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

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.

Automated summary

This study investigates the sequence of mineral transformations and changes in fluid composition during skarn formation in the Belka Pahar wollastonite deposit in India. Petrographic studies and major-trace element chemistry of skarn minerals were used to analyze the fluid-rock interaction. The results provide insights into the diffusion and advection of elements in the hydrothermal fluid and the shift in pH. The composition of different generations of garnet serves as a useful proxy for fluid composition and can indicate the associated ore type.