马达加斯加Maevatanana金矿床Re-Os同位素年龄及其地质意义

Re-Os isotope age of pyrite from the Maevatanana gold deposit in Madagascar and its geological significance

马达加斯加Maevatanana金矿床是一个产在绿岩带中的石英脉型金矿床,其成因备受争议。本文对其进行了元素地球化学分析和测年,探讨了金矿床的成矿时代和成矿过程。马达加斯加Maevatanana片麻岩金含量为1.11×10^(-9)~80.8×10^(-9),平均值19.55×10^(-9);片麻岩中黄铁矿为主要载金矿物,黄铁矿Re-Os同位素等时线年龄为752±27 Ma。含金石英脉金品位26.44×10^(-6)~132.22×10^(-6),黄铁矿为主要载金矿物,含金石英脉中黄铁矿Re-Os同位素等时线年龄为535±26 Ma,代表了Maevatanana金矿床的成矿年龄。两期黄铁矿Re-Os同位素等时线年龄与泛非造山事件两次碰撞的时间相对应。片麻岩金含量远远高于地壳平均品位,在黄铁矿中可见包裹金,黄铁矿Re-Os同位素等时线年龄表明752±27 Ma花岗岩、辉长岩侵入,马达加斯加地体发生了广泛的绿片岩相到角闪岩相的变质作用,火山沉积岩中的金被活化、迁移、富集形成矿源层。含金石英脉的成矿年龄535±26 Ma和马达加斯加中北部寒武纪后碰撞A型花岗岩的年龄一致,野外证据和地球化学数据表明Maevatanana金矿床和寒武纪后碰撞A型花岗岩有密切的成因联系。在岩浆上升过程中,岩浆逐渐演化形成成矿流体,这些成矿流体进入容矿构造,富集成矿。

The Maevatanana gold deposit in Madagascar is a quartz-vein type gold deposit hosted in a greenstone belt,and its genesis is highly controversial.This article conducts elemental geochemical analysis and dating of the Maevatanana gold deposit,and explores the mineralization age and process of the Maevatanana gold deposit in Madagascar.The gold content of gneiss is 1.11×10^(-9)~80.8×10^(-9),with an average of 19.55×10^(-9);pyrite is the main gold-bearing mineral in gneiss,with a Re-Os isotopic age of 752±27 Ma.The gold grade of the gold-bearing quartz vein is 26.44×10^(-6)~132.22×10^(-6),and pyrite is the main gold-bearing mineral.The Re-Os isotopic age of pyrite in the gold-bearing quartz vein is 535±26 Ma,representing the mineralization age of the Maevatanana gold deposit.The Re-Os isotope isochron ages of two phases of pyrite correspond to the time of two collisions during the Pan African orogeny event.The gold content in gneiss is much higher than the average grade of the crust,and encapsulated gold can be seen in pyrite.The Re-Os isotopic age of pyrite indicates intrusion of granite and gabbro at 752±27 Ma.The Madagascar terrane underwent extensive metamorphism from greenschist to amphibolite facies,and gold in volcanic sedimentary rocks was activated,migrated,and enriched to form a source layer.The mineralization age of the gold-bearing quartz vein is 535±26 Ma,which is consistent with the age of the Cambrian post collisional A-type granite in central northern Madagascar.Field evidence and geochemical data indicate a close genetic relationship between the Maevatanana gold deposit and the Cambrian post collisional A-type granite.During the process of magma ascending,the high-Si melts entered the magma and gradually evolved into ore-forming fluids,which enter the host structures and enrich mineralization.