滇西北衙金矿含矿斑岩H2O的来源及富集机制

One of the key factors that control the formation of porphyry type deposits is the enrichment of water (H2O > 4 wt. %) in the ore-bearing porphyries. Under magmatic conditions, the adding of H2O can effectively decrease the melting point of magma, thus enhancing the melting of Cu-, Au-bearing sulfides or leaching of Cu and Au from wall rocks. In addition, after the emplacement of water-enriched magma, saturation of H2O will soon be achieved and exsolution of fluids will take place in a large scale, during which metals (i.e., Cu, Au, Mo) will preferentially fractionate into fluids.For such deposits which are formed in the arc environment, dehydration of the subducting oceanic crusts contributes the majority of water. However, the origination of H2O in the ore-bearing porphyries emplaced in the post-collision environment remains to be unclear. Key issues regarding whether H2O comes from the mixing of contemporaneous, spacially coexisting mafic magma, or the fractional crystallization of ore-bearing magma, still need to be further investigated. Therefore, in this study, we choose Beiya Porphyry Au deposits, which is widely thought to be emplaced after the collision of Asia and India continents, as a typical case to throw light on the origination and enrichment of H2O in ore-bearing porphyries. Here we plan to systematically study the petrological and geochemical characteristics of the porphyries and the mafic enclaves. The project will focus on the the composition of H2O-enriched minerals of porphyries and enclaves, combined with whole-rock geochemical analyses to evaluate the extent of magma mixing and the enrichment of H2O, in order to help illustrate the H2O origination and enriching processes in the continental scale of collision-induced porphyry emplacement and ore generation.

含矿斑岩富水（H2O＞4 wt.%）是形成斑岩型矿床的关键因素。岩浆条件下，H2O的加入可以降低岩浆的熔点，促进源区含铜、金硫化物重熔或萃取岩石中铜、金成矿元素；并且含H2O高的含矿岩浆就位后，易于达到H2O饱和，大量流体的出溶使得金属元素（如Cu，Au，Mo）优先向流体中分配。弧环境下形成的斑岩矿床，俯冲洋壳脱水贡献了大量的H2O。但在后碰撞环境下形成的斑岩矿床含矿斑岩H2O的来源尚存争议，成矿所需的H2O是来自矿区内同时代、共空间产出的镁铁质岩的混合注入，还是含矿岩浆自身的分异结晶导致富H2O，目前尚未得到回答。为此，本项目选择印亚大陆碰撞后形成的北衙金矿为研究对象，对这一该区域内探明资源量最大的金矿中的含矿斑岩及镁铁质包体开展研究，以揭示其含矿斑岩H2O的来源及富集机制。为实现目标，项目拟通过矿区斑岩体及包体中矿物的组分分析，结合全岩地化分析估算岩浆混合作用和结晶分异作用中H2O的富

陆内碰撞背景下斑岩矿床含矿斑岩H2O的来源与富集机一直以来存在争议。本项目通过对金沙江-哀牢山造山带中的北衙超大型斑岩-矽卡岩金多金属矿床中斑岩体和微粒包体（MMEs，包括辉长岩包体和闪长岩包体）的研究，为斑岩矿床含矿岩浆形成过程中H2O的来源及富集机制提供了证据。锆石LA-ICPMS U–Pb年龄显示了矿区始新世酸性斑岩和同期MME包体的形成（~36Ma）。MME包体具有球状的外形，针柱状的磷灰石，以及斜长石的震荡环带，说明MME为多期镁铁质岩浆注入并与酸性主岩浆混合形成的球状包体。其中辉长岩包体具有高MgO(5.6 wt.%)、高TiO2 (1.7 wt.%)的特点，富集大离子亲石元素，亏损高场强元素， Nb、Ta、Ti呈现负异常，表明辉长岩包体与滇西同期形成的钾质镁铁质岩石类似，都起源于板片流体改造的交代地幔。闪长岩包体具有钾质及埃达克岩的特点，含有较高的MgO (4.4–4.9 wt.%)， K2O(6.4–7.4 wt.%)和微量元素 (Ni: 115–142 ppm； Cr: 214–291 ppm)，以及较高的Sr/Y 比值。闪长岩包体εHf (t)为-1.9-+3.6，与斑岩体类似。岩相学、元素地球化学和同位素研究表明北衙闪长岩包体的形成与分别起源于交代地幔和下地壳的钾质镁铁质熔体和埃达克的熔体混合有关。而北衙斑岩体角闪石斑晶发育、轻稀土富集而重稀土亏损、具有高Sr（>300×10-6）、低Y（<10×10-6）和高（La/Yb）N(6.19~26.8）的地球化学特征，暗示在岩浆结晶早期有角闪石斑晶的晶出，角闪石矿物组分计算表明岩浆熔体中水。北衙斑岩体中的MME包体起源于含水镁铁质岩浆，较高的岩浆水含量可能与被元古代俯冲板块流体交代的富集地幔有关，岩浆混合作用的发生导致了含矿的斑岩水含量的升高。这种具有高水含量的岩浆在源区岩石部分熔融的过程中，水的加入可以降低其熔点，促进源区含Cu、Au硫化物重熔，或萃取岩石中Cu、Au成矿元素，或聚集岩浆中分散分布的金属元素，形成富金属、富水的岩浆。