Investigation of geology, mineralogy and genesis of Mohammadabad-Oryan boron index, south Sabzevar

Mohammadabad-Oryan index is the only borate index in the north-east of Iran that is embedded in the formationscontainingtuff marl, tuffaceous marl, volcanic breccia and tuff brecciawhichbelongs to cretaceous. This index is located about 50 km south of Sabzevar. Two NE-SW trending thrust and reverse faults have occurred in the area. These faults were pathways for ore forming solutions and created open spaces for ore deposition. Magnesium, magnesium–calcium, calciumhydrated borates and hydroboracite (Mg-Ca Borate) are the most important minerals of this index. The nodules of ulexite can be clearlyobserved in the area. In addition, tincalconiteand meyerhofferitehave also been detected by XRD.According to the field observations, this index is located in the rock units of clay, marl and tuff. Due to its similarity to other known borate deposits, it can be concluded thatborateminerals areprecipitatedin fractures aslens shaped orebodies, discordant with sedimentary clay-marl layers. Field observations on dug trenches in the area show that the mineralization intensity decreases with depth.It seems that meteorite waters penetrate to depth,through tectonic fractures, and go up after warming (due to the high geothermal gradient), creating some thermal convective circulating cells. The lower rock units are leached within cells and produced boron-rich fluids, then borate minerals deposited in fractures within clay-marl layers as result of reducing conditions.