内蒙古太平川铜钼矿成矿斑岩时代、地球化学及地质意义

Geochronology and geochemistry of the Taipingchuan copper-molybdenum deposit in Inner Mongolia,and its geological significances

内蒙古太平川Cu-Mo矿床位于得尔布干成矿带北段额尔古纳地区,为新近发现的斑岩型矿床。矿区内发育环带状热液蚀变,由内向外主要为硅化-绢云母化和泥化。热液蚀变围绕花岗闪长斑岩分布,Cu-Mo矿化主要受控于硅化-绢云母化蚀变,主要分布在花岗闪长斑岩中。本文获得成矿斑岩岩浆锆石U-Pb谐和年龄为202±5.7Ma,指示该矿床可能形成于晚三叠世。同时在样品中也发现继承锆石(784Ma),表明该地区可能存在晚元古基底。成矿斑岩的元素地球化学数据表明,主量元素SiO2(65.86%～68.84%)>56%、Al2O3(15.18%～16.28%)>15%、MgO(0.84%～1.06%)<3%、Na2O/K2O>1;微量元素亏损重稀土,Sr(471×10-6～513×10-6)>400×10-6、Y(15.0×10-6～17.9×10-6)<18×10-6、Yb(1.27×10-6～1.81×10-6)<1.9×10-6,表明该花岗闪长斑岩具有明显的埃达克质岩石的地球化学特征。同时成矿斑岩具有相对高的SiO2、Yb含量和Th/Nb、Ce/Nb比值,而相对低的Al2O3、TiO2、MgO、Sr、Th含量和Th/Ce比值,这些特征与源于俯冲板片的埃达克岩相似。然而成矿斑岩(87Sr/86Sr)i(0.70943～0.71019)较大,εNd(t)为-3.4～-3.9,我们推测额尔古纳地块在岩浆上升过程中贡献了部分物质。结合区域构造演化,我们认为该矿区成矿斑岩岩浆形成于俯冲洋壳的部分熔融,矿床形成背景为早中生代蒙古-鄂霍茨克洋向额尔古纳地块俯冲的陆缘弧环境。

Recently discovered Taipingchuan porphyry Cu-Mo deposit is located in the north of Derbugan metallogenic belt,Ergun city of Inner Mongolia.Concentric rings of hydrothermal alteration and Cu-Mo mineralization occurred surrounding the granodiorite porphyry.Two hydrothermal alteration zones from the host porphyry to wall rocks have been recognized in Taipingchuan mine:A silicified-sericite zone and an argillic alteration zone.The Cu-Mo mineralization is clearly controlled by the silicified-sericite alteration.By using the zircon U-Pb dating methods a precise age data of the host porphyry is obtained.The dating data show that the age of host porphyry is 202 ± 5.7Ma.The inherited zircon(784Ma) has also been found in the host porphyry,which indicates that a Proterozoic block was probably located in the study area.Geochemically,the host porphyry of Wunugetushan possesses some geochemical characteristics of adakite(i.e.,high SiO2,Al2O3,Na2O and Sr contents,as well as La/Yb ratios,low MgO,Y and HREE contents).Some geochemical characteristics of the host porphyry,including relatively high SiO2,Yb contents and Th/Nb,Ce/Nb ratios and relatively low Al2O3,TiO2,MgO,Sr,Th contents and Th/Ce ratio indicate that the porphyry derived from partial melting of the subducted slab.The Sr-Nd isotopic composition of the porphyry displays an initial(87 Sr/86 Sr) i ratio of 0.70943 to 0.71019,a negative εNd(t) values of -3.4 to -3.9.The Sr-Nd isotopic data indicates that Proterozoic block had some contribution to the magma sources of the host porphyry.Combining the regional geology,we infer that the subduction of Mongol-Okhotsk ocean slab under the Ergun block led to the host porphyry forming,and Taipingchuan Cu-Mo deposit was related to a continental arc environment.