Insights into the anatectic origin of granites parental to tungsten mineralization: A case study from the trans-Aravalli terrane, NW India

Abstract

Potential progenitors for W (±Sn) deposits include peraluminous granites of S-type affinity. The anatectic origin of such granites parental to W mineralization has received little attention. This study focuses on Balda Granite (BG), a peraluminous intrusion parental to W-rich ore bodies in the Sirohi region (NW India). We reflect upon the potential source for BG and investigate its anatectic origin through open-system phase equilibria modeling. On the prograde path, muscovite- and biotite-dehydration reactions at 675–745°C and 755–870°C yield ~10 and 13 wt.% melt, respectively. Si, K, Al, and Fe contents of the cumulative melt increased with progressive anatexis. Modeling results suggest high-T (>800°C) stability of the peritectic garnet, which is abundantly observed in the leucosome-dominated migmatitic patches. Cumulative melt extracted till 868°C was chosen to model the crystal fractionation along three polybaric gradients of 30, 45, and 60°C/kbar. As the modeled anatectic melt cooled, its peraluminosity and maficity decreased progressively. With the intermediate cooling gradient of 45°C/kbar, the melt achieved complete crystallization at ~7 km, the depth at which the BG had been emplaced and evolved into a W-rich residual (fractionated) model melt. In terms of peraluminosity, and major and trace element (Lu, Sc, Dy, Y, Yb) chemistry, the fractionated (residual) model melt compares well with BG. This study also models the W concentration in the anatectic melt during its generation and fractional crystallization. We argue for the origin of BG through high-T anatexis of Sirohi Group metapelites and cooling (and fractional crystallization) of the parent anatectic melt at the maximum gradient of 45°C/kbar. Thus, a high-T anatectic origin of granites parental to W deposits may be more prevalent than so far inferred.