Retrograde fluid evolution at the Rampura Agucha Pb-Zn-(Ag) deposit, Rajasthan, India

Abstract

Fluid inclusions in mineralized graphite-sillimanite-mica schist from the Rampura-Agucha Pb-Zn-(Ag) deposit, Rajasthan, northwest India, have been investigated by microthermometry and Raman microspectrometry. Three different main types of fluid inclusions in quartz can be distinguished: (1) gaseous (CO2, partially mixed with CH4-N2), (2) low salinity aqueous inclusions (0–8 eq. wt% NaCl) and (3) high salinity aqueous inclusions (NaCl ± MgCl2-CaCl2). Low density CO2-rich and low salinity H2O inclusions are contemporaneous and occur, together with CH4-N2 inclusions, in close association with sulfide mineral inclusions. This indicates immiscibility between the gaseous and aqueous phase and participation of these fluids during the deposition or remobilization of the ore, which occurred over a wide P (1220 to 200 bar) and T (450 to 250 °C). Raman spectra of graphite indicate upper greenschist-facies metamorphic conditions, although host rocks have been metamorphosed at upper amphibolite-facies metamorphic conditions. This indicates that graphite re-equilibrated with the CO2-rich phase during retrograde metamorphism.