Phosphate rock is a kind of sedimentary rock rich in phosphorus, which is mainly formed by biochemical sedimentation. The phosphorite in phosphorite belongs to fluorocarbon apatite isomorphic series, and its crystalline state is amorphous, aphanitic, flaky and columnar. The minerals associated with apatite are calcite, dolomite, glauconite, siliceous and organic matter. The structure of phosphorite ore is mainly massive and banded, and there are also nodular, layered and reticulate vein structures. Ore structure generally includes gel structure, internal clastic structure, spherulite structure and bioclastic structure.
The phosphorite deposits found at present are all marine deposits. Although the continental deposits also contain phosphate, none of them meet the mining requirements. According to the occurrence, phosphorite can be divided into layered phosphorite and nodular phosphorite. Phosphate deposit is the most important industrial type of all kinds of phosphate rock, and its reserves account for 80% of the total reserves of phosphate rock in the world. Yunnan, Guizhou, Hubei, Sichuan, Shaanxi, Shanxi, Henan and Gansu are all distributed in China, and many foreign countries such as the United States and Australia also belong to this type.
Phosphorite deposits have a wide geological age. From Proterozoic to the present, almost all phosphate deposits are distributed in all ages. The main times of phosphorite formation in the world are Sinian, Cambrian, Permian, Cretaceous and Tertiary. The main periods of sedimentary phosphorite in China are Late Sinian and Early Cambrian, followed by Devonian, Carboniferous and Ordovician.
In addition to phosphorite deposits, diatomite deposits, sedimentary natural sulfur deposits and sedimentary pyrite deposits also belong to biochemical sedimentary deposits. Some of them are directly accumulated by biological remains, while others are accumulated by certain elements due to biological life activities or biodegradation. Modern research shows that the formation of multi-element metal deposits, including black shale, is related to biological action. Biological mineralization is an important aspect of modern metallogenic research.
Two. Important mineral deposits
1. Yunnan Kunyang Phosphate Mine
The deposit is located in the west of Yangtze platform and the east of Kangdian axis. Phosphate rocks occur in Zhong Yi Village, Yuhucun Formation in the early stage of Meishucun in the Early Cambrian (Figure 7-30). The rock assemblage of phosphorus-bearing rock series is siliceous rock-dolomite-phosphorite. Phosphate rock is divided into upper and lower layers, with grayish white phosphorus-containing hydromica clay rock in the middle, with a thickness of about 1.6m, a coal seam length of 8km from east to west, a mining area of 20.86km2, and an upper coal seam thickness of 1.97 ~ 14.85 m, which is relatively stable. The thickness of the lower coal seam is 0 ~ 6.87 m, with an average thickness of 3.5m, which varies greatly. The average P2O5 content of ore in the whole region is 26.24%, and the grade of surface oxidized ore is high, with P2O530% or more and less harmful impurities. The content of P2O5 in deep primary ore decreased significantly (20% ~ 25%), while the contents of CaO and MgO increased. The ore types are mainly blue-gray phosphate-rich rocks and light-gray dolomite phosphate rocks. The ore minerals are mainly collophanite with a small amount of dolomite, calcite, chalcedony, glauconite and chlorite. The ore structure is mainly granular structure (internal debris, spherulite and bioclastic structure). The ore structure is massive, banded, nodular and gravelly.
Figure 7-30 Stratigraphic Histogram of Yuhucun Formation in Xiaowaitoushan, Kunyang Phosphate Mine
(quoted from Tu Guangchi, etc., 1988)
The deposit is located on the gentle slope (platform) of carbonate rocks on the southeast edge of the Kangdian axis, and is formed in the changing sedimentary environment of subtidal zone-intertidal zone-supratidal zone-intertidal zone-subtidal zone. The bedding types of phosphorite are mainly horizontal bedding, wavy bedding and granular bedding, followed by cross bedding. The mining area enters Shaanxi through Kunming, Emei and northern Sichuan, forming an important phosphorite metallogenic belt with a length of 800 kilometers from north to south and a width of 40 ~ 60 kilometers from east to west.
2. Shifang Phosphate Deposit in Sichuan
Located in the middle section of Longmenshan platform fold belt in the western margin of Sichuan Basin. The phosphorus-containing profile belongs to the middle and upper Devonian series, and consists of gravelly phosphorite, phosphostrontium-aluminum ore, phosphorus-containing kaolin, phosphorus-containing timely sandstone and dolomite from bottom to top. The ore-bearing profile is parallel unconformity on algae dolomite of Dengying Formation of Upper Sinian.
The ore bodies are phosphorite ore bodies and aluminum phosphate ore bodies. Phosphorite ore bodies are mainly layered and lenticular, and their shape and thickness are controlled by the characteristics and development degree of floor paleokarst landform. In karst depression, the ore body thickens and the bulge becomes thinner. The thickness of the ore body is 0 ~ 75.3m, generally 6 ~ 10m, and the length of a single ore body is 4500m, generally 1300~3200m ~ 3200m. Phosphate rock-strontium aluminum ore body is layered and lenticular in the middle of phosphorus-bearing section, which is located on the phosphorite layer, and the two are in a rapid transition state. In most cases, it is the same as the clastic phosphorite below. The ore body is 70 ~ 3 100 meters long, usually hundreds to thousands of meters, and 0 ~ 23.50 meters thick, usually 2 ~ 5 meters.
Phosphate rocks in the ore are mainly fluorocarbon apatite and aluminum phosphate, while other minerals are mainly clay minerals and siliceous. The ore structure mainly includes internal clastic structure and colloid structure, and the ore structure mainly includes breccia structure, massive structure, layered structure and banded structure. The deposit is a weathered-redeposited phosphorite deposit, which is an early (Late Sinian-Early Cambrian) phosphorus-bearing layer, and was deposited by long-term weathering, leaching and transportation in Devonian.
Three. Genesis of phosphorite deposit
The content of phosphorus in the crust is 0. 13%, which is considered as a typical biological element. Animals absorb phosphorus in their life cycle to form bodies, such as bones, teeth, crustaceans and so on. For example, the bones of vertebrates contain P2O5 as high as 53.3 1%, and some species of tongue-shaped shellfish and round shellfish contain P2O5 580% ~ 9 1.5%.
Phosphorus in phosphorite mainly comes from the weathering release of apatite in magmatic rocks. Phosphorus is easily dissolved in surface water containing CO2 and organic acids, which is taken to the sea basin and absorbed by animals and plants. Some geologists believe that phosphorus dissolved in seawater is mainly caused by submarine volcanic eruption.
At present, there are different understandings about how phosphorus is enriched through biological and biochemical processes to form ore deposits, mainly including biological, biochemical and chemical causes.
According to the theory of biogenesis, phosphorite deposits were formed by accumulation in seawater after a large number of organisms died. South of the Cape of Good Hope in South Africa, the equatorial warm current meets the Antarctic cold current, and a large number of creatures die, and their remains accumulate on the seabed, forming a large number of phosphate nodules. In the Early Silurian phosphorite deposit in Estonia, there are three layers of phosphorite, almost all of which are composed of the shells of mineralized scallops.
Biochemical genetics theory holds that the enrichment of phosphorus is related to plankton in the ocean. In tropical shallow waters, a large number of plankton reproduce and absorb phosphorus in seawater. When organisms die, the debris sinks into the mud on the seabed, which can be interpreted by bacteria to release phosphorus. Therefore, the mud is rich in phosphorus, which is 70 ~ 150 times higher than the bottom seawater. For example, the content of phosphate in Caspian water is 3 ~ 7 mg/m3, while the content of phosphate in seabed sludge is as high as 200 ~ 1 100 mg/m3. Sludge water with high phosphorus content diffuses to the bottom layer with low concentration. In the process of diffusion, phosphate accumulates around small particles (such as sand, mineral particles, biological debris, etc.). ) forms phosphate nodules. In this way, due to the long-term deposition of silt rich in organic matter, thicker phosphorite deposits can be formed.
The theory of chemical genesis was put forward by a b Kachakov (1937). According to the data of marine hydrochemistry, he studied the distribution of phosphorus in modern seawater and the phase equilibrium relationship of P2O5-CaO-HF-H2O, and then expounded the chemical deposition process of phosphorus reasonably. He divided the seawater into four layers (Figure 7-3 1). The first layer is from the water surface to the depth of 60m, which is the active breeding layer of plankton, or photosynthetic layer; In this seawater layer, organisms absorb a lot of phosphorus from seawater. Therefore, the phosphorus content in this layer of seawater is very low. The maximum content of P2O5 is 10 ~ 15 mg/m3, usually lower than 2 ~ 5 mg/m3. In addition, the partial pressure of CO2 in this area is less than 30.39Pa(3× 10-4 atm). The water depth of the second layer is from 60m to 300~400m, which is the water layer through which dead organisms pass, and the phosphorus content is slightly higher. The water depth of the third layer is from 300 ~ 400 m to about 1000 ~ 1500 m, which is a water layer for dead organisms to decompose; Due to the decomposition of phosphate, the concentration of P2O5 in seawater is 300 ~ 600 mg/m3, or even higher. The partial pressure of CO2 is increased to 12× 10-4 atmospheric pressure. The water depth in the fourth layer is below 1000 ~ 1500 m, and the phosphorus content decreases again.
Fig. 7-3 1 schematic diagram of phosphate precipitation in seawater of shelf area under the condition of deep ascending cold water flow.
(According to the information of Kazakov, 1937)
1-pebble bed facies and sand facies along the coast; 2- phosphorite facies; 3- Calcareous sedimentary facies; 4- Sedimentation of plankton residues; 5 —— Current direction
In view of the fact that the concentration of CO2 in seawater plays an important role in phosphate dissolution, CO2 and CO2-3 increase with the increase of seawater depth. Therefore, when the rising ocean currents bring the deep-cooled seawater rich in phosphorus and CO2 to the edge of the shelf, the water temperature rises and the water column pressure decreases, resulting in CO2 escaping from the water, or being absorbed by organisms, or forming CaCO3 precipitation. In this case, the partial pressure of CO2 in water is significantly reduced, and the solubility of calcium phosphate in water is also reduced. When supersaturated, phosphorus deposits in the form of calcium phosphate, forming phosphate deposits at the edge of the shelf. The key to this conclusion is that there must be an ascending ocean current, so it is also called the theory of phosphorus formation by ascending ocean current. Kachakov's hypothesis of chemical genesis well explains the absence of animal fossils in many large phosphate deposits, as well as the absence of phosphate rocks in terrestrial freshwater basins and shallow closed basins. This theory has long been regarded as a classical theory to explain the formation of phosphate deposits.
Phosphate nodules in modern marine sediments occur in deep sea and shallow sea bottom in many parts of the world. Phosphate deposits in shallow water are mostly distributed in the area with water depth of 30 ~ 300 m, which are formed by deep circulation ocean currents flowing into shallow water.
A large number of facts and studies show that huge marine phosphorite deposits are formed after phosphate deposition and often undergo transformation. The formation of large-scale phosphate deposits from Late Sinian to Early Cambrian in southwest China has roughly gone through two metallogenic stages. The first is colloid aggregation. In the relatively stable and low-energy shallow water environment, phosphorus is saturated and precipitated from phosphorus-containing seawater, and colloid aggregates and precipitates, which is the main mineralization of marine phosphorite. The second stage is transformation or selection. The agglomerated phosphorus gel is stirred up under the action of ocean currents and strong storm waves, and after winnowing and migration, particles with different particle sizes (gravel, sand, aggregates, biological debris, etc.) are formed. ) formed and redeposited in different energy environments. This is an important metallogenic model and stage for further enrichment of phosphorus and formation of a large number of high-quality phosphate deposits. In addition, in the diagenetic stage, the precipitation of phosphorus solution in pores will also enrich phosphorite.
Four. Key points of exploration and evaluation
The geological age of phosphate deposits is very long, but in a region or a structural unit, there are often only one or two phosphorus-bearing horizons, such as Yuhucun Formation of Lower Cambrian in eastern Yunnan and Doushantuo Formation of Upper Sinian in central Guizhou and western Hubei. Therefore, it is the primary work to study the stratigraphic profile and find ore along the phosphorus-bearing horizon.
The formation of phosphorite is related to paleogeographic environment. At present, all the phosphorite deposits found are deposited in marine environment. Although the phosphate precipitated by phosphorus-rich ocean current mainly occurs at the continental shelf edge, the scope of phosphate redeposition is much wider due to the scouring and ore lifting of storm waves and ocean currents, which can be deposited in the coastal and shallow sea areas of the continental shelf (continental shelf) and also in the sub-deep sea basin at the edge of the continental shelf. In the phosphorite concentration area in the southwest of China, phosphorite is especially concentrated in islands, carbonate platforms, reefs, tectonic highlands and other "submarine highlands" and their nearby semi-limited basins.
Phosphate rock is the product of abnormal marine conditions, which is accompanied by or adjacent to a group of abnormal sediments and can indicate the formation environment and origin of phosphate. The main types of rock assemblage in phosphorite sequence are: ① phosphorite-dolomite-magnesia clay-flint; ② Phosphate rock-black shale-flint; ③ Phosphate rock-banded iron oxide-flint; (4) Phosphorus ore-oxides of iron and manganese.
Phosphate rock is a kind of sedimentary rock rich in phosphate. Phosphate minerals is mostly amorphous, aphanitic and extremely fine apatite crystals, which are difficult to be recognized by naked eyes. Therefore, there is often no obvious difference between phosphorite and other sedimentary rocks such as carbonate rocks. During exploration, we should strengthen the work of testing and microscopic observation, or adopt simple chemical methods.
In addition, many phosphate deposits contain radioactive elements such as uranium, so the gamma spectrometer can be directly used for prospecting in the field.