胶东三山岛金矿床巨斑花岗岩的形成时代、成因及对金成矿的启示

Formation Age,Petrogenesis,and Implications for Gold Mineralization of Giant Porphyritic Granite in the Sanshandao Gold Deposit in Jiaodong

胶东地区三山岛金矿床的主要赋矿围岩是中生代玲珑花岗岩和郭家岭花岗岩。通过对三山岛巨斑花岗岩进行岩相学、岩石地球化学和锆石U-Pb同位素年代学等研究,分析其地球化学类型、形成时代、岩浆源区及成因,讨论了成岩与成矿的关系。岩石地球化学特征表明:三山岛巨斑花岗岩的SiO_(2)、Al_(2)O_(3)和全碱(Na_(2)O+K_(2)O)含量较高;铝饱和指数A/CNK为1.34,属过铝质—钙碱性系列岩石;稀土元素含量较低,轻稀土相对富集,重稀土相对亏损;岩石富集Rb、Sr、和Ba等大离子亲石元素,亏损Ta、Nb、P和Ti等高场强元素;岩石地球化学特征与富钠花岗岩和年轻的TTG(<3 Ga)相似。巨斑花岗岩的锆石U-Pb加权平均年龄为(127.05±0.41)Ma,属于早白垩世郭家岭期花岗岩。岩石中金丰度值较早前寒武纪变质岩系明显降低。综合研究认为,巨斑花岗岩是下地壳酸性岩浆与少量幔源基性岩浆混合作用的结果,形成于太平洋板块俯冲和华北克拉通破坏背景下的伸展构造环境,岩体中的钾长石巨晶是岩浆期后钾化作用的结果;具有较高背景金丰度的早前寒武纪变质基底岩石部分熔融过程中,地球化学元素重新调整,金质被活化、迁移,形成富金流体库,并产生贫金花岗岩;花岗岩快速侵位和地壳强烈隆升产生的拆离断层系统,以及钾化作用导致岩石体积膨胀而破裂,为成矿流体聚集和矿化富集提供了良好的物理圈闭空间。

The primary host rocks of the Sanshandao gold deposit in the Jiaodong region are the Mesozoic Linglong granite and Guojialing granite.Through the application of petrographic analysis,geochemical assays,and zircon U-Pb geochronology,the geochemical classifications,formation ages,magma source regions,and petrogenesis of the granite were investigated.Additionally,the relationship between diagenesis and mineralization was examined.The geochemical characteristics of the rocks reveal a relatively high content ofSiO_(2),Al_(2)O_(3),and total alkali(Na_(2)O+K_(2)O).The aluminum saturation index(A/CNK)is 1.34,classifying these rocks within the Peraluminous-Calcium alkaline series.The concentration of rare earth elements(REE)is relatively low,with a relative enrichment of light rare earth elements(LREE)and a deficiency in heavy rare earth elements(HREE).The rocks are enriched in large-ion lithophile elements(LILE)such as Rb,Sr,and Ba,while high field strength elements(HSFE)such as Ta,Nb,P,and Ti are notably deficient.The geochemical characteristics of the Sanshandao giant porphyritic granite are comparable to those of sodium-rich granites and young tonalite-trondhjemite-granodiorite(TTG)complexes(younger than 3 Ga).The zircon U-Pb weighted average age of the giant porphyritic granite is determined to be(127.05±0.41)Ma,corresponding to the early Cretaceous Guojialing period granite.The gold concentration in the giant porphyritic granite is markedly lower compared to that in early Precambrian metamorphic rocks.Integrating these findings with prior research,it is inferred that the giant porphyritic granite predominantly comprises lower crustal acidic magma,with a minor contribution from mantle-derived basic magma,formed through magmatic mixing.The formation of the rock occurred within an extensional tectonic environment characterized by the subduction of the Pacific Plate and the destruction of the North China Craton.The presence of large feldspar crystals within the rock can be attributed to post-magmatic potassium mineralization.Additionally,Early Precambrian metamorphic rocks,which exhibit relatively high background values of gold abundance,undergo partial melting.During this process,geochemical elements are redistributed,leading to the activation and migration of gold,which results in the formation of a gold-rich fluid reservoir and the subsequent production of gold-poor granite.The rapid intrusion of granite,coupled with the detachment fault system induced by significant crustal uplift and potassium mineralization,collectively contributed to the initial expansion and subsequent rupture of rock volume.This process created an optimal physical trap space conducive to the accumulation and enrichment of ore-forming fluids.