Prospectivity of Mesoproterozoic magmatism in the northern Karbi Hills, NE India for porphyry copper mineralization

Abstract

The Kuthori porphyry copper prospect in the Mesoproterozoic Kaziranga magmato-sedimentary domain of the northern Karbi Hills of Shillong Plateau, NE India, is a recently discovered granitoid-hosted Cu-Fe-S occurrence. We present here a preliminary geological account to appraise the petrography, geochronology and bulk geochemistry of the occurrence, and propose a geological model for copper mineralization in the Precambrian granitiods of Karbi Hills. These largely anorogenic (A-type) granitoids show evidences of multiple injections of magma of varying compositions ranging from granite-granodiorite-quartz monzonite. The granitoids have undergone hydrothermal alteration, resulting in the development of alteration mineral zones characterized by a central zone of potassic alteration, containing quartz–K-feldspar-biotite-sericite, followed by phyllic and argillic alteration showing typomorphic mineral growth away from the central alteration zone. The granitoids have wide range of silica contents (54.69–73.07%); are dominantly metaluminous and calc-alkaline; contain high total alkalis and Fe-rich biotite; and show high abundances of Y, Nb, Zr, high FeO/(FeO + MgO) ratios and low CaO, MgO and Sr abundances. The incompatible trace element distribution pattern, positive zircon εHf(t) (+3.2 to + 9.7), initial whole rock 87Sr/86Sr ratio (0.703) and sulphide minerals with 34S/32S ratio (+4.5 ± 0.05‰), indicate magma and sulphide derivation from an enriched mantle source at varying temperatures (828 °C-688 °C). The overall evidence suggests that granitoid magma production and emplacement took place in postcollision back arc basin, creating avenues for ore deposition due to a rapid diapiric rise of magma, sudden decrease in pressure and temperature, resurgent boiling marked by crackle brecciation and increase in the oxidation state of magma.