内蒙古中部克布铜镍硫化物矿床岩石成因与成矿过程

Petrogenesis and metallogenicprocess of the Kebu magmatic Cu-Ni sulfide deposit in the central Inner Mongolia

内蒙古中部地区横跨中亚造山带与华北克拉通北缘,区内岩浆铜镍硫化物矿床(点)广泛发育。克布矿床位于该区白云鄂博裂谷西南侧,是近年来新发现的品位较高的铜镍化物矿床。因此,准确厘定克布镁铁-超镁铁质岩体的形成时代,查明岩石成因与矿床成矿过程对于进一步指导克布矿床深部铜镍找矿突破以及区域类似镁铁-超镁铁质岩体含矿性评价具有重要意义。克布岩体主要岩相由主体辉长岩相与赋矿橄榄岩相组成,其中辉长岩相属于主体岩相而橄榄岩相则是主要赋矿岩相。岩石地球化学分析结果表明,克布镁铁-超镁铁质岩体各岩相具有轻稀土富集、重稀土亏损的右倾型稀土配分模式,富集Rb、Sr和Ba等元素,亏损Nb和Ta等元素。本次研究获得克布岩体橄榄辉长岩的LA-ICP-MS锆石U-Pb年龄为267.5±1.8Ma,为中二叠世岩浆作用的产物,形成于古亚洲洋闭合后的碰撞后伸展环境。同位素组成研究结果表明,克布岩体锆石ε_(Hf)(t)值为-4.67~2.84,(~(87)Sr/~(86)Sr)_(i)值为0.7062~0.7071,ε_(Nd)(t)值为为-10.6~-5.65。铂族元素(PGE)组成研究结果表明,克布铜镍硫化物矿床矿石具有相对较低的Ni/Cu值,较高的Pd/Ir值和Cu/Pd值。综合上述研究结果,初步查明克布镁铁-超镁铁质岩体形成于古亚洲洋闭合的碰撞后伸展环境,岩浆源区为经历了俯冲交代作用的EMⅠ型岩石圈地幔,原生岩浆主要为高镁玄武质岩浆。在幔源岩浆侵位过程中经历了一定程度的地壳物质混染,其中地壳硫的加入是促进克布镁铁-超镁铁质岩浆发生硫化物熔离并最终富集成矿的关键因素。结合最新探矿工程控制情况,本文认为克布矿床深部铜镍成矿潜力较高,应将晚期超镁铁质岩及深部岩浆通道作为下一步找矿工作重点。

The central Inner Mongolia stretches across the Central Asian Orogenic Belt and northern margin of the North China Craton,where magmatic Cu-Ni sulfide deposits are widely distributed.The Kebu Cu-Ni sulfide deposit,located in the southwest part of the Bayan Obo rift zone in central Inner Mongolia,is a newly discovered high grade Cu-Ni deposit.It is important to determine the formation age of the Kebu mafic-ultramafic intrusion and identify its petrogenesis and mineralization process,so as to guide the deep prospecting breakthrough of the Kebu deposit and mineralization evaluation of similar mafic-ultramafic intrusions within the region.The Kebu intrusion is mainly composed of dominant peridotites and ore-bearing gabbros,both of which show enrichment of light rare earth elements,Rb,Sr,Ba but depletions of heavy rare earth elements,Nb and Ta.The olivine-gabbro of the Kebu pluton yields a LA-ICP-MS zircon U-Pb weighted-mean age of 267.5±1.8Ma,indicating that it was generated by Middle Permian magmatism.The zirconε_(Hf)(t)values,(87 Sr/86 Sr)i values andεNd(t)values of the Kebu intrusion range from-4.67~2.84,0.7062~0.7071 and-10.6~-5.65,respectively.PGE geochemical characteristics show that the Kebu ore samples have relatively low Ni/Cu values and high Pd/Ir and Cu/Pd values.The above suggests that the Kebu intrusion was formed under a post-collision extensional setting after the closure of the Paleo-Asian Ocean.The source magma was high Mg basaltic magma that derived from the subducted-metasomatic EMI type lithospheric mantle and experienced crustal material mixing during emplacement process.It is worth noting that the addition of crustal sulfur played a key role in promoting sulfide segregation and final mineralization of the Kebu mafic-ultramafic magma.Combining with the latest prospecting finding,we propose that the Kebu deposit has high potential for Cu-Ni mineralization in its deep section,and the later stage ultramafic rocks and deep magma channel should be the focus of the following mineral exploration work.