The industrial minerals of chromium are highly concentrated in the sub-group minerals of chromium spinel. The general chemical formula is (RO R2O3), that is, (Mg, Fe2+)(Cr, A 1, Fe3+)2O4 or (Mg, Fe2+) O (Cr, A 1, Fe3+)2O3. The content of Cr2O3 in chromium ore with industrial value is generally above 30%, which is mainly composed of the following chromium-containing minerals: the content of chromite [(Mg, Fe) Cr2O3] Cr2O3 is 50% ~ 60%; The content of chromite [(Mg, Fe)(Cr, A 1)2O4]Cr2O3 is 32% ~ 50%; The content of chromium-rich spinel [(Mg, Fe)(Cr, A 1)2O4]Cr2O3 is 32% ~ 38%.

According to different industrial uses, chrome ore can be divided into four industrial types: metallurgical grade, refractory grade, chemical grade and cast stone grade, and the industrial standards of each type are also different:

1) Metallurgical grade chrome ore: mainly used for smelting various ferrochromium alloys, and then used as an important raw material for smelting chrome steel, chrome alloy steel and stainless steel. Metallurgical grade chromium ore can also be used to smelt metal chromium, and the content of chromium can exceed 98%.

At present, the smelting methods of ferrochromium alloy in China mainly include electric furnace method and vacuum carbon reduction process. Chromium ore needs rich ore or concentrate. See table 2- 1 (Yao peihui et al., 1996) for industrial indicators.

Table 2- 1 Industrial index of rich ore (or concentrate) for smelting ferrochromium alloy

2) Refractory chrome ore: chrome ore is mainly used to manufacture magnesia-chrome bricks, chrome bricks and chrome-aluminum bricks. In addition, it can also be used to manufacture various unshaped refractories. There are two main manufacturing methods: sintering method and electrofusion method.

This ore can be divided into two grades:

First-class products: Cr2O3 content ≥35%, SiO2≤8%, CaO≤2%. Can be used as a natural refractory.

Second-class products: Cr2O3 content ≥30%, SiO2≤ 1 1%, CaO≤3%. Can be used for manufacturing chrome bricks, chrome-magnesium bricks and the like.

3) Chemical grade chrome ore: chrome ore is mainly used to produce dichromate and other chromium compound products as raw materials. The common methods are sulfuric acid method and carbonization method. For this purpose, the chemical composition of chromium ore requires Cr2O3≥30%, Cr2O3/(FeO) ≥ 2 ~ 2.5, and contains a small amount of SiO2.

4) Cast-stone grade chrome ore: it can be used to manufacture diabase cast stone and chrome magnesia products. Its chemical composition requires Cr2O3 ≥ 10% ~ 20% and SiO2≤ 10%.

Two. Genetic types of chromium deposits

The chrome ores in the world are mainly primary chrome ores, all of which occur in basic-ultrabasic rocks without exception, and generally appear as independent minerals, showing strong metallogenic specificity. The characteristics of these two genetic types are briefly described as follows:

The differentiation of magmatic crystallization in layered basic-ultrabasic rocks is the main type of chromite in the world, with large scale and reserves, accounting for more than 70% of the world's chromite reserves (Yang Jingsui, 20 10), but the output is less than 50%. Pod-type chromium ore is favored in the world because of its high quality and easy beneficiation and metallurgy. According to the statistics of Shi Junfa (20 10), the output reached 55%. Layered chromite generally occurs in the extensional continental rift environment or continental margin in the ancient stable craton. The ore-bearing stratum is a layered complex of basic-ultrabasic rocks with multi-cycle characteristics. The most famous deposits are Bushveld in South Africa and Stilwalt in Zimbabwe. Chromium ore is distributed in layers in the strata with the highest or higher alkalinity (Figure 2- 1).

Figure 2- 1 Schematic diagram of Stilwalt chromite deposit in Zimbabwe.

(Quoted from Shi Junfa, 20 10, slightly modified)

The genetic types of ophiolite-type podiform chromium deposits are complex, and scholars at home and abroad have done a lot of research, such as: the theory of early magmatic crystallization accumulation founded by European and American scholars; Later, the late magmatic differentiation-residual magma metallogenic theory appeared; China scholar Wang Hengsheng and others put forward the theory of detachment mineralization (the battle of chrome ore in 1970s) and so on. Searle first named the chrome ore formed in the Alps as "pod chrome ore" in 1964. After 1970s, the theory of plate tectonics rose, especially the definition of ophiolite was defined at the Penrose meeting in 1972. Because of its close relationship with plate tectonics, it has attracted the attention of geologists and structural geologists all over the world. During the period of 1979 ~ 1990, four international seminars on ophiolite and oceanic lithosphere were held. China also held two consecutive symposiums on ophiolite in 1996 and 1997, and published corresponding papers. The progress of ophiolite research has opened up a new idea for the theoretical study of the genesis of chromium ore. Many scholars have realized that the podiform chromite produced in a specific environment is closely related to ophiolite, and put forward some genetic explanations, but there is a lack of systematic analysis and summary. In 1980s, Chinese scholars Wang Xibin and Bao Peisheng (1999), together with the first-line geologists, put forward the viewpoint that the pod-type chromium deposits in ophiolite construction were highly melted and transformed from primitive mantle rocks, and made a systematic summary of the pod-type chromium deposits in China. In the ideal profile of ophiolite (Figure 2-2), pod-like chromium formed by local melting of mantle rocks occurs in metamorphic mantle peridotite below Moho (Gass & J.D. Smeving, 1973), that is, plagioclase peridotite with dunite lens in some areas. Ophiolite enters the continental crust in the form of randomly intercepted blocks (rocks) through subduction and collision of plate movements. Due to the intense and complex tectonic movement, the in-situ characteristics of ophiolite are destroyed, the complete sequence is very rare, and the occurrence horizon of chromium ore is difficult to identify. Chromite bodies in metamorphic mantle peridotite (that is, the middle mantle tectonic rocks in Figure 2-2) often have the characteristics of plastic deformation, irregular shape, group appearance, banded distribution and segmented enrichment. A mining area is often composed of dozens to hundreds of small ore bodies. Although there are not many large ore bodies, they have good industrial value because of their good quality and high grade.

Fig. 2-2 Schematic diagram of occurrence position of chromite in ophiolite profile

(Quoted from Chen Zheng et al., 2004, slightly modified)

Chromium mine in China is no exception. One is "layered" chromium ore, which occurs in basic-ultrabasic rocks near ancient land masses or rift zones. Its characteristics are different from those of giant layered chromite ore abroad, and the rhythmic layer is often not obvious and the scale is much smaller. It mainly occurs in the eastern China land block and near the deep fault of the land block. Ore-bearing rock bodies are mostly distributed in belts with a large area, and the relationship between ore bodies and surrounding rocks is transitional. Representative deposits are chromium deposits such as Pingdingshan and Fangmayu in Beijing. Another type of chromium ore is located in small basic-ultrabasic rocks near deep faults. Rock mass has obvious ring structure, and the basic degree increases from the edge to the center, often forming a central pure peridotite facies belt. Chromium ore is mainly distributed in the lithofacies belt with the highest basicity, such as Gaositai in Hebei and Dacao in Sichuan. The chemical composition of ore-forming chrome spinel in this kind of chrome ore is often characterized by high iron (high titanium). It is speculated that the chromium deposit may be formed by the differentiation of magma crystals from continental mantle, and the magma structure is normal, which can coexist with iron, copper, nickel and platinum group elements. Its industrial value is not high.

Ophiolite-type chrome ore is the main type of chrome ore in China, such as Luobusha large chrome ore in Tibet, salto Sea medium chrome ore in Xinjiang and Hegaola medium chrome ore in Inner Mongolia. The ore bodies with industrial value are mainly produced in partially melted mantle peridotite (plagioclase with dunite); The ore bodies of a few deposits occur at the bottom of the accumulative rock (the most basic lithofacies) representing magma chamber. In the third chapter, the metallogenic mechanism of ophiolite-type chromium deposits will be expounded in detail in combination with the genesis of typical deposits.