小兴安岭西北部与永新金矿有关岩浆岩的年代学和地球化学及成矿构造环境

摘要

The Yongxin gold deposit is a large-scale fracture-controlled alteration type deposit found in Xiao Hinggan Mountains in recent years.The dioritic porphyry intersected with the orebodies and intruded into the hanging wall volcanic rocks of the Longjiang Formation.The LA-ICP-MS U-Pb zircon dating of the andesite and dioritic porphyry yield concordant ages of (113.7 ± 1.8) Ma and (114.8 ± 1.9) Ma,respectively,implying that they formed at the Early Cretaceous.The geochemical data of the andesite and dioritic porphyry show large variations in Na and K contents,intermediate concentrations of Ca and Mg and enrichment in A1 and Fe.From the volcanic rocks to the dyke,the petrochemical features indicate a transition from calc-alkaline to alkaline.They display uniform REE distribution patterns with ores,but the ores exhibit relatively lower REE concentration.The trace elements of the samples generally enrich in Zr,Hf,Nd and U,and deplete in Nb,Ta,Th,P and Ti.The samples are plotted into the field of classic island arc rocks in Sr/Y vs.Y and (La/Yb) N vs.(Yb) N discrimination diagrams,and the field of volcanic arc granite in Nb vs.Y and Ta vs.Yb diagrams,and the crust-mantle granite field in the (La/Yb)N VS.δEu diagram.Combined with the S,Pb and Fe isotopes of pyrite,spatial and temporal distribution of regional gold deposits and geochemical features of rocks and ores,we suggest that the formation of the Yongxin gold deposit are genetically related to the volcanic-magmatism associated with upwelling of mantle materials trigged by the rollback of subducted paleo-Pacific plate in the Early Cretaceous.%永新金矿是近年发现的大型蚀变岩型脉状金矿床,闪长玢岩脉等与金矿脉相互穿切并侵入于上盘龙江组火山岩中.锆石LA-ICP-MS U-Pb测年显示,龙江组安山岩结晶年龄为(113.7±1.8)Ma,闪长玢岩结晶年龄为(114.8±1.9)Ma,均形成于早白垩世晚期.二者钠钾含量变化相对较大,相对富铝富铁、钙镁含量中等,从火山岩到脉岩表现为钙碱性向碱性系列过渡特征.火山岩、脉岩与矿石的稀土元素配分曲线有很好的一致性,矿石稀土含量低于岩石.岩石微量元素总体显示Zr、Hf、Nd、U等明显富集,而Nb、Ta、Th、P、Ti等明显亏损.在Sr/Y-Y和(La/Yb)N-(Yb)N图解中,所有样品均落入经典岛弧岩石范围;在Nb-Y、Ta-Yb的构造环境判别图中均处于火山弧岩浆岩区,在(La/Yb)N-δEu变异图上落在壳-幔型岩浆范围.结合矿床稳定同位素及区域金矿时空分布规律等综合特征,认为永新金矿的形成与早白垩世古太平洋板块俯冲所引发的(火山)岩浆活动有关.