西秦岭地区金矿类型及其成矿作用

Classification and mineralization of the gold deposits in the western Qinling region,China

西秦岭地区是中国最重要的金矿矿集区之一,除产出少数夕卡岩型金矿床外,几乎所有的其他金矿床都可归并为造山型、卡林型和类卡林型3种类型。研究表明,西秦岭地区中生代花岗岩主要形成于中-晚三叠世,而金矿成矿主要集中在晚三叠世,它们都是华北板块与华南板块碰撞导致的秦岭造山作用的产物。西秦岭地区造山型金矿床主要赋存在泥盆系和石炭系一套复杂的构造变形和区域变质的绿片岩相岩中,主要受北西西向脆-韧性剪切带控制,成矿元素组合主要为Au-Ag。矿石中含有大量显微自然金、银金矿,明金可见。成矿流体主要为变质流体。由造山作用引起的强烈构造运动为成矿流体提供了运移通道,为矿质沉淀提供了有利的场所。虽然一些造山型金矿床与中酸性岩体相邻,但矿化与岩浆活动不具直接的成因关系。西秦岭地区卡林型金矿床主要产于轻微变质的寒武系至三叠系沉积岩中,明显受地层、岩性和构造控制。金矿床中的金以超显微金和存在于含砷黄铁矿与毒砂晶格中的固溶体金为主。成矿元素组合为Au-As-Hg-Sb-Ba。成矿流体由早期形成的地层水被后期大气降水补给活化形成,也有部分岩浆水或变质水的加入。在伸展背景下大气降水通过循环演化形成了较浅层次的流体系统,导致Au等成矿元素发生沉淀而形成浸染状矿石。西秦岭地区类卡林型金矿床主要产于浅变质沉积岩建造中,受脆-韧性剪切带的控制,并形成于花岗岩岩体附近。与造山型、卡林型金矿床最大的不同之处在于,类卡林型金矿床的形成与同时期的岩浆活动有密切的成因关系。矿石中存在显微自然金,载金矿物主要为黄铁矿、含砷黄铁矿和碲化物。成矿热液主要是岩浆水与变质水、建造水的混合流体。与造山型金矿床类似,流体不混溶导致类卡林型金矿床的形成。

Western Qinling is one of the most important gold producing regions in China.Except for a few skarn-type gold deposits,almost all gold deposits are classified as orogenic,Carlin-type or Carlin-like gold deposits.Mesozoic granitoids in the region emplaced during the Middle-Late Triassic while gold deposits formed during the Late Triassic-both are the product of Qinling Orogenesis caused by the final collision between the North and South China Blocks.Orogenic gold deposits are confined to NWW-trending brittleductile shear zones in Devonian and Carboniferous greenschist-facies metasedimentary rocks that were highly-deformed and regionally-metamorphosed.The deposits show a close Au-Ag association.Ores contain mainly microscopic gold,electrum and visible gold,along with pyrite.The ore-forming fluids are mainly metamorphic fluids.Intensive tectonic movements caused by orogenesis create fluid-migrating channels as precipitation sites.Although some orogenic gold deposits occur adjacent to granitoids,mineralization is not synchronous with magmatism.The Carlin-type deposits are mostly hosted in slightly metamorphosed Cambrian to Triassic sedimentary rocks,showing stratigraphically,lithologically and structurally controlled features.The deposits mainly contain submicroscopic and solid solution gold in arsenian pyrite and arsenopyrite,with characteristic ore-forming element of Au-As-Hg-Sb-Ba.The ore-forming fluids are earlystocked formation water and later-recharged meteoric water,and partially from the magmatic water or metamorphic fluids.Meteoric water apparently evolved in ore-forming fluids by circulation,indicating the extensional setting;it led to the deposition of Au and other elements in cool reactive permeable rocks at shallow levels to form the disseminated ores.Carlin-like gold deposits mainly formed from shallow sedimentary rocks through metamorphism.They are controlled by brittle-ductile shear zone and occur adjacent to granitoid plutons.The most important characteristics of Carlin-like gold deposits,differing from orogenic and Carlin-type gold deposits,is their genetic relationship with synchronous magmatism.Gold occurs as microscopic gold.Pyrite,arsenian pyrite and telluride can be recognized as gold-bearing minerals.The ore-forming fluids are mainly magmatic water mixed with metamorphic and/or formation water.Similar to orogenic gold deposits,fluid immiscibility led to the formation of Carlin-like gold deposits.