The geochemistry and distribution patterns of Boron in Sabalan geothermal fluids, NW Iran

 The manifested geothermal fluids around Sabalan volcano show different compositions. These compositional differences are observable in both the anionic composition and the concentration of trace elements. Regarding to the geographic distribution of geothermal fluids and hot springs, Na-Cl fluids (NW Sabalan), Acid Sulfate fluids (East of Sabalan), and HCO3 fluids (South of Sabalan) have been recognized. These fluids are also considerable regarding to the trace element and heavy metal contents. Boron as a common component in geothermal fluids shows relatively high concentrations in some hot springs of Sabalan area (South of Sabalan) and the obtained samples from the drilled wells (NW Sabalan). The studied samples of South of Sabalan area show 19-33 mg/l Boron content and the well samples show 22 mg/l Boron values in on average. With a regard to theIn generals there is a linear and positive relation between Boron and temperature in many geothermal fluids, this positive relation is also seen in the studied samples. The correlation of B-pH for the studied samples displays a special pattern. The highest concentration of Boron can be seen in 6-9 pH range. In this pH range, B(OH)4 is the dominant species. The high concentration of Boron can be dangerous from the environmental point of view. The contamination of high-Boron geothermal fluids with in the rivers, lakes, dams, and drinking waters can may cause health problems. This phenomena currently happens currently in some rivers such as Khiyav chay (7 mg/l Boron content) around the Sabalan volcano.  

Boron is the 51th element in of the earth’s crust with 8 ppm average concentration (Cotton et al., 1999; Jansen 2003). Boron is a non-metal element and is usually found with Oxygen compositions and never found as a free element (Cotton et al., 1999). More thane 200 Boron bearing minerals have been found till now. and Borax (Na2[B4O5(OH)4]•8H2O), Kernite (Na2B4O7•4H2O), Colemanite (Ca2B6O11•5H2O), and Ulexite (NaCaB5O9•8H2O) are the main Boron minerals. This element is a basic micronutrient for most of plants, animals, and even humans (Rainey et al., 1999, 2002).     
Method of investigation
In order to investigate the Boron content and distribution patterns of this element in geothermal fluids in Sabalan area several stages were anticipated and carried out.
The chemical analyses of these samples for determination of major and trace elements as well as stable isotopes were carried out in hydrogeochemistry lab at Bremen University (Germany).
The stable isotope analyses (δD and δ18O) were carried out by using LGR DLT-100 Laser Spectrometer (Los Gatos Research). The analytical precisions for δ18O and δD were ±0.2‰ and ±1‰, respectively.

From the geological point of view the study area is located in a volcanic setting which belongs to the Cenozoic era. This area is a small part of western Alborz-Azerbaijan geologic zone. The main lithological units of Sabalan geothermal system consist of igneous rocks such as andesite, basalt, dacite, trachyte, and monzonite.
The geothermal manifestations such as hot springs are observable in most parts around Sabalan volcano and include different types of geothermal fluids such as Na-Cl, Acid Sulfate, and Bicarbonate waters. These waters show a relatively high Boron contents especialy in the southern Sabalan geothermal field.    

Several conclusions have been obtained from our investigation:-  

         The studied samples show different Boron concentrations which include 20-33 mg/l in Southern Sabalan and 20-22 mg/l Boron in the drilled wells in NW Sanalan.
-           There is a positive and strong relation between temperature and Boron content of most studied geothermal fluids. In some of the studied samples, this positive relation is not seen which can be thought as the occurrence of mixing process.
-           The high content of Boron in Khiyav-Chay  can be dangerous for the environment and may cause health problems in the area.