Original paper

Geology, geochemistry and microthermometry of Mn-bearing hydrothermal veins near Birjand, east Iran

Barghi, Behnaz; Calagari, Ali Asghar; Zarrinkoub, Mohammad Hossein; Simmonds, Vartan

Abstract

Mn-bearing veins in three prospect (Beshgaz, Sehchangi, and Basiran) near Birjand, east Iran are hosted by Paleogene andesitic lavas (Eocene) and dacitic-rhyodacitic tuffs (Eocene-Oligocene). The major opaque minerals in the veins are pyrolusite, cryptomelane, psilomelane, hollandite, hematite, goethite, and limonite mainly having colloidal and open-space filling textures. The major non-opaque gangue minerals are gypsum, halite, barite, calcite, and silica. Alteration halos (mainly argillic and silicic) developed in the wall rocks of the veins and veinlets. X-ray diffraction and geochemical data indicate that the primary manganese minerals were deposited in as amorphous oxy-hydroxides. Recrystallization to psilomelane and hollandite, and then to pyrolusite occurred at the expense of the primary amorphous minerals. The average Mn/Fe ratios of the ores in Beshgaz, Sehchangi, and Basiran are 26.31, 48.55, and 1.81, respectively, and indicate that the Mn ores formed from hydrothermal fluids. Fluid inclusions study demonstrates that the ore-forming solutions had salinities within the range 0.5-5.5 wt% NaCl eq., and homogenization temperatures in the range 120-220°C. The pressure is estimated to be about 50 bars, corresponding to a depth of ore vein formation of ~150 meters.

Keywords

mn veinspsilomelanebasiransehchangifluid inclusionbirjandhydrothermalhollanditepyrolusitebeshgaz