Platinum enrichment processes in ultramafic massifs of the Uralian Platinum belt, Russia - evidence for mineralization at the magmatic to hydrothermal transition

铬铁矿和原生铂通常被认为是原始玄武岩熔体的早期高温相。因此，当在原始岩浆堆积中发现富集铬铁矿和Pt-Fe合金时，可以认为这些相是在岩浆温度下富集的。乌拉利铂带地块中的铬铁矿-铂包裹体显示出许多反对纯岩浆成因的特征。与玄武岩熔体中各自的液相相相比，乌拉利岩体中的铬铁矿和Pt-Fe合金可以粗几个数量级。含水硅酸盐是常见的，而铬铁矿在遵循铬铁矿晶体对称的自面体腔内携带了许多代的晚期含水相。问题是，流体相是直接来自堆积物的母体熔体，还是反映了随机叠印。原始岛弧熔体-超镁质乌拉尔地块的潜在母体-含有~ 4wt .%的水。该H2O浓度足以使衍生熔体及其累积物饱和，形成含水流体相。工作假设是岩浆铬铁矿和Pt-Fe微晶是否以及在多大程度上可以被从岛弧ankaramites结晶深成根中分离出来的积云间熔体和流体动员和浓缩。了解富集过程的一个重要方面是确定成矿的年代。乌拉尔铂带所有含铂超镁铁质块体均平行于乌拉尔山脉前志留系会聚板块边缘的古缝合线。因此，如果它们勾勒出了化石火山弧，它们应该都返回了相似的就位年代。选择的测年方法是190Pt-186Os衰变方案，在这里测试了来自Kondyor地块的Pt-Fe合金。如果UPB中所有含铂侵入体返回的铂矿化年龄在误差范围内相同，则乌拉尔地块的侵位和冷却历史与其铂矿化之间必然存在遗传联系。

Chromite and native Pt are usually considered to be early high temperature phases of primitive basaltic melts. Consequently, when chromite and Pt-Fe alloy are found enriched within primitive magmatic cumulates, it is assumed that these phases were enriched at magmatic temperature.The chromite-Pt enclaves in the massifs of the Uralian Platinum belt show many features that argue against a purely magmatic origin. Compared with the respective liquidus phases in basaltic melts, chromite and Pt-Fe alloys in the Uralian massifs can be orders of magnitude more coarse-grained. Hydrous silicates are commonplace, and chromite carries many generations of late hydrous phases within euhedral cavities that follow the crystal symmetry of chromite. The question is whether the fluid phases were derived directly from the parental melts of the cumulates or whether they reflect a random overprint. Primitive island arc melts - the potential parents to the ultramafic Uralian massifs - contain ~ 4 wt.% H2O. That H2O concentration is sufficient to render derivative melts and their cumulates saturated with a hydrous fluid phase. The working hypothesis is whether and to what extent magmatic chromite and Pt-Fe microliths can be mobilized and concentrated by intercumulus melts and fluids segregating from the crystallizing plutonic roots of island arc ankaramites.An important aspect to understand the enrichment processes is to date the mineralization. All platiniferous ultramafic massifs of the Uralian Platinum Belt are aligned parallel to the old suture marking the former Silurian convergent plate margin in the Ural mountains. Hence, if they outline a fossil volcanic arc, they should all return similar emplacements ages. The dating method of choice is the 190Pt-186Os decay scheme, tested here on Pt-Fe alloys from the Kondyor massif. If all platiniferous intrusions of the UPB return Pt mineralization ages identical within error, a genetic link must have existed between emplacement and cooling histories of the Uralian massifs and their Pt mineralization.