Chlorine-rich amphibole and biotite in the A-type granites, Rajasthan, NW India: Potential indicators of subsolidus fluid-rock interaction and metallogeny

摘要

The A-type granites in the northern Delhi fold belt, Rajasthan, India, comprise Cl-enriched amphibole and biotite. These hydrous phases, in general, show significant variation in the Cl and F contents on the scale of individual intrusions and also at times on thin-section scale. This variation of the hydrous phases that equilibrated with the fluid is principally controlled by the variation in the fluid activity ratios of a(Cl)-/a(OH)- and a(F)-/a(OH)-. The activity gradients in the fluid developed due to the interaction between the granites and an externally derived Cl-bearing fluid coupled with limited integrated fluid flux. The fraction of the fluid that reached the least albitised granite was highly Cl-rich as a consequence of preferential extraction of water from the grain-boundary fluid for the formation of hydrous phases like epidote and sericite. The occurrence of Cl-rich amphibole and biotite coupled with the variation in their halogen contents in the A-type granites can act as potential indicators of hydrothermal fluid activity and the subsequent mineralisation in a region.