Multistage dolomitization in the cretaceous carbonates of the east Shahmirzad area, north Semnan, central Alborz, Iran

Abstract

Different types of Cretaceous limestone and dolostone are the host for lead–zinc deposits and occurrences in the northeast of Shahmirzad, central Alborz. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include (1) very fine-crystalline, non-planar, xenotopic-a (Rd1); (2) fine- to medium-crystalline, planar-e dolomite rhombs, idiotopic-e, scattered dolomite crystals in micrite matrix (Rd2); (3) fine- to medium-crystalline, planar-e (porphyrotopic) (Rd3); (4) medium-crystalline, planar-s, subhedral to euhedral mosaic dolomites, idiotopic-s (Rd4); they post-date early submarine cements. Two types of dolomite cements were identified: medium- to coarse-crystalline, planar-e, euhedral to subhedral dolomite cements, idiotopic-c (Cd1); coarse-crystalline, non-planar, saddle dolomite cements (Cd2). They post-date replacement dolomites and predate late-stage calcite cements that line mouldic vugs and fractures. The association of Rd1 with stromatolites and evaporites, combined with δ18O positive values, postdating early diagenetic features such as micrites and marine cements, could evidence the importance of evaporitic brines for the dolomitization in sabkha (salt flats) environment. The emersion of the carbonate successions at the end of Early Cretaceous caused an ingression of waters of mixed meteoric-marine composition. This caused a wide dissolution of the carbonates and the generation of a first karstic phase. Rd2 and Rd3 dolomites were formed in this environment, as well. A second generation of karstic dissolution happened during the Laramide orogeny in Late Cretaceous, when the carbonate successions were uplifted till emersion. This stage wsas followed by a shallow burial phase and by the hydrothermal formation of Rd4, Cd1 and Cd2 dolomites. These dolomite types are associated epigenetic base-metal mineralization (MVT). The δ18OPDB and δ13CPDB isotopic ratios of the dolomites range from −5.80 to 2.14‰ and from −0.03 to 2.94‰, respectively. The δ18OPDB values are suggesting that all the studied dolomites have been deposited in low-temperature conditions. The measured δ18O values of fine-grained dolomites are heavier than those occurring in the hydrothermal types. This is pointing to lower formation temperatures in respect to other dolomite types in the study area. The remaining porosity of vugs and veins was filled by banded calcites and/or macrocrystals of sparry calcite. The δ18OPDB values of these calcites vary between −12.46 and −7.45‰ and δ13CPDB is in a range between −3.28 and 1.56‰. The O-isotopic values of calcites are lower than those of dolomites and may therefore indicate higher temperatures of precipitation. The different generations of dolomite contain also variable amounts of calcium carbonate, ranging from 49.5 to 55.5%. Fe, Mn and Sr in the dolostones are also variably concentrated: 280–1,500, 75–101 and 71–178 ppm, respectively. Fe, Mn and Sr values can also reflect the complexity of the diagenetic processes and the hydrothermal activity, which was responsible for Pb–Zn mineralization throughout the area as well as for the late dolomite generations. The hydrothermal flow might have been caused by Eocene volcanism, which affected the Cretaceous succession. Late dolomite distribution suggests that the dolomitizing fluids were probably diffused from the tectonic network and were expanded laterally following the permeability conduits.