The studied manganese prospects are located in ophiolitic complex of Birjand, east of Iran. 11 The ophiolitic sequence in this region hosts manganese ores occurring as lens associated with radi-12 olarian cherts and as veinlets with shales. The main gangue minerals are calcite, silica and gypsum. 13 The major manganese ore minerals are pyrolusite, braunite, bixbyite, ramsdellite, and romanechite 14 showing replacement, colloidal, and brecciated textures. High concentration values of Si, Fe, Mn,

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... 15 Ba, Zn, Sr, and As in the studied manganese ores can be applied as evidence for sea-floor Mn-rich 16 hydrothermal exhalatives responsible for the formation of the ores. Investigations on fluid inclu-17 sions of calcites demonstrated that the ore-forming solutions had salinities within the range of 0.5-18 4.5 wt% NaCl eq., homogenization temperature range of 100-220˚C and density about 0.8-1 g/cm 3 . 19 The pressure was estimated to be about 50 bars which corresponds with a depth of ~150 meters at 20 the time of formation manganese prospects that fluid inclusions datas approved hydrothermal pro-21 cesses in the studied manganese prospects can be applied as evidence for sea-floor Mn-rich hydrothermal 22 exhalatives responsible for the formation of the ores. The geological and geochemical results also 23 revealed that deposition of the ores occurred on upper parts of the ophiolitic sequence by submarine 24 exhalatives. The hydrothermal activities caused leaching of elements such as Mn, Fe, Si, Ba, As, and 25 Sr from basaltic lavas (spilites) through concurrent faults with deposition, elements entered to sed-26 imentary basin and during the retreat of the sea with the expansion of oxidation conditions are 27 formed the primary manganese minerals were Mn oxides and hydroxides which have gradually been con-28 verted to psilomelane and finally pyrolusite, braunite and bixbyite. 29