西藏自治区改则县多不杂斑岩铜金矿床岩浆演化与成矿背景研究

Duobuza porphyry copper-gold deposit is a newly discovered deposit, which is located in the west of Bangonghu-Nujiang metallogenic belt, and together with Bolong porphyry deposit makes up Duolong metallogenic district. In these years, the exploration in Duobuza porphyry deposit gains considerable progress, but its metallogenesis and metallgoenic setting are incompletely understood. According to the mapping result, with typical porphyry copper deposit alteration zoning, pre-mineral basaltic andesite, diabase, volcanic breccia, intermineral granodiorites and postmineral dacite are occurring in Duobuza deposit, providing a good opportunity to study the magmatic evolution of Duobuza deposit. Magmatic rocks in Duobuza are panned to sample, and their geochemical composition, zircon U-Pb dating, 39Ar-40Ar dating and Sr-Nd-Pb-Hf isotopic composition analysis will be performed, aiming to disclose their petrogenesis and source regiones. Based on the rock -forming sequence with these rock-forming ages, combined with the geochemical data and Sr-Nd-Pb-Hf isotopic composition, magmatic evolution and metallogenesis of duobuza porphyry deposit will be discussed. Many porphyry intrusions are widespread in the Duolong metallogenic district, and the differences between mineralized porphyries and barren porphyries are also designed to study in this program by comparasion of their geochemistry, Sr-Nd-Pb-Hf and fluid inclusion features. Combined with the regional tectonic evolution, tectonic-magmatic process of duobuza porphyry deposit will be also researched, in order to discover the metallogenic dynamic process and guide the exploration in Duolong metallogenic district and Bangonghu-Nujiang metallogenic belt.

班公湖-怒江成矿带是青藏高原中部新发现的成矿带，多不杂斑岩铜矿是带内发现的第一个斑岩铜矿。近几年找矿勘探工作取得重大进展，但多不杂矿床成因、成矿背景等研究未持续深入开展。地表填图结果显示多不杂矿床具有典型的斑岩铜矿蚀变分带，矿床内发育成矿前的玄武安山岩、辉绿岩，成矿期的三期花岗闪长斑岩，成矿后的英安岩等，多种岩浆岩发育为多不杂矿床岩浆演化研究和深部成矿动力学探索提供了良好的契机。本申请拟对矿床内发育的岩浆岩进行系统的岩石化学、年代学和Sr-Nd-Pb-Hf同位素组成分析研究，揭示各期次岩浆岩的源区及岩石成因，并结合野外确定的成岩序次，建立多不杂矿床岩浆演化序列，探讨矿床成因。在此基础上，在矿区内开展成矿斑岩和非成矿斑岩对比研究；结合区域内构造演化，研究多不杂矿床成矿地质背景和地球深部成矿动力学过程，为多龙矿集区和班公湖-怒江西段找矿提供指导。

西藏改则县多不杂斑岩铜金矿床位于藏北班公湖怒江结合带北侧，是藏北发现的第一个斑岩铜矿，其所处的多龙矿集区内目前已经发现5个大型斑岩铜矿，成为藏北最有前景的找矿远景区。本项目在开展野外地质特征调查的基础上，重点对多不杂矿床出露的系列岩浆岩进行了岩相学、年代学和岩石化学研究。多不杂矿床在成矿前发育辉绿岩和枕状玄武岩、玄武安山岩，成矿期主要发育两期花岗闪长斑岩和较晚的闪长斑岩，紧随成矿作用，在多龙矿集区内发育玄武岩-安山岩。.辉绿岩的锆石U-Pb年龄为169～170Ma；成矿前的玄武岩-玄武安山岩成岩40Ar/39Ar年龄集中于141～148Ma，具有富集高场强元素的特征，属于高Nb碱性玄武岩。多不杂斑岩铜矿中第二期侵位的花岗闪长斑岩是多不杂矿床的主要成矿斑岩。锆石U-Pb测年结果显示，三期斑岩在ca. 120Ma集中侵位。较早侵位的两期花岗闪长斑岩均富集轻稀土、大离子亲石元素，亏损重稀土、高场强元素，Eu异常不明显，显示出岛弧岩浆岩的特征，均具有高Sr低Y的特征， (87Sr/86Sr)i值0.7057~0.7062和0.7059~0.7064，εNd(t)值为较小的负值（-2.5~-1.7和-6.2~-4.0），第二期花岗闪长斑岩的εHf(t)值为3.7~7.5，显示两期花岗闪长斑岩均属于类埃达克质岩石，可能起源于新生下地壳角闪岩相，有幔源物质混入。最晚侵位的闪长玢岩与两期花岗闪长斑岩岩石化学特征类似，但具有富钠特征，其MgO、Y、Yb等含量相对较高，可能表明闪长玢岩与花岗闪长斑岩源区相同，有更多幔源物质混入。成矿后的玄武岩-安山岩形成年龄为成矿末期在多龙矿集区内发育玄武岩-安山岩系列火山岩，火山岩沿北东向断裂产出，具有柱状节理构造，显示这些火山岩产于陆相环境，其形成年龄为111～118Ma。.班公湖-怒江洋的闭合是一个持续的过程，时间范围为125~109Ma。成矿末期在多龙矿集区内发育玄武岩-安山岩系列火山岩产于陆相环境，形成年龄为111～118Ma，略晚于多不杂矿床的成岩-成矿-蚀变年龄。因此，幔源岩浆的持续注入可能是多不杂斑岩铜矿成矿的一个重要过程，多不杂斑岩铜矿可能形成于班公湖-怒江洋向北俯冲末期，形成于陆缘弧环境。