海沟金矿地质-地球化学特征及其成矿机制探讨

海沟金矿床产于海沟花岗岩体中,受NE-NNE向断裂控制,为大型贫硫化物石英脉型金矿床.微量元素研究表明成矿作用以充填作用为主,矿床主要指示元素为Au、Ag、Hg、Pb、Bi、Mo,由上至下的轴向分带序列为Ba-Hg-Ag-Bi-Au-As-Sb-Pb-Mo.同位素和流体地球化学资料表明成矿物源与色洛河群变质岩系和海沟花岗岩有关,成矿流体由岩浆水和古大气降水组成,流体属CO₂-NaCl-Na₂SO₄-H₂O型.在区域地质构造演化背景下,深大断裂的活动导致富金的色洛河群变质岩系发生深熔作用形成海沟花岗岩及富矿质和矿化剂的岩浆热液,同时也产生了以大气降水为主的含矿热液.在143 Ma左右,随区内应力场转变为张性,并在岩体和周边地层中形成大量NE-NNE向次级断裂系统,富矿质和矿化剂的岩浆热液和以大气降水为主的含矿热液的平衡被打破,导致两种流体向NE-NNE向次级断裂系统的扩容带迁移,混合作用、沸腾作用以及物化条件的改变导致金的沉淀,从而造就了海沟金矿床.

GEOLOGICAL AND GEOCHEMICAL CHARACTERISTICS AND THE ORE-FORMING MECHANISM OF HAIGOU GOLD DEPOSIT

Haigou gold deposit, occurring in Haigou granite body, is a large low-sulfide quartz-vein type of deposit, controlled by NE- to NNE-trending faults. Analysis on trace element geochemical characteristics of ores, altered rock and wall rocks shows that infilling mineralization is dominant in the ore-forming process. The main indicator elements are Au,Hg, Pb, Bi and Mo. The axial zonal sequence of the deposit, from upper to lower, is presentedAg, as Ba-Hg-Ag-Bi-Au-As-Sb-Pb-Mo. The data of isotope and fluid geochemistry show that the ore-forming elements are derived from the metamorphic rocks of Seluohe group and Haigou granite. The ore-forming fluid is composed of magmatic water and paleo-meteoric water, which belongs to CO2- NaC1- Na2SO4- n20 type. Under the background of regional geological evolution, the movement of lithospheric faults causes the anatexis of the metamorphic rocks of Seluohe group in the deep,forming Haigou granite and magmatic fluid rich in ore-forming elements and mineralizers, as well as the ore-forming fluid dominated by paleo-meteoric water. With the conversion of the stress field in the studied area to tension, and the forming of many secondary NE- to NNE-trending faults in Haigou granite body and its periphery at about 143 Ma, the equilibrium between magmatic fluid and paleo-meteoric water is disturbed, leading the migration of the two kinds of fluids to the dilation zones of the fault system. The mixture and ebullition of fluid and the change of physicochemical conditions result the deposition of gold from fluids, and form the Haigou gold deposit.