镁铁-超镁铁岩的全岩和单矿物钴镍分布特征及其成矿指示

Distribution of Co and Ni in mafic-ultramafic rocks and minerals and mineralization implications

本文对不同成因类型的镁铁-超镁铁岩石中全岩及主要造岩矿物中Co、Ni元素分布特征进行了总结,并探讨了不同类型地幔源区及其矿物元素含量差异对Co-Ni成矿的控制作用。结果表明,科马提岩的全岩Co(15×10^(-6)~250×10^(-6))、Ni(28×10^(-6)~5045×10^(-6))含量均高于苦橄岩(Co=11×10^(-6)~177×10^(-6)、Ni=29.9×10^(-6)~2069×10^(-6))和玄武岩(Co=1.1×10^(-6)~300×10^(-6)、Ni=0.5×10^(-6)~2837×10^(-6)),表明地幔高程度部分熔融形成的熔体更加富集Co和Ni。地幔橄榄岩捕掳体的全岩Co(21×10^(-6)~220×10^(-6))和Ni(2.1×10^(-6)~4060×10^(-6))含量均显著高于辉石岩捕掳体(Co=7×10^(-6)~131×10^(-6)、Ni=4.58×10^(-6)~2311×10^(-6)),表明部分熔融过程中橄榄岩地幔源区相较于辉石岩地幔源区更容易形成富Co、Ni的玄武质岩浆。矿物学特征方面,地幔捕掳体中橄榄石的Co、Ni含量分别为41×10^(-6)~437×10^(-6)和148×10^(-6)~8970×10^(-6),尖晶石的Co、Ni含量分别为141×10^(-6)~745×10^(-6)和282×10^(-6)~9587×10^(-6),均高于共生的斜方辉石(Co=10×10^(-6)~364×10^(-6)、Ni=5.02×10^(-6)~4439×10^(-6))、单斜辉石(Co=4.09×10^(-6)~385×10^(-6)、Ni=11.1×10^(-6)~5931×10^(-6))和石榴子石(Co=0.47×10^(-6)~205×10^(-6)、Ni=0.09×10^(-6)~644×10^(-6))。同时,橄榄石和尖晶石的Co含量与其Ni/Co比值呈较好的负相关关系,而斜方辉石和单斜辉石的Co含量与其Ni/Co比值无明显相关性。这些特征表明部分熔融过程中尖晶石相地幔比石榴石相地幔更容易形成富Co、Ni的熔体。侵入岩中主要造岩矿物具有与地幔捕掳体中各矿物相一致的Co、Ni含量变化特征。这表明地幔部分熔融或岩浆结晶分异过程中,橄榄石和尖晶石是控制岩浆中Co、Ni含量的主要矿物相,并且可能造成岩浆中Co-Ni解耦,而斜方辉石和单斜辉石的熔融或结晶过程中Co-Ni倾向于共生。此外,蛇绿岩作为风化壳型镍矿床的成矿母岩,其橄榄岩和铬铁岩中尖晶石和橄榄石具有较高的Co和Ni元素含量,橄榄石的Ni(1280×10^(-6)~5789×10^(-6))含量明显高于尖晶石(229×10^(-6)~2616×10^(-6)),而其Co(41.2×10^(-6)~193×10^(-6))含量则明显低于尖晶石(176×10^(-6)~781×10^(-6)),表明风化过程中富集橄榄石的岩石更容易形成富Ni的风化壳型矿床,而富集尖晶石的岩石更容易贡献于风化壳型镍矿床中的Co矿化。

This paper summarizes the distribution characteristics of Co and Ni elements in whole rocks and major rock-forming minerals of various mafic-ultramafic rocks,and discusses controlling effects of different types of mantle source and their mineral element contents on Co-Ni mineralization.Our results indicate that the whole rock Co(15×10^(-6)~250×10^(-6))and Ni(28×10^(-6)~5045×10^(-6))contents in komatiites are generally higher than those in picrites(Co=11×10^(-6)~177×10^(-6),Ni=29.9×10^(-6)~2069×10^(-6))and basalts(Co=1.1×10^(-6)~300×10^(-6),Ni=0.5×10^(-6)~2837×10^(-6)),suggesting melts formed by partial melting of the mantle at high degrees are more enriched in Co and Ni.The Co(21×10^(-6)~220×10^(-6))and Ni(2.1×10^(-6)~4060×10^(-6))contents in mantle peridotite xenoliths are significantly higher than those in pyroxenite xenoliths(Co=7×10^(-6)~131×10^(-6),Ni=4.58×10^(-6)~2311×10^(-6)),indicating that peridotite sources are more likely to generate Co-and Ni-rich magmas than pyroxenite sources during partial melting.Mineralogical characteristics reveal that olivine in mantle xenoliths has Co and Ni contents of 41×10^(-6)~437×10^(-6) and 148×10^(-6)~8970×10^(-6),respectively,while spinel has Co contents ranging from 141×10^(-6) to 745×10^(-6) and Ni from 282×10^(-6) to 9587×10^(-6),both of which are higher than those in orthopyroxene(Co=10×10^(-6)~364×10^(-6),Ni=5.02×10^(-6)~4439×10^(-6)),clinopyroxene(Co=4.09×10^(-6)~385×10^(-6),Ni=11.1×10^(-6)~5931×10^(-6)),and garnet(Co=0.47×10^(-6)~205×10^(-6),Ni=0.09×10^(-6)~644×10^(-6)).Meanwhile,olivine and spinel show negative correlations between their Co contents and Ni/Co ratios,whereas such correlation is not evident in orthopyroxene or clinopyroxene.These features indicate that spinel-facies peridotites could supply more Co and Ni to partial melts during partial melting than garnet-facies peridotites.Major rock-forming minerals in intrusive rocks exhibit similar Co and Ni variations compared to the counterparts in mantle xenoliths,indicating that olivine and spinel are main phases controlling Co and Ni contents in magmas during partial melting and magmatic differentiation,leading to Co-Ni decoupling,whereas orthopyroxene and clinopyroxene tend to make Co and Ni co-variations.Moreover,as parent rocks of laterite Ni deposits,peridotites and chromitites in ophiolites show higher Co and Ni contents.Ophiolitic olivine shows higher Ni contents(1280×10^(-6)~5789×10^(-6))than spinel(229×10^(-6)~2616×10^(-6)),whereas its Co contents(41.2×10^(-6)~193×10^(-6))are significantly lower than that of spinel(176×10^(-6)~781×10^(-6)),indicating that olivine-rich rocks during weathering are more likely to form Ni-rich laterite deposits,while those spinel-rich ones are more likely to contribute to Co mineralization in lateritic Ni deposits.