辽宁树基沟铜锌矿床黑云斜长角闪片麻岩锆石U-Pb年龄、Hf同位素组成及其地质意义

Zircon U-Pb Age and Hf Isotopic Composition of the Biotite Plagioclase Hornblende Gneiss of Shujigou Cu-Zn Deposit,Liaoning Province

树基沟铜锌矿床是太古宙花岗-绿岩带中与火山岩有关的块状硫化物矿床(VMS)。本次对赋矿围岩黑云斜长角闪片麻岩开展了锆石LA-ICP-MS U-Pb定年,主量元素、微量元素和Hf同位素分析。锆石U-Pb定年结果表明,黑云斜长角闪片麻岩原岩结晶年龄为(2 549.4±5.6)Ma,可近似代表树基沟铜锌矿床成矿时代。地球化学研究表明,岩石富集大离子亲石元素(LILE)、亏损高场强元素(HFSE)、轻稀土富集、重稀土亏损,为一套岛弧环境的火山沉积岩。黑云斜长角闪片麻岩εHf(t)值为5.0~8.7,单阶段模式年龄TDM 1值为2 527~2 649 Ma,指示其源区主要亏损地幔物质,树基沟地区可能存在新太古代地壳增生事件。结合前人研究成果,认为树基沟铜锌矿床成矿于板块俯冲环境,地幔部分熔融形成富含贱金属岩浆向上运移、汇聚、喷发、固结成岩,后经海水淋滤下渗成矿元素富集二次喷发成矿,并经后期鞍山运动变质变形作用的产物。

Located in the Archean granite-greenstone belt,the Shujigou copper-zinc deposit belongs to the volcanic-associated massive sulfide deposit(VMS).In this study,the authors carried out a comprehensive study including zircon U Pb dating,petro-geochemical investigation,and Hf isotope analysis of the biotite plagioclase hornblende gneiss from the Shujigou copper-zinc deposit output.The LA ICP MS zircon U Pb chronology indicated that the biotite plagioclase hornblende gneiss was formed at ca.(2 549.4±5.6)Ma,representing approximately the ore-forming age.Geochemically,the biotite plagioclase gneiss is characterized by its LREE enrichment,HREE depletion,and also the LILEs enrichment and HFSEs depletion,and the original rocks were of a set of island arc environment volcanic-sedimentary assemblage.According to the zircon Hf isotope analysis,theεHf(t)values of biotite plagioclase gneiss are 5.0 8.7,and the single stage model age(T DM1)varies from 2 527 Ma to 2 649 Ma.These characteristics suggest that the primary magma was derived from a depleted mantle,and the Shujigou area occurred in crustal growing during the Late Neoarchean.Combined with the geochemical characteristics and the evolution of the host rock,the mineralization involved in partial melting of the mantle and forming of a rich base metal magma,and went through plate subduction,magma migration,convergence and eruption til the formation of base metals igneous rocks in the study area.The percolation of seawater down into these igneous rocks and leaching the metals from them into feeder zones formed the VMS deposit system.The fluids within this feeder system were erupted at the seafloor;these magmas focused along the faults,and the minerals were precipitated at the contact zone between these metal-laden fluids and the bottom cold seawater,and subsequently underwent the metamorphism and deformation.