PETROLOGY, GEOTHERMOBAROMETRY AND C-O-H-S FLUID COMPOSITIONS IN THE ENVIRONS OF RAMPURA-AGUCHA ZN-(PB) ORE DEPOSIT, BHILWARA DISTRICT, RAJASTHAN

The massive Zn-(Pb) sulfide ore body at Rampura-Agucha in Bhilwara dis trict, Rajasthan, occurs within graphitic metapelites surrounded by ga rnet-biotite-sillimanite gneiss containing concordant bodies of amphib olite. These rocks and the sulfide ores have been studied to estimate the pressure, temperature and fluid composition associated with upper amphibolite facies metamorphism. Geothermobarometric calculations invo lving garnet-biotite and garnet-hornblende pairs, as well as sphalerit e-hexagonal pyrrhotite-pyrite and garnet-plagioclase-sillimanite-quart z assemblages indicate that the most pervasive P-T condition during pe ak of regional metamorphism was 650 degrees C and 6 kb, and was attain ed between the first and second deformations in the region. Some tempe rature-pressure estimates also cluster around 500 degrees C-5.lkb whic h probably represent retrograde cooling during unloading. Consideratio n of devolatilization equilibria in the C-O-H-S system at the pervasiv e metamorphic conditions mentioned above shows that the metamorphic fl uid was H2O-rich (X-H2O = 0.52) but also had a substantial component o f CO2 (X-CO2 = 0.39). H2S(X-H2S = 0.043) and CH4 (X-CH4 = 0.025) were the other important phases in the fluid. CO(X-CO = 0.002) and H-2 (X-H 2 = 0.002) were the minor phases in the fluid. It is probable that a p art of this aqueous fluid was consumed by re-/neocrystallization of hy drous silicate phases like chlorite during the retrogressive metamorph ic path, so that fluid entrapped in quartz below 450 degrees C was ren dered CO2-rich (Holler et al 1996).