【作者】 许东青；

【导师】 聂凤军；

【作者基本信息】 中国地质科学院 ， 矿物学、岩石学、矿床学， 2009， 博士

【摘要】 内蒙古四子王旗北部的苏莫查干敖包地区是我国北方重要的单一萤石矿集区,位于中亚晚古生代构造带上,在下二叠统大石寨组中酸性火山-沉积岩与燕山中期花岗岩的内外接触带上产出有苏莫查干敖包超大型萤石矿床和中型敖包吐萤石矿床以及伊和尔、贵勒斯泰和西里庙等诸多萤石矿化点。苏莫查干敖包萤石矿床以及区域上的温多尔努如和瑙尔其格萤石矿化点产出于大石寨组第三性段的底部;相比之下,敖包吐萤石矿床产出在大石寨组第三岩性段的顶部,贵勒斯泰萤石矿点产出于燕山中期花岗岩之内。依据矿物组合可以将萤石矿石划分为3种类型,即萤石-石英型、硫化物-萤石型和碳酸盐-萤石型;根据矿石的结构和构造特征划分为纹层状、条带状、细晶致密块状、伟晶状、皮壳状、混合伟晶状和泥沙状,其中早期成矿阶段类型主要为条带状、纹层状和细晶块状,而晚期为伟晶状和混合伟晶状。蚀变类型为硅化、绢云母化和高岭石化,其中蚀变矿物绢云母和伊利石的钾-氩年龄分别为141.5±1.2Ma和137.6±1.1Ma。鉴于萤石和绢云母和伊利石具有共生结构关系,因此,萤石的成矿作用发生在早白垩世。同位素测年结果表明,大石寨组火山岩锆石SHRIMP铀-铅年龄为276±10Ma,MSWD为1.9。属早二叠世。岩石地球化学研究表明,大石寨组火山岩为高硅（SiO₂含量在63.66%～78.02%之间,样品数为12）、富碱（（Na₂O+K₂O）的含量为5.55%～9.86%,样品数为12）、过铝质（Al₂O₃的含量为11.78%～21.33%,铝饱和指数/CNK为1.0～2.0,样品数为12）的钙碱性到高钾钙碱性岩石系列。研究区与萤石成矿作用关系密切的侵入岩类为早白垩世卫境花岗岩,主要岩石类型为中细粒含黑云母似斑状花岗岩。花岗岩的锆石SHRIMP铀-铅年龄为138±3.8Ma,MSWD为2.3,属早白垩世。岩石地球化学研究表明,花岗岩主要表现为高硅（SiO₂含量为70.18%～75.62%,样品数为8）、富碱（全碱（Na₂O+K₂O）含量为8.25%～9.34%,样品数为8）、准铝-弱过铝质（Al₂O₃的含量为12.69%～14.68%,铝饱和指数A/CNK为0.92～1.05）的特点,属高钾钙碱性岩石系列。卫境岩体中含萤石细粒花岗岩脉的锆石SHRIMP铀-铅年龄为137±2Ma,MSWD为0.25,与卫境花岗岩基本同期,同时与苏莫查干敖包萤石矿脉中的蚀变矿物绢云母和伊利石的钾-氩年龄基本接近。所有上述证据表明研究区的萤石成矿作用与早白垩世的岩浆活动具有密切的成因联系。苏莫查干敖包萤石矿床地球化学研究结果表明,各成矿期次萤石单矿物的镍含量较高,其变化范围为（28.60～38.60）×10^-6,远高于上地壳的镍的平均含量。成矿作用早期形成的纹层状、细晶块状、条带状萤石矿石的稀土元素配分型式为轻稀土元素略显富集、基本没有铕异常;相比之下,成矿作用晚期形成的伟晶状、混合伟晶状萤石,为轻稀土元素亏损和重稀土元素富集,铕既有正异常也有负异常。萤石的包裹体研究结果表明,成矿流体主要属H₂O-CO₂-NaCl体系,各类型萤石矿石中存在3种包裹体类型:气-液2相包裹体,富CO₂包裹体和气相-液相-固相共存的3相包裹体。高温和高盐度与低温和低盐度包裹体均可见及,前者的均一温度变化范围为396～436℃,盐度为29.47%～47.95%NaCleq.,集中分布在36%～40%NaCleq.之间;后者的均一温度在140～160℃,盐度在0.18%～10.98%NaCleq.集中分布在2.0%NaCleq.附近。萤石矿石中黄铁矿的δ³⁴S为-5.2‰～+7.0‰,与典型的岩浆热液矿床大体相似。成矿早期阶段萤石δD为-119.0‰～-102.0‰,δ¹⁸O为-7‰～-11.6‰,反映了δ¹⁸O交换的特征;成矿晚期阶段萤石δD为-128‰～-104‰,δ¹⁸O为-21‰～-14.8‰,分布在大气降水线附近。在萤石的成矿作用的早期阶段,成矿流体主要是高温和高盐度的岩浆流体,流体与大石寨组流纹岩和流纹质凝灰岩以及大理岩透镜体的高比例的水-岩反应是萤石从含CaF₂的成矿流体中沉淀析出的主要机制,萤石矿脉普遍发育的高岭石化是这一反应的结果;成矿晚期阶段的成矿流体主要是大气降水为主要来源的混合流体,流体温度的降低是CaF₂从流体中沉淀的主要因素。苏莫查干敖包萤石矿床的锶-钕同位素研究结果表明,萤石单矿物的锶同位素初始比值(⁸⁷Sr/⁸⁶Sr)_i为0.70861～0.71045。萤石的钕同位素初始比值(¹⁴³Nd/¹⁴⁴Nd)_i为0.512203～0.512341,εNd（t）为-2.33～-4.77,二阶段模式年龄T_2DM为1121～1329 Ma。萤石的锶-钕同位素特征表明苏莫查干敖包萤石矿床的成矿物质来源主要与早白垩世卫境花岗岩有关,萤石高的εNd（t）反映了成矿物质来源中有相当比例的幔源物质组分（或幔源流体）的加入。萤石的二阶段模式年龄与南蒙古乌呼塔格的结晶基底年龄接近,暗示了部分成矿物质来源与南蒙古陆块有着紧密的关联。更多还原

【Abstract】 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.更多还原