gems, beryl, emerald, India, Rajasthan, geology

The emerald deposits of Rajasthan, in northwest India, are located within a narrow NE–SW-trending belt in the Aravalli Mountains. These deposits formed through metasomatic reactions between muscovite (± garnet ± tourmaline) pegmatites and lenticular bodies of altered ultramafic rocks hosted within the Delhi Group gneisses. This process produced phlogopite schists containing exometasomatic emeralds, consistent with other granite-related emerald occurrences. Endometasomatic alteration of the pegmatites is reflected in muscovite geochemistry (phengitic substitution) and in feldspar alteration (disappearance of potassic feldspar and calcification of plagioclase). K–Ar dating of syngenetic phlogopite from the phlogopite schists and muscovite from the pegmatites yields an age of approximately 790 Ma, corresponding to the final major orogenic event in the region. This age aligns with the formation timing of other granite-related emerald deposits, all of which are associated with active orogenic settings. Biotite yields younger ages than muscovite, suggesting minor radiogenic argon loss due to deformation.