橄榄石成分对东昆仑夏日哈木超大型镍钴矿床岩浆过程的约束

Constraints of Olivine Composition on the Magmatic Process of the Xiarihamu Super-Large Ni-Co Sulfide Deposit in East Kunlun Orogenic Belt

橄榄石是与镁铁-超镁铁质岩浆有关的铜镍硫化物矿床中最主要的造岩矿物之一,其成分记录了丰富的镍成矿信息,并可反映母岩浆成分组成、结晶分异过程及后期物质交换等。东昆仑夏日哈木镍钴矿床是全球铜镍硫化物矿床勘查20余年来最为重要的找矿发现之一,将逐步成为中国重要的镍钴资源基地。本文在大量镜下岩相学观察的基础上,对夏日哈木矿床橄榄石进行系统电子探针分析。分析数据表明,夏日哈木橄榄石Fo值为83.01~90.12;Ni含量为676×10^(-6)~2845×10^(-6),Mn、Ca含量均较低,最大值分别为2457×10^(-6)和650×10^(-6)。“巨型”斜方辉石包裹的橄榄石成分自核心到边部Fo值和NiO含量均增高,表明含矿岩相形成时存在新鲜岩浆的补充,至少存在2次岩浆活动。橄榄石最高Fo值为90.12,反映夏日哈木镍钴矿床的母岩浆是Mg#值为0.73的高镁玄武质岩浆。硫饱和始于深部岩浆房母岩浆中少量橄榄石的结晶分异,早期熔离量较小,估算的橄榄石与硫化物熔体质量比大致为20∶1。本次研究利用橄榄石成分约束了夏日哈木矿床岩浆的多期次活动,限定了母岩浆的成分性质与硫化物的熔离时机,可为矿床成因研究提供借鉴。

Olivine is one of the most important rock-forming minerals in Ni-Cu sulfide deposits related to mafic-ultramafic magmas.Its composition records abundant nickel mineralization information,and can also reflect the composition of parent magmas,crystallization differentiation process and later material exchange between olivine and magmas.The Xiarihamu Ni-Co deposit in East Kunlun orogenic belt is one of the most significant prospecting discoveries of Ni-Co sulfide deposits in the world over the past two decades.The large-scale resource reserves will make it gradually become an important nickel-cobalt resource base in China.This work conducted a large number of microscopic petrographic observations,and performed systematic electron probe analysis of olivines from the Xiarihamu deposit.The analysis shows that the Fo value of the Xiarihamu olivine is 83.01~90.12;the Ni content is 676×10^(-6)~2845×10^(-6);the content of Mn and Ca is relatively low,with the maximum content being 2457×10^(-6) and 650×10^(-6),respectively.The Fo and NiO content of olivines wrapped by"giant"orthopyroxene increased from the core to the edge,indicating that there is fresh magma supplement and at least two magmatic pulses during the formation of ore-bearing lithofacies.The highest Fo value of olivine is 90.12,reflecting that the parent magma of Xiarihamu is a high-Mg basaltic magma with Mg#value of 0.73.Sulfur saturation started from the crystallization differentiation of a small amount of olivine in the parental magma of deep magma chamber,and the scale of early sulfide segregation was relatively small.The estimated mass ratio of olivine to sulfide melt was approximately 20:1.The olivine composition was used to constrain the multi-pulse activity of magma in the Xiarihamu deposit,and to limit the composition properties of the parental magma and the timing of sulfide melting,which can be used as a reference for the study of the deposit genesis.