摘要
中亚造山带在中国境内可分为东、中、西三段,东段和西段主要为洋内型汇聚板块边缘,中段主要为安第斯型汇聚板块边缘,均发育大量志留纪-三叠纪的镁铁-超镁铁质岩体。但是,赋含具经济价值的铜镍硫化物矿床的岩体主要集中在早二叠世-晚三叠世,笔者对中亚造山带早二叠世-晚三叠世镁铁-超镁铁质岩体的地幔源区性质、辉石岩组分参与地幔部分熔融的作用、地幔源区和幔源岩浆氧逸度、岩浆硫化物饱和机制,以及差异性成矿的控制因素等开展系统研究,总结出汇聚板块边缘铜镍硫化物矿床形成的几个主要特点。首先,通过对比西段额尔齐斯构造带的构造演化过程和喀拉通克地区含矿岩体的形成过程,发现成矿作用具有爆发性,表现在含矿岩体的形成时代与区域平行造山带伸展构造完全同期,其形成与持续碰撞挤压过程中局部地区出现的短暂伸展构造有关,促使幔源岩浆快速侵位至地壳浅部,抑制深部橄榄石分离结晶造成的岩浆镍丢失,从而有利于成矿。这一认识与通常认为这些含矿岩体形成于碰撞后长期伸展阶段不同。其次,通过对不同岩体超镁铁质岩相中橄榄石的成分研究,发现辉石岩组分参与地幔部分熔融有利于幔源岩浆富Ni,但其参与程度与熔融压力有关:在洋内型汇聚板块边缘,约10%的辉石岩组分参与地幔熔融,就能明显提高岩浆Ni含量,有利于成矿;而在安第斯型汇聚板块边缘,古老交代岩石圈地幔的部分熔融虽然有可能造成更多辉石岩组分的参与,但其地幔部分熔融程度低且熔出的岩浆量少,仍不利于成矿。第三,通过计算不同岩体的岩浆氧逸度,发现含矿岩体普遍具有高于MORB地幔的氧逸度值,这与交代地幔部分熔融形成的岩浆普遍含水有关。通过硫化物原位S同位素和矿石C同位素组成分析,发现这种高氧逸度岩浆可能携带了足够成矿的幔源硫,其硫化物饱和主要是由于外来的地壳有机碳加入所造成,氧化的岩浆被有机碳还原从而导致岩浆中硫溶解度降低。同时,这种富水岩浆易于形成特殊的网脉状和角砾状矿石构造。总之,汇聚板块边缘背景下产出的铜镍硫化物矿床与板内拉张背景下产生的同类矿床具有不同的特点,总体上表现为矿体规模相对较小、含矿岩体成带分布、成矿具爆发性、矿石构造特殊。汇聚板块边缘背景形成的造山带构造演化复杂,有些地区覆盖严重。因此,在汇聚板块边缘寻找含矿岩体应具有不同的思路和方法。
The Central Asian Orogenic Belt(CAOB)in China can be divided into the eastern,central,and western segments based on convergent margin type:the eastern and western segments represent intra-oceanic convergent margins,while the central segment represents an Andean-style continental convergent margin.There are large numbers of mafic-ultramafic intrusions in the belt with ages spanning from the Silurian to the Triassic,however,economically important intrusions are from the Early Permian to the Late Triassic.In this study,we carried out a systematic study on the nature of the mantle source,the role of pyroxenite components in the partial melting of the mantle,the oxygen fugacity of the mantle and mantle-derived mafic-ultramafic magmas,the triggers of sulfide saturation of magmas,and the controls on metallogenic differences of the mafic-ultramafic intrusions in the CAOB to understand the origin of Ni-Cu sulfide deposits in the convergent margin settings.We summarized a few major features of Ni-Cu sulfide mineralization in convergent margin settings.We compared the formation ages of sulfide-bearing and barren intrusions in the Kalatongke area in the western segment of the CAOB along a major suture zone of the Irtysh shear zone,and found all sulfide-bearing intrusions were emplaced in a short-lived metallogenic epoch,which is consistent with the syn-collisional extension stage on a regional scale.This may lead to the rapid ascent and emplacement of mantle-derived mafic magmas at a shallower crustal level,preventing significant Ni loss from the magmas due to olivine crystallization at depths.Note that the stage of syn-collisional extension here cannot simply be considered as a post-collisional setting,which cannot explain the limited age range of Ni-Cu sulfide-bearing mafic-ultramafic intrusions.We examined the trace-element compositions of olivine from representative sulfide-bearing and barren intrusions in the CAOB and found that the involvement of pyroxenite during partial melting of the mantle sources is important for producing Ni-rich mafic magmas.However,the degree of involvement of pyroxenite components may be related to the pressure of partial melting of the mantle:in the intra-oceanic convergent margin,the addition of 10%pyroxenite during partial melting of the mantle would dramatically increase the Ni content of mantle-derived mafic magmas,whereas magmas produced by low degrees of partial melting of the mantle at depths in the Andeans-style continental convergent margins still have less potential to Ni-Cu sulfide mineralization even with greater pyroxenite involvement during partial melting of the mantle.We calculated the magma oxygen fugacity of representative sulfide-bearing mafic-ultramafic intrusions in the CAOB,the integrated S isotopic compositions of sulfides in the ores,and the C isotopic compositions of the ores.We found that their parental magmas are highly oxidized and can contain sufficient S to form medium-to small-scale Ni-Cu sulfide deposits.Sulfide saturation of such oxidized magmas was likely triggered by crustal organic carbon materials derived from external sources.The oxidized magmas were likely derived from a metasomatized mantle source and were therefore highly hydrated,such hydrated ore-forming magmas can lead to special ore textures such as globular,net-work,and breccia ores.We conclude that the Ni-Cu sulfide deposits in the convergent margin settings were likely formed from the metasomatized mantle-derived oxidized mafic magmas and were emplaced into the lower crust in a very short period against a long history of collision,and they have different formation mechanism from those in within-plate,mantle plume-related mafic magmatism.The exploration of Ni-Cu sulfide deposits in convergent margin settings needs new thoughts and methods.
作者
王焰
魏博
曹勇华
高剑峰
暴宏天
杨军辉
WANG Yan;WEI Bo;CAO Yonghua;GAO Jianfeng;BAO Hongtian;YANG Junhui(State Key Laboratory of Deep Earth Processes and Resources,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;Guangdong Provincial Key Laboratory of Mineral Physics and Materials,Guangzhou 510640,Guangdong,China;School of Geosciences and Info-physics,Central South University,Changsha 410012,Hunan,China;Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,Guizhou,China;University of Chinese Academy of Sciences,Beijing 100049,China)
出处
《大地构造与成矿学》
北大核心
2025年第4期753-776,共24页
Geotectonica et Metallogenia
基金
国家自然科学基金重大研究计划集成项目(92462305)
国家自然科学基金重点项目(41730423)
中国科学院前瞻战略科技先导专项(A类)课题(XDA0430303)
广东省科技计划项目(2023B1212060048)联合资助。
关键词
铜镍硫化物矿床
镁铁-超镁铁质岩体
中亚造山带
汇聚板块边缘
magmatic Ni-Cu sulfide deposits
mafic-ultramafic intrusions
Central Asian Orogenic Belt
convergent margin setting
