Early Cretaceous magmatism and associated polymetallic mineralization in South China: the Tiantang example

The Tiantang Cu–Pb–Zn polymetallic deposit in western Guangdong, South China, is hosted in the contact zone between the monzogranite porphyry and limestone of the Devonian Tianziling Formation. Orebodies occur in the skarn and skarnized marble as bedded, lenses, and irregular shapes. In this study, we performed LA-ICP-MS zircon U–Pb dating, zircon trace elements, and Hf isotopic analyses on the Tiantang monzogranite porphyry closely related to Cu–Pb–Zn mineralization. Twenty-two zircons from the sample yield excellent concordia results with a weighted mean ²⁰⁶Pb/²³⁸U age of 104.5 ± 0.7 Ma, which shows that the emplacement of the monzogranite porphyry in the Tiantang deposit occurred in the Early Cretaceous. The zircon U–Pb age is largely consistent with the sulphide Rb–Sr isochron ages, indicating that both the intrusion and Cu–Pb–Zn mineralization were formed during the Early Cretaceous in South China. The <i>ε</i>_Hf(<i>t</i>) values of three inherited zircons from the monzogranite porphyry are 13.1, 11.9, and 12.9, respectively, and the two-stage Hf model ages are 1096 Ma, 1087 Ma, and 1055 Ma, respectively. Except for the three inherited zircons, all <i>ε</i>_Hf(<i>t</i>) values of zircons are negative and have a range of −7.6 to −3.4, with the two-stage model ages (T_DM2) of 1380–1643 Ma, which indicates the rock-forming materials were mainly derived from the partial melting of Mesoproterozoic to Neoproterozoic crust rocks, and probably included some Neoproterozoic arc-related volcanic-sedimentary materials. In this study, the monzogranite porphyry from the Tiantang deposit has calculated Ce⁴⁺/Ce³⁺ ratios of zircon ranging from 91 to 359, indicative of a more oxidized signature and significant prospecting potential for ore-related magmatism. Based on ore deposit geology, isotope geochemistry, and geochronology of the Tiantang Cu–Pb–Zn deposit and regional geodynamic evolution, the formation of Early Cretaceous magmatism and associated polymetallic mineralization in South China is believed to be related to large-scale continental extension and subsequent upwelling of the asthenosphere.

华南粤西天堂铜铅锌多金属矿床（Tiantang Cu–Pb–Zn polymetallic deposit）赋存于二长花岗斑岩（monzogranite porphyry）与泥盆系天子陵组灰岩的接触带中。矿体赋存于矽卡岩（skarn）及矽卡岩化大理岩中，呈层状、透镜状及不规则状产出。本研究对与铜铅锌成矿密切相关的天堂二长花岗斑岩开展了LA-ICP-MS锆石U-Pb定年（LA-ICP-MS zircon U–Pb dating）、锆石微量元素（zircon trace elements）及铪同位素分析。样品中的22颗锆石获得了极佳的谐和年龄结果，其加权平均²⁰⁶Pb/²³⁸U年龄为104.5 ± 0.7 Ma，表明天堂矿床中二长花岗斑岩的侵位时代为早白垩世。该锆石U-Pb年龄与硫化物铷锶等时线年龄基本一致，指示该岩体侵位与铜铅锌成矿均形成于华南早白垩世。该二长花岗斑岩中的3颗继承锆石的ε_Hf(t)值分别为13.1、11.9和12.9，其二阶段铪模式年龄分别为1096 Ma、1087 Ma和1055 Ma。除这3颗继承锆石外，其余锆石的ε_Hf(t)值均为负值，范围为-7.6至-3.4，其二阶段模式年龄（T_DM2）为1380~1643 Ma，表明成岩物质主要来源于中元古代至新元古代地壳岩石的部分熔融，并可能混入了部分新元古代弧相关火山沉积物质。本研究中天堂矿床二长花岗斑岩的锆石Ce⁴⁺/Ce³⁺比值计算值为91~359，指示其具有较强的氧化特征，且与成矿相关的岩浆作用具有良好的找矿潜力。综合天堂铜铅锌多金属矿床的矿床地质学、同位素地球化学、地质年代学数据及区域动力学演化背景，认为华南早白垩世岩浆作用及伴生多金属成矿作用与大规模大陆伸展及随后软流圈上涌密切相关。