内蒙古苏莫查干敖包特大型萤石矿床地质特征及成因

Geological features and origin of Sumoqagan Obo superlarge independent fluorite deposit,Inner Mongolia

苏莫查干敖包矿床是迄今为止在全球范围内找到的最大规模单一萤石矿床。萤石矿体大多呈层状、似层状和透镜状沿下二叠统碳泥质板岩(夹灰岩透镜体)与流纹质火山岩接触带分布,并且与中生代花岗岩类侵入岩体具有密切的空间分布关系。流纹质火山岩和花岗岩的锆石SHRIMPU-Pb年龄值分别为(276±10)Ma和(138±4)Ma,它们分别是海西晚期和燕山中期酸性岩浆活动的产物。萤石矿体主要由4种类型矿石所构成,即石英-萤石型、石英-硫化物-萤石型、萤石-石英型和方解石-石英-萤石型。矿物组分有萤石、石英(蛋白石、玉髓)、方解石、绢云母和绿泥石,个别样品中有磁铁矿、黄铁矿、磁黄铁矿、黄铜矿、闪锌矿、毒砂和锡石。围岩蚀变自矿体中心向外分别为绢云母化、硅化、碳酸盐化和高岭石化。钕、锶和铅同位素分析结果表明,萤石矿床是多期次和多阶段岩浆热液活动的产物,成矿作用分别发生在海西晚期和燕山中期,并以后者为主。海西晚期富碱质酸性岩浆喷发活动为层纹状和条带状矿体以及矿源层的形成创造了有利条件。相比之下,燕山中期花岗质岩浆侵入活动不仅为细晶质、伟晶质和角砾状矿体的形成提供了物质、动力和热力来源,而且是成矿流体对流循环的"发动机"。苏莫查干敖包矿床属于与花岗岩类深成侵入岩体有关的热液型萤石矿床,成岩(矿)物质来自壳、幔混合源,其中壳源组分占有绝对优势。

The Sumoqagan Obo deposit, located on the northern margin of the North China Craton, is the largest independent fluorite deposit in the world. Exploratory drilling has defined reserves in excess of 20 million tons of CaF2 with a mean grade of 78 %. Fluorite mineralization occurs mainly along the contact zones between Lower Permian carbonaceous-argillaceous slate and rhyolite of Xilimiao Group in stratiform, stratoid and lenticular forms, but is spatially and temporally associated with the Mesozoic Yanshanian Weijing graniotid batholith. The batholith, dated at （138 ± 4） Ma （Zircon SHRIMP）, consists mainly of K feldspar granite in the central phase and porphyritic granite in the marginal phase. It intruded into Lower Permian volcano-sedimentary sequences of the Xilimiao Group along the deep-rooted fault zone on the northern margin of the North China Craton. Four major styles of fluorite mineralization have been identified based on mineral assemblages： （1） Quartz-fluorite ore： It is mainly distributed in the central part of the deposit, and occurs on the surface or at a depth less than 100 m. The ore consists of fluorite and quartz, with a certain amount of chlorite, calcite, sericite, opal and pyrite. The CaF2 content varies from 77% to 99%, with an average value of 95% ;（2）Quartz-sulfide-fluorite ore： It is mainly located in the southwestern part of the deposit and has a buried depth of more than 100 m. The ore comprises mainly fluorite, quartz and pyrite together with a certain amount of chalcopyrite, sphalerite, pyrrhotite, arsenopyrite, cassiterite, chlorite, calcite and sericite. The CaF2 content varies from 30% to 70%, with an average value of 56 % ; （3） Calcite-quartz-fluorite ore： It is mainly distributed in the central-southern part of the deposit, and has a buried depth ranging from 30 m to 100 m. The ore consists mainly of fluorite, calcite and quartz, with a certain amount of chlorite, calcite, sericite, opal and pyrite. The CaF2 content varies from 40 % to 77%, with an average value of 55% ; （4）Fluorite-quartz ore： It is mainly located in the central part of the deposit, and occurs on the surface. The ore is composed mainly of quartz and fluorite, with a certain amount of chlorite, calcite, kaolinite, opal, chalcedony, and pyrite. The CaF2 content varies from 40% to 85%, with an average value of 62%. The minable part of the deposit can be traced along the strike for 2900 m, and down dip for 600 to 800 m （up to 1200 m）, ranging from 0.5 to 22 m in thickness. δ^34S values of the pyrite separates from the fluorite ores range from - 8.63‰ to - 6.75‰, with an average of - 7.52‰. All these data are plotted in the range between the δ^34S values of pyrite separates from the Yanshanian granitoid batholith and Early Permian carbonaceous-argillaceous slate. εNd（138 Ma） values of the fluorite ores vary from -4.61 to -4.07 with an average of -4.34. All these data are also plotted in the range between the εNd（138 Ma） value fields of the K-feldspar granite and carbonaceous-argillaceous slate. In addition, the fluorite ores have a similar εNd（138 Ma） value to porphyritic granite and ancient continent. Fluorite ores are characterized by high radiogenic lead isotopic composition similar to that of Early Permian carbonaceous-argillaceous slate and Yanshanian granitoid batholith. These overlapping lead isotopic data of fluorite ore, slate and granite are interpreted as being indicative of mixing between lead from the Yanshanian granitoid magmatism and lead from the Precambrian metamorphic basement and/or Permian sedimentary rocks. The combined isotope data and petrological observations indicate that the metallogenic processes of the Sumoqagan Obo deposit can be divided into two stages： （1） Late Hercynian preparatory stage （also called ground preparation） and （2） Middle Yanshanian main ore-forming stage. The deposit was formed from the mixing of magmatic fluids with meteoric water. The 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 northern margin of the North China Craton.