Layers rich in chromian spinel (Cr-spinel) occur in numerous differentiated and layered intrusions. These layers are often characterized by elevated and even economic concentrations of platinum-group-elements (PGEs), but only scarce sulfide mineralization. One particular type of such lithology occurs in the roof parts of the Norilsk-type differentiated intrusions (Russia) and is referred to as the “sulfide-poor PGE ores”. We investigated rocks containing variable enrichments in Cr-spinel, sulfides, and platinum-group minerals (PGMs) from two sections of the upper zone of the Norilsk-1 intrusion, with a focus on Cr-spinel. The rocks are dominated by two lithological types: (1) leucogabbro/troctolitic and (2) olivine-gabbro. Fine-grained (5–100 μm) disperse disseminations with varying modal abundances of Cr-spinel are characteristic for the rocks studied. Those abundances range from scarce mineralization through to very dense (up to 30 vol.% Cr-spinel) cloud-like accumulations. However, compact-grained accumulations and cumulate-like textures, which are typical for chromitites of layered intrusions, are not characteristic for the studied rocks. Instead, the disseminations exhibit chain- and trail-like alignments of Cr-spinel grains, which cross the boundaries between enclosing silicates, and sub-circular arrangements. The study revealed millimeter-scaled patchy distribution of Cr-spinel compositions within a given dissemination with Cr-spinel chemistry being strongly correlated with a kind of the enclosing silicate. (1) In unaltered rocks, plagioclase hosts more magnesian Cr-spinel (Mg# 30–60), while Cr-spinel in mafic minerals is less magnesian (Mg# 18–35). (2) In altered rocks, more magnesian Cr-spinel is hosted by less altered silicates, while strongly altered silicates mainly host less magnesian Cr-spinel. Systematics of trivalent cations exhibits divergent trends, even on a scale of a thin section, and depends on a kind of hosting lithology. Leucogabbro/troctolite lithologies contain Cr-spinel with anomalously low Fe³⁺ and extremely high Ti contents, whereas Cr-spinel from olivine-gabbro lithologies have moderate Fe³⁺ and moderately-high Ti contents. It is envisaged that crystallization of Cr-spinel and their host rocks occurred from viscous mingled magmas, which had different compositions and redox state. Subsequent processes involved (1) high-temperature re-equilibration of Cr-spinel with enclosing silicates and (2) post-magmatic alteration and partial recrystallization of Cr-spinel. During these processes, Cr-spinel was losing Mg and Al and gaining Fe and Ti. These chemical trends are generally coincident with those established for other intrusions worldwide, but the upper zone of the Norilsk-1 intrusion seems to possess an exceptional variety of Cr-spinel compositions, not recorded elsewhere.

中文翻译：

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在Norilsk-1矿床（俄罗斯北西伯利亚）的“硫化物含量低的铂族金属矿石”中铬铁尖晶石的原位结晶和连续改性

富含铬尖晶石（Cr-spinel）的层出现在许多不同且分层的侵入体中。这些层的特征通常在于铂族元素（PGE）的浓度升高，甚至达到经济浓度，但硫化物矿化很少。这种岩性的一种特殊类型出现在Norilsk型差异化侵入岩（俄罗斯）的顶部，被称为“硫化物贫乏的PGE矿石”。我们研究了来自Norilsk-1侵入岩上部区域两个部分的Cr-尖晶石，硫化物和铂族矿物（PGM）富集富集的岩石，重点是Cr-尖晶石。岩石以两种岩性类型为主：（1）白带/微晶石质和（2）橄榄石-藻类。Cr-spinel的模态丰度不同的细粒度（5–100μm）分散分布是所研究岩石的特征。这些丰度从稀少的矿物质到非常稠密（高达30％（体积）的Cr-尖晶石）的云状堆积。但是，对于层状侵入体的铬铁矿来说，典型的致密颗粒状堆积物和类似堆积的纹理并不是所研究岩石的特征。取而代之的是，散布显示出Cr-尖晶石晶粒的链状和尾状排列，该排列穿过包围的硅酸盐和亚圆形排列之间的边界。研究表明，在一定的传播范围内，Cr-spinel成分的毫米级斑片状分布与Cr-spinel的化学作用与一种封闭的硅酸盐密切相关。（1）在未变的岩石中，斜长石拥有更多的镁镁铬尖晶石（Mg＃30–60），而镁铁矿质中的铬尖晶石镁含量较低（Mg＃18–35）。（2）在蚀变的岩石中，镁含量较高的Cr-尖晶石由较少变化的硅酸盐构成，而强烈蚀变的硅酸盐主要具有较少的镁质Cr-尖晶石。三价阳离子的体系学表现出不同的趋势，即使是在薄截面的规模上也是如此，并且取决于一种宿主岩性。Leucogabbro /云母岩岩性含Cr-尖晶石，铁含量异常低 并取决于一种宿主岩性。Leucogabbro /云母岩岩性含Cr-尖晶石，铁含量异常低 并取决于一种宿主岩性。Leucogabbro /云母岩岩性含Cr-尖晶石，铁含量异常低³⁺和极高的Ti含量，而橄榄石-鹅卵石岩性的Cr-尖晶石具有中等的Fe ³⁺和中等高的Ti含量。可以设想，Cr-尖晶石及其宿主岩石的结晶是由粘性混合岩浆产生的，该岩浆具有不同的组成和氧化还原状态。随后的过程涉及（1）用封闭的硅酸盐对Cr-spinel进行高温重新平衡，以及（2）Cr-spinel的岩浆蚀变和部分重结晶。在这些过程中，Cr-尖晶石损失了Mg和Al，而获得了Fe和Ti。这些化学趋势通常与世界范围内其他入侵的趋势一致，但是Norilsk-1入侵的上部似乎具有异常丰富的Cr-尖晶石成分，这在其他地方没有记录。