Y/Ho ratio as genetic indicator of sparry magnesites in south Urals, Russia
用Y／Ho比值指示俄罗斯乌拉尔南部晶质菱镁矿矿床的成因

There are two types of sparry magnesite deposits in the Southern Urals province: 1) layer-like bodies in the stratigraphic dolostone horizons; 2) lens-like bodies in the dolomitized limestones. The sparry magnesite deposits are located in the dolostones of the Lower and Middle Riphean series and are absent in dolostones units of the Upper Riphean. These two types of magnesite are distinguishing by the shape of ore bodies and size of crystals, quantity of quartz and dolomite admixtures. The resources of the first type are huge; magnesite has a coarse-grained structure with crystal size of more than 10 mm (up to 150 mm) , as a rule, sharp boundaries of ore bodies with host dolomites. This type of deposits is typical for the Lower Riphean sequence. The second type of deposits has a very irregular shape of ore bodies with impregnation of magnesite crystalls into host dolomites, with relatively small size of magnesite crystals (1-5 mm). The second type predominates in the Middle Riphean sections. All the deposits possess the features of metasomatic origin. The two types of magnesite ores show distinct features of some major elements and REE distributions. Magnesites of the first type have lower contents of FeO, CaO and SiO2 in comparison with magnesites of the second type. The first type magnesites are LREE depleted with La/Lu < 1 in comparison with country rocks of dolomites (La/Lu > 1) . Magnesites of the second type have a low degree of fractionation and differences in REE distributions between magnesites and country rocks of dolomites. In this paper the importance of Y/Ho ratio is discussed, because the resemblance of this ratio in magnesites and host rocks makes it possible to separate hydrothermal and early diagenetic metasomatic processes of magnesite formation. We suggest the first type magnesites, such as the Satka and Bakal deposits with high Y/Ho ratios belong to early diagenetic stage of development of the rock-forming basin. The second type magnesites, such as the Ismakaevo and Katav-Ivanovsk deposits with low Y/Ho ratios, are related to hydrothermal processes of fluid migration.