Сommercial-genetic types of talc and magnesite deposits
V.V. Nasedkin, S.V. Efremova & N.M. Boeva
Institute of Geology of Ore deposits, Petrography, Mineralogy and Geochemistry of Russian Academy of Sciences, Moscow, Russia
Institute of Geology and Geochemistry of Russian Academy of Sciences, Yekaterinburg, Russia
Institute of Non-metallic Raw Materials, Kasan, Russia
Genetic types of magnesite and talc deposits are considered by authors with position fundamental geochemistry of magnesium. The most essential importance in our conseption has substantiation of sedimentary nature of magnesite in carbonate series and subsequent Mg influense on processes of metamorphism of rock associations.
1. ENDOGENETIC DEPOSITS
The joint occurence of talc and magnesite deposits demonstrates close relations in their genesis. Analysis of geologic situation within Onotskoe, Kirgiteiskoe, Alguiskoe, South Urals group deposits and many others has shown that all of them were formed as a result of progressive or regressive metamorphism of carbonate-terrigene primary sedimentary series, terrigene series with ultrababasic intrusions and carbonate series with primary sedimentary magnesite. The first case is characterized by the presence of Mg carbonate rocks: dolomites and magnesite. Talc was originated from the interaction between magnesian and silicate rocks. The formation of a) massive steatite ores, b) disseminated talc-magnesite (talc-dolomites) ores,
c) fine-vein lets ores is possible. Metasomatic zones can be formed on the contact between Mg-carbonate and silicate rock. As a rule talc is characterized by low Fe content. Magnesite forms separate bodies: lenses, beds which have different length (100-5000 m) and thickness (10-50 m). The deposits of this type are complex- talc-magnesite: Onotskoe, Alguiskoe, Kirgiteiskoe a. o.
The second case is characterized by combination of primary sedimentary terrigene series and tabular stratiform ultrabasic intrusions. The formation of deposits of this type occurred in several stages. At an initial stage the serpentine minerals formed as a result of hydration of forsterite and enstatite. At the second stage, minerals of talc-antigorite facies were formed. The deposits of the second type are presented by :a) talcites in zonal reaction-metasomatic columns, b) talc-magnesite stones as result isochemical metamorphism of serpentinites by increased partial CO2 pressure. Talc in these complexes is characterized by high Fe content.
Shabrovskoe deposit is a classic example of talc and talc-magnesite bodies of this type. The talc-magnesite lens extends up to 2 km. The thickness is 50-350 m. The talc bodies as a
rule are located on the contact between serpentinite and serpentinous peridotites with aplite and albitite dykes. The length of bodies is 50-200 m, the thickness –5-20 m. The resources of talc vary from 20 to 30 mln. tons, ones of magnesite – from 50 to 60 mln. t.
The third case is represented by magnesite bodies located in metamorphosed carbonate series. The magnesite rock consist of coarse-grained or sparry magnesite (for example Savinskoe deposit) . Magnesite occurs as beds and lenses between dolomites. The thickness of bodies is 10-100 m. They extend from 0.2 of km to 10 km. The main commercial mineral is crystalline magnesite. Additional minerals: talc, quartz, chlorite, dolomite. Resources - tens mln. tons.
2. EXOGENETIC DEPOSITS
Talc and magnesite of the weathering crust and redeposited ones are present in a few deposits. The powder talc ores from Kirgiteiskoe deposit are of the most commercial importance. They have the highest brightness (Fe content is no more 0.1-0.2%) . The thickness of talcites in crust of weathering Alguiskoe deposit reaches 200-150 m.
Magnesite deposits of weathering rust (Khalilovskoe and others of Ural) are presented by veins and bodies of irregular form. They are formed by chemical weathering of serpentinous ultrabasits. For Russia these deposits have no great commercial importance.
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