Mesozoic gold metallogenic system of the Jiaodong gold province,eastern China
Mesozoic gold metallogenic system of the Jiaodong gold province,eastern China
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作者：
Yang Li
胶东半岛是我国最重要的金矿集中区，境内已发现金矿床150多处，探明金矿储量达4000吨。金矿床数量和资源量巨大，金矿床分布广泛，金矿赋存和成矿类型各异，但成矿地球动力学背景、围矿岩环境、金矿赋存条件和成矿特征总体相同：（1）胶东地区是由前寒武纪基岩和超高压（UHP）组成的内生热液金富集区。 中生代变质岩、构造作用和岩浆作用频繁发生,130~110Ma金成矿事件发生时间比区域变质作用晚2000Myr左右;(2)区域性金成矿系统形成于早白垩世陆缘伸展构造背景下,大规模金成矿事件发生在区域性北西伸展向北东伸展转变的过程中。 北东东向挤压，对应华东岩石圈缩减、华北克拉通破坏和大陆裂谷高峰期；(3)金矿聚集在北北东向玲珑、雀山、昆玉山变质核杂岩周围，主要沿前寒武纪变质岩与中生代花岗岩接触带发育的区域性北北东向拆离断层；(4) 控矿断裂带经历了早期韧脆变形和晚期脆性变形构造叠加，在三维空间上呈平滑波形延伸，控制了金矿体的横向走向和分段富集；（5）成矿类型主要有碎屑蚀变（角砾岩）岩型、（硫化物）石英脉型和复合脉带型，结构为 矿岩中破碎结构、晶粒结构、间隙结构以及浸染结构、脉状结构、块状结构、块状结构等结构丰富，表明成矿环境由韧脆性条件向脆性条件转变；(6)金属矿物主要有黄铁矿、黄铜矿、方铅矿和闪锌矿，非金属矿物主要有 石英、绢云母、钾长石、方解石；金矿物主要有金金矿、自然金和少量的紫锂辉石，主要以可见金的形式赋存于黄铁矿和石英的裂隙中，晶隙中较少，以包裹体形式存在；热液蚀变类型主要有黄铁矿化、硅化、绢云母化和碳酸盐化；成矿元素主要为Au-Ag(-Cu-Pb-Zn)组合；上述蚀变和成矿组合具有中浅成热液组合特征；(7)成矿流体来自地壳和地幔，主要是壳源变质流体；成矿物质来源于中生代重新活化的前寒武纪变质基岩体，并混有少量浅部地壳、地幔成分。区域成矿特征的一致性表明，胶东金矿集区早白垩世大规模金矿成矿受均匀地质事件控制，金矿成矿属于表生中温热液-浅成热液脉状金矿成矿系统。这些金矿床具有明显的时空集群分布特征，主要分布在三个变质核杂岩周围不同岩相的接触带上。从西向东，金矿成矿时代由旧变新。由此可划分出胶北隆起蚀变岩-石英脉型、苏鲁超高压变质带硫化物-石英脉型和胶莱盆地北缘蚀变角砾岩型3个金子系统。矿化样式由浸染状脉状、脉状网状、石英脉型转变为硫化物石英脉型、蚀变角砾岩型。矿石结构和结构表现为以细脉浸染状构造为主，到带状构造和梳状构造，再到角砾岩构造，表明矿化分别发生在脆性—韧性转变带（深约15km）、脆性拉张剪切断裂带和脆性角砾岩带（深约5km）。蚀变和矿化的规模和强度减小，浅层增加。 成矿物质中的地壳成分,可能与矿床所处位置越来越远离物源区有关。成矿P-T条件逐渐降低,成矿流体中大气水和/或盆地卤水比例逐渐增大,分别对应成矿深度变浅和趋向扩展的成矿构造环境。成矿特征的区域规律性变化反映了滑脱断层韧脆过渡带和脆性角砾岩带之间地壳不同垂直深度的地壳成矿连续体。胶东金省中生代金矿成矿系统不同于典型的“侵入型金矿床”、“造山型金矿床”或全球其他已知的金矿床类型,不能归入已知的成矿模式。为合理解释独特的地球动力学背景、围矿岩环境及成矿特征,提出了对“胶东型”金矿床及成矿模式的新认识。认为古太平洋伊邪那岐俯冲板块的回滚可能是导致区域前寒武纪变质基岩体成矿物质大规模复兴的主要驱动机制,成矿流体主要来自俯冲板块的变质脱水作用。金主要以Au(HS)2络合物形式存在，在矿液中沿滑脱断层系运移。滑脱断层系脆性转变带和脆性角砾岩带周围构造空间急剧增大，成矿温度和压力急剧降低。因此，矿液中CO2和H2S的损失以及硫化作用会导致Au(HS)2-等金络合物的稳定性下降，进而导致金的大规模沉淀。
Jiaodong Peninsula is the most important gold concentration area of China,more than 150 gold deposits within it have been found and the proven gold reserves add up to 4000 tons. The amount of gold deposits and gold resource are huge,gold occurrence and mineralization type vary with the widely distributed gold deposits,however,the metallogenic geodynamics background,ore-host rock environment,gold occurrence conditions and metallogenic characteristics are identical as a whole:( 1) Jiaodong area is an endogenic hydrothermal gold concentration area consisting of Precambrian base rocks and ultra-high pressure( UHP) metamorphic rocks,tectonism and magmatism happened frequently in Mesozoic,130 ~ 110 Ma gold metallogenic events occurred 2000 Myr or so later than the regional metamorphism;( 2) regional gold metallogenic system formed in the Early Cretaceous continental-margin extension tectonic background,large-scale gold metallogenic events happened in the process of regional NW extension changing to NE extension followed by the NEE compression,which corresponded to the lithosphere reduction in East China,North China craton destruction and the peak of continental rifting;( 3) gold deposits clustered around the NNE Linglong,Queshan and Kunyushan metamorphic core complexes,mainly along the regional NE-NNE detachment faults developing along the contact zone of Precambrian metamorphic rocks and Mesozoic granites;( 4) the ore-controlling fault belts went through the early ductile-brittle deformation and late brittle deformation structure superposition,extended in smooth-out waveforms in 3D space and controlled the lateral trending and subsection enrichment of gold orebodies;( 5) the mineralization types mainly include clastic altered( breccia) rock type,( sulfide-) quartz vein type and compound vein-belt type,textures like the crush texture,crystalline-granular texture,interstitial texture and structures like the disseminated structure,vein structure,massive structure,crumby structure are abundant in the ore rocks,which indicates the oreforming environment changed from ductile-brittle conditions to brittle conditions;( 6) metallic minerals mainly include pyrite,chalcopyrite,galena and sphalerite,non-metallic minerals mainly include quartz,sericite,potash-feldspar and calcite; gold minerals mainly include the electrum,natural gold and a small amount of kustelite,which mainly occur in the fractures of pyrites and quartz in the form of visible gold,lesser in crystal gap and as inclusions; hydrothermal alteration types mainly include pyritization,silicification,sericitization and carbonatation; ore-forming elements mainly consist of the Au-Ag(-Cu-Pb-Zn) assemblage; the alteration and mineralization assemblage mentioned above show the characterisics of mesothermal-epithermal assemblage;( 7) ore-forming fluids came from both the crust and mantle and are mainly the crust source metamorphic fluids; metallogenic materials derived from the Precambrian metamorphic basement rock mass which reactivated in the Mesozoic,mingling with a small amount of the shallow crustal and mantle components. The consistency of regional metallogenic characteristics indicate that the Early Cretaceous large-scale gold metallogenesis in Jiaodong gold concentration area is controlled by the uniform geological events,and the gold metallogenesis belongs to an epigeneticmesothermal-epithermal hydrothermal vein gold metallogenic system. These gold deposits have obvious characteristic of spatio-temporal cluster distribution and lie mainly along the contact zones of different lithofacies around three metamorphic core complexes. From west to east,the gold mineralization age changes from older to newer. Therefore,three gold subsystems can be divided,which are the altered rock-quartz vein type in Jiaobei Uplift,the sulfide-quartz vein type in Sulu UHP metamorphic belt and the altered breccia type in north margin of Jiaolai Basin. The mineralization style changes from disseminated-veinlet,veinlet-stockwork and quartz vein type,to sulfide-quartz vein type,to altered breccia type. The texture and structure of ores are characterized by veinlet-disseminated structure dominated,to band structure and comb structure,and then to breccia structure,indicating mineralization occurred respectively in brittle-ductile transformation zone( ca. 15 km deep),a brittle extension-shear fault zone,and a brittle breccia zone( ca. 5km deep).The decrease of the size and strength of alteration and mineralization,and the increase of shallow crustal components in metallogenic materials,may be related to the deposits' location which is more and more far away from the source area. The ore-forming P-T conditions gradually decreased and the meteoric water and / or basin brine ratio in the ore-forming fluid gradually increased,respectively,which corresponds to the shallower metallogenic depth and more and more extended trending mineralization tectonic environment. All the regional regular changes of metallogenic characteristics reflect a crustal continuum metallogenic in different vertical depth of crust,between the detachment fault ductile-brittle transition zone and the brittle breccia zone. Mesozoic gold metallogenic system of the Jiaodong gold province is distinct from typical"intrusion-related gold deposit","orogenic gold deposit"or other known gold deposit type around the globe,and can't be classified into the known metallogenic model. To reasonably explain the unique geodynamic background,environment of ore-host rock and mineralization characteristics,we put forward the new understanding of the"Jiaodong type"gold deposit and metallogenic model. We conclude that the rollback of ancient Pacific Izanagi subduction plate may be the main drive mechanism leading to large-scale revitalizing of the metallogenic materials in regional Precambrian metamorphic basement rock mass,and ore-forming fluids mainly came from metamorphic dehydration of the subducting plate. Gold is mainly in Au( HS)2-complex and transported along detachment fault system in ore fluids. The tectonic space increases sharply as well as the metallogenic temperature and pressure decreases suddenly around the brittle-ductile transformation zone of detachment fault system and brittle breccia zone. Therefore,CO2 and H2S lossing from the ore fluids and sulfidation leads to a stability decreasing of Au( HS)2-and other gold complexes,and subsequent large-scale gold precipitation.