Zhangjiakou area

1. Deep earth structure

Recent research shows that the deep part of Zhangjiakou-Xuanhua area is similar to a mantle sub-thermal column structure or a mantle branch structure (Li Hongyang et al.,1996; Cattle, etc., 1996). The main evidences are as follows: (1) The thickness of the lithosphere from Paleozoic to Mesozoic to Cenozoic is 170→ 100→80km, showing an obvious thinning trend; (2) Moho isobath map shows that the isobath in northwest Hebei is distributed in a ring shape, and the thinnest part of the crust is just in Zhang Xuan area (Figure 6-1); ③ The Mesozoic metamorphic core complex structure is mainly composed of thermal dome of metamorphic basement of supracrustal rocks, detachment zone (unconformity surface) of detachment fault and caprock thrust zone.

2. Fracture system

Figure 6-2 shows the geological structure and deposit distribution in Zhangjiakou area. It can be clearly seen from the figure that there are mainly two groups of fault systems in this area. One group is the east-west Kangbao-Weichang-Chifeng fault and Shangyi-Chongli-Pingquan fault; The other group is the northeast Dadian-Kangbao fault and the northern part of Taihang Mountain fault (source-Chicheng fault). Guyuan, Zhangbei, Chongli and Chicheng in this study are all within the control range of the above four fault zones. According to research, these faults are all deep and large faults in the ultra-crust lithosphere that were formed in Archean and are still active today (regional geology of Hebei Province, 1989). Some important gold, copper and polymetallic minerals in China are concentrated near the fault zone, especially at the intersection. It is also the earthquake-prone area in China.

Figure 6- 1 isobath of Moho surface in northwest Hebei (quoted from Li Hongyang, 1996)

3. Rock system

Figure 6-2 Geological Structure and Deposit Distribution Map of Zhangjiakou Area

There are two main types of rocks in the whole area. One is Archean metamorphic rocks, and the main types are pyroxene granulite, gneiss, amphibolite, felsic gneiss and migmatite. The other is Mesozoic Jurassic volcanic rock formation, lacustrine coal-bearing sedimentary formation, and intermediate-acid granite homologous to volcanic rock. If the dam crest (watershed) is taken as the boundary, the rocks in this area can be divided into three parts from south to north (Figure 6-3): ① In the south, metamorphic rocks and granite are mainly distributed in the border area of Xuanhua-Chongli-Chicheng County; ② andesite and basaltic andesite are mainly distributed in the first-line area of Guyuan-Zhangbei County in the north; The dam crest (watershed) belongs to the border areas of Guyuan-Zhangbei and Chicheng-Chongli counties, where rhyolite and dacite are mainly distributed, and basaltic andesite or andesite is gradually exposed on both sides.

Figure 6-3 Rock Distribution Map of Zhangjiakou Research Area

4. Heat flow system

Active in Xuanhua-Chongli-Chicheng area in the south. Main performance; ① Alkaline metasomatism of rock mass (such as Dongping rock mass); ② Placement of remelted granite slurry; (3) acid time-dependent veins and carbonate veins containing gold and copper; The former is considered to be the potassium metasomatism of rocks by alkaline fluid in the mantle, while the latter two are the magma and its gas-liquid activity after the crust rocks are remelted again, forming a concentrated area of gold, silver and copper deposits in this area (Qiu Xiaoping,1996; Li Hongyang, 1996).

5. The distribution of selenium in rocks and its relationship with mantle plume, fault system and thermal fluid system.

In the third chapter, we analyzed and counted the selenium content of various rock types (Table 3-2). The selenium content in metamorphic rocks and intermediate-basic volcanic rocks is the highest, which is 0. 1 18 and 0. 1 10μ g/g respectively. The selenium content of granite and intermediate-acid volcanic rocks is the lowest, which is 0.048 ~ 0.065 and 0.070μg/g respectively. Judging from the distribution of rock types, most of the southern part of this area is exposed metamorphic rocks, and the selenium content of rocks is relatively high; Middle-basic volcanic rocks are mainly exposed in the north, and the selenium content in rocks is high; The top of the dam and its vicinity (watershed) are mainly intermediate-acid volcanic rocks, so the selenium content in the rocks is low.

In the south, the deposit concentration area is also an active area of deep thermal fluid, with a large number of Mesozoic granites exposed and alkaline rock masses formed by mantle alkaline fluid metasomatism everywhere. Time veins, carbonate veins and altered rocks containing gold, silver and copper constitute large and small ore bodies and mineralized bodies in this area. Some sporadic data show that the content of arsenic, gold, antimony and mercury related to selenium is low in metamorphic rocks, but high in chronological vein ore bodies or alkaline rocks. For example, gold in metamorphic rocks is only 4 ~ 8 ng/g, while gold in timely veins can reach 0.42 ~ 9.93 μ g/g. Similarly, arsenic in metamorphic rocks is only 0.2 ~ 0.4 μ g/g, while arsenic in altered rocks and veins can reach 6 ~10 μ g/g. Selenium in pyrite can reach/kl. Qiu Xiaoping et al., 1996). These phenomena show that selenium in high-content metamorphic rocks in this area, like its closely related elements Au and As, has undergone many forms of migration and enrichment in the deep part of the earth, and finally enriched in gold-bearing and copper-bearing bodies (possibly produced in pyrite and quartz). Therefore, although the content of Se in the molten magma of deep metamorphic granite and intermediate-acid volcanic rock is low, the background value of Se in metamorphic rock and gold deposit is not low.

From the comprehensive figure 6- 1 ~ 3, it can be seen that the high selenium rock background in Xuanhua-Chongli-Chicheng area in the south of this area is obviously coupled with mantle plume, cross-fault system and ore deposit intensive area in space. It is not difficult to explain this coupling phenomenon from tectonic dynamics, fluid dynamics and geochemical field theory. During Meso-Cenozoic, due to the subduction collision between Siberia plate and North China plate from north to south and the subduction of Pacific plate from southeast to northwest to Eurasia plate, an east-west fault system and a north-east fault system were formed in the east of China, and Xuanhua-Chongli-Chicheng area in the south of this area is the intersection of these two fault systems. This triggered the upwelling of the mantle plume, and the alkaline hot fluid at its top entered the crustal rocks to replace the basement metamorphic rocks to form alkaline rock masses. The thermal energy of the mantle plume melts deep rocks to form heavy lava slurry, which becomes basic-intermediate acid magma after deep part differentiation, and rises along the fault zone and ejects the surface to form basalt, basaltic andesite, andesite, dacite and rhyolite. Late intermediate-acid magma rose to the shallow crust along the fault zone, forming granite or granite porphyry; After the magmatic stage, ore-bearing hydrothermal solution flowed along the fault zone, metasomatism and alteration formed a time-dependent vein ore body. It can be seen from the above process that there are two ways to extract selenium from metamorphic rocks, one is alkali replacement and the other is remelting. The former directly enters the ore-bearing hydrothermal solution, and the latter is enriched by complex magmatic differentiation, and the selenium decreases from the time pulse to the intermediate-basic volcanic rocks, intermediate-acid granites and volcanic rocks in turn.

In the area far from the intersection of mantle plume and fault, the influence of selenium is mainly manifested in the spatial distribution of rock types. From south to north, volcanic rocks erupting from the center of the mantle plume form intermediate-acid lava and a small amount of intermediate-basic lava at the top of the dam; Intermediate-basic lava was formed in Guyuan county, and their selenium backgrounds are different.

(2) Supergene system of the earth

1. Topographic landscape

Taking the crest of the dam (the east-west watershed) as the boundary, the area is divided into two distinct natural geographical landscape areas. The northern part is plateau plain and hilly landform, which is called Bashang; The south is mountainous and hilly landform, which is called Bashang.

Although the elevation under the dam is lower than that on the dam, there are many metamorphic rock series in the exposed basement due to deep cutting. On the other hand, the dam is flat and mainly covered by volcanic lava. The rocks under the dam are exposed, the valleys are vertical and horizontal, and only multi-level river alluvial terraces are formed in the intermountain basin. On this basis, residual slope soil and alluvial soil are developed. On the dam, there are hills and valleys near the watershed, which eases the ups and downs and gradually evolves into a vast plain in the north. Residual slope soil with volcanic rocks as its bedrock is developed on hills, aeolian loess and alluvial soil are developed in valleys, and alluvial soil and saline soil are developed on plains.

2. Weather and climate system

This area is located in the transitional climate zone between warm temperate zone and cold temperate zone. With the dam crest as the boundary, the climate on the dam belongs to the cold temperate zone, with cold and dry wind and sand in winter and spring, warm and humid in summer and autumn, and abundant rainfall. The annual average temperature is 0 ~ 3.5℃, the annual precipitation is 300 ~ 420 mm/a, and the frost-free period is 90 ~ 1 10 days. The climate under the dam belongs to warm temperate zone. Although the overall climate characteristics are similar to those on the dam, the annual average temperature is 3 ~ 5℃ higher than that on the dam, and the annual precipitation is much richer than that on the dam. The frost-free period is also 20 ~ 30 days longer than that on Bashang.

3. Surface water system

Surface water system can be divided into groundwater system and surface river system. The surface water system takes the dam crest as the watershed, forming a lightning water system in Guyuan County, Bashang, which belongs to a mature river with little river flow. There are also some inland lakes in the basin. The juvenile river is mainly developed in the mountain area under the dam, which flows into the Daqinghe River and Baihe River systems respectively.

The groundwater system in this area is developed, and spring water and well water are the main drinking water for people. Villages with thick loess layers have deep water levels. In other places, the water level of wells is shallow. Except for a few cases, selenium in drinking water is basically less than 1ng/ml. Obviously, the impact on the human body can be ignored.

4. Soil system

The soil types developed in this area are sandy soil, cinnamon soil, calcareous soil and meadow soil. The parent material layer of sandy soil is generally alluvial layer, the parent material layer of calcareous soil is mostly alluvial layer+loess, and the parent material of cinnamon soil and meadow soil is mainly diluvium. Meadow soil is black or grayish brown, which is distributed near both sides of the watershed and the hillslope on the north side. The primary rocks in the parent rocks are mostly rhyolite and quartz porphyry. Cinnamon soil is also distributed near the north side of the basin and in the north of Guyuan County. The parent rocks are mostly basaltic andesite and andesite. Calcareous soil is distributed in the valleys on both sides of the watershed, with no bedrock exposed, and sometimes the loess is several meters to tens of meters thick. Sandy soil is produced in the river beach on both sides of the watershed, mainly in the valley terrace on the south side. To sum up, the soil in this area can be roughly divided into: ① The southern part of this area is mainly sandy soil, and its soil-forming parent material mostly comes from the debris accumulation of metamorphic rocks and granite in the upstream. ② The northern part of this area is dominated by cinnamon soil and sandy soil, and the parent bed is the residual slope deposit of intermediate-basic volcanic rocks and river alluvium. ③ The basin at the top of the dam is composed of meadow soil and calcareous soil, with a small amount of cinnamon soil and sandy soil. The parent rocks are acidic volcanic residues and loess, and there are also complex intermediate-basic volcanic residues and alluvial deposits.

According to the incidence of Keshan disease, the average total selenium in soil samples collected from villages with severe, moderate and non-endemic areas was 0.179 → 0.133 → 0.097 μ g/g, showing a downward trend. Compared with the above-mentioned soil distribution areas, non-diseased villages are mainly distributed in the south and north of this area, and the soil types are mostly sandy soil and cinnamon soil, and the corresponding total selenium is low; The river basin (dam crest) and its vicinity are seriously ill villages, and the soil types are mostly meadow soil and calcareous soil, corresponding to high total selenium; The hilly valley areas on both sides of the basin are middle and low disease villages, and the soil types are mostly cinnamon soil and sandy soil, corresponding to the selenium content in the middle. Therefore, it can be simply concluded that Keshan endemic area corresponds to high selenium meadow soil, lime soil, cinnamon soil and some sandy soil, while non-Keshan endemic area corresponds to low selenium sandy soil and cinnamon soil.

In Table 3-7 and Table 3-8 in Chapter 3, the total selenium in meadow soil and cinnamon soil in seriously ill villages is > 0. 152 ~ 0.266 μ g/g, and the total selenium in sandy soil in non-seriously ill villages is 0.062 ~ 0.16 μ g/g, in which plants can absorb it. Because the soil MnO _ 2+Fe2O3 and organic carbon in seriously ill villages are higher than those in non-seriously ill villages, and the pH is lower than that in non-seriously ill villages, many available selenium in seriously ill villages cannot be absorbed by plants, resulting in the selenium content in grains in seriously ill villages being lower than that in non-seriously ill villages. Therefore, from the point of view of soluble available selenium in soil, seriously ill villages are in a relatively low selenium state, while non-ill villages are in a high selenium state. Contrary to the total selenium in soil.

5. Plants and food systems

The plants and food crops under the dam obviously reflect two different climatic characteristics. Grassland vegetation is dominant on the dam, and shrubs and trees are dominant under the dam. The crops planted on the dam are mainly wheat, naked oats, potatoes and broad beans, with short growth period and cold tolerance, and warm crops such as millet, sunflower and corn can be planted under the dam.

The results of sampling analysis showed that the average values of selenium in wheat, naked oats and millet were 0.00 16μg/g, 0.00 15 and 0.00 13μg/g, respectively, which were very close. The selenium content of grain in seriously ill villages, moderately ill villages and non-ill villages is 0.0081→ 0.015 → 0.0235μ g/g respectively. The selenium content of cereals in meadow soil, calcareous soil and cinnamon soil in diseased villages is low, while the selenium content of cereals in sandy soil and cinnamon soil in non-diseased villages is high. Compared with the soil total selenium, it was found that the soil with high total selenium in the diseased village grew low selenium grain; The low total selenium soil in non-diseased villages grows high selenium grains. This is mainly because the sandy soil and part of cinnamon soil in non-diseased villages provide more effective selenium for grain absorption than the meadow soil and lime soil in diseased villages.

6. People's lifestyle, food structure and selenium nutrition level

This area belongs to the poverty-stricken counties in the whole country and Hebei Province. Due to economic backwardness, local residents still live in a relatively closed environment. For generations, local food and vegetables have been the main food, and foreign food has little influence on local residents. This lifestyle and simple food structure are the ideal environment for us to study the nutritional status and function of selenium in human body.

This book collects the data of selenium content in hair of people in villages with severe, middle and non-endemic areas to reflect the selenium nutrition status of human body. The results showed that the human hair selenium in severe-moderate-non-diseased villages was 0.191→ 0.21→ 0.237 μ g/g, and the seriously ill villages were very close to the moderately ill villages. The selenium content of people in sick villages is low, while that of people in non-sick villages is high, which is consistent with the conclusion of grain selenium.