北祁连西段小关山金矿岩石年代学、火山岩地球化学特征及其地质意义

PETROCHRONOLOGY AND GEOCHEMICAL CHARACTERISTICS OF XIAOGUANSHAN GOLD MINE IN THE WESTERN SECTION OF NORTH QILIAN AND ITS GEOLOGICAL SIGNIFICANCE

小关山金矿位于北祁连造山带中段,通过岩石年代学研究,测得小关山一带玄武岩、玄武质安山岩铷—锶年龄分别为(455.8±1.9)Ma、(453.5±2.6)Ma,代表了火山岩的成岩年龄,花岗闪长岩LA-ICP-MS锆石U-Pb年龄为(433±2)Ma,代表了岩体侵位年龄属早志留世,其成矿时代应晚于花岗闪长岩侵入年龄(433±2)Ma。地球化学特征表明分异性元素Pb、W、As、Sb、Au分布极不均匀,成矿概率极大,显示火山岩分异程度较高,属于玄武岩—玄武安山岩的范围,具有低铁、高铝特征,具岛弧和洋脊火山岩的特点。玄武岩稀土总量平均值为117.31×10^(-6),∑LREE/∑HREE比值1.8~5.07,∑Er-Lu=5.0×10^(-6)~15.0×10^(-6),∑Sm-Ho=18.0×10^(-6)~30.0×10^(-6),∑LaNd=55.0×10^(-6)~77.0×10^(-6),微量和稀土元素特征与弧后盆地玄武岩的特征相似,Ba/Th为11.3~40.7,表明岩浆过程中存在俯冲流体的加入,样品^(87)Sr/^(86)Sr的初始比值为0.7054047和0.705669,La/Yb(0.97~3.77)和Sm/Yb(0.97~2.16)比值表明小关山火山岩的形成与富集地幔高程度熔融作用有关。说明其形成在弧后和岛弧环境,受俯冲造山作用和碰撞造山作用的双向构造机制制约。岩体的侵位与含矿流体的运移过程中,含矿热源形成富集,石英细脉、蛋白石等的析出伴随着金属硫化物的带出,赋矿岩石明显发生硅化、黄铁矿、毒砂化,金元素的再次运移富集形成金矿体。

Xiaoguanshan gold mine is located in the middle part of the northern Qilian orogenic belt.It is a tectonic alteration rock type gold deposit,and the mining volcanic rocks are mainly base-medium base.According to the rock dating study,the LA-ICP-MS zircon U-Pb age is(433±2)Ma,which represents the age of rock mass invasion in early Silago,and the age of rubidium-strontium of basaltic and basalt andesite is(455.8±1.9)Ma and(453.5±2.6)Ma,which represents the diagenetic age of volcanic rocks,and the mineralization age should be later than that of granodiorite invasion age(433±2)Ma.The background geochemical parameters of Pb,W,As,Sb,Au have uneven distribution and great minerenic probability;the geochemical characteristics of rock show high degree of volcanic rock differentiation,which belongs to the range of basalt-basalt andesite,with low iron and high aluminum characteristics,showing the characteristics of island arc and ocean ridge volcanic rock.The average value of total rare earth in basalt was 117.31×10^(-6),LREE/HREE ratio ranges from 1.8 to 5.07,Er-Lu=5.0~15.0,Sm-Ho=18.0~30.0,La-Nd=55.0~77.0,belonging to the heavy rare earth enrichment type.The rock combination in the mining area has typical ophiolite characteristics,which indicates that its formation is formed after the arc and the island arc environment,and is restricted by the bidirectional structural mechanism of subduction orogeny and collision orogeny.In the process of rock mass penetration and the migration of ore-containing fluid,ore-bearing heat sources are formed and enriched.With the precipitation of quartz veins and opal,accompanied by the outflow of metal sulfide,there is obvious silicification,pyrite and solidification of ore-bearing rocks,and the migration of gold elements is enriched again to form gold ore body.