Fluid-Rock Interaction in Calc-Silicate Rocks and the Role of Halogens in W Mobilization and Precipitation: Clues from Wollastonite Skarn at Belka Pahar, Rajasthan, Western India

Abstract:

The Aravalli Craton in western India witnessed several episodes of granite magmatism during the late Proterozoic (900-700 Ma). Some of these granites are associated with world-class skarn wollastonite deposit at Belka Pahar. In this study, we attempt to use the skarn minerals in the Belka Pahar wollastonite deposit to constrain fluid-rock interaction processes during skarn formation. Minerals such as garnet in the skarn rocks interact continuously with hydrothermal fluid and record any change in physicochemical conditions and fluid composition. Textural observations and chemical analysis reveals two generations of garnet. The texturally earlier garnet (Grt₁; And_65–70Grss_35–30) is present as remnant core and is mantled by later oscillatory zoned garnet (Grt₂). It has higher Y-concentration and HREE-enriched REE pattern typical of garnets. In contrast, Grt₂ has high F, low Y- and an its REE pattern shows significant HREE-depletion relative to Grt₁. The Grt₂ also appears as separate oscillatory zoned grains with andradite-rich core (And_60–65Grss_40–35) and grossular-rich rim (And_27–30Grss_73–70). Scheelite (CaWO₄) is included within the rim of Grt₂. The results of this work are in contrast to the general perception of assuming andradite formation as an indicator of late stage hydrothermal events. The crystallization of F-rich Grt₂ rim, vesuvianite and fluorite indicate an increase in F-concentration fluid rock interaction progressed. Textural observations reveal that these F-rich minerals have grown together with the scheelite suggesting that the removal of F from the fluid probably created favorable conditions for W-precipitation. The increase of W & Sn-concentration in Grt2-rim also suggest its possible link with the tungsten mineralization and demands further attention.