岩浆氧化状态、含水性、挥发份特征: 对西藏冈底斯朱诺斑岩型铜成矿的约束

Many models about the genesis of porphyry copper mineralization in the Gangdese collisional zone of southern Tibet have been advanced based on abundant elemental and isotopic analyses. However, some issues remain unclear including the source of magmatic water and contribution of ultrapotassic and high-Mg dioritic magmas to porphyry copper mineralization. The Zhunuo porphyry Cu deposit in the western part of the Gangdese copper belt is characterized by the occurrence of many Miocene igneous rocks including fertile high Sr/Y monzogranite and monzogranite porphyry, high-Mg dioritic rocks (enclaves and diorite porphyry), ultrapotassic rocks (lamprophyre), and barren low Sr/Y granite porphyry, and by the enriched or curst-like Nd-Hf isotopes of high Sr/Y rocks. These features are rarely present in the porphyry copper deposits in the eastern Gangdese, and thus the Zhunuo deposit offers a new window into the genesis of porphyry Cu mineralization in continental collision zones. This project is aimed at analyzing compositions of some minerals such as zircon, apatite, amphibole, and plagioclase by Electron microprobe analysis (EMPA) and LA-ICP-MS, and in-situ sulfur isotopes of sulfides, studying magmatic oxidation state, water content and volatile (e.g, S, F, Cl) and their evolution during magma mixing, and constraining the source of magmatic water and contribution of different magmas for porphyry copper mineralization. The results will not only be helpful for further understanding porphyry copper mineralization and evaluating the fertility of igneous rocks in the Gangdese belt but also provide new insights into improving porphyry copper metallogenic theory in collisional zones.

西藏冈底斯碰撞造山带中新世斑岩铜矿带已开展大量的元素和同位素地球化学研究并提出了多种成矿模型，但仍不能全面解释成矿岩浆水来源及超钾质和高镁闪长质岩浆对成矿的贡献。该矿带西段朱诺铜矿发育中新世含矿高Sr/Y二长花岗岩和二长花岗斑岩、高镁闪长质岩（包体和闪长玢岩）、超钾质岩（煌斑岩）、贫矿低Sr/Y花岗斑岩，且含矿岩石具有富集Nd-Hf同位素特征，这些特点明显不同于矿带东段其他铜矿，从而为解决上述关键科学问题提供了契机。项目开展朱诺中新世岩浆岩中锆石、磷灰石、角闪石、斜长石等矿物的电子探针和原位微区元素分析，结合金属硫化物原位硫同位素分析，探讨岩浆的氧化状态、含水性、挥发份（包括S、F、Cl）等特征及其在岩浆混合过程中的变化规律，进而约束成矿岩浆水来源以及各种岩浆对成矿的贡献。研究成果对深入理解冈底斯斑岩铜成矿作用、评价岩体含矿性具有重要意义，也可能为完善碰撞造山带斑岩铜成矿理论提供新的视角。

西藏冈底斯中新世后碰撞环境斑岩铜矿含矿岩浆的形成已提出多种模型,但仍不能全面解释成矿岩浆水的来源，以及幔源超钾质或高镁闪长质岩浆对成矿的具体贡献。项目以朱诺矿床进行典型解剖，构建了矿区岩浆岩年代学格架与成因,提出二长花岗岩（14.7±0.3 Ma）、二长花岗斑岩（14.5±0.2 Ma）及其所含暗色镁铁质包体(MMEs；14.9±0.3 Ma)，以及闪长玢岩（14.2±0.2 Ma）为成矿期产物，而煌斑岩（12.2±0.1 Ma）为成矿后产物。煌斑岩具有超钾质和钾质岩特征，前者源区为古老沉积物（特提斯洋壳和印度陆壳）交代的富集地幔，后者为超钾质煌斑岩岩浆混染了含矿斑岩；闪长玢岩和MMEs具有高镁闪长岩特征，岩浆源自地幔，经历了特提斯洋壳沉积物交代（印度物质的贡献相比超钾质岩浆少）；二长花岗岩及二长花岗斑岩具有高Sr/Y（> 40）特征，是俯冲改造的青藏下地壳部分熔融与高镁闪长质岩浆混合的产物。通过对二长花岗岩、二长花岗斑岩、闪长玢岩及MMEs样品中角闪石、斜长石和磷灰石等矿物的精确测定，进一步证实了超钾质岩浆/高镁闪长质岩浆与成矿斑岩岩浆发生混合，并理清了混合过程中岩浆温度、压力、氧逸度等条件的变化。高镁闪长质岩（闪长玢岩与MMEs）中磷灰石记录了初始熔体具有富Cl、富S特征，且记录了流体出溶过程（XF/XCl值逐渐变大，Cl含量逐渐降低，SO3含量波动大），暗示高镁闪长质岩浆在与二长花岗斑岩岩浆混合过程中向后者注水并可能提供S、Cl等挥发分，也可能直接向后者出溶的流体中注入S、Cl。岩浆混合程度很低的二长花岗岩中磷灰石记录了初始熔体具有富Cl、富S特征，而岩浆混合程度较高的二长花岗斑岩中磷灰石记录了初始熔体具有与二长花岗岩初始熔体类似的富Cl特征，但S的含量比后者更高。同时，二长花岗岩及斑岩中磷灰石记录到了流体出溶过程，Cl含量逐渐降低，但SO3含量波动大，这可能与高镁闪长质岩浆的多次注入有关。此外，本项研究也提出可通过岩相学特征、全岩Nd同位素和锆石Eu/Eu*、U/Yb值来识别朱诺成矿斑岩及贫矿斑岩的含矿性评价指标。