Crystal-chemical, mineralogical and geochemical features of dolomites of the Avzyan formation of the Middle Riphean (Southern Urals)

Abstract

Introduction: The crystallochemical characteristics and the reasons for their variations in the dolomites of the Avzyan Formation of the Middle Riphean of the Bashkir meganticlinorium, weakly and strongly modified by epigenetic processes, have been determined. Methods: X-ray diffraction, X-ray fluorescence analysis, electron microscopy with energy-dispersive microanalysis. Results and discussions: In the dolomites of the Avzyan formation, increased parameters of the crystal lattice are recorded (аav=4.8105 Å, сav=16.0211 Å) and decreased values of the substructural reflection (Кav=0.66) compared to stoichiometric dolomites. The average Fe (II) content is 4545 g/t, Mn – 423 g/t, Sr – 124 g/t. An increase in the crystal lattice parameters of dolomites of the Avzyan formation directly correlates with an increase in the contents of Fe and Mn, and the values of the substructural reflection have an inverse relationship with them. The presence of two generations of dolomite and calcite was discovered in the rocks of all studied sections. The formation of calcite-dolomite vein occurred at temperatures of 260– 530°С and pressures of 0.3–2.3 kbar. In dolomites of different subformations, the average values of the crystal lattice parameters а and с decrease from early (Kataska subformation) to late Avzyan (Tulmenskaya subformation). The same trend of increase was revealed for average values of the coefficient К of ordering the structure of dolomites. Conclusions: Dolomites of different stratigraphic levels of the Avzyan formation are subject to intense epigenetic transformations caused by the impact of Fe- and Mn-containing medium- and high-temperature fluids on the rocks. The change in the crystallochemical characteristics of the dolomites of the Avzyan formation is associated not so much with the process of regional metamorphism, but with the activity of post-magmatic (sections near the village of Veselovka and on the Tulmen River) and catagenetic (sections near the village of Islambaevo and B. Avzyan River) fluids, the circulation of which occurred in faults.