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The study area is located in the NNE of Tehran, at Fasham area (west of Zaigun Village) in the central Alborz Mountain.
Barut Formation, with a total thickness of 270 m, composed of coloured shale, very fine sandstones, siltstones and over 30 interbeded calcareous and dolomitic rocks. The age of Barut Formation is probably Precambrian.The carbonate sequence of Barut Formation consist mainly of stromatolitic rocks with stratiform, laterally link hemispheroid (LLH) and stacked hemispheroid (SH) shapes.
Based on petrographic (size and fabric), elements and isotopic studies, four dolomite types were recognized.Variation in dolomite types is mainly related to early to late diagenetic processes leading to changes in composition of dolomitizing fluids. Geochemical studies also indicate that medium to coarse grain dolomites formed in shallow to deep burial setting under reducing conditions. Sea water was the major source of Mg for early diagenetic dolomite (type1),while Mg for late diagenetic dolomites (type2,3,4) were sourced by shale pressing processes.Decrease in MgO % from 37 to 84 % in montmorillonite and chlorite,support that source of Mg for dolomitization in burial setting was mainly provided by leaching of Mg from Mg – bearing clay minerals. Calculation of organic matter in carbonate samples indicates that the amount of organic matter in dolomites are three times more than limestones.As dolomitization processes increased by increasing organic matter contents,part of dolomitization in Barut Formation can be related to availability of organic matter.
Palaeotemperature calculation (based on ?W of –3 ± 1 and heaviest ?18o value of –5.05‰) indicate that early diagenetic dolomite formed in seawater ranging in temperature between 42 – 50 °C.