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

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