Sulfide Rb-Sr and Zircon U-Pb Dating of the ~825 Ma Liujiaping Cu-Zn deposit in the Northwestern Yangtze Block (China) and its tectonic implications

The Neoproterozoic Liujiaping Cu-Zn deposit is a typical VMS (volcanic massive sulfide) deposit in the northwestern Yangtze Block, which is located at the convergent region of different blocks in China. The tectonic setting could provide some implications for the Neoproterozoic tectonic evolution of the Yangtze Block. Both the metallogenic chronology and isotope geochemistry of the Liujiaping deposit are tested. Sulfides and zircons from this deposit were analysed to determine its mineralization age. The sulfide Rb-Sr isotope dating of ores yields an age of 821.0 ± 5.0 Ma (MSWD = 0.2). The zircon U-Pb SIMS dating of volcanics yields as 825.5 ± 5.1 Ma (MSWD = 0.3). These dating results indicate that the metallogenesis is synchronous with the volcanic eruption. The Rb contents of the sulfides range from 0.0714 × 10^–6 to 0.7986 × 10^–6, the Sr contents range from 0.6002 × 10^–6 to 15.49 × 10^–6, and the initial Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr)_i range from 0.70784 to 0.70911, with an average of 0.70815. The Pb isotopic compositions of the ores and volcanics are (²⁰⁶Pb/²⁰⁴Pb) = 16.751–18.051, (²⁰⁷Pb/²⁰⁴Pb) = 15.363–15.597 and (²⁰⁸Pb/²⁰⁴Pb) = 36.885–38.441. These isotopic geochemical characteristics indicate that the ore-forming materials of the Liujiaping deposit might originate from the mixture of continental crust and mantle. Other Meso-Neoproterozoic VMS deposits along the northwestern Yangtze Block are also summarized. It is concluded that the Liujiaping volcanics and other VMS deposits were formed by the partial melting of lower crust during the subduction of oceanic crust, and the northwestern margin of the Yangtze Block was located in an arc setting at ~820 Ma rather than intra-continental rift.

新元古代刘家坪铜锌矿床是扬子地块西北部典型的VMS（火山成因块状硫化物，Volcanic Massive Sulfide）矿床，该区域位于中国不同地块的汇聚地带。其形成的构造背景对揭示扬子地块新元古代构造演化具有重要启示意义。本次研究对刘家坪矿床的成矿年代学与同位素地球化学特征开展了测试分析：通过对矿床中硫化物与锆石的分析以限定其成矿年龄。矿石硫化物的铷-锶同位素定年结果为821.0±5.0 Ma（MSWD（平均标准加权偏差，Mean Square of Weighted Deviates）=0.2）；火山岩的SIMS（二次离子质谱，Secondary Ion Mass Spectrometry）锆石U-Pb定年结果为825.5±5.1 Ma（MSWD=0.3）。上述定年结果表明，成矿作用与火山喷发活动同期发生。硫化物的铷（Rb）含量范围为0.0714×10^–6至0.7986×10^–6，锶（Sr）含量范围为0.6002×10^–6至15.49×10^–6，初始锶同位素比值(⁸⁷Sr/⁸⁶Sr)i介于0.70784至0.70911之间，平均值为0.70815。矿石与火山岩的铅同位素组成分别为：²⁰⁶Pb/²⁰⁴Pb=16.751~18.051，²⁰⁷Pb/²⁰⁴Pb=15.363~15.597，²⁰⁸Pb/²⁰⁴Pb=36.885~38.441。上述同位素地球化学特征表明，刘家坪矿床的成矿物质可能来源于大陆地壳与地幔的混合作用。本次研究同时总结了扬子地块西北部其他中元古代-新元古代VMS矿床的相关资料，最终得出如下认识：刘家坪火山岩及其他VMS矿床形成于洋壳俯冲过程中下地壳的部分熔融；扬子地块西北缘在~820 Ma时期处于弧构造环境，而非陆内裂谷。