0016-7622/2015-86-3-253/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.86, September 2015, pp.253-262 Petrography and Microthermometry of Fluid Inclusions in Apatite in the Turamdih Uranium Deposit, Singhbhum Shear Zone, Eastern India - An Insight into Ore Forming Fluid DIPAK C PAL 1* and TRISROTA BHOWMICK 1,2 1 Department of Geological Sciences, Jadavpur University, Kolkata - 700 032, WB, India 2 Present address: Mineral Processing division, National Metallurgical Laboratory, Barma Mines, Jamsedpur - 831 007, Jharkhand, India Email: dipak.pal@gmail.com; dcpal@geology.jdvu.ac.in Abstract: The Singbhum shear zone is one of most important polymetallic mineralized zone, characterized by uranium, Cu and apatite-magnetite mineralization. Although there is unanimity regarding the hydrothermal nature of different ores, the fluid characters, particularly related to apatite-magnetite mineralization, are not very well-c onstrained. This study aims at deciphering the fluid character involved in apatite-magnetite and associated mineralization through fluid inclusion studies in apatite from apatite-magnetite-bearing uranium ores in the Turamdih uranium deposit. The studied host rock is quartz-chlorite schist comprising predominantly of quartz, and chlorite with magnetite, apatite, uraninite, monazite, allanite chalcopyrite, and pyrite. Textural and micro-structural relations of apatite indicate that the studied apatite along with uraninite and monazite crystallized prior to or at the early stage of shearing. Based on the content of fluid inclusions in ambient room temperature, the primary inclusions are classified in to two groups, namely type-I and type-II. Aqueous bi-phase inclusions, defined as type-1, are most common and abundant. The type-II polyphase inclusions are characterized by the presence of aqueous liquid, vapor and one or more solid phases. Fluid inclusion microthermometric experiments suggest that apatite crystallized from highly saline fluid and the fluid composition can be best expressed as H 2 O-NaCl-CaCl 2 (± MgCl 2 ) brine. The salinity varies between ~ 22 to 43 wt % NaCl equivalents. Although, the final melting temperature of hydrohalite could not be determined due to very small size of the inclusions, the minimum concentrations of CaCl 2 was calculated considering the final ice melting temperature to be the hydrohalite melting temperature. The NaCl and CaCl 2 content ranges between ~ 2 to 21 and ~ 4 to 28 wt. % respectively and the CaCl 2 :NaCl ratio are mostly above 1:1 indicating a calcic brine. The temperature of final homogenization (T h ) of type-I inclusion (L+V→L) ranges mostly between 240 o to 450 o C. This study suggests that apatite started crystallizing at a higher temperature (T h ~ 450 o C) from a high salinity brine (~35 wt% NaCl equivalent). Subsequently, this fluid mixed with a fluid of lower temperature (T h ~ 300 o C) and somewhat lower salinity (~ 25 wt% NaCl equivalent). The associated uraninite and monazite likely precipitated along with apatite from the same fluid. However, the temperature of fluid entrapment and hence apatite crystallization must be higher as T h provides the minimum temperature of entrapment. Based on this study and several other lines of evidence, we propose that the mineralizing fluid was derived from basinal brine or from evaporite dissolution. Keywords: Fluid inclusion, Apatite, Uranium deposit, Turamdih, Singhbhum shear zone, India. INTRODUCTION The Singhbhum shear zone (SSZ), eastern India is well known for its uranium and copper mineralization. Several small apatite-magnetite deposits also occur along the shear zone. Additionally, recent studies have demonstrated that the host rocks of the polymetallic mineralization can be potential source of rare earth elements (Pal et al., 2011a). Most authors proposed that the polymetallic mineralization in the SSZ is hydrothermal in origin (Dunn and Dey, 1942; Sarkar and Deb, 1971; Sarkar, 1982; Pal et al., 2009, 2011a, b; Pal and Rhede, 2013, Ghosh et al., 2013). Recent studies also (Pal et al., 2009, 2010, 2011a, b; Pal and Rhede, 2013; Ghosh et al., 2013) suggest that most ore minerals (sulfides, uraninite, magnetite, monazite, allanite) formed prior to or at the early stage of shearing. It therefore stands to reason that characterizing the hydrothermal fluid involved in pre-/