华北北部大型-超大型矿床的特征及其形成的宏观地质背景

CHARACTERISTICS AND GEOLOGICAL BACKGROUND OF FORMATION OF THE LARGE AND GIANT ORE DEPOSITS WITHIN THE NORTHERN MARGIN OF THE NORTH CHINA PLATFORM

华北北部经历了太古宙—元古宙克拉通基底形成演化、中新元古宙裂谷作用、古生代“软”造山、中生代伸展构造体制以及新生代盆 -岭构造演化 .中元古宙和中生代是华北北缘的两个大规模成矿时期 .华北北部中段从南往北依次分布着 :华北克拉通北缘 Au- Cu矿化集中区 ;大兴安岭 Cu- Sn多金属矿化集中区 ;北邻蒙古和俄罗斯境内 Fe- Au- U -WSn( Mo,Be)集中分布 .Mo矿化主要出现在克拉通内部 ,Sn矿化主要出现在大兴安岭地区 ,成矿物质可能来源于微地体 ,Cu矿化局限于西拉木仑断裂以北附近 ,成矿物质可能来源于尚未消耗饴尽的洋壳物质 .成矿元素组合取决于基底组成 ,矿床类型取决于区域大地构造动力学背境 .华北北缘西段的总体构造格局为一系列逆冲推覆构造 ,西段大型、超大型矿床表现出“地层 -构造 -岩浆”三位一体化特征 ,断裂构造起着先决性条件 ,控制着成矿断陷盆地的形成和发展 ,构成岩浆和成矿流体的运移通道 ,并且控制着地层褶皱的形成以及矿体的赋存空间 .西段大型、超大型矿床经历了中元古宙热水沉积主成矿期、加里东和华力西矿化再富集时期 .大兴安岭矿化集中区可以鉴别出两类成矿流体 :黄岗梁式高温高盐度水熔体 ,大井中低温较低盐度富水成矿流体 .大井矿床的流体包裹体的均一温度。

The northern margin of the North China platform has experienced four geotectonic evolution stages which include the formation of the Archean Proterozoic basement, rifting in the middle to late Proterozoic, 'soft' orogenesis in the Palaeozoic, extensional regime in the Mesozoic, and evolution of basin ridge geotectonic regime in the Cenozoic. The middle Proterozoic and Mesozoic are the two metallogenetic epochs when the large and giant ore deposits were formed within the northern margin of the North China platform. In the central section of the northern margin of the North China platform, from the south to the north there occur, in succession, the Au Cu metallogenetic belt within the northern margin of the North China craton, the Cu Sn polymetallic mineralization belt within the southern section of the Da xing an ling mountain, and the Fe Au U WSn (MoBe) mineralization region within the peripheral region of Mongolia and Russia. Mo mineralization occurs dominantly within the craton, whereas Sn mineralization occurs dominantly in the Da xing an ling mountain, and has possibly derived from the micro terrain. However, Cu mineralization is limited to the northern side of the Xilamulun fault and has possibly derived from the remnants of oceanic crust which have not been consumed yet. Metal species depends on the composition of basement, whereas the genetic type of ore deposits depends on the regional structure and geodynamic conditions. Within the western section of the northern margin of the North China platform, the overall structural framework is characterized by the thrust system. Within the western section the large and giant ore deposits exhibit polygenetic character of 'strata structure magma' multiple controls among which the fault constitutes a prerequisite. The fault led to the formation and development of basins, channelized the magma and mineralizing fluids, caused the folding of strata, and provided space for the deposition of ore materials. The large and giant ore deposits in the western section of the northern margin of the North China platform experienced a major metallogenetic epoch of hot water deposition in the middle Proterozoic, and the epoch of re enrichment during the Caledonian and Variscanian. Two kinds of mineralizing fluids are identified in the Da xing an ling mineralization belt that include the Huang gang liang type of fluid of melt water mixture with high temperature and high salinity, and the Da jing type of fluid composed dominantly of water with medium to low temperature and relatively low salinity. The homogenization temperature, salinity of fluid inclusions, from the Da jing ore deposit, and their hydrogen isotopic composition exhibit the characteristics of meteoric water, whereas the stable isotopic compositions of sulphur and carbon suggest a magmatic origin. In combination, these features indicate that the mineralizing fluids of the Da jing ore deposit already have the characteristics of the down stream fluid, i.e. the magmatic fluid diluted with large amounts of meteoric water.