Mineral chemistry and geochronology of the Rajasthan emerald deposits, NW India

The emerald deposits in Rajasthan, northwest India, are situated in a narrow NE–SW belt in the Aravalli Mountains. The studied deposits were formed by the metasomatic reaction between muscovite (± garnet ± tourmaline) pegmatites and lenticular bodies of altered ultramafic rocks that are hosted by the Delhi Group gneisses. This reaction produced phlogopite schists containing the exometasomatic emeralds, as in all other granite-related emerald deposits. Endometasomatic changes of the mineralogy of the pegmatites is indicated by the geochemistry of the muscovite (phengitic substitution) and the feldspars (disappearance of the potassic feldspar and calcification of the plagioclase). The K-Ar analyses of syngenetic phlogopite (from the phlogopite schist) and muscovite (from the pegmatites) give an age of ca. 790 Ma, close to that of the last major orogeny affecting the region. This is in accordance with the ages of other granite-related deposits, which all formed in conditions of active orogeny. The ages of the biotite are lower than those of the muscovite, indicating limited radiogenic argon loss as a result of deformation.