Formation of magnesite and siderite deposits in the Southern Urals-evidence of inclusion fluid chemistry

World-class deposits of magnesite and siderite occur in Riphean strata of the Southern Urals, Russia. Field evidence, inclusion fluid chemistry, and stable isotope data presented in this study clearly proof that the replacement and precipitation processes leading to the formation of the epigenetic dolomite, magnesite and hydrothermal siderite were genetically related to evaporitic fluids affecting already lithified rocks. There is, however, a systematic succession of events leading to the formation of magnesite in a first stage. After burial and diagenesis the same brines were modified to hot and reducing hydrothermal fluids and were the source for the formation of hydrothermal siderite. The magnesites of the Satka Formation as well as the magnesites and the siderites of the Bakal Formation exhibit low Na/Br (106 to 222) and Cl/Br (162 to 280) ratios plotting on the seawater evaporation trend, indicating that the fluids acquired their salinity by evaporation processes of seawater. Temperature calculations based on cation exchange thermometers indicate a formation temperature of the magnesites of 130 A degrees C. Considering the fractionation at this temperature stable isotope evidence shows that the magnesite forming brines had delta O-18(SMOW) values of +1 aEuro degrees thus indicating a seawater origin of the original fluid. Furthermore it proves that these fluids were not yet affected by appreciable fluid-rock interaction, which again implies magnesite formation in relatively high crustal levels. In contrast to the magnesites, the siderite mineralization was caused by hydrothermal fluids that underwent more intense reactions with their host rocks in deeper crustal levels compared to the magnesite. The values of Sr-87 Sr-/86 in the siderites are substantially higher compared to the host rock slates. They also exceed the Sr-87 Sr-/86 ratios of the magnesites and the host rock limestones indicating these slates as the source of iron as a consequence of water-rock interaction. The siderites were formed at temperatures of 250 A degrees C indicating a relatively heavy fluid in equilibrium with siderite of 13 aEuro degrees delta O-18(SMOW), which is in the range of diagenetic/metamorphic fluids and reflects the A +/- complete equilibration with the host rocks. Carbon isotope evidence shows that the fluid forming the siderites underwent a much higher interaction with the host rocks resulting in a lowering of the delta C-13 numbers (-3,3 to -3,7 aEuro degrees). The light carbon was most probably derived from decaying hydrocarbons in the Riphean sediments. In a very early stage after sedimentation of the Satka Formation ( 1,550 Ma) magnesite was formed by seepage reflux of evaporitic bittern brines at the stage of riftogenic activity in the region (1,380-1,350 Ma). Sedimentation of the Bakal Formation ( 1,430 Ma) and intrusion of diabase dykes (1,386 A +/- 1,4 Ma) followed. Diagenetic/epigenetic mobilization of these buried fluids at 1,100 Ma resulted in the formation of hydrothermal siderite bodies.