The deposit occurs in marine sedimentary rock series strata. The ore body is marl, chalk or sand and clay layered, which is often related to phosphorus-bearing layer, glauconite layer or volcanic eruption rock. The thickness of the seam can reach several hundred meters, and the composition is relatively pure. Diatomite is a soft powdery or chalky loose rock. In foreign countries, this kind of deposit is the most important type of diatomite. Most of the deposits occurred in Tertiary, and marine diatomite deposits have not been found in China. The Lombok diatomite deposit in California, USA, belongs to this type and occurs in tertiary strata.
Two. Lacustrine diatomite deposit
Deposits occur in lacustrine strata, often interbedded with lacustrine sediments such as sand and clay, and some deposits have multilayer volcanic rocks (mainly basalts). The thickness of sediments is small and the composition is complex. Ore is not as good as the former type in quality and quantity, but it is widely distributed, so it also has great industrial significance. Diatomite mines in Zhejiang, Jilin, Shandong and other provinces in China all belong to this category. The following focuses on Maanshan diatomite deposit in Changbai Autonomous County, Jilin Province (Figure 7- 1).
(1) ore-bearing strata
The deposit occurs in the fourth member (N 1m4) of the upper part of the Miocene Ma 'anshan Village Formation (N 1m) of Neogene. Ma 'anshan Village Formation is covered by phyllite of Liaohe Group or Jurassic volcanic rocks, and they are unconformity contact. Ma 'anshan Village Formation is covered by Pliocene basalt, and there is also unconformity contact between them. Ma 'anshan Village Formation consists of four layers of sedimentary rocks and three layers of basalt alternately (Huang et al., 1993). From top to bottom:
Fig. 7- 1 Second Bid Section of Maanshan Diatomite Mine
(According to Chen Hongcai 1980)
1-grayish black olivine basalt; 2- sand and clayey silt; 3-gray-black spotted olivine basalt; 4 seams; 5- Drilling
Overburden: Pliocene basalt.
inconformity
(9) Loose sediment is 3.71~11.56m.
② The thickness of medium-coarse sand layer is 0.36 ~ 8.56m.
(1) brownish red sandy clay layer, sometimes containing diatom clay layer 0.20 ~ 3.60m.
crack
(8) Diatomite layer (n1m 4)13.65 ~ 47.58m.
⑧ Gray-green diatom clay mixed with yellow is the upper indicator layer of the deposit.
⑦ The white diatomite layer is the first ore-bearing layer in this mining area, with a local maximum thickness of 3.04m..
⑥ grayish yellow-grayish green diatom clay, oblique bedding, and coal seam stability of 0.4 ~ 7.92m..
⑤ Grayish white diatomite is the second ore-bearing layer and the main ore-bearing layer in this mining area,1.19 ~10.43m.
④ Gray-dark gray diatom clay, massive, which is relatively stable and rich in plant fossils1.69 ~ 8.57 m.
(3) Grayish white diatomite, the third ore-bearing layer in this mining area, is thick-thin 1.26 ~ 6. 10m.
② Gray-green diatom clay, micro-layered, containing silty-silty clay, locally mixed with carbonaceous clay, rich in plant fossils, and 0.30 ~ 3.25m thick.
① Brown flaky diatomaceous earth, layered, micro-bedded, rich in organic matter, and partly carbonaceous claystone with a thickness of 0.20～2.32m m.
crack
(7) Sandy and silty clay layer1.20 ~ 23.23m.
③ Fine-coarse sand layer, yellowish brown, mainly composed of timely and feldspar, with good roundness and great thickness variation.
② Silty clay, grayish green, containing 2.38 meters of carbon.
① The bottom is medium-coarse grained sandstone with great thickness variation.
inconformity
(6) Dark gray and porphyritic olivine basalt (N 1β3) (third basalt) 7 ~ 21m.
inconformity
(5) Sand, clay silt and silt layer (N 1m3)
③ Gray silty clay, containing diatom clay and diatom layer 2.25 ~ 3.25m.
② Yellow-brown medium-coarse grained sand layer, mixed with fine sand or green silty clay 8.44 ~ 10m locally.
① Clayey silty sand interbedded with sand, containing 0.7~7.2m diatom sandy clay locally.
inconformity
(4) gray-black porphyritic olivine basalt (N 1β2) (second basalt)19.58 ~ 21m.
(3) Sand and silty clay layer (N 1m2)
(3) grayish yellow-grayish black layer, containing silty sand and silty diatomite, with micro-bedding, containing plant fossils and ferromanganese nodules, with a thickness of 4.63 ~ 9.54m.
② Gray-green-dark gray silty clay, containing diatoms, plant fossils and 8.83 ~ 2 1m blue iron ore nodules.
① Dark gray-grayish white silty sand, medium fine-medium coarse gravel sand layer, containing phyllite and medium basic basalt gravel, with poor sorting14.52 m.
inconformity
(2) The gray-black spotted olivine basalt (N 1β 1) (the first basalt) is only found in the low-lying area of about 20m.
inconformity
(1) Gravel layer (N 1m 1)
The upper part is yellowish brown medium-coarse sandstone, about 10m.
The lower part is gravel, which consists of quartzite, granite gneiss, phyllite and volcanic lava. , the separation is good, and the gravel diameter is about 10m, ranging from 5 ~ 6cm to 6 cm.
inconformity
Underlying stratum: phyllite of Liaohe Group or Jurassic volcanic rocks.
It can be seen from the above that there are diatomite layers in the 8th layer (N 1m4) of this group, among which there are 3 ore-bearing layers. The second ore-bearing seam is the main ore seam, in which the thickness of Grade I ore seam is 0.34 ~ 4.33 m, and that of Grade II ore seam is 0.4 ~ 4.66 m; The first ore-bearing layer is also an industrial ore-bearing layer, in which the thickness of grade I ore-bearing layer is 0.44 ~ 2.39 m; The thickness of the third ore-bearing layer changes greatly, so it is not an industrial mining layer.
According to the vertical lithofacies changes of ore-producing sedimentary strata, it can be seen that the grain size of sediments ranges from coarse to fine, from no bioclastic to thick diatomite deposits. The lithologic combination of ore beds is mainly silty layer, silty clay layer, clay layer and diatom clay layer, and horizontal bedding and micro-bedding are developed in sediments. Diatoms have a small rhythmic structure formed by periodic deposition, which reflects the characteristics of sedimentary types of freshwater lakes. Diatoms are formed in a relatively quiet water environment, when terrestrial biomass is rarely mixed with diatom remains with water flow. In this case, the color of diatomite formed is grayish white or white.
(2) Chemical composition and physical properties of diatomite
Huang et al. (1993) systematically tested the chemical composition of the diatomite, in which the silica content was 68.97% ~ 86.99%, the alumina content was 2.94% ~ 12.30%, and the ferric oxide content was 0.55% ~ 6.4 1%. According to the relative content and appearance characteristics of material components (mainly diatomaceous earth, clay and mineral debris) in the ore, the ore can be divided into four categories: diatomaceous earth, clayey diatomaceous earth, clayey diatomaceous earth and diatom clay. According to the chemical composition and other characteristics of the ore, the classification of the original soil grade of the mine is shown in Table 7-2.
Table 7-2 Chemical Composition and Other Related Components of Diatomite in Maanshan Mine and Classification of Diatomite
(3) Composition of diatom species in diatom layer
Through the analysis and identification of diatoms in the seam, it is determined that there are 29 genera 120 species of diatoms (Huang et al., 1993). It is found that the content of SiO _ 2 in each seam is related to the diatom species in the same seam. For example, the content of SiO _ 2 in diatomite layer is high, and the species of diatoms are relatively single; When the impurity content in the seam increases and the silica content decreases, the diatom species increase.
(4) Geological age of diatomite layer.
Among the three main species in diatomite deposits, namely Cyclotella striata, Alternaria grooved and filamentous algae particles, the first two are fossil species of Late Tertiary, and filamentous algae particles are the dominant and common species in non-marine diatomite deposits of Late Tertiary all over the world, and they are also the main species in some Miocene diatomite deposits in China, so the geological age of deposit formation should be Miocene. In addition, other plant fossils collected in mining areas are also common in Miocene strata in China.
(5) Genesis of the deposit
1. ore-bearing buildings
The lithologic combination of the ore bed is mainly composed of silt layer, silty clay layer, clay layer and diatom clay layer, and there are small rhythmic structures formed by horizontal bedding, micro-bedding and diatom periodic deposition. The common diatom species in the seam are mostly freshwater species, so the ore-bearing formation belongs to the freshwater lake facies clastic rock formation. In view of the multi-layer basalt sandwiched in the building, the ore-bearing building can also be called lacustrine fine clastic basalt building.
2. Climatic conditions and sedimentary environment
The occurrence of diatomaceous earth seam is gentle, and the seam inclines from the periphery of the basin to the center, with a small dip angle, some of which are nearly horizontal, showing relatively stable lacustrine sedimentary characteristics, which is basically unaffected by the later structural changes, indicating that diatomaceous earth was formed in a relatively calm water environment.
There is a small amount of plant debris in the ore, which indicates that the climate is warm when it is deposited, which is suitable for the survival of a large number of microorganisms such as diatoms. The ore also contains a small amount of Porphyridium nodosum (Fe3 [PO4] 2 8H2O) and pyrite, which indicates that the reducing environment and phosphorus-containing water body during deposition are favorable for diatom reproduction.
3. Source of ore-forming materials
Basalt exists above and below the seam, which shows that volcanic activity provides necessary SiO2 _ 2 for diatom survival and good preservation conditions for the seam after deposition.
The above results show that the diatom is a weakly cemented deposit formed by partial accumulation of diatom remains and preliminary diagenesis, and belongs to the biochemical deposit of freshwater lake.
(vi) Industrial importance
Maanshan-Xidapo in Changbai Autonomous County is a large-super-large diatomite deposit, and diatomite is famous for its high quality. Maanshan Mine can not only produce diatomite ore, but also process more than five kinds of filter AIDS, and the products are exported at home and abroad.