Chemistry of paleofluids forming the magnesite and siderite deposits in the Soutern Urals

摘要

This paper deals with "world class" magnesite and siderite deposits in the southern Urals, 300 km south of Ekaterinburg. The magnesite orefields of Satka and the siderite deposits of Bakal have been ex-loited for more than 100 years. The stratiform Satka magnesite deposits belong to the "Veitsch Type" of mag-nesites and is hosted in shallow marine dolomites with algal mats and desiccation cracks of the Lower Riphean. The Bakal siderite deposit is located in carbonate and clay dominated series and forms elongated bodies with an alteration halo of dolomite. The investigation focuses on the significance of the inclusion fluid chemistry for the ore forming processes and the question of the relationship between the magnesite- and the siderite- forming fluids. The analytical results suggest formation of both the magnesites and the siderites from moderately to high salinity, evaporitic brines, not long after sedimentation. This was accompanied by the formation of alteration haloes of hydrothermal dolomites exhibiting basically the same fluid characteristics, whereas hostrock limestones were not affected by the hydrothermal event and still show marine signatures.