Nature of fluids and regional implications for Lesser Himalayan carbonates and associated mineralization

Abstract

Fluid inclusion data are presented for the successive stages of limestone, dolomite, magnesite and sulphide-bearing quartz veins in Proterozoic carbonate rocks of the Lesser Himalaya. Subsurface fluids were H 2O–NaCl–KCl ± MgCl 2 ± CaCl 2 and showed successive increase in salinity and temperature. The salinity of the pore fluid during limestone diagenesis was in the range of ∼7.5–15 eq wt.% NaCl and the magnesite-forming fluids had a salinity of about 9 to 19 eq wt.% NaCl. This progressive rise in salinity is attributed to a more saline fluid in the deeper zones. The inverse relation between homogenization temperatures and final melting temperatures suggests mixing of the fluids during diagenesis, and highly depleted δ 18O values rule out participation of magmatic fluid in the mixing. A late stage carbonic fluid is linked with talc formation. The low temperature of sulphide-forming epigenetic solutions, as obtained from fluid inclusions, is also substantiated by the chemical data from these sulphides. δ 34S values in galena infer that magmatic sulphur was probably not involved, and the sulphur of the galena is derived from an isotopically heavy source.