Water-rock chemical action in weathering process
After the formed rocks (including ores) enter the earth's surface, due to the changes in the physical and chemical conditions of the environment, in order to achieve a balance with the new environment, the original forms of material existence (including the content and combination of elements, the combination of elements, etc.) will undergo profound changes.
Supergene environment is a multi-component dynamic system under the action of solar energy and gravity energy. The temperature of supergene environment is not high but changes rapidly, and the pressure is normal pressure with little change. There is a vast free space and is rich in oxygen and carbon dioxide; Water can participate in the action in three phases: gas, liquid and solid. Biological activities and functions are extremely strong, especially in modern times with the participation of human activities; Supergene chemical processes are mostly exothermic reactions. Supergene organic action produces a large number of substances with large molecular weight and uncertain molecular weight, and the transitional combination forms between elements also appear in large numbers.
when the external static pressure is reduced, the rock first undergoes physical transformation. When the water in the environment, anions dissolved in water, CO2 and O2 react with the rock, the rock begins to undergo chemical changes, so that the composition and structure of the rock and minerals are completely transformed. Biological participation in physical and chemical processes will intensify the decomposition of rocks.
after chemical and biochemical processes, silicate minerals form a series of secondary minerals, mainly clay minerals and hydrous oxides of iron, manganese and aluminum. After chemical weathering, some elements enter secondary minerals, while others become soluble, such as alkali metals and alkaline earth metals K, Na, Ca, Mg, etc. in the form of cations, and others dissolve in water in the form of anions. Generally speaking, K+ and Mg2+ can be strongly adsorbed by clay, while Ca2+ and Na+ are more easily dissolved in water and migrated.
when carbonate rocks are subjected to strong chemical action, a large number of Ca and Mg are dissolved under the action of CO2-containing water, but almost all other heavy metal elements are left behind and enriched.
ultrabasic rocks subjected to water-rock interaction are characterized by being rich in Cr and Co, and the content of Ni is also higher than other rocks. In humid areas, most of the elements in intermediate-acid rocks are taken away by rain, and the residual components form silicon-aluminum weathering crust. In arid areas, the formation of calcareous deposits or calcareous weathering crust makes the environment alkaline, which can form alkaline barrier.
Iron hat is a special product of supergene water-rock interaction, which is composed of iron-bearing sulfide and oxide weathered to form iron hydrous oxide in soil or nodule form. Many famous ore deposits were discovered through iron hat.
the activity of elements in supergene zone is related to pH-E h condition. For example, both copper and molybdenum have moderate or high activity in oxidizing and acidic media, but the activity of molybdenum and copper is very different under neutral and alkaline conditions. Therefore, copper and molybdenum are often associated in porphyry copper deposits, but molybdenum forms molybdate complex anions in the supergene zone and stably migrates into water, while copper forms basic carbonate and precipitates in situ. For another example, Pb and Zn often accompany each other under endogenous conditions, but under supergene conditions, the solubility of PbSO4, the oxidation product of Pb, is very small and easy to precipitate, while ZnSO4, the oxidation product of Zn, is very soluble and easy to migrate into water. Similarly, Ni and Co are closely associated with each other in the endogenous process. Under the condition of supergene oxidation, NiSO4 _ 4 migrates stably in water, while Co is oxidized to form Co _ 3+,which is quickly adsorbed by Fe (OH) _ 3, showing the enrichment of CO..
The main factors affecting weathering are:
(1) Chemical composition of parent rock and weathering resistance of minerals. Generally, the order of mineral stability during weathering is oxide > silicate > carbonate and sulfide, and the more simple the mineral composition, the more difficult it is for rocks to be weathered. Homomorphism exists in minerals, which may often play a catalytic role in weathering process and accelerate the oxidation and dissolution of minerals. For example, the weathering rate of sphalerite rich in Fe, Mn and Cd is much higher than that of sphalerite with almost no isomorphic characters. The relative stability of minerals in weathering process also depends on the particle size, aggregate structure and permeability of minerals.
(2) environmental conditions. Besides water, the main physical and chemical factors affecting supergene action are temperature, oxygen and carbon dioxide in the atmosphere, pH of water medium, and redox conditions of the environment.
3.4.1.1 Zoning of Weathered Crust and Differential Evolution of Silicon, Aluminum and Iron
3.4.1.1 Stages of Formation and Development of Weathered Crust
After weathering, unstable minerals are decomposed, soluble substances are lost with water, and the remaining substances remain in place and form on land with soil formed by biological weathering. Typical chemical reactions in weathering crust mainly include dissolution reaction and redox reaction (Table 3.12). The migration of elements in weathering crust includes leaching and accumulation. The leaching speed of different elements fluctuates greatly, such as Cl and S, which are brought out nearly a thousand times faster than Al, Fe and Ti, so that Al, Fe, Ti and other elements with low activity are accumulated in the weathering crust.
the stages of mineral weathering are very prominent in the weathering process, and the transformation of a primary mineral into the final product is usually not directly completed. For example, the general process of silicate weathering transformation is: potash feldspar → sericite → hydromica → kaolinite; Pyroxene → amphibole → chlorite → chlorite → montmorillonite → kaolinite → kaolinite; Biotite → vermiculite → montmorillonite → kaolinite.
Weathered crust is zonal, with fresh original rocks (parent rocks) at the bottom, semi-decomposed rocks at the top, and completely decomposed parts at the top and near the surface. Ginsburg(1947) divided the weathering profile into three zones from bottom to top: ① Semi-decomposed zone and partially leached bedrock. The leached material is basic silicate, which contains a large number of primary residual minerals, hydrated substitutes (mica, chlorite, hydromica, chlorite), and infiltrated substances. The pH value of this zone is generally 8.5 ~ 9 or higher. ② The incompletely weathered zone or aluminosilicate (clay mineral) zone contains montmorillonite and kaolinite minerals generated under weak alkaline, neutral and weak acidic conditions, and the pH value of this zone fluctuates from 5 to 8.5; ③ Weathered residual zone. Usually, the compounds in this zone are aluminum oxide, iron oxide, manganese oxide, etc. The medium is obviously acidic, and the pH value is < 5.
table 3.12 typical examples of chemical weathering reaction
3.4.1.1.2 chemical evolution of silicon, aluminum and iron during laterization
the core of the genetic mechanism of laterite or latosol is the stability of clay minerals in tropical soil. Tropical soils can be divided into two categories: one is clay minerals-based soils; The other is iron laterite and aluminum laterite, in which clay mineral content is less. Iron laterite contains hematite, goethite, gibbsite, boehmite, etc., and diaspore is rarely seen. If the final products of laterization are silica-free alumina and alumina-free silica caused by silica-alumina separation, there should not be a large number of clay minerals with silica-alumina separation in tropical areas. However, this is not the case in fact. The tropical climate characteristics of "half-year rainy season and half-year dry season" may be the main reason for laterization. In the rainy season with high temperature and high humidity, silicon dioxide will be leached out. In the dry season, the groundwater level drops obviously, and the aqueous solution rises along the capillary, bringing the lower iron oxide and aluminum oxide to the surface, and the water is evaporated under dry conditions to precipitate iron-rich laterite and aluminum-rich laterite. Because the aqueous solution causing rock weathering is nearly neutral (pH value is 6 ~ 8) and contains less oxidants such as ferric sulfate, colloidal migration of silicon and participation of microorganisms and plants in tropical weathering are of great significance in laterization.
Experiments show that silica in silicate minerals (including clay minerals) can be decomposed in both acidic and alkaline solutions. According to the research results of Okamoto et al. (Figure 3.11), when the temperature is ℃ and the pH value of water is between 5.5 and 8, the solubility of amorphous silica is about 1 mol/L; When the temperature is raised to 22℃ and the pH is 6.5 ~ 7.5, the solubility is about 2 mol/L; When pH is 9.5, its solubility is 2 mol/L(℃) and 3 mol/L(22℃) respectively. Figure 3.12 shows the solubility of iron, aluminum and silicon oxides in aqueous solutions with different pH values. Iron oxide is only dissolved in acidic solution with pH < 3, so it is practically impossible to transfer it out of the weathering zone. Alumina is dissolved in weakly acidic and alkaline solutions, but it cannot migrate when the pH value of the aqueous solution is between 4 and 9 in the weathering zone. Therefore, silicon should be separated from iron and aluminum under the condition of deep weathering.
the zoning of several ore deposits is as follows. From the oxidation zoning of the weathered leaching iron deposit in San Isidro, Venezuela, we can see the further enrichment process of iron ore under oxidation conditions. The zoning of the oxidation zone from bottom to top is as follows: ① The weathered iron-bearing zone, the groundwater is stagnant, pH > 7, and the rock has no obvious change; ② In the oxidation leaching zone with iron-bearing layer, groundwater moves, and the pH value is 7 or slightly lower; (3) In the false hematitization zone, groundwater does not play a major role, and its pH value is less than 7; ④ In the hydration zone, the hard shell of iron hydroxide is often formed, and the water contains organic compounds, and the pH value is about 6.
Figure 3.11 Solubility of amorphous silica
Figure 3.12 Relationship between solubility of iron, aluminum and silicon oxides and pH
Weathered and leached iron-rich ore deposits in Tiequadrilateral area of minas gerais, Brazil have similar zoning from top to bottom. Among them, the grade of iron ore in strip-shaped iron-silicon building (BIF) has increased by 1.6 ~ 1.8 times after the oxidation in laterization process. Gongchangling Iron Mine in Anshan, China also has hundreds of millions of tons of rich iron ore, but the proportion of rich iron ore is greatly reduced because of the lack of supergene oxidation enrichment.
The weathering profiles of Penglai in Hainan and Zhangpu-Longhai in Fujian are examples of aluminum-rich weathering crust formed by iron-rich basalt. The weathering crust can be divided from bottom to top (Li Wenda et al., 1995): ① Weakly weathered basalt layer with pH value of 6.7 ~ 7.5; (2) Clay layer containing nodules, the main minerals of which are kaolinite, goethite and Shi Ying detritus, etc., containing .3% ~ .7% organic matter, and the pH value of clay is 5.4 ~ 6; ③ Nodular clayey bauxite layer, the main minerals are gibbsite, goethite, kaolinite, etc., and the pH value of clay minerals is 4.7 ~ 5.6; ④ Red soil layer, mainly composed of kaolinite, goethite and/or bauxite pisolite, with a pH value of 5 ~ 5.4.
The above two laterite weathering profiles have good contrast. Only because of the difference in the material composition of the original rock, the two finally evolved into aluminum-rich weathering crust and iron-rich weathering crust respectively.
The above weathering profile is characterized by increasing acidity of aqueous solution from bottom to top, increasing solubility of silicon and increasing intensity of oxidation. This zoning also reflects the different stages of weathering. In fact, the decomposition of silicate minerals has undergone a series of clay minerals evolution before it transited to bauxite. The equilibrium reaction between potash feldspar, mica, kaolinite and gibbsite at room temperature is as follows:
Geochemistry
Not all weathering crusts are the products of modern weathering. For example, rich manganese oxide ore bodies exist under the intermediate-acid volcanic rocks of Upper Jurassic in Liancheng, Fujian Province. This manganese oxide ore body was originally a manganese-bearing rock of Qixia Formation of Lower Permian, and later, after strong weathering, it formed an ancient weathering crust in Mesozoic (Chen Huacai, 1989).
3.4.1.2 Supergene oxidation of sulfide deposits
Sulfide deposits are extremely widely distributed metal deposits in the earth's crust, and almost all non-ferrous metals, precious metals and some ferrous metals occur in such deposits. Since 195s, many mineralogists have focused their attention on the study of modern mineralization and metallogenic simulation experiments, and put forward many important metallogenic models.
The oxidation zone of sulfide deposits is a copper deposit dominated by pyrite and chalcopyrite, especially the secondary enrichment law of veinlet disseminated porphyry copper deposits. Later, it was extended to the study of oxidation zones of sulfide deposits such as lead, zinc, gold, silver, cobalt and nickel, and the basic principles of chemical thermodynamics and dynamics were applied to analyze and discuss the supergene ore-forming process.
3.4.1.2.1 Zoning of sulfide deposit oxidation zone
The sulfide deposit oxidation zone is mainly the product since Neogene. The formation of the oxidation zone of sulfide deposits has experienced a long-term redox process, so the oxidation zone and the primary sulfide deposits below it should be studied as a unified system.
The vertical change of oxidation zone from top to bottom is actually a process of redox reaction. With the extension to the depth, the oxidation gradually weakens or even disappears, and the reduction gradually strengthens, resulting in the emergence of distinctive mineral assemblages in different depths of the oxidation zone, which can also be subdivided into some sub-zones. The two zoning models of North American School (Emmons, 1981) and Russian School (Smirnov, 1965) (Figure 3.13) are all bounded by the groundwater surface, with the oxidation sub-zone above, the oxidation-reduction sub-zone below near the water surface, and the primary sulfide ore zone below. From the analysis of the dynamic conditions of water-rock interaction, it is found that above the water table, atmospheric precipitation and surface water percolate downward, and the water is rich in dissolved oxygen and carbon dioxide, and the aqueous solution has great oxidizing activity and strong dissolving power, and the atmosphere may also directly penetrate into this zone, resulting in a strong oxidation sub-zone. Below the groundwater level is the water-rock exchange area, and the diving slowly moves sideways, so the dissolved oxygen content in the water decreases, the atmosphere has been isolated, and the oxidation ability of the aqueous solution has decreased sharply, which is in a situation of stalemate in oxidation and reduction, which is conducive to the emergence of secondary sulfide enrichment sub-zones. Figure 3.13 shows the theoretical zoning of oxidation zone of sulfide deposits.
fig. 3.13 schematic diagram of oxidation zone zoning of sulfide deposits
Chen Jianping et al. (1998) proposed that Yulong copper deposit in Tibet has undergone multiple oxidation and mineralization, forming complex supergene oxidation zoning. The oxidation zone of the deposit is divided into the following zones: secondary oxide enrichment zone, secondary sulfide enrichment zone and primary sulfide ore zone. According to the mineralization stages of oxidation zone, it can be roughly divided into early vertical oxidation mineralization and late lateral migration superimposed enrichment mineralization. The copper ore produced by late lateral enrichment ore has high grade and large thickness, which is the most important metallogenic stage of Yulong copper mine.
3.4.1.2.2 oxidation mode of sulfide in oxidation zone
Although there are various types of sulfide deposits, the main sulfide minerals are similar, and the redox conditions on the surface are also similar. Therefore, the oxidation of sulfides in the oxidation zone is mainly in the following ways:
Geochemistry
All sulfides can be oxidized into soluble, slightly soluble or insoluble sulfates. Table 3.13 shows the maximum concentration (activity) when some common ligands and transition metals reach equilibrium in the oxidation zone (Kranskoph, 1979). Iron and manganese can hardly enter the solution under the given Eh and pH conditions, because their oxides and hydroxides are in a high oxidation state and cannot be dissolved. These components are left on the outcrop to form a brown-black "iron hat".
table 3.13 some metals and common