含硫化物镁铁质-超镁铁质侵入体中橄榄石Mg、Ni含量的制约:原理、模式及Voisey's Bay侵入体样品研究

镁铁质-超镁铁质岩浆结晶分离早期形成镁铁矿物,镁铁矿物中的Ni和Mg是相容元素。随着结晶分离作用的进行,Ni、Mg在硅酸盐岩浆及后形成的硅酸盐物质中的丰度下降。橄榄石中Ni含量及硅酸盐物质MgO/FeO比值都与母岩浆的相关值相关,据此可推断母岩浆的信息,它们之间可由实验测得的系数相联系。当岩浆饱和硫化物时,在结晶分离过程中硫化物珠滴会与镁铁硅酸盐物质一道析出,同时,与硫化物非饱和岩浆相比,过多的Ni会随之析出。这也反映在Ni、Mg含量比无硫化物分离时有更迅速的降低上。Ni、Mg含量变化值可以在VoiseysBay侵入体的模式曲线上反映出,加拿大Labrador的这一侵入体赋存了一个世界级的Ni-Cu-Co硫化物矿床。过去的作法是将侵入体中橄榄石的Ni、Mg含量与Simkin和Smith得出的各种火成岩中橄榄石的Ni、Mg含量相比较以确定Ni亏损,进而假定橄榄石来自硫化物饱和、有经济价值的岩浆。现在的研究显示这种简单的对比会导致错误。将样品数据与模式曲线对比并反映出侵入体矿物结晶堆积特征是重要的方法。使用这一方法,样品数据能很好地被模式曲线拟合。以在VoiseysBay的研究为例,当硫化物液相与硅酸盐矿物被去除后,硫化物非饱和的分离作用期就会显现出来,随后是硅酸盐结晶作用期。

CONTROLS ON THE Fo AND Ni CONTENTS OF OLIVINE IN SULFIDE-BEARING MAFIC-ULTRAMAFIC INTRUSIONS: PRINCIPLES, MODELING AND EXAMPLES FROM VOISEY'S BAY

Both Ni and Mg are compatible in mafic minerals that form early during the fractional crystallization of mafic-ultramafic magma. Thus both decrease in abundance in the silicate magma, and thus in later-forming silicates as fractionation proceeds. The concentration of Ni in silicates such as olivine and the MgO/FeO ratio of the silicates are related to values in the magma from which the olivine crystallized by coefficients which have been experimentally determined and therefore can be used to infer information about the magma. If the magma is saturated in sulfide, so that sulfide droplets are removed along with mafic silicates during fractionation, additional Ni will be removed in comparison with the sulfide-absent situation. This will be reflected in a more rapid decrease in Ni with Fo than if sulfides were not separating. Variations of Ni with Fo are examined in the light of model curves for the Voisey's Bay Intrusion that hosts a world-class Ni-Cu-Co sulfide deposit in Labrador, Canada. In the past it has been the practice to compare the Ni and Fo contents of olivines from a given intrusion with the field determined by Simkin and Smith for a wide variety of igneous olivines in order to identify those that are Ni-depleted. The objective is that these are presumed to have come from sulfide-saturated, and therefore economically-interesting magma. Several studies cited in this review have shown that this simple comparison can lead to errors. It is important to compare natural data with model curves that have been generated for, and reflect the cumulus mineralogy of each intrusion in question. Using this approach, the natural data can be closely duplicated by model curves which, in some cases, place additional constraints on possible petrologic interpretations. For example, at Voisey's Bay a period of sulfide-unsaturated fractionation can be shown to have been succeeded by removal of a sulfide liquid plus silicate minerals, followed by a period of silicate crystallization.