鄂东南鸡笼山矽卡岩型金矿床花岗闪长斑岩的成因:地球化学和锆石U-Pb年代学约束

Petrogenesis of granodiorite porphyry in Jilongshan skarn Au deposit of southeast Hubei Province:Geochemical and zircon U-Pb dating constraints

矽卡岩型金矿床是近二十年国际上新的一类矽卡岩型矿床,鸡笼山是鄂东南地区大型矽卡岩型金床之一,成矿与花岗闪长斑岩岩体密切相关,前人对该矿床及外围花岗闪长斑岩成因的研究相对较少。文章对鸡山和白果树2个岩体进行岩石学、地球化学和年代学研究,探讨其成因和形成时代。LA-ICP-MS锆石U-Pb测年果表明鸡笼山和白果树岩体的分别形成于(151.6±0.7)Ma和(142.4±0.7)Ma,即晚侏罗世—早白垩世,与鄂东地区花岗闪长斑岩的成岩年龄在误差范围内基本一致。鸡笼山和白果树花岗闪长斑岩岩体具有中等富集硅、铝、碱的特征,类似于丰山洞铜钼矿床的含矿岩体。它们具有富集大离子亲石元素Rb、Ba、Sr,亏损高场强元素Y、N Ti,并富集轻稀土元素及无负Eu异常的特征。研究表明,鸡笼山含矿岩体为准铝质高钾钙碱性I型花岗闪长斑岩可能是富集岩石圈地幔,经历了不同程度的部分熔融,同化混染了不同比例下地壳的产物。结合区域地质和年代的资料,显示鸡笼山花岗闪长斑岩岩体的形成是在晚侏罗世—早白垩世鄂东南地区岩石圈伸展的地球动力学背下岩浆活动-成岩事件的一部分。

Skarn Au deposits have become a new type o{ skarn deposits in the past twenty years. Jilongshan is a large typical skarn Au deposit in southeastern Hubei Province, whose mineralization and granodiorite porphyry are closely related to each other; nevertheless, the study of the genesis of granodiorite porphyry in the ]ilongshan Au deposit has been very insufficient. The authors studied petrology, geochemistry and geochronology of two small rock bodies, i.e., Jilongshan and Baiguoshu, and investigated their formation ages and petrogenesis. LA-ICP- MS zircon U-Pb age suggests that the rock-forming ages of Jilongshan and Baiguoshu are （151.6±0.7） Ma and（ 142.4±0.7） Ma respectively, belonging to the Late J urassic-Early Cretaceous period and being consistent with the rock-forming ages of the granodiorite porphyries in southeastern Hubei Province. Jilongshan and Baiguoshu granodiorite porphyries are characterized by moderate enrichment of silicon, aluminum and total alkali, similar to things of ore-bearing rocks in the Fengshandong Cu （Mo） deposit. They aim have the features of enrichment of large ion lithophile elements Rb, Ba, Sr and compatible elements K, depletion of Y, Nb, Ti, with no negative Sr anomaly. In addition, they are rich in light rare earth elements, with no obvious negative Eu anomalies and negative Ce anomalies. Researches suggest that the Jilongshan ore-bearing rock body is a metaluminous, high-K calc-alkaline type I granodiorite porphyry, and the marie magma which was formed by the enriched lithospheric mantle experienced different degrees of partial melting, thus belonging to the product of the lower crust. The re- gional geological and geochronological data suggest that the Jilongshan granodiorite porphyry belonged to a part of the magmatism-diagenesis event in the lithosphere extensional geodynamic background in southeast Hubei Province in the Late Jurassic-Early Cretaceous period.