西秦岭李坝金矿田脉岩的成因及其对金成矿作用的制约

It is universal existence in many gold deposits that granite and lamprophyre dikes are temporally and spatially associated with gold deposits. However，there are different views on petrogenesis of dikes and its constraints on gold mineralization. This project will take the Liba gold ore field in western Qinling with related granite and lamprophyre dikes as an research subject to carry out the following studies:（1）to make clear the petrology of dikes and geological characters of the deposits by systematic field trip and indoor study and achieve diagenetic and metallogenic geochronology by in situ U-Pb dating of zircon by LA-ICP-MS and Re-Os dating of arsenopyrite or pyrite，respectively. By doing this to confirm the relationship of the spatial and temporal distribution regularities between dikes and gold deposits；（2）to confirm the relationship between metallogenic material source and different kinds，different stages of dikes and make sure the role that dikes played during the formation of gold deposits based on the study of rock geochemistry，mineral geochemistry and isotope tracer. These achievements would provide crucial information for mineral exploration in western Qinling，at the same time，the research production would also provide new data on enrich the metallogenic theory of gold deposit，the evolution and interaction ofregional crust and mantle as well as the identification and determination of tectonic environment.

煌斑岩脉和花岗岩脉与金矿床在时空上紧密依存在国内外许多金矿床中普遍存在，但脉岩的成因及其对金成矿作用的制约存在不同认识。本项目选择西秦岭地区李坝金矿田和矿田发育的各类脉岩为研究对象，拟开展详细的野外地质调查，系统的脉岩岩石学和矿床地质特征研究，应用原位微区锆石U-Pb法和毒砂或黄铁矿的Re-Os同位素法精确测定成岩成矿时代，理清脉岩与金矿床的时空分布规律；开展不同种类、不同期次脉岩的岩石地球化学、矿物地球化学和同位素地球化学示踪研究，理清成矿物质来源与脉岩的关系，揭示脉岩在金矿形成中的作用，为该区域进一步找矿预测提供新的信息，同时可以为丰富热液型金矿床的成矿理论，地区性壳幔演化及其相互作用研究以及构造环境的识别和判定提供新的资料。

煌斑岩脉和花岗岩脉与金矿床在时空上紧密依存在国内外许多金矿床中普遍存在，但脉岩的成因及其对金成矿作用的制约存在不同认识。本项目研究显示，西秦岭李坝金矿区脉岩与金矿（化）主要呈近EW、NE、NW向密切共生产出，脉岩的岩石类型主要有花岗（斑）岩、闪长（玢）岩、石英闪长岩、花岗细晶岩、闪长细晶岩、云斜煌斑岩、斜闪煌斑岩，根据其相互穿切关系可划分为成矿前脉岩、成矿期脉岩和成矿后脉岩三类。LA-ICP-MS锆石U-Pb定年显示，李坝金矿区花岗岩脉的年龄为210Ma±1Ma，MSWD=0.15，与中川岩体同属晚三叠世构造岩浆活动的产物，5件载金黄铁矿Re-Os同位素等时线年龄为168.1±9.1Ma，MSWD＝0.47，为李坝金矿床主成矿期的时代。主成矿期可能晚于脉岩及其他岩浆活动和广泛的热液蚀变作用时代。花岗（斑）岩富碱、高钾，属于过铝质、高钾钙碱性系列，煌斑岩脉属高钾钙碱性系列。Nd-Hf同位素结果显示，李坝金矿区花岗斑岩脉的143Nd/144Nd比值为0.512084±0.000007，147Sm/143Nd比值为0.0903，εNd(t)为-7.98，Nd同位素亏损地幔模式年龄TDM2为1.64Ga，显示出亏损地幔来源岩浆的特征；花岗斑岩脉的εHf(t)=-6.30～+1.47，TDMc为1.65～1.15Ga，仅有一个点的εHf(t)为+3.32，TDM为1.08Ga。煌斑岩脉的εHf(t)=-6.44～+1.38，TDMc为1.67～1.17Ga，显示了源区主要为古老下地壳物质。硫同位素结果显示，李坝金矿床中的黄铁矿δ34S 值变化于4.3‰～11.6‰，平均值为8.45‰，接近地幔硫同位素值，也显示地层硫特点，说明成矿物质混染了一定比例的地层组分。结合年代学与Nd-Hf、S同位素约束可得，研究区早中生代花岗岩体的源区物质组成较为复杂，但总体上以地壳物质为主，混有少量的年轻幔源组分，矿区煌斑岩脉的源区应该具有幔源特征。它们是起源地壳的岩浆与幔源岩浆混合的结果。研究区脉岩与金矿在空间上紧密共生，时间上成矿晚于脉岩，脉岩为成矿物质上移提供了通道，成矿同期脉岩可能为金矿提供成矿物质来源。