Genesis of Two Zoned Pegmatites of the Bihar Mica Belt: A Fluid Inclusion Study

Abstract

Two pegmatites, namely, Bandarcbua and Kharonia, show ideal mineralogical zoning with a massive quartz core surrounded by a muscovite·plagioclase-perthite·quartz and muscovite-quartz zone. Aqueous two phase inclusions with 40% vapour and three phase inclusions with C0₂(liquid)+CO₂ (gas)+H₂0(liquid) occur as primary inclusions in quartz from the feldspathic zone. Aqueous three phase inclusions with NaCl daughter mineral coexisting with gaseous inclusions with 60-80% vapour occur as pseudosecondary inclusions in quartz from the feldspathic zone and as primary inclusions in quartz from the muscovite-quartz zone. Quartz from migmatitic gneisses show the common occurrenee of monophase carbonic inclusions. Heating-freezing data assign CO₂ densities of 0.81-0.90 g/cm³ for the peak metamorphic fluids which were entrapped at 4.2 Kb pressure at a temperature of 680°C. The feldspathic zone crystallized from moderate density (0.6 g/cm³) and salinity (10 wt, % NaCI) fluids. Coexisting H₂0 and CO₂-H₂0 inclusions record a temperature of 630°C and a pressure of 2.4 Kb for the equilibration of the feldspathic zone. Subsequent adiabatic decompression resulted in the boiling of fluids with enhanced density (1.O g/cm³) and salinity (32 wt, % NaCl). Boiling, which precipitated bulk of the mineralization, occurred at temperatures of 360°C to 320°C and at pressures of 110-90 bars, corresponding to 1100-850 metres depth. The fluid characters suggest a probable relationship between regional metamorphism and the genesis of the mineralized pegmatites of the Bihar mica belt.