Fluid evolution in quartz vein-hosted tungsten mineralization in Chhendapathar, Bankura District, West Bengal: Evidence from fluid inclusion study

Tungsten mineralization in Chhendapathar area is hosted by quartz veins tha t traverse mostly the metasediments in and around Jikhu Nala. Fluid inclusi on microthermometric experiments reveal the presence of four distinct types of inclusions. These are: aqueous biphase, monophase carbonic, aqueous-car bonic and halite-bearing polyphase inclusions. Salinity-temperature variati on points towards the presence of two fluids of contrasting salinities and both independently followed simple cooling paths without any indication of fluid mixing. The P-T of mineralization was calculated from the intersectio n of coexisting and coeval aqueous biphase, carbonic and halite-bearing inc lusions. The deduced values range from 1.63 kb/361 degrees C to 2.30 kb/385 degrees C, However, the initial temperature must have been much higher as indicated from the high dissolution temperature (> 450 degrees C) of halite . Transportation of tungsten in the high saline fluid was facilitated by ca tion-tungstate ion pairing, i.e., with the help of Na2WO4 and/or NaHWO4 com plexes. A rapid fall in solubility in such fluid with falling temperature t in the range of 300-400 degrees C), and by occasional fluid-rock interactio n triggered precipitation of wolframite.