Lithological and Structural Control of the Emerald Occurrences South of Singhbhum Shear Zone within the Singhbhum Crustal Province, Eastern Indian Craton

English Abstract:

Emerald is the chromium and vanadium bearing variety of beryl (Be3 Al2Si6O18) with a "grass green" primary hue and a slender yellow or blue secondary hue. Globally, this gem variety is known to be hosted by Pegmatites, schists with or without pegmatites and within veins and breccia of black shale. In this work we have reported new area of emerald occurrences, where mineralization occur within the schist with or without pegmatities as intrusive, along a north-south oriented 25km long belt near Singhbhum Shear zone within the Eastern Indian Craton. Emerald occurrences in the study area is confined within biotite schist, talc-actinolite schist and show evidences of lithological and structural control. The detailed microstructural and chemical analysis of beryl, emerald, amphibole, mica, tourmaline, quartz and chlorite indicate that the syn to post tectonic mineralization may be correlated with magmatic activities, post-magmatic hydrothermal activities and regional-metamorphic event. These rocks contain 3000–5000 ppm Cr, and behave as source rocks to supply the Chromium for the crystallization of emeralds. The basement rocks within the area is an ensemble of migmatised gneisses, mica schists, granite gneisses and amphibolites along with widely scattered small bodies of ultramafic rocks, intensely invaded by pegmatite and quartz veins. This work reveal that regional metamorphism and tectono-metamorphic processes during shear zone deformation might have played a significant role in the formation of emerald deposits. The Singhbhum shear zone provides an ideal condition for emerald crystal growth, where the beryllium bearing hydrothermal fluid interacts with chromium bearing mica schist or ultramafic rocks.

Key word: Emerald, Pegmatites, Singhbhum, Lithology, Structure, Singhbhum Shear Zone