Dolomitization model of the Soltanieh Formation in the south-west of Zanjan

In this study, to determine a dolomitization model for the Soltanieh Formation a suitable outcrop 35 km southwest of Zanjan city has been studied. The Soltanieh Formation with a total thickness of 985 m sharply laid on the shales and sandstones of the Bayandor Formation and was covered by shales and red-sandstones of the Barut Formation. Distribution and extension of the dolomite layers in the Soltanieh Formation, is parallel with the sedimentary layers and bedding and also has considerable lateral extension. Abundant relict of blue-green algae (stromatolites) and nodules and bands of cherts, have been recognized into the dolomitic layers of the Soltanieh Formation. Based on fieldwork, petrography and geochemistry evidences (ICP-OES and XRD), four different types of dolomite have been distinguished in Soltanieh Formation. These dolomites include: (1) fine crystalline dolomites or dolomicrite; (2) subhedral, dense and medium crystalline dolomite; (3) subhedral to anhedral coarse crystalline dolomite and finally (4) pore-filling dolomite cements. In the studied dolomites of Soltanieh Formation, saddle dolomite were not observed. The results of this research indicate that these dolomites have been formed in shallow to relatively deep diagenetic environments by moderately saline marine fluids (mean Na 435 ppm), via seepage reflux of evaporate seawater into the Soltanieh Formation platform in the study area. Low Sr values (mean 47 ppm) and relatively high Fe (mean 3088 ppm) and Mn values (mean 453 ppm) in coarse crystalline dolomites, likely indicate increase in dolomite crystal size and recrystallization process during the burial. The very low Ba concentration (mean 12 ppm) and the absence of saddle dolomite into the dolomitic sequences of the Soltanieh Formation, indicates ineffectiveness of hydrothermal fluids acting during dolomitization process of the Soltanieh Formation carbonates into the study area.