内蒙古霍各乞海西期侵入岩形成时代、成因及其与铜矿体的关系

Formation epoch and genesis of intrusive rocks in Huogeqi orefield of Inner Mongolia and their relationship with copper mineralization

文章通过对狼山霍各乞矿区侵入岩中锆石U-Pb年代学和铪同位素组成、岩石化学及空间上毗邻铜矿体地球化学的研究,探讨了其岩石成因及与铜矿体的关系。结果表明,岩体岩性为辉长-闪长岩,岩体中锆石阴极发光图像具明显的振荡环带结构,Th/U比值均大于0.1(0.20~0.77),指示岩浆成因特征;岩体中锆石加权平均206Pb/328U年龄为(273.9±1.2)Ma,表明岩体形成于海西晚期;锆石的εHf(t)值变化范围为-4.1^-10.0;Hf模式年龄分布于1.56~1.92 Ga之间,其峰值与Nd两阶段亏损地幔模式年龄一致,这说明成岩物质主要来自古老地壳物质的熔融,并非来自亏损地幔源区;铜矿体钕模式年龄变化比较大,变化范围为1.78~2.37 Ga,反映了成矿物质多来源的特征;野外地质特征和爆裂测温数据表明铜矿体受后期改造,与海西期侵入岩有关。认为海西期辉长-闪长岩体应由太古代结晶基底部分熔融并混入少量幔源组分形成,该岩体对霍各乞铜矿床没有提供大量成矿物质,但岩体的形成对矿体局部有明显的富集改造作用。

ingle zircon MC-ICP-MS U-Pb dating and lithogeochemical studies of gabbro-diorite intrusives in the Huogeqi orefileld were conducted for the purpose of constraining their petrogenesis and their relationship with copper mineralization.Zircons collected from gabbro-diorite display oscillatory zoning and high Th/U ratios（0.20～0.77）,indicating their magmatic origin.A weighted average U-Pb age of（273.9±1.2） Ma for Huogeqi intrusive rocks was obtained,suggesting that the formation of gabbro-diorite should be associated with the collisional event of the North China Plate and Siberia Plate in late Hercynian period.Their initial Hf isotope ratios and Hf two-stage depleted mantle model ages range from-4.1 to-10.0 and from 1.56 to 1.92 Ga,respectively.Their peak model ages are consistent with the bulk rock Nd two-stage depleted mantle model age of 1.77Ga, implying that the rock-forming materials were mostly derived from the paleo-crust instead of from the depleted mantle.Geological and geochemical characteristics of the gabbro-diorite intrusives reveal that they were derived from partial melting of Archaean basement and interacted with mantle-derive magmas by about 17%,as estimated by simulated calculation.A comparison of geochemical characteristics between gabbro-diorite intrusives and the copper ore body suggests that Hercynian diorite failed to provide significant metallogenic elements.Furthermore,Hercynian diorite intruded in the compressive orogenic environment 273.9 Ma ago,whereas copper mi-neralization occurred in Middle-Late Proterozoic,suggesting the absence of direct genetic relationship between the plutonic intrusion and the formation of the copper ore body.Based on field observation and decrepitation curves of quartz and chalcopyrite,the authors have reached the conclusion that the Huogeqi copper deposit was reformed by magmatic hydrothermal fluid along the contact zone between the gabbro-diorite and the copper ore body.