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

铬铁矿和自然铂通常被认为是原始玄武岩熔体的早期高温相。因此，当铬铁矿和Pt-Fe合金被发现富集在原始岩浆堆积物中时，可以假定这些相是在岩浆温度下富集的。与玄武质熔体中的液相线相相比，乌拉尔玄武岩中的铬铁矿和铂铁合金的粒度可以粗几个数量级。含水硅酸盐是常见的，铬铁矿在自形腔中携带许多代晚期含水相，这些自形腔遵循铬铁矿的晶体对称性。问题是流体相是否直接来自于堆积物的母体熔体，或者它们是否反映了随机的叠印。原始岛弧熔体-超镁铁质乌拉尔玄武岩的潜在母体-含有约4重量%水该H2O浓度足以使衍生物熔体及其累积物饱和含水流体相。工作假设是，是否以及在何种程度上可以动员和富集岩浆铬铁矿和Pt-Fe细石的堆间熔体和流体分离的结晶深根岛弧ankaramites。所有含铂超基性岩的乌拉尔铂带平行排列的旧缝合线标志着前志留纪收敛板边缘在乌拉尔山脉。因此，如果它们勾勒出一个化石火山弧，它们应该都返回类似的侵位年龄。所选择的测年方法是190 Pt-186 Os衰变方案，在这里对来自Kondyor Bronze的Pt-Fe合金进行了测试。如果UPB的所有含铂侵入体返回铂矿化年龄误差内相同，一个成因联系必须存在之间的侵位和冷却历史的乌拉尔火山岩和他们的铂矿化。

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.