The retrograde evolution of F-rich skarns: Clues from major and trace element chemistry of garnet, scheelite, and vesuvianite from the Belka Pahar wollastonite deposit, India

Abstract

Skarn settings host many ore deposits and are ideal for studying the mobility of elements in hydrothermal fluids. Belka Pahar, in the Sirohi district of Rajasthan (NW India), is the largest wollastonite deposit in India (estimated wollastonite reserve of 56 million tonnes). No detailed textural and geochemical study has been conducted on these skarn rocks. In this study, we use petrographic studies, and major-trace element chemistry of skarn minerals from Belka Pahar to deduce the sequence of mineral transformations and to characterize the changes in the composition of the fluid with the progress of fluid-rock interaction during skarn formation. Two major rock types from the exoskarn have been considered for detailed study: prograde skarn rocks comprising pyroxene, garnet, wollastonite, quartz, and calcite, and retrograde skarn rocks containing garnet, scheelite, vesuvianite, fluorite, and sphalerite. Two types of garnet are recognized in the rocks-texturally earlier prograde garnet (Grt 1 ) and texturally late retrograde oscillatory-zoned Grt 2 , which is further sub-divided into three generations Grt 2a , Grt 2b and Grt 2c . The Grt 1 (And 27–47 ) and Grt 2a (And 50–60 ) have high Y-concentrations, and high HREE/LREE ratio, which is in stark contrast to the late Grt 2b (And 40–50 ) and Grt 2c (And 27–40 ), which is characterized by high F, low Y-concentrations, and HREE-depleted REE patterns. The chemistry of different generations of garnet suggests a shift from diffusive to more advective metasomatic conditions and increasing water/rock ratio, accompanied by a shift in pH of the fluid from neutral to acidic to weakly acidic. The crystallization of F-rich garnet, vesuvianite, and fluorite indicates an increase in F-concentration in the later-stage fluid. An increase of W concentration in Grt 2b and co-precipitation of scheelite and fluorite also suggests its possible link with scheelite crystallization. The high grossular component of Grt 2b and Grt 2c suggests transport of Al in the hydrothermal fluid as F-complexes. The hydrothermal fluid composition extrapolated from scheelite chemistry agrees with the LREE enriched patterns exhibited by all skarn minerals. Principal Component Analysis done on a compiled dataset of skarn garnet shows that its trace element composition can be a useful proxy of fluid composition and can be used as an indicator of associated ore type. • Garnet composition is a useful proxy of fluid chemistry in the skarn system. • High F concentration in fluid enhanced Al mobilization, forming grossular-rich garnet. • Scheelite precipitation is linked to interaction of F-rich fluid with calc-silicate rocks. • PCA shows control of fluid composition on garnet chemistry. • Garnet trace element chemistry is a robust proxy of ore-genetic association.