I. Overview
Black shale is a common lithostratigraphic unit in marine strata and an important type of ore-hosting sedimentary rocks. The earliest reported deposits in black shale are copper deposits, such as Permian copper-bearing shale in Mansfield, Germany. In the 195s, China learned that black shale is often a part of rock series containing manganese and phosphorus. It has only been discovered since the 196s that black shale in many areas contains vanadium and uranium, which can be enriched into industrial deposits, often accompanied by a variety of other metals. In the 197s, it was affirmed that there were important gold deposits in black shale, and the enrichment of platinum group elements also appeared in some places.
Some black shales studied at first are mostly considered as carbonaceous shales or asphaltene shales. An international comparison project named Metallic Black Shales and Related Deposits defines black shales as black (gray or black) fine-grained (below silt grade), usually argillaceous sedimentary rocks with quite high organic carbon. The data of the regions that have been studied in detail show that the rock composition of black shale is actually different. For example, Mansfield's "copper-bearing shale" is actually marl, and the lithology of black shale in several regions of China also includes organic argillaceous rocks, carbonate rocks, siliceous rocks, silty rocks and transitional rocks between them, which form a certain rock combination, so it is also called black rock series. Another characteristic of black shale composition is that it often contains a small amount of pyrite, in which case it may contain a variety of trace metal elements. In most cases, the content of these elements is far below the industrial grade, but they occur frequently and have many kinds of metals. Therefore, black shale was regarded as an important source bed or a potential resource in the study of stratabound deposits.
the important feature of black shale deposits is that they are obviously controlled by horizons. Ore bodies are usually layered, and sulfides such as carbon, asphaltene and pyrite in ore-bearing rocks are mostly produced as particulate disseminated bodies or striations. The rich metals in the deposit can be in various combinations, such As Cu-Pb-Zn, U-V, Co-Mn, Ni-Mo, Au-Ag and Pt, accompanied by Ba, As, Sb, Hg, Tl, Se, Te and other elements. The distribution of black shale ore-bearing rock series is very wide, but the areas where metals are enriched to industrial grade are often only a few. The sulfide particles in the deposit are fine, some metal sulfides are even in aphanitic state, and the composition is complex, so there are often some special problems to be solved in ore dressing and metallurgy.
black shale deposits are widely distributed, and the formation times are Neoproterozoic, Cambrian, Silurian, Devonian, Permian and Triassic, and there are Jurassic and Cretaceous abroad. The environmental characteristics during the formation of ore-bearing rock series are the primary controlling factors of metal enrichment, usually in a closed and semi-closed stagnant basin environment, including lagoons, or some deep-water basin. Some black shales are produced in large quantities in a wide area (Figure 6-22). For example, the widespread distribution of early Paleozoic black shales in southern China is considered to be related to an ocean anoxic event. Seen from the tectonic environment, the metal-rich black shale is one of the representative sedimentary types of passive continental margin.
Figure 6-22 Schematic diagram of distribution and biogeographic division of Cambrian stone coal, vanadium and multi-element deposits in southern China (according to Lu Yanhao, 1979)
II. Important deposits
1. Vanadium and uranium deposits
Black shale vanadium deposits and uranium deposits are found in western Hubei, northeastern Jiangxi, western Hunan, eastern Guizhou and northern Sichuan.
( 1) Yangjiabao vanadium deposit, Shiyan, Hubei. Ore-bearing rock series are slightly metamorphic clastic rocks and argillaceous rocks of the lower CAMBRIAN, with a thickness of about 8 m. The lower member is in false conformity contact with the Sinian Dengying Formation, and it is gray-black siliceous limestone. The middle section is the main ore-bearing section, with a thickness of 16 ～28 m, in which there are three ore-bearing layers from bottom to top. The first layer is a thin layer of siliceous slate with carbonaceous siltstone, containing a small amount of phosphocalcareous nodules, with a thickness of 24 m and a V2O5 content of .6% ~ .9%. The second layer is dominated by coal seam, which can contain vanadium coal mixed with thin siliceous rocks, containing more phosphorus nodules, 2.7 ~ 5.8 m thick, and generally containing .8% ~ 1.1% V2O5. The third ore-bearing bed is mainly carbonaceous slate, with stone coal or thin siliceous rocks, containing a small amount of phosphorus nodules locally, generally 2. 44 m thick, and generally containing . 7% ~ .9% V2O5. It is composed of slate, carbonaceous slate and siliceous rocks in the upper part of Lower Cambrian, and contains no ore. Minerals found in the deposit include chromium-vanadium garnet, chalcopyrite, tennantite, sphalerite and nickel-bearing pyrite. In addition to vanadium, the deposit is accompanied by uranium, molybdenum, copper, silver, platinum, palladium and yttrium. The ore deposits are distributed in a strip shape with a large scale, and the ore-bearing strata extend westward to Yunyang, Zhushan and other places.
( 2) zhengfang vanadium deposit in Zhangcun, northeast Jiangxi. The ore-bearing rock series is also lower CAMBRIAN, and the ore-bearing rocks are special black feldspathic rocks and carbonaceous clay feldspathic rocks, which form two rock combinations in the ore-bearing strata. One is the combination of schistose feldspathic rocks and siliceous rocks, and the rocks are all black and form interbedded belts, and pyrite and barite-rich belts often appear in them, and vanadium is mainly produced in schistose rocks. The other is the combination of schistose feldspar-carbonaceous schistose clay feldspar and siliceous rocks, and the three kinds of rocks also form banded interbeds, and the clay mineral is mainly K-illite. The content of SiO2 _ 2, Al _ 2O _ 3, V, K2O and BaO in black feldspathic rocks is 46% ~ 8%, 4% ~ 14%, n‰, 27. 5% and .9% ~ 6% respectively. The main chemical composition of black clayey feldspathic rocks is similar to that of feldspathic rocks, with richer organic matter content and higher Mg and Ca content, while the black siliceous rocks are low in SiO2 9. 96% ~ 97.51%, K2O, V2O5 and BaO. Minerals in rocks are fine in particle size, and vanadium-bearing minerals appear in rocks as inclusions. The above characteristics indicate that vanadium ore is syngenetic, and the characteristics rich in Ba and K may indicate the origin of hydrothermal deposition. The location of the deposit is in the marginal sea basin near the Yangtze block in the ancient island arc of the south of the Yangtze River.
( 3) Uranium in black shale. It is also a widely distributed uranium deposit in South China. The ore-bearing strata are Doushantuo Formation of Upper Sinian, Liuchapo Formation (or Dengying Formation and Laobao Formation) and Xiaoyanxi Formation of Lower Cambrian. Ore-bearing rocks include carbonaceous argillaceous rocks, siliceous rocks and transitional rocks between them, as well as argillaceous rocks, dolomite, phosphorite and so on.
92 deposit is a typical example of uranium deposit in black shale in South China. The exposed core in the mining area is Proterozoic, and the wing is a folded stratum composed of Sinian and Cambrian, and an interlayer fracture zone is developed in the steep occurrence section. Uranium mineralization is controlled by strata and fault structures, and occurs in all strata of the Upper Sinian Doushantuo Formation, Liuchapo Formation and the middle of Xiaoyanxi Formation, among which the mineralization in the middle of Xiaoyanxi Formation is the best, and there are also two layers of Doushantuo Formation with continuous mineralization. The mineralized area along an interlayer fault structure is 1, meters long, with a depth of 1 ～2 m and a width of tens of centimeters to 6 m. There are several layered and lenticular ore bodies in each mineralization section, and most of them are blind ore bodies (Figure 6-23). The ore-bearing rocks are carbon-rich mudstone and carbonaceous slate, and the mineralization is disseminated, breccia, banded and vein-like. Uranium mainly exists in the form of adsorption, and there are also a few uranium minerals with extremely fine particles, including pitchblende and uranium black, as well as other uranium-bearing minerals such as autunite, P-Ca-Uranium and P-Uranium. Associated minerals include pyrite, sphalerite and white iron ore, and elements associated with U in the ore include V, Mo, Ni and Cd, among which Cd can be comprehensively utilized. As for the genesis of this kind of deposit, it is generally believed that a primary enriched source bed was formed in the early stage of sedimentary diagenesis, and after being buried for a certain period of time, it rose due to tectonic action and formed a broken zone. Subsequently, groundwater caused oxidation and reduction, forming some residual ore bodies under the oxidation zone and leaching-induced uranium-rich ore bodies.
52 deposit is a uranium deposit in Silurian siliceous rocks and siliceous limestone in northwest Sichuan. Similar deposits are rare, but it is of great industrial significance. Silurian in this area is a set of shallow metamorphic terrigenous clastic rocks with volcanic sedimentary rocks at the bottom. There are uranium ore-bearing zones in thick carbonaceous siliceous rocks and slate fracture zones of the Lower Silurian, and in slate and phyllite mixed with siliceous limestone of the Upper Silurian. The ore-bearing zone is composed of siliceous rocks, limestone and carbonaceous slate, and various transitional types of siliceous rocks and limestone are developed to form layered and lenticular rock bodies with different sizes (Figure 6-24). The transitional rocks are composed of Shi Ying, calcite and carbonaceous clay minerals with varying contents, and also contain a small amount of dolomite, chalcedony, sericite, pyrite, apatite, barite and tourmaline. According to the occurrence and composition characteristics, they are considered to be the products of limestone replacement by SiO2 _ 2. The source of SiO2 is probably related to the submarine hot water activity. The occurrence of ore bodies is generally stratabound, distributed in groups along interlayer structures, with great changes in shape and continuity. There are pitchblende, pyrite, sphalerite, harthite, goethite and chalcopyrite in the ores, and most of the uranium is still adsorbed, with high grade. There are different views on the genesis of the deposit, such as sedimentary reconstruction, leaching, hydrothermal sedimentary mineralization and supergene transformation.
Figure 6-23 Schematic Diagram of Section 216 of 92 Deposit (according to Beijing Institute of Uranium Geology, 1976)
2. Gold (platinum) deposit
Gold deposits in black rock series have become an important type of gold deposits in China. The Laerma gold deposit in Gansu Province is one of the large-scale gold deposits recently identified and studied. It occurs in the Paleozoic fold belt in the western part of South Qinling at the junction of Gansu and Sichuan provinces together with the previous 52 uranium deposit. The ore-bearing strata of the gold deposit are the Taiyangding Group of Sinian-Cambrian, and the main rock types are gray-black thick-bedded siliceous rocks mixed with black medium-thin carbonaceous slate. Siliceous rocks and slate have rhythmic characteristics, and the SiO2 _ 2 content in siliceous rocks is above 95%, and the trace elements As, Sb, Ag and Au are generally high. Most people think that it was formed by hot water transformation in the early stage of syngenesis or diagenesis. Gold mineralization is controlled by east-west strike faults, and the ore bodies are layered, lenticular and vein-like, and distributed in groups. The ore-bearing rocks can be siliceous rocks and slate, and silicification, barite and pyritization are often seen. The ore has breccia, vein, crystal cave and cluster structure, and the main minerals are native gold, stibnite, pyrite, Cinnabar, tetrahedrite, tennantite, sphalerite and gray selenium mercury ore. Among them, Shi Ying, barite, dickite and stibnite are the main gold-bearing minerals, followed by pyrite and other sulfides. Gold mainly exists in the form of microscopic gold and submicroscopic gold, and extremely uneven grain groups are wrapped in the above minerals, and can also be distributed among grains. The composition and fabric characteristics of gold ore show that mineralization is formed in multiple stages.
Songpan Dongbeizhai is another large-scale gold deposit discovered and proved in recent years at the junction of South Qinling fold belt and Songpan Ganzi fold belt. The ore deposit is directly controlled by the compound folds and fault structures in the north-south structural belt of Minjiang River. Gold deposits occur in the fractured zone of black rock series composed of Carboniferous-Permian carbonaceous slate and phyllite. The ore body is bedded and lenticular, with reticulated breccia and disseminated structure. The following mineral assemblages can be divided in the ore: ① pyrite-arsenopyrite-Shi Ying; ② Realgar-Shi Ying; ③ Realgar-calcite (containing gold); ④ Realgar-a small amount of stibnite and natural arsenic; ⑤ Calcite. Gold appears in the form of fine disseminated visible gold, and its content is positively correlated with organic matter, and the gold grade can reach above 5 g/t. It can be seen from the above that all these gold deposits have macroscopic occurrence controlled by black rock series horizon, and gold mineralization has obvious characteristics of epigenetic hydrothermal mineralization.
Figure 6-24 Profile of Exploration Line of 52 Deposit in Northwest Sichuan (according to Guiyang Institute of Geochemistry, China Academy of Sciences, 1972)
Table 6-1 Average chemical composition (WB) of Ni-Mo multi-element metal sulfide layer in South China
Note: Except for Pt, Pd, Os, Ru and Au, the others are all 1-2 units. (According to Joy Chen Sheng et al., 1982)
Gold and platinum group elements contained in black shale of Lower Cambrian in southern China have long attracted the attention of mineral deposit experts. Chen Nansheng first published the data of chemical composition of multi-element metal sulfide layers in six mining areas in Yunnan, Guizhou, Hunan and Zhejiang provinces (Table 6-1). A recent geochemical study of platinum group elements in the Lower Cambrian black rock series in Hunan and Guizhou (Li Shengrong, 1994) shows that different rock types in the black rock series in the two places contain precious metal elements, among which the black shale rich in metal sulfide has the highest content, and the concentration coefficients of each element are Os 67. 5, Rn 5, Rh 1. 92, Ir 1. 8, Pt 37 and PD 27. Therefore, there is no doubt that the black rock series is a precious metal rich body. At the same time, the gradient of the content of precious metal elements in black shale in the vast areas of the two places shows high stability and good utilization prospects. According to the relative enrichment of Pd-Pt in platinum group elements, it is speculated that the source of metal is probably related to the submarine hot water activity.
fig. 6-25 histogram of cobalt-manganese deposits and boreholes in Huidong, Sichuan
3. Cobalt-manganese deposits and nickel-molybdenum deposits
Cobalt-manganese deposits and nickel-molybdenum deposits in black shale are black shale multi-element metal deposits discovered and studied earlier in China. Cobalt-manganese deposits were discovered in Hanyuan and Huidong in western Sichuan. Hanyuan mining area is located in the eastern edge of Kangdian axis, and the ore-bearing rock series is Silurian shallow-sea carbonate rock and the lower part of clastic rock series. The lower part of the ore-bearing bed is dominated by manganese-bearing limestone and manganese carbonate, and the upper part is dominated by sandstone and carbonaceous shale. Black carbonaceous shale generally contains scattered or lumpy pyrite, and the black shale interlayer in manganese ore layer is the most rich in cobalt, and cobalt either forms cobalt-nickel sulfide (mainly sulfur-nickel-cobalt ore) or occurs in pyrite and white iron ore as isomorphism; Sulfide pellets and tubular and leaf-like plant tissue remains commonly considered as mineralizing bacteria in ores. Huidong deposit occurs in the same horizon. There are two layers of manganese carbonate ore bodies in the ore-bearing rock series, and cobalt ore bodies occur in the black carbonaceous shale between the two layers of manganese ore bodies (Figure 6-25), including sulfur-nickel-cobalt ore, cobalt-bearing pyrite, white iron ore, and a small amount of particulate sphalerite, chalcopyrite, tetrahedrite, galena, goethite, etc. The ore has disseminated and banded structure, and the residual structure of bacteria and algae is also seen. The ore body is layered without obvious wall rock alteration.
Ni-Mo deposits are mainly distributed in Dayong, Cili, Hunan, Zhuji, Fuyang and Tonglu, Zhejiang, and the ore-bearing strata are the Lower Cambrian black rock series. Stone coal and vanadium were first explored in this horizon, and then nickel and molybdenum were discovered. The horizon of black rock series is stable and distributed in a wide area. The main ore-bearing horizon is black dolomitic shale and silty shale, and there is a thin layer of phosphorite at the bottom. The scaly black shale on it is rich in siliceous and phosphorus nodules, and metal minerals are produced in thin strips in black shale. The whole ore body is also thin and lenticular, and the thickness is generally not large. The ore is mainly composed of carbonaceous argillaceous, dolomite, quartz silt, pyrite and nickel-molybdenum sulfide, and the ore has extremely fine banded, clastic, colloidal and disseminated structures. Fine mineral particles, nickel disulfide (NiS2), nickel pyrite and glue.