交代地幔源区与造山带铜镍成矿作用

Genetic correlation of metasomatized mantle source with Ni-Cu mineralization in orogenic belt

造山带具有复杂的动力学环境、俯冲交代历史和热力学状态,其地幔源区物质组成、熔融机制和岩浆产物明显不同于地幔柱和大陆裂谷带,因此造山带环境中与镁铁-超镁铁质岩浆有关的铜镍成矿作用有别于板内环境。本文综述了交代地幔源区与造山带铜镍成矿关系的新进展,重点聚焦于:(1)岩石圈与软流圈的贡献。成矿岩浆来源于上涌的软流圈驱动交代岩石圈地幔不同程度的混合熔融,其中后俯冲阶段岩浆产物的成矿作用最为普遍、规模最大;岩石圈地幔经历不同比例俯冲板片沉积物熔体(富钙沉积物)和低温蚀变洋壳流体交代,具有显著的富挥发分和碳酸盐特征。(2)幔源岩浆氧化还原状态对成矿的约束。显生宙造山带成矿初始岩浆(硫化物未发生饱和)可能普遍为氧化性岩浆,岩浆体系硫化物饱和前后存在氧逸度骤降,导致岩浆体系硫化物饱和时硫的溶解度呈指数级下降,直接触发硫化物熔离,或者为达到熔离创造低的硫溶解度条件,其中岩浆还原作用由碳质物在岩浆期的有效混染造成。(3)挥发分对岩浆硫化物的运载汇聚。挥发分显著降低熔体粘度,从而减小粘滞阻力对岩浆超压的消耗;岩浆在上侵减压过程中挥发分能够发生出溶-成核作用,形成大量低密度气泡并与硫化物液滴形成复合液滴,促进硫化物液滴聚结,提高其成矿元素含量,并向上输运硫化物。总之,全球部分典型铜镍成矿省与交代地幔时空耦合关系表明,地幔具交代属性可能一定程度上促进了岩浆硫化物成矿。尽管交代地幔对造山带铜镍成矿的贡献方面取得了诸多重要进展,但前寒武造山带幔源岩浆氧化还原状态对成矿的影响、造山带镍铜钴成矿多样性、经历不同交代作用的地幔源区熔融过程中亲铜元素的行为机制、壳幔圈层内挥发分循环对亲铜元素迁移-富集的控制作用等领域仍需深入研究。

Orogenic belts normally have complex dynamic environments,subduction history and thermodynamic conditions,hence,their mantle source components,melting mechanisms and magmatic products are significantly different from those of mantle plumes and continental rift zones.For this reason,mafic/ultramafic magmatism-related Ni-Cu sulfide mineralization in orogenic belts is different from those in intra-plate environments.This article reviews the new progress in the relationship between metasomatized mantle sources and Ni-Cu mineralization in orogenic belts,focusing on the following three aspects:(1)Contributions of lithosphere and asthenosphere:Ore-forming mafic/ultramafic magmas are generated through the combination of decompression melting of asthenospheric mantle and hydrous melting of metasomatized lithospheric mantle.The metasomatized mantle,showing volatile-and carbonate-rich characteristics,was previously modified by variable proportions of slab sediments/melts(Ca-rich sediments)and fluids derived from low-temperature altered oceanic crust.The most important mineralization commonly forms in the post-subduction stage.(2)Constraints on mineralization from the redox state of mantle-derived magmas:A rapid drop in oxygen fugacity exists in the ore-forming magmas before and after sulfide saturation,leading to exponentially decrease in sulfur solubility in the magma system of the Phanerozoic orogenic belts,which may result in sulfide segregation or create low-sulfur solubility conditions for further sulfide saturation.The magma reduction process has proven to be caused by efficient assimilation of carbonaceous matter during magma emplacement.(3)Transport and concentration of sulfide droplets by volatile components:Volatiles significantly reduce the viscosity of the silicate melt,decreasing the viscous resistance to magma overpressure.The exsolution and nucleation of volatiles during magma ascent produce many low-density bubbles which combine with sulfide droplets to form compound drops.This process promotes the aggregation of sulfide droplets,increases their metal content,and facilitates the upward transport of sulfide droplets.These understandings in combination with the spatial-temporal coupling between some typical Ni-Cu metallogenic provinces and the underlying metasomatized mantle indicate that the metasomatized mantle may promote magmatic sulfide mineralization.Although significant advances have been achieved in recent years in the study of contribution of metasomatized mantle to Ni-Cu sulfide mineralization in orogenic belt,further research is needed on the effect of the redox state on the ore-forming magmas in the pre-Cambrian orogenic belt,the diversity of Ni-Cu-Co mineralization in orogenic belts,the effect of different metasomatism on the behavior of chalcophile elements during mantle melting,and the controls of volatile recycling within mantle-crust on the migration and enrichment of chalcophile elements.