【作者】 刘阳；

【导师】 孙景贵；

【作者基本信息】 吉林大学 ， 矿物学、岩石学、矿床学， 2022， 博士

【摘要】 小兴安岭地区是中国东北部重要的金、钼、银、铅、锌等多金属成矿区带之一,以广泛发育斑岩型钼、矽卡岩型铁铜多金属、浅成热液型金矿床为特色。其中,斑岩型钼、矽卡岩型铁铜多金属主要发育在侏罗世花岗岩隆起区,浅成热液金矿床发育在早白垩世火山盆地。近年来,我们课题组先后对该区发育的团结沟、三道湾子、东安,高松山和永新等浅成热液矿床进行了研究;本项工作在已有研究基础上,针对浅成热液金成矿作用科学问题,选择东安、高松山和团结沟三个典型矿床为代表,系统开展了地质、岩相学、流体包裹体和岩石-矿物的元素-同位素地球化学以及高精度年代学等方面取证,从成岩成矿时代、岩浆作用与含矿流体起源、演化角度揭示成岩成矿动力学背景、成矿机制,建立适于该区的成岩成矿综合模式与动力学模型,结合定量计算的热液沉淀古深度,评价了矿床就位深度与形成后的保存或剥蚀程度。获得的主要进展与新认识如下:1.矿床地质研究揭示,研究区的浅成热液金（银）矿床均发育在中生代火山盆地内部和边缘火山机构内,赋矿围岩为早白垩世安山岩-流纹质火山岩以及次火山岩;矿体类型以石英脉型为主,其次为角砾岩型,矿体分布主要受断裂构造控制;矿石在垂向上分带明显,包括浅部的热液充填角砾岩、中部的石英脉和热液角砾岩以及深部的富硫化物石英细脉,随着深度增加,黄铁矿-白铁矿含量明显增多;以矿体为中心向外由绢英岩化过渡为中等泥化和最远端的青磐岩化+碳酸盐化蚀变;依据脉系穿插关系和矿化时间顺序,流体活动可划分为石英-黄铁矿-赤铁矿阶段（I）、乳白色致密块状石英阶段（II）、灰白色石英-硫化物阶段（III）和石英-碳酸盐±萤石阶段（IV）。2.流体包裹体显微测温结果表明研究区内贵金属Au-Ag沉淀于低温、低盐度、近中性-还原流体;脉石矿物—石英内流体包裹体类型相对单一,以富液相包裹体为主,发育少量富气相包裹体;除团结沟金矿床（167～353℃）外,流体温度绝大多数低于300℃,盐度普遍低于10 wt.%Na Cl equiv;结合岩相学观察、拉曼成分分析和稳定（H-O）同位素测试结果,浅成热液金成矿系统初始含矿流体起源于挥发分饱和的岩浆流体出溶,为弱氧化的H₂O-Na Cl±CO₂体系;存储于岩浆房顶部的流体逐渐被释放,并沿构造薄弱带上升、与大气降水流体发生混合;矿石组构和矿物学特征表明沸腾作用、水岩反应、脱硫作用和流体混合的共同作用降低了流体中H₂S含量以及Au元素溶解度,破坏了Au稳定的物理化学环境,最终导致Au⁺与HS^-络合物失稳而沉淀。3.锆石U-Pb定年和热液矿物放射性同位素定年的结合精确限定研究区内浅成热液金（银）矿床形成于早白垩世晚期（110～100 Ma）;高松山银金矿区内安山岩-流纹岩-流纹斑岩-花岗斑岩火成岩组合锆石U-Pb年龄介于102～108 Ma,接近热液石英Rb-Sr同位素年龄（98±1.6 Ma）;东安银金矿区内安山岩-流纹岩-花岗斑岩组合锆石U-Pb年龄介于107～110 Ma,与热液绢云母Ar-Ar坪年龄年龄（107.24±0.55Ma）吻合;团结沟金矿区内英安岩-花岗闪长斑岩组合锆石U-Pb年龄介于103～110Ma,与黄铁矿Rb-Sr同位素年龄（113.8±4.0 Ma）在误差范围内叠加。4.早白垩世安山质-流纹质火山岩的全岩元素地球化学揭示该火山岩组合为钙碱性弧岩浆喷发、就位的产物,明显富集轻稀土元素（LREEs）和大离子亲石元素（LILEs）,亏损重稀土元素（HREEs）和高场强元素（HFLEs）;结合亏损的全岩Sr-Nd同位素和锆石Hf同位素结果,（玄武）安山质母岩浆起源于壳幔源岩浆的混合作用,而硅酸盐矿物的结晶分异控制了岩浆演化的深部动力学过程,熔体-流体的分离贡献了最初的成矿流体。5.相关矿物地质氧压计（锆石、角闪石和黑云母）研究揭示:东安和团结沟热液成矿系统内高演化的花岗质岩浆为富水岩浆（>4.0 wt.%H₂O）,具有氧化属性（FMQ>2.0±2.0）,有利于Au等元素在熔体内携带和聚集;高松山热液成矿系统内相关岩浆为贫水（≥2.5 wt.%H₂O）、还原的（FMQ 0.12±1.26）流纹质岩浆;此外,团结沟花岗闪长斑岩母岩浆存储于浅部地壳（3.42～4.31 km）,达到了挥发分饱和,能够释放足够的含矿热液来补给后续的成矿过程。6.将岩浆-热液演化与区域构造演化相结合,该区早白垩世浅成热液金矿床的成岩成矿地质过程概括如下:在早白垩世地史时期,古太平洋板片西向欧亚大陆高角度俯冲或回撤,在压力释放构造背景下小兴安岭北麓盆地发生幔隆、地壳伸展,岩石圈地幔受俯冲流体交代作用减压熔融,产生的镁铁质岩浆底侵于等密度的壳幔过渡带,加热使新生玄武质下地壳岩石发生部分熔融产生长英质岩浆并发生了混合;伴随着岩浆上侵和硅酸盐矿物的分离结晶,岩浆逐渐向长英质成分演化,（可能）伴有不相容的Au元素逐渐富集;高演化的岩浆最终存储于浅部地壳（～4 km）并达到挥发分的饱和而发生初始含矿流体的出溶;低密度的挥发分流体沿构造薄弱带快速上升,继而发生沸腾作用、水岩反应、混合作用和脱硫作用等,流体物理化学环境的骤变导致Au和沸腾指示矿物（冰长石-刃片状方解石）等的卸载和沉淀。7.依据沸腾流体包裹体压力计和岩浆磷灰石低温热年代学,确定了小兴安岭北麓地区典型浅成热液金（银）矿床的古就位深度:在静水压力环境下,高松山地表矿石沉淀压力范围为2.3±0.9 MPa,对应古深度为200～300 m;东安地表矿石沉淀压力范围为3.6±1.1 MPa,对应古深度为300～400 m;团结沟地表矿石沉淀压力范围为7.6±1.9 MPa,对应古深度为～900 m;古深度计算结果与磷灰石低温热年代学反演模拟结果高度吻合,如东安花岗斑岩就位深度明显低于0.7 km;团结沟花岗闪长斑岩最大就位深度为1.0 km。更多还原

【Abstract】 The Lesser Xing’an Range is one economically important area of northeast China,with endowed mineralization of Au,Mo,Ag,Pb,and Zn etc.,and a spectrum of deposit types comprise porphyry,skarn,and epithermal types etc..Porphyry Mo and skarn polymetallic deposits generally occur in areas where granitic intrusions are prodominant,whereas epithermal gold deposits are developed in volcanic basins.We recently did a lot of works on epithermal gold deposits in Lesser Xing’an Range area,as Tuanjiegou,Sandaowanzi,Dongan,Gaosongshan,and Yongxin.Considering the issue on Au mineralization of epithermal system,exemplified by the Dongan,Gaosongshan,and Tuanjiegou,a combination of field mapping and sampling with geochronology,fluid microthermometry,and geochemistry has been conducted in this study,with an aim to decipher the magmatic-hydrothermal processes that have essential controls on the mineralizing capability and eventually ore depositions at shallow-crust levels.We also quantified the paleodepths of ore deposition,which indirectly provide constraints to the erosion and preservation degrees of individual deposit.1.Ore veins are generally hosted in the coeval andesitic to rhyolitic suite and felsic porphyry of early Cretaceous in age,and the distribution of veins is largely controlled by the normal faults related to the volcanic activities,and locally by the brecciaed zones being in association with hydrothermal eruption.Ore zonation vertically grades from upper thin vein-filling breccias through intermediate quartz bands and hydrothermal eruption breccias to lower sulfide-rich quartz veinlets,with increasing proportions of pyrite and marcasite with increasing depths.Alteration system centered on the ore veins comprises the innermost pervasive phyllic alteration,outer intermediate argillic alteration,and distal propylitic shell.Multiple events of fluid pulses are recognized based on the crosscut relationships,including quartz-pyrite-hematite stage（I）,ivory massive quartz（II）,gray quartz-sulfides stage（III）,and clear quartz-carbonate±fluorite stage（IV）.2.Inclusion microthermometry results show the Au and Ag precipitation from low temperature,dilute,near-neutral,reduced hydrothermal fluids.The inclusion types in hosting gangue minerals are relatively simple and dominated by liquid-rich inclusions with minor vapor-rich ones.Except for Tuanjiegou system associated with relatively high temperature fluids（167～353℃）,the fluids in epithermal systems have temperatures of<300℃and salinities of<10 wt.%Na Cl equiv.They are initially exsolved from volatile-saturated melts,and are weakly oxidized.After these stored fluids are liberated from reservoir,a transition to reduced state occurred during magmatic fluids ascent along fractures and mixing of meteoric water.Evidence of textures and mineralogy suggests the cooperation among boiling,water-rock interaction,desulfurization,and mixing of two fluid components that drove the Au precipitation due to the lower metallic solubility in the late near neutral,reduced fluids compared to the early weakly oxidized,acidic magmatic fluids.3.Dates of magmatic zircon and hydrothermal minerals suggest that the epithermal precious metal mineralization occurred in a short duration between 110 Ma and 100 Ma.Andesite-rhyolite-granite porphyry assemblage at Gaosongshan have ages of 108-102Ma,overlapping with an age of 98±1.6 Ma of hydrothermal quartz within errors.The emplacement timing of Andesite-rhyolite-granite porphyry suite is at 110-107 Ma,coincident with the hydrothermal sericite of 107.24±0.55 Ma.Dates of dacite-granodiorite porphyry suite at Tuanjiegou span a range from 110 Ma to 103 Ma,consistent with an age of 113.8±4.0 Ma of pyrite collected from ores.4.A genetic bond of Au-Ag mineralization with andesitic to rhyolitic eruption is emphasized here.These resulting volcanic rocks have calc-alkaline affinities and are arc-related,characterized by enriched LREE and LILEs and depleted HREE and HFSEs.A combination of depleted both Sr-Nd isotope and Hf isotope and whole-rock geochemistry suggests that andesitic magmas are sourced from the mixing between crust-and mantle-derived melts and subsequently experienced fractionation of silicate minerals to produce the more felsic melts during the magmas ascent.The segregation of fluids from melts contributes the primary fertile solutions into hydrothermal system.5.We determined that the highly evolved granitic melts are hydrous（>4.0 wt.%H₂O）and oxidized（FMQ>2.0±2.0）using the mineral oxybarometers,facilitating the transports and concentrations of Au in melts.By contrast,the ore-related rhyolitic magmas in the Gaosongshan system is relatively dry（≥2.5 wt.%H₂O）and reduced（FMQ 0.12±1.26）.Magmas parental to the Tuanjiegou hydrothermal system were stored at 3.42-4.31km levels and became volatile-saturated,ensuring abundant hydrothermal fluids to be liberated.6.A framework of magmatic-hydrothermal processes in Creataceous is summarized here.The decompressional setting is related to the deep subduction of Paleo-Pacific slab downward Eurasian Continent at Early Cretaceous,and as a result,the fluid-modified lithosphere mantle was remelted to produce the mafic magmas that then underplated at the crust and mantle boundary due to the density similarity.A mixing between heat mafic melts and felsic melts generated by the heating-derived remelting of lower crust materials must have occurred to produce the intermediate（andesitic）melts.Magmatic evolution totally towards silicic compositions is dominated by the fractional crystallization of silicate minerals during ascent,and the incompatible Au would have been concentrated during hosting melts ascent.High-silica melts were eventually stored at shallow-crustal levels（～4 km）and reached the H₂O-saturation boundary to exsolve hydrothermal solutions.Low-density fluids rapidly transported upward along the fractures and experienced the extensive boiling,water-rock interaction,mixing with cool meteoric waters,and desulfurization.These processes led to the dramatic changes in physicochemical environments and thus the deposition of Au and boiling-indicator minerals（e.g.,adularia and bladed carbonate）.7.Fluid geobarometer and apatite low temperature thermochronology are used to determine the paleodepths of ores in individual low sulfidation deposit in the Lesser Xing’an Range area.Under hydroclastic pressures,fluids in the Gaosongshan system is deposited at pressures of 2.3±0.9 MPa,equivalent to a hydrostatic depth of about 200-300 meters below paleowater stable,whereas the Dongan fluids at 3.6±1.1 MPa pressures,with a paleodepth between 300 m and 400 m.An elevated pressure of 7.6±1.9MPa characterize the Tuanjiegou ores,corresponding to a～900 m paleodepth.These paleodepth estimates are consistant with the time-temperature modelling of apatite low temperature thermochronology,such as the maximum emplacement depths of<0.7 km for Dongan granite porphyry and of 1.0 km for Tuanjiegou granodiorite porphyry.更多还原