rich ore. Chondrite normalized REE distribution pattern shows strong enrichment of LREE and no Eu anomaly, which is consistent with Carbonatite distribution in other parts of the world. The ratio of NLa/Yb is 139~1776, with an average of 814, indicating that Carbonatite has heavy fractionation between light rare earth and heavy rare earth elements. In addition, the content of rare earth elements in different samples is nearly 14 times, indicating that REE distribution in Carbonatite dykes is extremely uneven. This is consistent with the results of the local height concentration (up to 15% to 20% (volume fraction)) of rare earth minerals such as barecerium ore discovered by rock slice observation. In addition, calcite, although rich in Sr and Mn, is characterized by a typical igneous Carbonatite, but its REE content is below the detection limit of the electron probe, indicating that REE mainly exists in the rare earth minerals such as cerous, cerium and other minerals.
The original mantle normalized trace element spider diagram of the representative samples of the Baiyunebo Carbonatite rock wall (original mantle normalized data from Wood etc., 1979; samples 90/39, 90/42, 90/43, 90/44, 90/45, 90/52, 93/149 are Carbonatite; 90/40, 90/46/90/55, and 56 is neon).
The original mantle normalized trace element spider diagram of the representative samples of the Baiyunebo Carbonatite rock wall (original mantle normalized data from Wood etc., 1979; samples 90/39, 90/42, 90/43, 90/44, 90/45, 90/52, 93/149 are Carbonatite; 90/40, 90/46/90/55, 56 is neon) Atlas
Trace elements: the results of XRF analysis are shown in Table 2. The original mantle normalized Carbonatite trace element spider diagram shows that Carbonatite is rich in Ba, Th, LREE and Sr, the variable Nb and P, and the lower Rb, K and Ti, although the Zr is low, but the abnormality is not significant. This trace element spider diagram is exactly the same with the common fine calcite Carbonatite. It is worth noting that they overlap with trace element spiders of fine grained dolomite marble (H8f) in Baiyunebo REE-Nb-Fe deposit. The whole rock Sm-Nd isotope dating results indicate that the age of Carbonatite dyke formation is t=1223 + 65 (2 sigma) Ma, INd=0.510926 + 35 (2 sigma), and epsilon Nd (T) =-2.63 + 0.68. This is in the error range, which is consistent with the isochronous age of Sm-Nd in Baiyunebo rare earth ores, and the initial 143Nd/144Nd ratio is also very close.
Compared with the normal sedimentary quartz sandstone, the SiO2 content of the neenite decreases, and the Na2O and Fe2O3 increase significantly, indicating that the neon granitization is brought into Na and Fe, and has Si. In addition, the microelement spider diagram of the neon rock shows a higher Rb, K and Zr, retaining the characteristics of the general sedimentary quartz sandstone; however, Ba, La, Ce, Nd, Sr are obviously enriched, which is the characteristic of the superposition of the neon rock caused by the Carbonatite rock wall. From the time of formation, the neon rock is at the same time with Carbonatite, or a little late. Therefore, it is reliable to use neigite and Carbonatite as the Sm-Nd isotopic dating samples.

The enrichment mechanism of rare earth elements:
Carbonatite has reached the grade of REE rich ore. It is mainly contained in the minerals such as BIC and CFC, and the rock structure analysis shows that they are in contact with calcite, indicating that it is a primary mineral from Carbonatite pulp. This rock structure relationship has also been found in Mountain Pass rich rare earth Carbonatite. It is worth pointing out that the distribution patterns of rare earth elements in Carbonatite and BIC ore are very similar to those in the ore bearing dolomite marble of the Baiyunebo deposit. Carbonatite's trace element spider diagram and formation age are almost the same as ore bearing dolomite marble and rare-earth ore. These geochemical characteristics indicate that the formation of Carbonatite may be related to the genesis of Baiyunebo deposit. In addition, O, C and Sr research results have supported this inference.
Compared with calcite in the normal sedimentary limestone, it can be seen that calcite in REECarbonatite is rich in Sr and Mn; the Sr and Mn mapping of different REECarbonatite samples of Chinese calcite show that they have an inverse correlation, reflecting the separation and crystallization of Carbonatite. And the diagrams made by La/Sr-La/Nd and Ba/Sr-La/Sr show that Carbonatite has separated crystallization. Therefore, it can be speculated that the crystallization of massive calcite leads to highly enriched REE in residual magma. The enrichment process of REE is very similar to that of Mountain PassCarbonatite. Wyllie etc. pointed out that the highly enriched process of CarbonatiteREE should occur in the crustal environment, but the initial concentration of REE in the Carbonatite magma derived from the mantle is not too low, otherwise REE will enter the main carbonate minerals and other minerals, so that there is no opportunity to form a REE mineral. If the REE concentration of Carbonatite magma is higher than that of this lowest level, the most important factor to exclude the formation of REECarbonatite is that in the crystallization of Carbonatite magma, the crystallization temperature of REE minerals (such as apatite, monazite, perovskite, etc.) is separated from the magma, and with a considerable amount of REE. Such a result must not form REECarbonatite, and form apatite magnetite Carbonatite.
Since there is no silicic acid unsaturated alkaline rock exposed to Carbonatite, it is difficult to judge the possibility that the Dula Hara Carbonatite is formed by liquid immiscibility in the crust under the low degree of melting of the mantle soft ring, but it can not exclude the existence of this mechanism. In the Mesoproterozoic, Baiyunebo is located in the rift environment of the north margin of the North China platform, which is obviously different from the tectonic environment formed by -Carbonatite in alkaline rocks in Africa. Therefore, there may be a difference in the rock composition of the igneous Carbonatite. Even if the unobserved liquid immiscibility in the crustal environment, REE will not be enriched in the melted Carbonatite melts. It can be assumed that Carbonatite pulp is directly formed in the partial melting of the lithospheric mantle. After deducting the effect of high concentration of REE caused by separation of crystallization, it is assumed that the sample 90/39 represents the original Carbonatite pulp, then the mantle source area of the mantle must be required to be an enriched mantle. A large number of high temperature and high pressure experiments show that the partial melting of mantle peridotite can produce Carbonatite melts under the conditions of the upper mantle of 2.1 ~ 3.1 GPa and 930~1080 C. The simulation results show that the initial Carbonatite melt with a concentration of about 1000 REE g.g-1 can be formed by the partial melting of the rich mantle enriched by 10~20 times of the original mantle, with a low degree of partial melting of 1%. Residue of mineral phase garnet is not less than 20%. The Carbonatite melt that has been preliminarily enriched by REE can undergo the separation and crystallization process in the crust environment to achieve the observed REE concentration level of Carbonatite.
The study shows that the Dali rock mass is a Carbonatite intrusive body, followed by the fine granulation of coarse dolomite marble, resulting from the tectonic action and mylonitization, which resulted in the recrystallization of the hydrothermal fluid, and the REE-Nb-Fe mineralization was related to the Carbonatite magmatism. The possible mode of dolomitization is that the intrusion of Carbonatite pulp causes the resurfacing of the convection in this area to be readjusted, resulting in the activation of magnesium from the Baiyunebo group of sedimentary rocks, especially shale, to the Carbonatite body resulting in dolomitization. Because of the small amount of heat generated by the Carbonatite rock wall, the convective hydrothermal system of a certain scale can not be formed, so most of them have no dolomitization. The other possibility is that the coarse dolomite marble is formed by the cooling and crystallization of the primary dolomite Carbonatite pulp and is a product of different stages of the homologous magma of the calcareous Carbonatite rock wall. The direct basis for this speculation is that the other fine grain Carbonatite rock wall of the H9 slate overlying the coarse dolomite marble is calcareous and has no dolomitization; and he is consistent with the trace elements of dolomite marble and the geochemical characteristics of C and O isotopes.
The trace element spider diagram of mantle fluid is also similar to the common Carbonatite. It is one of the important media to cause mantle metasomatism. It is assumed that the mantle fluid is metasomatism to sedimentary limestone or dolomite to form ore dolomite marble, which has the rock geochemical characteristics of Carbonatite, then the symbiotic assemblage of Mineral Minerals and the sequence of mineral formation should follow the relation of metasomatism. In fact, the sequence of rock structure and mineral production is consistent with mineralized Carbonatite.