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

Abstract

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 aCl‐/aOH‐ and aF‐/aOH‐. 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.