Enshi-Lichuan area

1. Deep earth structure

Figure 6-6 is the contour map of Moho surface in Hubei Province. As can be seen from the figure, Enshi area is a part of the mantle flat area in western Hubei, which is located in the shoulder mantle uplift area of Chengkou mantle depression and Xianfeng mantle depression. However, in general, the western mantle plateau is still in a state of subsidence, and the eastern part of it is Wuling mantle ridge or mantle terrace connected with Taihang Mountain mantle ridge belt, which has obvious characteristics of steep crustal dip and gravity gradient belt. The western part of this area is the famous Sichuan Manchu area.

Figure 6-6 Moho isobath map of Hubei Province (unit: km) (according to the regional geology of Hubei Province, 1984)

2. Structure and fracture system

Corresponding to the western mantle flat area, the crust is characterized by slow mantle uplift, forming the upper Yangtze platform fold belt. It is adjacent to Sichuan Depression Basin of Yangtze Platform in the west and Wuling Folding Mountain System in the east. With the evolution of the tectonic history of the upper Yangtze platform fold belt, this area has experienced three stages of geological development: ① platform cover deposition stage (∈—T2), ② platform activation stage (T3—K), and ③ neotectonic movement stage (N—Q) (see Section 2 of Chapter 5). According to Lu Jiren's research (1996), the activation of the Yangtze platform originated from the activity of Emei mantle plume in the west of the Yangtze platform and the large-scale eruption of Permian Emei basalt. At that time, the area was in the sedimentary stage of platform depression. According to his speculation, huge mantle thermal movement and volcanic eruption will have an impact on the deposition in this area. According to the stratigraphic records of Maokou Formation of Lower Permian and Wujiaping Formation of Upper Permian in this area, the black rock series composed of siliceous rocks, carbonaceous shale and coal measures in bioclastic limestone may reflect the characteristics of wet delta facies and tidal flat facies at that time. Among them, siliceous rock belt and siliceous carbonaceous shale are likely to have the characteristics of submarine hot water deposition. In a word, they reflect a hot and humid reductive sedimentary environment. The enrichment of selenium-related elements such as vanadium, nickel, phosphorus and sulfur and the corresponding vanadium, phosphorus and pyrite layers were found in black rock series. Recently, the first independent selenium deposit was discovered in Enshi.

The faults in this area belong to Wulingshan fault system, and there are Qiyueshan-Daloushan fault, Xianfeng fault and Jianshi-Enshi fault from west to east, which are arranged in northeast or northeast parallel. A series of inclined fault basins are formed between faults. But there is no volcanic magma activity.

3. Rock system

The rocks exposed in this area are a set of Paleozoic continuous sedimentary sequences. The main selenium-bearing horizons are the lower Cambrian Shuijingtuo Formation, the upper Ordovician Wufeng Formation, the lower Permian Maokou Formation and the upper Permian Wujiaping Formation siliceous carbonaceous shale black rock series. These black rock series are characterized by limestone, dolomite, carbonaceous shale, siliceous rocks and coal measures, and are distributed in the east of Qiyue Mountain. Low-selenium rocks in this area are distributed in the west of Qiyue Mountain, and the main rock formations are sandstone, limestone, siltstone and shale from Late Triassic to Jurassic and Cretaceous. Low selenium rocks are Jurassic purple sandstone and siltstone. Among the rocks with high selenium background, only the Lower Permian Maokou Formation and the Upper Permian Wujiaping Formation are controlled by stratigraphic rocks, resulting in selenium poisoning of people and animals. Under the background of low selenium rocks, Keshan disease only occurred in the area controlled by Jurassic purple sand shale.

The results of rock sampling analysis show that the average selenium content of dolomite, carbonaceous siliceous rocks and siliceous carbonaceous shale are 3. 13, 52.40 and 247.707 μ g/g respectively, and the average selenium content of purple sandstone is 0.097 μ g/g. ..

To sum up, the thermal movement of mantle plume and the large-scale basaltic magma eruption in the western neighborhood of this area may be the main reasons for the high background values of V, Mo, Cr, Ni, S, P and Se in this area. They are mainly produced in carbonaceous black rock series. Similar rock formations have also been found in southern Sichuan, Yunnan, Guizhou, Guangxi and other areas adjacent to this area, and deposits and mineralization of vanadium, molybdenum, phosphorus, gold, silver, platinum and palladium have been found. It is considered that it is the result of basaltic magmatism related to mantle plume, because high Cr, Ni, V, P, Pt, Pd and other elements are usually closely related to basic-ultrabasic magmatism. Therefore, it can be speculated that under the control of the high heat flow anomaly of mantle plume activity since Paleozoic, siliceous and algal carbonate deposits have appeared on the seabed, which are rich in elements such as V, Ni, Cr, P, Se and S. After many cycles of transgression and regression unconformity, this enrichment reached its peak in the Middle and Late Permian.

(2) Supergene system of the earth

1. Topographic landscape

Qiyue mountain system is in the west of this area, and the residual vein of Wuling mountain system is in the east, extending in the northeast. There are large and small fault basins and valley basins between and within the two mountain systems. The mountain area is above 1000m, the basin is less than 300 ~ 500m, and the height difference is 500 ~ 700m m. To the west of Qiyue Mountain are hills and hilly basins, and to the east are low hills and basins. The vegetation in this area is dense, the mechanical weathering and denudation is not strong, and there are few huge flood accumulation and alluvial terrace landscapes.

2. Weather and climate characteristics

Affected by the monsoon in Yunnan-Guizhou Plateau and eastern China, this area belongs to the humid monsoon climate in mountainous areas. When the southeast maritime monsoon is weak, it is easy to form a dry climate. For example, the continuous dry climate of 1959 ~ 1963 has seriously affected the agriculture and grain production in this area, and selenium poisoning has also broken out from time to time in recent years.

3. Groundwater and surface water systems

There are permeable layers (such as sandstone and limestone) and barrier layers (such as peat shale and coal seam) in the rocks in this area. Therefore, the groundwater level is low and spring outcrops can be seen everywhere. Surface water system development. Qiyue Mountain can be divided into two water systems, the Qingjiang River in the east and the Yangtze River in the west.

The residents in this area mainly drink well water and spring water. According to the analysis of water samples from poisoned area-non-poisoned area-Keshan area, the average selenium in water is 24.4→3.4→0.20ng/ml, and the selenium in water in poisoned area has exceeded the national drinking water standard (Se < 10 ng/ml. The selenium content in drinking water in Keshan disease area is lower than the average value in selenium-deficient area (0.5ng/ml).

4. Soil system

With Qiyue Mountain as the boundary, brown soil and yellow brown soil are dominant in the eastern mountainous area, and swamp soil and paddy soil are developed in the valley basin; Purple soil and swamp paddy soil are developed in hilly areas of western China. The soil in this area is mainly diluvial residual soil, and alluvial soil only appears in the valley basin, so the inheritance relationship between soil and bedrock in this area is particularly obvious. Because the occurrence of rocks in most areas of this area is relatively gentle, when the occurrence is close to the slope direction and gradient, the genetic effect of soil properties (including selenium) on a certain rock is particularly prominent. The fifth chapter emphasizes that the high-selenium siliceous carbonaceous shale of Wujiaping Formation exposed in Luojiaba and two villages with high selenium poisoning and high selenium poisoning in sandy land is distributed on a gentle slope consistent with the slope direction, and the soil formed in situ is in a high-selenium state (20.56 and 18.54μg/g). In other non-toxic villages with high selenium (such as Xiaoguan and Zishengqiao). ), dolomite and siliceous dolomite with relatively low selenium content are exposed in a large area, and the selenium content in the soil generated by them is low (2. 19 and 3.99μg/g). Similarly, in areas with low selenium (such as Changping and Zhiluo), the soil developed by regional purple sandstone with low selenium is also in a low selenium state (0. 12 and 0.073μg/g). However, in areas with steep occurrence of folded rocks (such as Yutangba and Xintang), because of the steep slope, the residues are easy to be transported, and gray-brown sandy clay (i.e. swamp soil and paddy soil) with alluvial origin will appear in low-lying areas or river valleys. Due to the complex composition of these sediments, the total selenium content in the soil is not high (4.99 and 4.96μg/g), but the water-soluble selenium content is high (23.5 1 and 66.38μg/g).

Speciation analysis of soil selenium in Yutangba poisoned village showed that the sum of soil organic selenium, sulfide selenium and residual selenium reached more than 95%, while the sum of water-soluble selenium and exchangeable selenium was only 24 ~ 75 ng/g, accounting for 1% ~ 2% of total selenium. Most exchangeable selenium is Se (Ⅳ) (Table 5- 13). Its solubility is also controlled by pH value, iron manganese oxide and organic carbon. Therefore, only water-soluble selenium and exchangeable selenium may not meet the absorption needs of plants. Generally speaking, organic selenium may be more effective for plants.

5. Plants and food systems

Because of the differences in topography, climate and soil between the east and the west of this area, its vegetation also has its own characteristics. Coniferous forests and shrubs are produced in the eastern mountainous areas, such as Cyclobalanopsis glauca, Pinus tabulaeformis, Cunninghamia lanceolata and Pinus massoniana. Xishan produces coniferous forests and broad-leaved forests such as conifers, Chinese fir, Pinus massoniana and Cyclobalanopsis.

Corn, tobacco, tea and citrus are mainly planted in the eastern mountainous areas, and rice, corn and wheat are planted in the basin. Rice, wheat, corn and citrus are the main agricultural products in western hilly areas.

According to the analysis of grain sampling in high selenium poisoning villages and high selenium and low selenium non-toxic villages, the average selenium content appears at three levels: 0.78 ~ 3.30 (high selenium), 0.05 ~ 0. 18 (medium selenium) and 0.00 147 ~ 0.0029 (low selenium), which is in the high selenium level. Compared with the data of previous research and analysis, the results of this analysis show that the selenium content of grain in high selenium areas has been decreasing year by year since 1960s. Since 1980s, the selenium content in cereals has dropped to a relatively stable level (see Chapter 5, Section 3).

Comparing the selenium content in poisoned rice and corn with the results of this analysis, they are 3.96, 8.66 and 0.8 16,1.645μ g/g respectively. The selenium content in corn is twice that of rice. The selenium content of rice and corn in this experiment is about 4 ~ 5 times lower than that in the poisoning period. Even so, it is estimated that the current selenium level in cereals can still make people in a state of high selenium poisoning, unless rice is the staple food to increase food diversity (see Chapter 5, Section 3).

6. People's lifestyle, food structure and human selenium status.

Enshi is located in a remote mountainous area, with inconvenient transportation and backward economy. People in valley basins and hilly areas are mainly rice, while residents in mountainous areas are mainly corn grains, and meat is mainly self-raised, forming a relatively closed ecological eating environment. With the improvement of life, mountain residents are mainly eating rice, and corn is mainly used as feed to feed livestock. Therefore, in recent ten years, human selenium poisoning has gradually disappeared, but livestock selenium poisoning has occurred from time to time.

The sampling analysis results of selenium poisoning villages-non-poisoning villages-low-selenium villages also showed three levels of selenium, high selenium 20.80~62.90μg/g/g/g, medium selenium 0.70 1 ~ 7.63μ g/g and low selenium 0.262~0.339μg/g/g/g, which basically corresponded to villages with high selenium poisoning risk. However, there are two special situations: first, human hair selenium in Luojiaba and Bajiao, two non-toxic villages, falls into the dangerous villages with high selenium poisoning like selenium in grain; 2. The human hair selenium of Yutangba Village is 5.4 1μg/g, which belongs to the selenium-enriched non-toxic village. Because the residents here live in rotation every year, the hair selenium level is lower than that of other toxic villages, but higher than that of general non-toxic villages. This situation illustrates an important fact, that is, normal people with low selenium or living in high selenium environment will soon increase the selenium level in human body; The opposite may also happen.

In the same selenium environment, children and the elderly are more sensitive to selenium absorption than young adults (Figure 5- 1 1). This may be related to the weak metabolic function of selenium in children and the elderly, and selenium accumulates in the human body for a long time. Therefore, children and the elderly are more prone to selenium poisoning than young people. However, it may be another situation in low selenium areas. Children absorb less selenium from low selenium environment than adults, and selenium can not meet the needs of various organs in the process of body development.