Petrography and microthermometry of fluid inclusions in apatite in the Turamdih uranium deposit, Singhbhum shear zone, eastern India - An insight into ore forming Fluid

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-constrained. 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 H2O-NaCl-CaCl2 (+/- MgCl2) brine. The salinity varies between similar to 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 CaCl2 was calculated considering the final ice melting temperature to be the hydrohalite melting temperature. The NaCl and CaCl2 content ranges between similar to 2 to 21 and similar to 4 to 28 wt. % respectively and the CaCl2: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 240A degrees to 450A degrees C. This study suggests that apatite started crystallizing at a higher temperature (T-h similar to 450A degrees C) from a high salinity brine (similar to 35 wt% NaCl equivalent). Subsequently, this fluid mixed with a fluid of lower temperature (T-h similar to 300A degrees C) and somewhat lower salinity (similar to 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 Th 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.