川西黑牛洞铜矿年代学及物质来源研究

Although has been researched for decades, genesis of stratiform deposits is still the bone of contention. The Jianglang dome situates in the southern margin of the Songpan-Ganzi orogen. The high-grade Cu deposits in its core provide an opportunity to solve the above-mentioned problem. Their Cu orebodies are stratiform and hosted by the Liwu group metamorphic rocks. The mineralization types are mainly massive, banded and disseminated, and the metal sulfides are composed chiefly of chalcopyrite and pyrrhotite. Based on field geological survey and microscopic observation, this project selects Heiniudong as a typical ore deposit, where significant prospecting results have been achieved in recent years. The main researches include: (1) biotite Ar-Ar, monazite U-Pb and chalcopyrite Re-Os dating methods, which could precisely determine deformed, metamorphic and metallogenic ages, and their coupling relation; (2) chalcopyrite He-Ar and in situ S-Pb isotopes, together with S-Pb isotopes of hosting rocks and granites, which could analyze the contribution of mantle-derived components and indicate their genetic relationship. Combined with prior studies, our research can uncover genesis of the high-grade Cu deposits, discuss its significance on regional mineralization and deepen metallogenic theories of stratiform deposits.

长期以来，层状矿床成因是地质学家争论焦点。江浪穹窿位于松潘–甘孜造山带南缘，其核部发育的富铜矿床为解决该问题提供契机。矿体呈层状赋存于里伍岩群变质岩系之中，矿化类型以块状、条带状及浸染状为主，金属硫化物主要是黄铜矿与磁黄铁矿。基于野外地质调查及室内镜下观察，本项目针对近年取得重大找矿成果的黑牛洞铜矿开展典型矿床研究：（1）采用黑云母Ar-Ar定年、独居石U-Pb定年及黄铜矿Re-Os定年方法，精确厘定变形–变质–成矿地质过程的时间序列，准确判别成矿事件与变形变质作用的耦合关系；（2）运用黄铜矿He-Ar同位素及原位S-Pb同位素示踪技术，结合里伍岩群变质岩与穹窿北侧花岗岩的S-Pb同位素分析结果，查明成矿过程是否摄入幔源物质，研究成矿事件与里伍岩群/花岗岩体的成因联系。结合前人研究成果，剖析富铜矿床形成机制，探讨区域成矿意义，深化层状矿床成矿理论。

（变）沉积岩容矿的层状铜矿床（简称SSC矿床）具有极高的铜品位（平均1.75%），提供了全球约23%的铜资源量，但其成因机制存在较大争议，包括同生至早期成岩矿化、晚期成岩矿化、后生矿化和叠加成矿系统等多种观点。江浪穹窿位于松潘–甘孜造山带东南缘，其核部地层中发育一系列高品位（平均1.75%）的铜多金属矿床，累计查明的铜金属量已近一百万吨。赋矿岩系为新元古代里伍岩群，岩性为变沉积岩夹少量顺层的变基性岩。这些矿床符合SSC矿床的基本特征，为解决上述科学问题提供了契机。.本项目以黑牛洞、里伍和中咀等典型矿床作为重点研究对象，采用地质年代学和同位素示踪等手段，剖析了江浪穹窿核部SSC矿床的形成机制。结果表明：（1）黄铜矿和磁黄铁矿Re-Os等时线年龄分为568 ± 52 Ma（n=4）和160.4 ± 7.0 Ma（n=5）两组（附件1–2），前者与赋矿岩系里伍岩群的形成时代（~553 Ma）一致，后者与邻近的花岗岩体结晶年龄（~164 Ma）吻合，指示了两期成矿作用；（2）二云石英片岩中的独居石U-Pb平均年龄为192.94 ± 0.67 Ma（n=50，附件3–4），代表了里伍岩群的峰期变质时代；（3）构造片岩中的黑云母Ar-Ar等时线年龄为141.6 ± 2.2 Ma、126.8 ± 2.9 Ma（附件5–6），指示了江浪穹窿在伸展构造背景下的主体变形时代；（4）黄铜矿和磁黄铁矿的3He/4He比值为0.187–0.052 Ra，幔源He比例仅2.96–0.70%（n=28，附件7–8），表明成矿物质以壳源为主；（5）黄铜矿原位S同位素组成δ34SV-CDT=9.50–3.87‰（n=72，附件9–10），矿石Pb同位素组成206Pb/204Pb=19.6826–18.0252、207Pb/204Pb=15.7943–15.6857、208Pb/204Pb=40.5011–38.2784（n=40，附件11–12），与里伍岩群变沉积岩、里伍岩群变基性岩和区域花岗岩基本一致。.综合已有成果资料，本项研究表明江浪穹窿核部SSC矿床的发育与区域变质作用、变形作用无关，而与富含成矿元素的古老变质岩系和壳源重熔型岩浆–热液流体具有亲缘关系。该认识支持SSC矿床“原始沉积+热液活化”的叠加成矿模型，亦为松潘–甘孜造山带东缘穹窿带的找矿勘查奠定了理论基础。