金川岩浆铜镍硫化物矿床中的镍钴分布规律及其控制因素

幔源岩浆形成与演化过程中镍(Ni)、钴(Co)具有相似的地球化学行为。金川岩浆铜镍硫化物矿床以Ni、铜(Cu)为主要矿种, Co为伴生, Ni、Co在金川矿床中的空间分布规律同步变化, 然而其Ni/Co比值(36.7)远高于地幔值(18.2)。这表明在金川矿床形成过程中Ni-Co发生了共生分离, 但Ni-Co分布特征尚不清楚、其控制因素尚不明确。本文对该矿床中主要矿石矿物的Ni、Co含量及分布进行了系统总结, 并与脉石矿物进行对比。结果表明矿石矿物镍黄铁矿是最重要的含Ni、Co矿物相, 其Ni、Co含量均远高于磁黄铁矿、黄铜矿及脉石矿物。对于脉石矿物, Ni在橄榄石、磁铁矿、铬铁矿内的含量依次降低, 在斜方辉石与单斜辉石中含量最低。Co则在铬铁矿、橄榄石内含量依次降低, 在斜方辉石、单斜辉石、磁铁矿中含量最低。在硫化物熔离过程中, Ni在硫化物熔体内相容性更强, 更加倾向于进入硫化物熔体, 使Ni显著富集于硫化物熔体内, 而Co则相对富集于硅酸盐熔体内, 由此导致Ni-Co解耦。硫化物冷却结晶过程中, Ni、Co倾向于进入最早结晶的单硫化物固溶体(MSS), 并在随后分解作用中集中进入镍黄铁矿内, 使镍黄铁矿成为金川矿床中最重要的含Ni-Co矿物相, 并使Ni、Co在金川矿床中具有相似的空间分布规律。在硅酸盐熔体结晶分异过程中, Ni在橄榄石中的相容性最强, Co在铬铁矿中相容性最强, 因此Ni倾向于进入橄榄石, 而Co倾向于进入铬铁矿, 由此导致Ni-Co发生解耦。硫化物熔离、橄榄石堆晶均会造成残余熔体Ni亏损程度高于Co, 且Ni在斜方辉石与单斜辉石中相容性高于Co将导致残余熔体随冷却结晶Ni/Co比值逐渐降低, 因此在粒间硅酸盐矿物结晶过程中Ni、Co倾向于共生。脉石矿物亚固相下与硫化物熔体反应对于Ni-Co共生分离的影响则与结晶作用完全相反。镍黄铁矿和磁黄铁矿出溶于硫化物矿浆结晶早期形成的MSS, 在不同岩/矿石类型内Ni、Co含量同步变化, 表明镍黄铁矿和磁黄铁矿成分可以用来指示岩浆铜镍硫化物矿床成矿过程中硫化物熔体成分的演变。

Distribution and controlling factors of nickel and cobalt in the Jinchuan magmatic Cu-Ni sulfide deposit

Nickel (Ni) and cobalt (Co) have similar geochemical behaviors during the formation and evolution of the mantle-derived magmas. Jinchuan magmatic Cu-Ni sulfide deposit contains Ni and Cu as the main economic commodities with Co as by-product. Despite the similar spatial distribution of Ni and Co, the curiously higher Ni/Co ratio (36.7) of the Jinchuan deposit compared to that of mantle (18.2) indicates the paragenesis and separation between Ni and Co occurred during the metallogenic processes, whose mechanism remains poorly understood. This paper summarizes the concentrations of Ni and Co of ore minerals comparing with gangue minerals in order to gain a better understanding of the distribution of Ni and Co, and its controlling factors. The results show that the pentlandite serves as the primary reservoir for Ni and Co, with substantially higher contents than pyrrhotite, chalcopyrite and the gangue minerals. Regarding the gangue minerals, olivine has the highest Ni contents followed by magnetite, chromite, orthopyroxene and clinopyroxene. Cobalt is richest in chromite followed by olivine, orthopyroxene, clinopyroxene and magnetite. Since Ni is much more compatible in sulfide liquids than Co during the sulfide segregation, Ni becomes significantly enriched in the immiscible sulfide melt while Co becomes relatively enriched in the silicate melt, resulting in the decoupling of Ni and Co. During the crystallization of sulfide liquids, both Ni and Co tend to be concentrated together in the early crystallized MSS. Subsequently, pentlandite exsolved from MSS, becoming the most important reservoir of Ni and Co and resulting the similar spatial distribution of Ni and Co. During the differentiation of silicate liquids, Ni is highly compatible in olivine, while Co is highly compatible in chromite. Therefore, fractional crystallization of cumulus olivine and chromite in the Jinchuan deposit results in the decoupling of Ni and Co. In contrast, the crystallization of orthopyroxene and clinopyroxene, in which Ni is more compatible than Co, is balanced by the depletion of Ni in the intercumulus residual melts. The sub-solidus diffusion of Ni and Co from olivine, chromite, orthopyroxene and clinopyroxene into sulfide liquids exerts effects on the Ni-Co distribution completely opposite to crystallization. Given that pentlandite and pyrrhotite exsolved from MSS crystallizing from sulfide liquids at early stage, the covariant Ni and Co contents of them between different rock/ore types prove that elements compatible with MSS in pentlandite and pyrrhotite can be used to indicate the evolution of sulfide liquids.