Geological Tectonic Characteristics And Genesis Of Vanadium Orefield In The Black Rock Series Of Lower Cambrian Shangnan-Shanyang County, Shaanxi

The early Cambrian is marked by the occurrence of anomalously organic-carbon-richblack rock series in large parts of the Southern Qinling. These sediments can be divided intothe northern and southern zones based on their spatial distribution. The northern zone islocated in Shanyang-Shangnan-Yunxi, occurring the large vanadium, gold deposits. Thesouthern zone is located in Ankang-Ziyang-Pingli-Zhenping, occurring the largewitherite-barite deposits and stone coal. Using structural geology, mineral deposit geology,petrology and geochemistry methods, this paper chose the vanadium deposits in the blackrock series of Shuigoukou Formation in the northern zone as the research object,systematically studied structural characteristics, occurrence of vanadium, vanadium depositgenesis, and reconstruct the tectonic evolution processes of research area, constructedmetallogenic model. The new advances and cognitions are proposed as follows.（1） The research area is located in the northern margin of the Wudang massif in themiddle part of the Southern Qinling Caledonian-Indochina fold belt, and the eastern part isclose to the Douling massif. Vanadium deposits are located in the two wings of theShuicaoping-Bailang reversed multiple syncline, in the middle of the research area. Theore-host strata is the Lower Cambrian Shuigoukou Formation. Shuigoukou Formation can bedivided into two lithologic members. The lithologic characters of the first lithologic memberare silicalite, carbargillite, intercalated with phosphatic nodule, baritic rock, and they haveobvious regularity from the bottom up, which are, the lower is dominated by silicalite, upwardcarbargillite gradually increased, silicalite gradually reduced. The lithologic characters of thesecond lithologic member are dominated by limestone, intercalated with little mudstone.（2） Vanadium ore bodies are controlled by the strata of black rock series, occurs in thefirst member of the Lower Cambrian Shuigoukou Formation. The ore-hosted rocks aresilicalite intercalated with carbargillite. The ore bodies are stratified and stratiform-like, arestable along the strike and dip. Vanadium mainly distributed in the carbargillite, but thelithologic association which occurs vanadium ore body, always intercalate with silicalite. Thevanadium enrichment in stratigraphic column has obvious regularity. From bottom up, silicalite intercalated with carbargillite–phosphatic-nodule-rich transition layer-carbargillite–carbonate rock, content of V₂O₅in the transition layer reached the highest.（3） The phase analysis show that vanadium mainly occurs in mica type mineral. ByX-ray diffraction and electron microprobe analysis, we found three vanadium-containingminerals, roscoelite, V-Fe oxide, V-Ti oxide.①Roscoelite which contains vanadium to amaximum of22.68%, is flake, ribbon, and its width is generally10μm, its length is generallygreater than100μm. Roscoelite has two typomorphic mineral. One is2M1, its chemicalformula is K（Al,V）₂（Si,Al）₄O₁₀（OH）₂or （K,Ba,Na）_0.75（Al,Mg,Cr,V）₂（Si,Al,V）₄O₁₀（OH,O）₂,vanadium has not only replaced aluminum of the six coordinate octahedral layer, but alsoreplaced aluminum of Si-O tetrahedral layer. Another is1M, its chemical formula isKV₂（Si₃Al）O₁₀（OH）₂, vanadium is located between the structure layer of roscoelite.②V-Feoxide showed strawberry-shaped, round-shaped, star-shaped, ring, its particle diameter is lessthan10μm, mainly in3～5μm, its orientation is roughly consistent with the direction of thebedding, which show that it is synchronous deposit mineral. The chemical compositionmainly conclude V, Fe, O, and a little of Ti, Mg. It contains vanadium to a maximum of10.09%, no water, is the colloid of FeO-V2O3.③V-Ti oxide disseminated in the matrix ofrock by dissemination and scattered specks. Its long axis direction is consistent with thebedding, which show that it is synchronous deposit mineral.At present, the dressing recovery of black rock series type vanadium ore is low, and thedressing cost is high, and dressing pollution is severe. This article on lower Cambrian blackrock series in the study of the occurrence and distribution of vanadium, contribute to theimprovement of mineral dressing technology, which have important practical significance forreducing the mine cost, improving dressing recovery and reducing the pollution.（4） The research area is located in the southern Qinling tectonic zone, is partitioned intofour subtectonic units by four nearly east-west north-dipping thrusts. The north is made up ofthree from north to south thrusting tectonic belts, which are Taiziping, Yaolinghe, Xianghe.The south is decollement structure zone in the north of Wudang massif, which is causing byWudang mount uplifting. Each tectonic zone has experienced multiple deformations. Studyingthe structure characteristics of each tectonic units, combining the characteristics of structuralevolution of Qinling orogenic zone, the research area has experienced five tectono-evolutionary stages. In Mesoproterozoic, the northern margin of Yangtze block riftedand erupted. In the early Jinning movement, Wudangshan Group folded. From early lateProterozoic era to early Paleozoic, passive continental margin in northern Yangtze block riftedagain. From late Hersynian to Indosinian, thrust nappe structure and extensional detachmentstructure are formed. In Yanshan period and Himalayan period, the research area extended,uplifted, fault depressed.（5） According to the research of stratigraphy, lithology, lithofacies and paleogeography,paleotectonic conditions, vanadium enrichment regularity and geochemical characteristics, weconclude that vanadium in the lower Cambrian black rock series of research area is derivedfrom submarine hydrothermal spring and volcanic activities, vanadium deposit belong toprimary sedimentary style, the genesis of vanadium deposit not only related to the backgroundof regional geological structure, submarine hydrothermal spring, volcanic activities,biological activity and organic degradation, but also related to sea-level fluctuation.In early Cambrian, which is expanding period of plate tectonics, research area waslocated in the passive continental margin of northern Yangtze block, developed rift. Thesubmarine hydrothermal spring and volcanic activities carried silicon, barite, vanadium goldand other metals to the seafloor. Extensive transgression made sea deepen in this period.Developed upwelling ocean current carried phosphorus, nutrients, etc into shelf area. Algaeand other plankton bloomed. Biological activity can enrich vanadium in seawater. Afterbiology died, during its body sinked, decomposition of organic matter consumed oxygen atthe bottom of water, so that the water at the bottom became anoxic state, which helped thevanadium-rich organic matter to store in the seabed. At the same time, because the biologicalactivity changed the water chemical environments, with the result that mud, silicon andvolcanic ash deposited in anoxic layers at the bottom, peserved with the organic matter, andformed vanadium deposit of black rock series type. It is concluded that the vanadium depositshould be of biological and hydrothermal sedimentary origin.（6） Integrating existing genetic evidence, combined with the lithofacies andpalaeogeography environment, this paper restores the formation of ore-bearing black rockseries of Shuigoukou Formation in three phases, namely, genetic model of vanadium depositsof black rock series in Southern Qinling. It has important reference significance for the study on genesis of vanadium deposit in Southern Qinling and even home and abroad.（7） Through the research on the genesis of black rock series type vanadium deposit, theauthor predicts that three stratigraphic divisions in the south of the Southern Qinling （theYunxi-Junxian, the Ankang-Pingli, the Gaotan-Bingfangjie） and the northern continentalmargin of Yangtze block to the south of Bashan arc fracture are the favorable target area toseeking black rock series type vanadium deposit.What is new?Firstly, the author new discoves two occurrence of vanadium: colloid of V-Fe, aggregatesof V-Ti oxide, and the former is the main. This provides direct evidence for vanadiumtransport and deposition, provides an important research data for the improvements forvanadium mineral dressing technology.Secondly, the author studies the orebody characteristics, regional tectonic background,lithology, lithofacies palaeogeographic, sedimentary environment, material sources, theenrichment regularity of vanadium, biological activity evidence and the geochemicalcharacteristics, puts forward that the vanadium deposit in the research area belongs tobiological-hydrothermal sedimentary origin, and sets up the genetic model of black rocksecies type vanadium deposit, which is reasonable to restore the formation of vanadiumdeposits.