Petrogenesis and mineralization potential of Bhilangana granitoid, Bhilangana Valley, Garhwal Himalaya, India

Bhilangana granitoid is a batholithic size body exposed in Bhilangana Valley and later thrusted over rocks of Lesser Himalaya. It comprises mylonitized porphyritic granite gneiss, psammitic gneiss, and its variants, containing primary biotite and muscovite, and S-type in nature. Bhilangana granitoid is intruded by basic intrusive. The variation in chemical signatures and associated petrogenetic issues of Bhilangana granitoid are dealt with detailed petrographic, geochemical and mineral chemical study. In the vicinity of basic intrusive, the porphyritic granite gneiss shows enrichment of MgO, whereas its evolved part, psammitic gneiss shows more boron enrichment with the presence of tourmaline nodules surrounded by quartzo-feldspathic rim. The major mineral assemblage in Bhilangana granitoid is quartz-K-feldspar-plagioclase-biotite-muscovite with tourmaline, epidote, zircon, ilmenite, apatite and monazite. The granite is peraluminous with ASI values always >1. Biotite shows variation from Fe-rich siderophyllitic to Mg-rich eastonitic composition, indicating involvement of both Al-rich crustal and Mg-rich mafic sources, inferring source heterogeneity or mixing of felsic-mafic magma. Granitoid melt was buffered below QFM to NNO and partly above HM, indicating a strong reducing to the oxidizing condition of melt. The melt was saturated with the presence of 3.3-6.8 wt.% H2O content. The crystallization temperature of Bhilangana granitoid (TZr) ranges between 697 degrees and 807 degrees C with a solidification pressure of about 2.92-4.31 kbar, implying crystallization at a depth of 8.76-13 km. The presence of greisens assemblage of quartz-mica-tourmaline-epidote, along with the presence of fluorite, is a representative greisens assemblage for Sn-W mineralization. Thus, the Bhilangana granitoid represents the fertile nature and potentiality of Sn-W minerals.