Geological Setting, Features And Origin Of The Sumochagan Obo Super-large Fluorite Mineralized District

Located in the northern of Siziwang Banner of Inner Mongolia and lying in the external contact zone between Early Permian Dashizhai Formation and Early Cretaceous Weijing granite bathlioth, the Sumoqagan Obo fluorite district is the most important fluorite producting areas in the North China and consist of the super-large Sumochagan Obo and the medium Obotu fluorite-only deposit,as well as studded fluortie occurrences, such as Yiher, Xilimiao,and Guilersty fluorite occurences.Exploratory drilling indicates that the Sumochagan Obo depopsit has a reserve in excess of 19 million tons of ores with a mean CaF₂ grade of 53.86% ,and the Obotu deposit with a reserve in excess of 1.15 million tons ores with a mean CaF₂ grade of 78%.The Somochagan Obo fluorite mineralization occurs mainly in the bottom of the third member of the Dashzhai Formation as stratiform, stratoid layers and lenes, and the Obotu deposit mainly in the top of the third Member of the Dashzhai Formation.All the mineralizations are spatially and temporally associated with the Yanshanian Weijing graniotid batholith.Metallogeny of the deposit can be divided into two stages.The early stage ores are mainly laminated,banded and fine-grained compact massive ores,whereas the late stage ores are mainly megacrystalline and mingled megacrystalline ores.According to the mineral compositions,the fluorite ores can be divided into 3 types,namely fluorite-quartz ores,sulfide-fluorite ores and calcite-fluorite ores.Ubiquitous alteration in flurite veins is the kaolinite alteration.Results from the K-Ar dating of alteration minerals of sericite and illite reveal that fluorite mineralization occurs in the early Cretaceous.Systematic petrological study and isotopic dating show that the intermediate to acid volcanic-sedimentary rocks of Dashizhai Formation is the result of arc volcanism.The mean age of zircon SHRIMP U-Pb of volcanic rocks is 276±10 Ma (with MSWD 1.9),showing volcanoes eruptived at the Early Permian. Petrologic study indicates volcanic rocks of the Dashizhai Formation have a characteristics of calc-alkaline to high potassium calc-alkaline rock series with high silicon content (SiO₂ ranging from 63.66% to 78.02% of 12 samples), alkaline-rich ((Na₂O + K₂O) ranging from 5.55% to 9.86% of 12 samples ) and peraluminous (Al₂O₃ ranging from 11.78% to 21.33%, A / CNK ranging from 1.0 to 2.0). Weijing granite batholith emplaced at the early cretaceous against the stretching tectonic background with zircon SHRIMP U-Pb age of (138±3.8) Ma (MSWD is 2.3), which associates with fluorite mineralization. Petrological research indicated the granite rocks are the high potassium calc-alkaline rock series, which have the characteristics of the high silica (SiO₂ ranging from 70.18% to 75.62%, n is 8), of the alkali-rich (contents of Na₂O + K₂O ranging from 8.25% to 9.34%) and of metalumininous to weakly peraluminous (Al₂O₃ ranging from 12.69% to 14.68%, A / CNK ranging from 0.92 to 1.05) .The fine-grained fluorite-bearing granite dyke in the Weijing granite batholith has a age of 137±2Ma (MSWD is 0.25),showing emplacement simultaneously with the Weijing granite batholith. The age consistency among the emplacement of the Weijing granite batholith and fine-grained granite veins, as well as the ages of alteration minerals in fluorite veins indicates that the early Cretaceous granite batholith has a close relation to fluorite mineralization.Comprehensive geochemistry studies have revealed all the representive fluorite separates of the Sumochagan Obo deposit have high nickle contents ranging from 28.60×10^-6 to 38.60×10^-6,which is far exceed the mean content of the upper crust.The early stage ores basiclly have poor-fraction REE patterns and exhibit low LERR-enrichment with no Eu anomaly,whereas the late stage ores exhibit HREE-enrichment REE patterns with positive or negative Eu anomaly.All ore types show obivious negative Ce anomaly indicating ore-forming fluid at oxided state.Fluorite inclusions studies show ore-forming fluids are maily NaCl-CO₂-H₂O system,where three type inclusions have been discerned, namely liquid-rich inclusions,CO₂-riched inclusions and halite-bearing multi-phase inclusions.Ore-forming liquids can be divided into two end-menbers,namely hyperaline brine with high temperatures associated with the early fluorite and low salinities with low temperatures associated with late stage fluorite.Hypersaline brine associated with eraly stage have a salinities of 29.47%～47.95% NaCleq. mainly ranging from 36% to 40% NaCleq. with homogeneous temperatures ranging from 396℃to 436℃,and formed at the lowest pressures of 270×10⁶Pa,equiivalent to a depth of 1.1km, under lithostatic conditions;whereas the low salinity fluids associated with late stage fluorites have a salinity of 0.18%～10.98wt%NaCl eq.,which mainly distribute round 2.0wt% NaCleq.with homogeneous temperatures ranging from 140℃to 380℃,and formed at presures of (4.5～160)×10⁶Pa, equivalent to a maxium depth of 1600m below the paleowater surface,under hyudrostatic condition. Sulfur isotopeδ³⁴S for pyrite in fluorite ores in the Sumochagan Obo deposit range from -5.2‰to +7.0‰,which is similar to those of the typical magmatic hydrothermal deposits.δD for fluorite associated with eraly metallogenic stage range from -119. 0‰to -102.0‰,whereasδ¹⁸O from -7‰to -11.6‰,which indicate the feature of oxygen isotope exchanges.δD for fluorite associated with late metallogenic stage range from -128‰to -104.0‰,andδ¹⁸O from -21‰to -14.8‰,which plot around meteroric line indicating mainly meteroric origins.The combided inclusions microthermal data and stable isotope data indicate the ore-forming fluids associated with early metallogenic stage are mainly magamatic hydrothermal fluids with high temperature and high salinties.The mechanisms of eraly fluorite metalloge are mainly fluids-rock interactions between eraly Permian volcanic rocks and marble lens.The CaF₂ precipitation from ore-forming fluids is the results of pH increases from the acid to the neutral.The ubiqitous kaolinized alterations in the Sumochagan Obo fluorite veins are the result of rock-fluids interactions.The ore-forming fluids associated with late metallogenic stage are mainly meteroic origins.The temperature decreases lead to the precipiatation of CaF₂ from the ore-forming fluids.Combined Sr-Nd-Pb isotope data of Sumocahgan Obo fluorites show that the ⁸⁷Sr/⁸⁶Sr initial ratios of fluorites calculated at 138Ma range from 0.70861 to 0.71045, ¹⁴³Nd/¹⁴⁴Nd initial ratios from 0.512203 to 0.512341,andεNd(t) from -2.33 to -4.77,two-stage modal ages T_2DM from 1121Ma to 1329Ma.All the results indicate the metallogenic matterials of the Sumochagan Obo super-large fluorite-only deposit have closely related to the emplacement of the eraly cretaceous Weijing granite batholith.The highεNd(t) of fluorite reflect the mantle materials possesse a definite proportion in metallogenic materials. The Sumochagan Obo super-large deposit is, therefore, believed to be a product of Mesozoic Yanshanian granitoid magma emplacement caused by coeval re-working of these deep-rooted faults occurring along the southern margin of the Siberian plate.The consistentency of the two-stage modal ages of fluorite with the that of the Southern Mongolia micro-continent crystalline basement imply a closely connections.This research project fill in the gap in the theoretic studies of super-large fluorite-only deposits in north China and will provide prospecting criteria of large and super-large fluorite deposits in the Sumuchagan Obo district and its neighboring area. Meanwhile, the project may also supply a technique support for China's fluorite mining industry to maintain its advanced position in the international mining circle.