Petrogenesis and Composition of Cretaceous Dolomite, Shahmirzad, Centeral Alborz

Different types of dolomitic carbonate rocks of Upper Cretaceous which are located in northeast of Shahmirzad, Centeral Alborz, hosted many lead-zinc deposits and occurrences. In this study 5 profiles of carbonate rocks of abandoned lead-zinc mines from Reza-Abad, Heydar-Abad and Reza-Barak have been selected. petrogenesis and composition of dolomites have been studied employing equation N CaCO3 = md + b, using stained thin sections, XRD graphs, Wet chemical analysis and XRF. Dolomites of this region are categorized in to 3 groups due to amount of calcium carbonate in which ranges from 49.5-51.5%, 51.5-53.5% and 53.5-55.5%.respectively Considering a/m studies, six types of dolomite textures have been recognized, which according to growth sequence of dolomite crystals in time, are consist of: Xenotopic A, Idiotopic E, P, S, C and Saddle dolomite. The decreasing of Na2+ and Sr2+ that create along diagenesis and increase crystal size, has not observed because of the influence of hydrothermal fluids on Cretaceous sequences. The amount of Fe2+ and Mn2+ are higher in dolosparite in comparison with dolomicrosparite. Considering the changes the amount of Fe2+, Mn2+ and Sr2+ in the composition of dolomites along profiles, it may be that there had been a flowing fluid from north to the south in accordance to Anzab Fault. Taking in to consideration the data gathered in the studies, the several-stage dolomitization in the region should be related to the early and late stage diagenesis. This may be due to sea and meteoric waters, basin waters, saline waters and influence of hydrothermal fluids. This phenomena occure during Cenozoic that may be associated with Eocene volcanic activities, which was affected to the Upper Cretaceous sequences.