Comparison of Lower and Middle Riphean sparry magnesite deposits of the Southern Urals province

Sparry magnesite deposits (SMD) of the Lower Riphean (LR) and Middle Riphean (MR) sequences in the western slope of the South Urals have some distinguished peculiarities, which allows to compare them and speculate about the mechanism of ore formation and Mg-source origin. LR magnesites are located in some stratigraphic levels and lie within widespread dolostone horizons. MR magnesites that occur in the lower carbonate member of Avzyan fm. are represented by dolomitized limestones. The shape of LR magnesite body is very often layer-like with sharp contacts. The shape of MR one is sometimes lens like, contacts of ore body are complecated with zones of impregnable magnesites near the hosting dolomites. The crystalline structures of LR magnesites are typically corse-grained with average size of crystalls 10 mm. The magnesite grains of the MR one have 1-3 mm of average size. The chemical composition of MR magnesite ore shows increase of SiO 2 , Al 2 O 3 , FeO and decrease of LREE fractionation in comparison with the LR magnesites. As we assume, metasomatic magnesites of the Lower Riphean time are connected with sedimentation/early diagenesis stage. The source of Mg came from a crust of weathering of basic and ultramafic rocks in humid climate. Magnesite occurrences in Middle Riphean time are connected with high Fe-contents metasomatic fluids, which were resulted of evaporite Mg-enriched solutions transformation during kathagenesis.