塔里木陆块周缘岩浆Cu-Ni-Co硫化物矿床形成的探讨

Discussion on the formation of magmatic Cu-Ni-Co sulfide deposits in margin of Tarim Block

因新发现的夏日哈木超大型岩浆铜镍钴硫化物矿床成岩成矿研究存在争议,对岩浆铜镍钴硫化物矿床的成因机制又引发新一轮的关注。中国造山带中铜镍钴硫化物矿床具有鲜明的分布特点,20世纪80年代东天山黄山东等一批岩浆铜镍钴硫化物矿床的发现,曾经提出蛇绿岩形成铜镍钴硫化物矿床的观点,而后出现了洋壳削减闭合陆-陆碰撞后新生陆壳裂陷槽环境形成铜镍钴矿的主流认识;进入21世纪后,随着塔里木早二叠世大火成岩省的提出,开始将东天山和新疆北山的岩浆铜镍钴硫化物矿床与塔里木大火成岩省的形成关联起来,并认为是地幔柱作用的结果。但随着研究的深入,由于含铜镍钴镁铁-超镁铁岩地球化学具有典型的岛弧特征,又将东天山铜镍钴矿床的形成与俯冲削减的板片再次联系了起来,导致岩浆型铜镍钴硫化物矿床形成背景还存在较大争议。我们认为岩石微量元素显示岛弧信息,是俯冲交代及地壳混染引起的地球化学屏蔽效应;铬尖晶石成分及所反映的氧逸度环境指示为张性环境,而非岛弧。塔里木陆块周缘3期岩浆铜镍钴硫化物矿床成矿代表了中国最重要的3期成镍事件,实际反映了塔里木陆块在全球大陆聚散演化中与超大陆之间关键的聚散事件:(1)新元古代金川Cu-Ni-PGE矿成矿代表了由于地幔柱作用以塔里木、扬子和西澳大利亚陆块之间的裂解为起点的罗迪尼亚超大陆裂解事件,直至早古生代形成原特提斯洋-古亚洲洋;(2)早古生代末夏日哈木Ni-Co矿则是代表冈瓦纳大陆南部裂解从而形成古特提斯洋的标志性事件;(3)晚古生代早二叠世坡-Cu-Ni矿应是地幔柱作用潘吉亚超大陆生长大火成岩省的深成相组成。本文系统总结了3个矿床的成矿特征,并对比了3个矿床的区别。尽管坡-在3个矿床中有基性程度最高的母岩浆,但坡-相对于夏日哈木、金川显示出低的地壳混染程度,特别是地壳硫混染程度,这可能是坡-成矿相对差的主要原因。此外坡-不仅地壳混染程度低,且其混染了较多的钙质,抑制了硫化物饱和。

Due to the controversy over the diagenesis and mineralization of the newly discovered Xiarihamu super-large magmatic copper-nickel-cobalt sulfide deposit,it started a new round of discussion on the metallogeny of the magmatic copper-nickel-cobalt sulfide deposit.The copper-nickel-cobalt sulfide deposits in the orogenic belt in China have a distinctive distribution feature.In the 1980 s,with the discovery of a series of magmatic copper-nickel-cobalt sulfide deposits in the mafic-ultramafic rocks in eastern Tianshan,it was proposed that these mafic-ultramafic rocks are ophiolites.Then,a mainstream viewpoint was formed that the copper-nickel-cobalt deposits in eastern Tianshan were formed in the continental crust rifting trough environment after the subduction of the oceanic crust and the closure of the continental collision.The discovery of the Early Permian Tarim Large Igneous Province in the 21 st century suggested that the magmatic copper-nickel-cobalt sulfide deposits in eastern Tianshan and the Beishan in Xinjiang are related to the formation of the Tarim mantle plume.However,based on the geochemistry of mafic-ultramafic wholerock with typical island arc characteristics,some researchers maintained that some magmatic copper-nickelcobalt deposit could be associated with subduction-reduction plates.We believe that the trace elements of whole-rocks that show island arc information are a geochemical shielding effect caused by the subduction and crustal contamination.The composition of the chrome spinel and the oxygen fugacity environment indicated that these deposits were formed in an extension environment,rather than the island arc.The three-period magmatic copper-nickel-cobalt sulfide deposits around the Tarim block represent the three most critical nickel-forming events in China,which reflect the vital convergent events between the Tarim block and the supercontinent.First,the Neoproterozoic giant Jinchuan Cu-Ni-Co-PGE deposit represents the Rodinia supercontinent break-up event,which led to the formation of the Paleo-Asian Ocean among the Tarim Block,Yangtze Block,and Western Australian Block.Second,the large Xiarihamu nickel-cobalt deposit in the Early Paleozoic was the product of the break-up event in the southern Gondwana land,leading to the formation of the Paleo-Tethys in the Paleozoic.Third,the Early Permian Poyi copper-nickel deposit should be a plutonic facies composed of mantle plumes and large igneous rocks grown in the Pangea supercontinent.This article systematically summarizes and compares the metallogenic characteristics of the three deposits.Although Poyi has the highest MgO content of parent magma among the three deposits,it shows a lower degree of crustal contamination,especially crustal sulfur contamination,compared with Xiarihamu and Jinchuan.This is probably the main reason why the Poyi intrusion has the lowest Cu-Ni mineralization among the three deposits.Furthermore,the Poyi intrusion has some contamination of calcium,which can hinder sulfide saturation.