Mineralogy and the genesis of fissure - ridge and vein type travertine (in Ab-e Ask) based on carbon and oxygen isotopes analysis

Travertine-bearing hot springs of Ab-e Ask are located at the far northeast margin of Tehran city and at the eastern side of Damavand volcano. Fissure-ridge and vein-type travertine are the main deposit types of these springs. Due to tectonic activities, fault zones and fracture systems were formed and provided migration paths for CO2-bearing hydrothermal fluids. Fissure-ridge and vein-type travertine that formed in these zones and systems are resulted from the intense degassing of CO2. Microscopic studies and XRD analyses showed that the fissure-ridge travertine is made from nearly pure calcite with some amounts of quartz mineral. This is also supported with SEM and microscopic studies that photosynthetic microorganisms such as green-blue algae and diatoms have an important role in the formation of travertine. Internal pressure of CO2-bearing hydrothermal fluids resulted in the formation of hydraulic fractures in these rocks. XRD analyses and petrographic studies are also resulted in the recognition of elongated blocky type calcite and other forms of calcite, pyrite, gypsum and a rather few amount of rancieite mineral. Based on stable isotope analyses (including δ13C and δ18O values), two different origins are recognized for these travertines: fissure-ridge type travertines are related to the de-carbonation of limestone and vein-type ones are related to the magmatic-de-carbonation origin. A considerable enrichment in δ13C values is attributed to the de-carbonation of limestone, algae activities and rapid degassing of hot springs.