紫金山含矿与不含矿岩体的氧逸度和H2O-F-Cl挥发分含量对比研究

Porphyry copper deposits are typically associated with volatile-rich and oxidized magmas, indicating a primary role for volatiles (water, F and Cl) and oxidation in driving mineralization. However, most arc magmas are volatile-rich (e.g. hydrous) and oxidized but do not produce economic deposits. It is not clear whether there is a clear contrast in oxygen fugacities and volatile contents of contemporaneous ore-bearing and ore-barren porphyry magmas in the same region. To address this issue, this project will take the famous Zijinshan Cu-Au ore field as a case study. Apatites and plagioclases from the ore-bearing and ore-barren intrusions will be analyzed by electron microprobe and laser ablation-inductively coupled plasma mass spectrometry. Mn content of apatite is a proxy of redox condition for the host magma. The SO3, F and Cl contents of apatite is important for understanding the formation of metal-rich fluid phase. Chemical composition of plagioclase is link to water condition of host magma. The results present in this project will be used to estimate oxygen fugacities, SO3, F, Cl and H2O contents of the Zijingshan magmas. The role of oxygen fugacity and volatile in driving mineralization will be revealed by a comparative study of late Mesozoic ore-bearing vs ore-barren magmas. This project will provide not only constraints on ore-forming processes of the Zijinshan porphyry Cu deposit, but also an example of mineral study of oxygen fugacity and H2O-F-Cl contents of magmatic rock.

对比研究含矿与不含矿岩浆的氧逸度和挥发分含量有助于更好的揭示斑岩矿床的成矿机理。磷灰石和斜长石是中酸性岩浆岩的常见组成矿物，磷灰石Mn和SO3含量对岩浆氧化还原条件有良好的指示，F、Cl含量记录了岩浆挥发分组成，斜长石的多项地球化学指标是岩浆水含量的探针。项目拟重点以我国著名的紫金山浅成热液-斑岩型铜多金属矿田为对象，在罗卜岭斑岩铜（钼）矿床、东南、西南斑岩矿化段及周边相关岩体详细的地质学、矿床地质、以及岩（矿）相学研究基础上，通过电子探针和LA-ICP-MS对岩浆岩中磷灰石和斜长石的化学成分进行原位分析，对比研究含矿与不含矿斑岩的岩浆氧逸度和F、Cl、水含量的异同，揭示斑岩系统中岩浆氧逸度、H2O含量和岩浆热液之间的内在关系及其对斑岩体成矿过程的控制作用。本项目不仅能为紫金山斑岩铜矿的成矿机理提供约束，还将为矿物化学方法系统研究斑岩矿床岩浆氧逸度和挥发分（H2O-F-Cl）含量提供实例。

岩浆的氧逸度和挥发分含量研究是揭示斑岩矿床成矿机理的关键之一。为了查明紫金山矿田早白垩世含矿的氧逸度及挥发分组成变化规律，揭示成矿与不成矿岩浆的差异性，项目对紫金山浅成热液-斑岩型铜多金属矿区的含矿和不含矿岩浆岩开展了全岩和矿物的主微量分析，对比研究了两类岩浆中磷灰石的变价元素、稀土元素和挥发分（H2O、F、Cl）含量组成，探讨了斜长石Al指数及其与全岩Sr/Y和Cu 富集程度之间的关系，利用湿度计和氧逸度计定量估算了成矿岩浆的含水性和氧逸度，约束了紫金山斑岩铜矿的成矿机理，获得了以下主要结论：.1、含矿岩浆具有高氧逸度和富F、Cl挥发分的组成：含矿斑岩体的岩浆磷灰石低Mn和锆石高Ce4+/ Ce3+的组成，表明成矿母岩浆具有较高氧逸度；岩浆磷灰石及其母岩浆富集F、Cl挥发分。阐述了高氧逸度和富集F、Cl的岩浆对于斑岩性Cu-(Au-Mo)的成矿的控制作用。.2、证实了含矿与不含矿岩浆含水量的差异性：含矿岩体与不含矿岩体中斜长石Al 指数（Al*）和 Ca/Na 比值差异较大，斜长石（-岩浆）湿度计估算结果显示早白垩世成矿岩浆是富水的，而不成矿的侏罗纪岩浆相对贫水，说明高水含量岩浆有利于斑岩矿体成矿。.3、总结归纳了紫金山斑岩型Cu-(Au-Mo)矿床的岩浆起源与成矿过程：古太平洋板块俯冲和后撤作用的诱发交代岩石圈地幔部分熔体形成富水、F、Cl的岩浆并上升底侵至壳幔边界，诱发加厚下地壳的部分熔融产生富F、Cl、高氧逸度、含水的埃达克质岩浆，这种性质的岩浆有利于成矿元素的迁移和富集。.研究成果表明矿物化学是约束岩浆氧逸度和挥发分含量的重要有效方法，深化了对于紫金山斑岩型矿床岩浆起源与成矿过程的认识。