华北克拉通北缘鸡冠山斑岩钼矿萤石及其流体包裹体研究

The large Jiguanshan porphyry Mo deposit is located at the southern part of the newly found Xilamulun metallogenic belt in northern margin of the North China Craton (NCC). Geochronology and geochemistry data indicated that Mo mineralization occurred at Late Jurassic (153-156 Ma), and that the host granite porphyry were characterized by high silicon, high differentiation, high oxidation and rich in alkaline. The Mo mineralization is closely related to the fluorite, and there are plentiful daughter mineral-bearing fluid inclusions present in the ore-bearing fluorite. In this project, we will study the geology, trace element and Sr、Nd isotope of the fluorite in order to discern its genesis and the relationship between F element and Mo mineralization. Based on preliminary petrography, laser Raman and microthermometry analysis, we particularly focus on the component and H-O isotope analysis of the fluid inclusion. Doing these, we aim to figure out the source and evolution process of the Jiguanshan ore-forming fluid, and also reveal Mo mineralization mechanism in order to provide information for metallogenic model of porphyry Mo deposit. Combined with regional information, the relationship between occurence of large-scale Mo mineralization in Late Mesozoic in northern magin of the NCC and destruction of the NCC will also be discussed.

鸡冠山斑岩钼(Mo)矿位于华北克拉通北缘新发现的西拉木伦Mo多金属成矿带南侧，为一大型斑岩Mo矿床。已有的年代学和地球化学研究精确限定了鸡冠山成岩成矿时代为晚侏罗世(153-156 Ma)；成矿斑岩具有高硅，富碱，高氧逸度，高分异的特征。矿区辉钼矿化与含F矿物萤石密切相关，且含矿萤石脉中出现大量含子矿物包裹体。本项目选择与钼矿化密切相关的萤石及其包裹体为研究对象，在以往矿区包裹体岩相学、激光拉曼和显微测温分析基础上， 重点对矿区不同类型萤石进行详细的地质，微量元素和Sr、Nd同位素分析，结合包裹体气液成分及H-O同位素分析，精细解剖萤石来源及成因；揭示F元素与成Mo矿之间的关系；查明鸡冠山斑岩Mo矿成矿流体来源、演化模式和成矿机制，为完善斑岩Mo矿成矿机制和成矿模式研究提供科学依据。同时，结合区域资料，探讨华北北缘晚中生代大规模Mo矿化事件的发生与华北克拉通破坏之间的关系。

鸡冠山大型斑岩钼矿是华北克拉通北缘造山带的大型单钼矿床。钼矿化主要以浸染状、网脉状产于侏罗纪成矿花岗斑岩中；以细脉状、脉状分布于成矿前花岗斑岩、泥盆纪火山岩和三叠纪石英斑岩中，与萤石密切相关。矿区萤石贯穿了钼矿化早期至晚期，早期以偏紫色萤石为主，晚期以偏绿色萤石为主。该项目对不同矿化期的萤石进行了系统采样，重点分析了萤石单矿物微量元素、Sr-Nd-Pb同位素特征。萤石单矿物微量元素数据表明紫色萤石微量元素含量远低于绿色萤石，但二者具有相似的稀土配分模式，并类似于成矿花岗斑岩稀土特征：具有明显的Eu负异常，且轻重稀土分馏不明显。萤石单矿物Pb同位素结果表明其Pb同位素组成与辉钼矿、成矿花岗斑岩相同，具有下地壳和造山带Pb同位素组成；同时萤石具有地壳Sr-Nd同位素特征（εNd(t)介于6.9-7.4之间），指示了萤石、辉钼矿和成矿花岗斑岩可能来自相同的地壳源区，受到部分造山带区域物质的混染。综合项目研究结果，我们提出含F的成矿流体主要来自富氟的花岗斑岩岩浆系统；并揭示出氟元素在Mo金属元素的溶解、运移和沉淀过程中均具有重要作用；完善了鸡冠山斑岩钼矿成矿模式：花岗质岩浆演化末期分异出携带了大量金属元素的高温、高盐度、高氧逸度及富F的H2O-NaCl流体，迁移到斑岩上部和围岩裂隙中充填-交代成矿。Mo金属元素的沉淀富集机制除了高温阶段的流体沸腾作用，后期温度下降、流体混合作用外，氟元素的沉淀也是导致矿化沉淀的关键因素。