流体出溶结构解密岔路口巨型斑岩钼矿初始热液流体过程

Hydrothermal fluid exsolution play an essential role in the formation of porphyry deposit, which controls the fertility of aqueous fluids and scale of deposit. Although some researchers have identified some geological textures of fluid exsolution and discussed their contributions to mineralization, the sparseness of associated samples and the lack of effective research techniques hinder our understanding of mechanism of fluid exsolution and its key factors controlling the formation of giant porphyry deposit. Considering that there are abundant geological textures recording fluid exsolution in Chalukou giant porphyry Mo deposit in northern Great Xing’an Range, NE China, including quartz eye, interconnected microcavities, unidirectional solidification texture and earliest quartz veins, this application will choose the quartz in fluid exsolution textures as research targets to constrain the genesis of fluid exsolution textures and properties of exsolution fluid. Based on the multiple generations of quartz revealed by scanning electron microscope-cathodoluminescence and back-scattered electron, we will carry out in-situ trace elements and oxygen isotope analysis by LA-ICPMS and SIMS respectively. The spatial and temporal variations in trace elements, oxygen isotope and fluid inclusions in quartz with different CL zones can be used to trace the system changes from beginning of fluid exsolution to fluid release. Furthermore, this proposal tries to decipher the linking factors between fluid exsolution and formation of giant porphyry Mo deposit and provide more detail ore-forming theories about porphyry deposit.

流体出溶过程决定了流体含矿性和矿化规模，是斑岩成矿至关重要的一环。以往研究识别出一些流体出溶结构，对其成矿作用有了初步认识，但受限于反映流体出溶现象的地质样品难以获得和有效研究手段的缺乏，未能深入认识斑岩矿床流体出溶机制及其对超大型斑岩矿床形成的控制作用。本申请拟选择大兴安岭北段岔路口巨型斑岩钼矿，以其中发育罕见且齐全的流体出溶结构（石英眼、显微连通晶洞、单向固结结构、早期石英脉）为研究对象，以图像分析（SEM-CL，BSE）揭示石英生长世代为基础，结合矿物颗粒尺度石英的原位微量元素（LA-ICPMS）和氧同位素（SIMS）分析及流体和熔-流体包裹体系统测试，研究该矿床流体出溶结构的成因和出溶流体性质，追溯流体从出溶到释放过程中温度、压力及流体成分的系统变化，精细刻画斑岩矿床流体出溶过程，限定流体出溶发生的早晚、深度和通量，打通这一超大型斑岩矿床形成的关键链条，可望丰富斑岩矿床成矿理论。

岔路口巨型斑岩钼矿中发育多种反应流体出溶过程的结构，如石英斑晶、单向固结结构（USTs）、高硅石英带和早期弯曲状石英脉（A veins）等，是研究斑岩矿床中“流体出溶过程如何控制成矿”这一科学问题的理想对象。以图像分析（SEM-CL）为基础，对上述流体出溶结构开展了石英原位微量元素、SIMS氧同位素分析和流体包裹体系统测试工作。结果显示流体出溶结构中石英具有复杂的生长历史，最早期CL明亮的石英代表最初形成的热液石英，中期CL灰色的石英是早期石英受改造的结果，而晚期CL黑色石英反映了后期热液流体的叠加作用。细晶斑岩体高分异、富氟的特征可以使Mo在残余熔体中富集，从而提高了出溶流体的金属含量。USTs记录了流体快速聚集的过程，而高硅石英带和早期石英脉形成于流体大量释放的阶段。流体出溶的深度约为4-5km。最早出溶的流体为高温、中低盐度、单一相近临界密度的气相流体，具有富金属、挥发分和高氧逸度的特征。斑岩体岩钟状的形态对岩浆热液流体起强烈的聚集和流体通道作用，以及流体多期次出溶所带来的大量金属和流体，是控制岔路口巨型斑岩钼矿形成的重要因素。流体出溶之后，随流体压力从静岩压力降低到近静水压力，天水流体与岩浆热液流体发生混合，导致辉钼矿大规模沉淀。在本矿床中识别出的矿质沉淀机制可能是斑岩钼矿形成的普遍规律，尚需进一步研究证实。