How is the salt mine made?

China has a vast territory, complicated geological conditions, staggered crustal activities, frequent transgressions and regressions, and arid climatic zones distributed in various geological ages. As a closed element of salt-forming basin, barriers exist widely, the geological conditions of salt formation are good, and salt resources are abundant and widely distributed.

(a) Temporal distribution of mineral deposits

The main salt-forming ages of salt deposits in China are Sinian, Triassic, Cretaceous and Tertiary, followed by Middle Ordovician, Early Permian, Jurassic and Quaternary.

(1) Sinian seawater invaded the Upper Yangtze Basin, forming a vast shallow sea area, partially semi-closed and dry climate, so a huge marine rock salt deposit was deposited in the Dengying Formation of the secondary structure in the southwest of Sichuan Basin. This is the earliest known age of salt deposit formation in the world.

(2) The Ordovician transgression was extensive and the climate was dry, and several first-order depressions were formed on the "paraplatform" in North China. Among them, marine thin-layer rock salt is deposited in Majiagou Formation of Middle Ordovician in Linfen, Shanxi and Fengfeng Formation of Middle Ordovician in Yanchang, Shaanxi.

(3) In Carboniferous, there were rock salt beds and secondary rock salt beds in the Lower Carboniferous in Yuntaishan, Zhongwei County, Ningxia; There are rock salt deposits and underground brine in the southern and northwestern margins of Tarim Basin in Xinjiang.

(4) In Permian, there was only underground brine in Pishan, Xinjiang.

(5) Early and Middle Triassic is one of the main salt-forming periods in China. Due to the dry climate, large-scale marine rock salts of Jialingjiang Formation and Leikoupo Formation were deposited in many secondary depressions with good sealing conditions in the upper Yangtze Basin. Moreover, most depressions have underground brine deposits with high salinity and rich elements such as potassium, bromine, iodine, boron and lithium.

Salt spring (used locally for cooking salt) is widely exposed in the salt-bearing section of Triassic Jiapila Formation in Mangkang County and Yanjing County, Tibet.

(6) In Jurassic, scattered early and middle Jurassic salt springs were found in western Yunnan, southern Yunnan and Tibet. Middle and Upper Jurassic glauberite deposits were discovered in Anning, Fumin and Luquan counties of Yunnan.

(7) The climate from Cretaceous to Tertiary was dominated by drought, which was one of the main salt-forming periods in China. After Yanshan movement and Himalayan movement, many fault basins were formed in China, and a large number of rock salt deposits were deposited, mainly in continental facies, with marine facies and alternating facies accounting for a few.

(8) Quaternary is one of the main salt-forming periods in China. Many modern lake salt deposits have been formed in Qinghai, Inner Mongolia, Xinjiang, Tibet, Ningxia, Shanxi and other provinces (regions) in China. There are also secondary and deformed rock salt deposits and coastal underground brine deposits.

(2) Spatial distribution of mineral deposits

The distribution of salt deposits in China is obviously controlled by tectonic conditions, and its temporal and spatial distribution law and Indosinian movement are the most prominent dividing lines.

Before Indosinian Movement (1), salt deposits mainly developed in ancient platforms, and salt deposits occurred in marine carbonate rock series of Yangtze platform and platform. Salt deposits are characterized by large development area, stable distribution of ore-bearing layers, few salt-bearing cycles and simple profile structure.

(2) After the Indosinian Movement, extensive and intense rifting activities took place in Chinese mainland, terrigenous clastic rocks-evaporated salt rocks were extremely developed, and salt deposits were widely distributed, covering almost all provinces (regions). The salt bed has small area, large thickness, many salt-forming cycles, complex profile structure, diverse material components and diverse deposit types. In addition to rock salt, there are gypsum, mirabilite, trona and potassium salt deposits. According to the control of Helan Mountain-Liupanshan Mountain-Longmenshan meridional structural belt and Kunlun Mountain latitudinal structural belt on salt formation elaborated by Li Siguang, China rock salt deposits can be divided into three salt-forming areas: east, northwest and southwest.

(III) Metallogenic regularity

The formation of (1) rock salt deposit has certain structural conditions. ① Most of them were formed after the Great Orogenic Movement, such as the Sinian Changning salt deposit after Jinning Movement, the huge Triassic salt deposit in Sichuan Basin after Hercynian Movement, and the Cretaceous-Tertiary rock salt deposits widely distributed in China after Yanshan Movement. ② Most of them are formed in depressions (or basins) with relatively stable crust and certain activity, such as Yanyuan salt mine (local trough salt mine) located at the eastern end of Bayankala geosyncline; Sichuan platform Sinian salt mine, early and middle Triassic salt mine (local platform salt mine), etc.

(2) The formation of salt deposits sometimes has the phenomenon of annular horizontal zoning. Salt substances with low solubility are deposited first, with a large distribution area, while salt substances with high solubility are deposited in turn, with a small distribution area; From the edge of the basin to the center: limestone, dolomite, dolomitic limestone (or variegated clastic rock series), anhydrite, gypsum (or glauberite), etc. ), stone salt (sometimes potassium magnesium salt); Rock salt and potassium magnesium salt are deposited in the center of the basin. For example, salt mines in central Sichuan, Dawenkou, Shandong and Dongying.

(3) Rock salt is closely related to other salt minerals, and sometimes it is rich in rare elements, which often has comprehensive utilization value.

(4) The formation of lake salt deposits has obvious rhythm: a complete sedimentary cycle is formed from clastic rock deposition, clay-silt deposition and evaporation salt deposition. There are single-cycle lake salt deposits, such as Jilantai Salt Lake in Inner Mongolia, and multi-cycle lake salt deposits, such as Qarhan Salt Lake in Qinghai and Yuncheng Salt Lake in Shanxi.

(5) Most of the lake salt deposits in China were formed in the Late Quaternary, which can be divided into three salt-forming periods: the first salt-forming period occurred in the Middle Pleistocene-Late Pleistocene, and the deposit of sodium sulfate and magnesium appeared in Yuncheng Lake Basin, Shanxi Province, which was the earliest salt-forming period in China. The second salt-forming period occurred from late Pleistocene to early Holocene, and many large salt-forming basins were formed, such as Qaidam basin, Qiangtang basin, Chagannuoer basin and Turpan basin. The third salt-forming period occurred in Holocene, which was the strongest and most common period for the formation of salt lakes in China, and the salt-forming process continued, such as Jilantai Salt Lake and Chaka Salt Lake in Margai.

(6) Arid-semi-arid climate is an important factor. Lake salt deposits in China are widely distributed in Inner Mongolia, Xinjiang, Qinghai, Tibet and other provinces (regions). The annual precipitation in this area is less than 400 mm, but the annual evaporation is as high as 2500 ~ 3000 mm. It is a typical continental arid climate zone with cold winter, dry heat in summer, many sandstorms and long sunshine time.

Two. Deposit type

Rock salt is an important part of salt. The types of salt deposits basically reflect the types of rock salt deposits. According to the geological age of deposit formation, salt deposits can be divided into ancient salt deposits and modern salt deposits According to the genetic characteristics and occurrence state of the deposit, rock salt deposits can be divided into three types: rock salt deposits, underground brine deposits and modern salt lake deposits.

(1) Types of rock salt deposits

The genetic type of (1) rock salt deposit belongs to ancient salt deposit, which is a solid deposit formed before Quaternary. According to the sedimentary environment and material sources when rock salt deposits were formed, they are usually divided into two genetic types: marine sedimentary rock salt deposits (referred to as marine salt deposits) and continental sedimentary rock salt deposits (referred to as continental salt deposits).

1) marine sedimentary rock salt deposit. Salt substances in this kind of sediments mainly come from seawater and are formed by long-term evaporation and deposition in closed or semi-closed bays and (mang) lake basins. Its salt-bearing rock series is mainly composed of carbonate except the coastal (Kongtong) lacustrine deposits, which are composed of sandstone and mudstone. Marine salt deposits are characterized by large scale, generally deep burial, high NaCl content, single composition, large single seam thickness, large metallogenic area and abundant reserves. The ore-bearing area can reach hundreds of thousands of square kilometers, the thickness of salt layer can reach thousands of meters, and the reserve scale can reach hundreds of billions of tons. Representative deposits are Sinian and Triassic salt deposits in Sichuan, Middle Ordovician salt deposits in Linfen, Shanxi and Yanchang, Shaanxi, and Cretaceous-Tertiary salt deposits in Tarim Basin, Xinjiang. China's marine salt reserves account for more than 85% of the total salt reserves in China.

2) Continental sedimentary rock salt deposit (also called continental lacustrine sedimentary rock salt deposit). This kind of sediment is formed by long-term evaporation and deposition after salt substances are carried by surface water or groundwater and gathered in inland basins. Its salt-bearing rock series are sandstone and mudstone strata. Continental salt deposits are shallow buried, the scale and distribution area of deposits are generally smaller than those of marine salt deposits, and the grade of NaCl is lower than that of marine salt deposits, which has the characteristics of many layers, small thickness of single layer, many biogenic components and large phase change. The continental salt deposits in China were mainly formed after Jurassic, and concentrated in Yunnan, Hubei, Hunan, Henan, Jiangxi and other provinces, with Yunying salt deposit in Hubei as the representative.

(2) According to the Code for Geological Exploration of Salt Deposits (draft for comment), the sedimentary types of salt deposits in China are divided into 2 salt formations, 6 deposit sedimentary types and 10 ore assemblage (Table 4.8.3).

(3) Industrial types of rock salt deposits According to the industrial significance of the deposits, the requirements of geological exploration and production technology, and considering the characteristics of deposit genesis, main ore components and structural deformation, China's rock salt deposits are preliminarily divided into three industrial types, including 9 subcategories (Table 4.8.4).

(2) Types of underground brine deposits

Underground brine deposits can be formed in both ancient and modern times.

1. The genetic types of underground brine deposits can be divided into three subtypes.

(1) Paleo-seawater sealed between sand grains during diagenesis or entering pores and fractures after diagenesis becomes high-salinity underground brine (also called primary sealed brine (2) leaching brine deposit) through concentration metamorphism. In the process of movement, groundwater dissolves salt substances or rock salt deposits in rocks, and then gathers to form underground brine.

(3) Sedimentation-leaching mixed brine deposit is an underground brine mixed by sedimentary brine and leaching brine during underground migration.

2. The industrial types of underground brine deposits can be divided into ancient underground brine and modern underground brine.

(1) Pore-type underground brine deposit Ancient underground brine refers to underground brine existing in the pores of overlying strata (sandstone or glutenite) of rock salt ore bodies or salt-bearing rock groups. This kind of brine is mainly confined water or running water, which is dynamic and stable and buried deeply. The halogen-containing layer is layered or quasi-layered, with stable-relatively stable horizon and weak-medium water abundance; The salinity is moderate, and the hydrochemical composition is mainly NaCl. For example, Qianjiang underground brine deposit in Hubei Province. Modern underground brine refers to Quaternary coastal underground brine in estuarine and tidal flat sediments (silt layer) and coastal sediments (fine sand, medium coarse sand and gravel layer), which is buried shallowly. The halogen-containing layer is layered and lenticular, and the horizon is relatively stable-unstable. The salinity is high, and the hydrochemical composition is mainly NaCl, which is rich in Mg2+, SO24-, K+ and Br-, such as the underground brine along the coast of Laizhou Bay in Shandong Province.

(2) The fractured underground brine deposit refers to the underground brine that occurs in the cracks of the surrounding rocks of salt-bearing rock series or rock salt ore bodies. Halogen-bearing layer is deeply buried, layered or quasi-layered, with the characteristics of interlayer sealing and enrichment and extension along the structural fracture zone. Mainly confined water or flowing water, the dynamic is relatively stable. The salinity is high and the hydrochemical composition is mainly NaCl. Such as Shuanghechang in Zizhong County, Sichuan Province and Dengguan structure in Zigong.

(3) Pore-fissure comprehensive underground brine deposit refers to underground brine existing in dissolved pores and fissures in the weakly weathered leaching zone of salt-bearing rock series. This kind of brine is shallow buried, and the brine layer is layered or lenticular. Mainly diving, with weak-medium water abundance. The salinity is generally low, and the hydrochemical composition is basically the same as that of the lower solid salt mine. For example, Baimuqiao nitrate deposit in Dan Ling County, Sichuan Province.

(3) types of modern lake salt deposits

The metallogenic age of the deposit is Quaternary. According to the occurrence state of the deposit, it can be divided into three types:

(1) solid lake salt deposits refer to completely dry dry salt lakes without brine and lake salt deposits with solid sodium salt deposits as the main mining objects. The former is like Lake Chaerhan Slatu in Qinghai; The latter, such as Jilantai Salt Lake in Inner Mongolia.

(2) Liquid lake salt deposits (also known as brine salt deposits) refer to salt lakes that have no sodium salt deposits or only a small amount of sodium salt deposits and cannot be mined separately. For example, Ebinur Lake in Xinjiang and Kunteyi Salt Lake in Qaidam Basin in Qinghai.

(3) Mixed-phase lake salt deposition with solid-liquid coexistence. The mining characteristics of this kind of deposits are both solid mining and liquid mining. For example, Ernuer Salt Lake in Inner Mongolia and Qijiaojing Salt Lake in Xinjiang.