Fluid inclusion and geochemical signatures of the talc deposits in Kanda area, Kumaun, India: implications for genesis of carbonate hosted talc deposits in Lesser Himalaya

Abstract

Talc deposits in the Deoban Formation of inner Lesser Himalaya in Kumaun are interpreted to have resulted from low-grade, regional burial metamorphism of the siliceous magnesium bearing carbonates consisting of magnesite and dolomite. These deposits are distributed over a large area, occurring in association with magnesite and rarely with dolomite. They are found as fine-grained, fibrous aggregates of talc restricted to small, irregular patches or pockets in carbonate host rocks. Their petrographic features represent different phases of reactions between magnesite and silica to produce talc, thus equilibrium conditions were attained by the assemblage of magnesite + quartz + talc. Scanning electron microscopy also demonstrates that magnesite has reaction margins, whereas dolomite has perfect grain boundary in the magnesite–dolomite–talc assemblage. The major and trace elements in magnesite/dolomite and talc rule out the possibility of any incursion of foreign material during talc formation. Early fluids in magnesite and dolomite were H₂O + NaCl + KCl ± MgCl₂ ± CaCl₂ in composition, their microthermometry data suggest mixing of the fluids. The fluid inclusion studies also imply that talc was formed under the condition of very low ${\text{X}}_{{{\text{CO}}_{2} }}$. A peak temperature of 300–340 °C and pressure of 2–2.2 kbar are estimated from the coexisting immiscible fluids in talc–magnesite assemblage. It is attributed that the estimated PT${\text{X}}_{{{\text{CO}}_{2} }}$ conditions in the Upper Proterozoic Deoban carbonate rocks, favored the talc formation from magnesite + quartz, and were not conducive to convert siliceous dolomite to talc on a large scale.