Occurrence of Native Gold and Gold-Silver Alloy in the Olivine Gabbro of Layered Cumulate Sequence of Naga Hills Ophiolite, India

Occurrence of native gold and gold-silver alloy formed in high-temperature olivine gabbro of a layered cumulate body is reported 2 km northwest of Sutsu. It lies close to a major fault in the central part of Late Cretaceous-Eocene Naga Hills ophiolite (NHO). The amphibole-bearing olivine gabbro is composed of serpentine (MgO-28.91), anorthite (An93), clinopyroxene (En46Fs8Wo46), edenite (Mg# 76), magnesiohornblende (Mg# 78), accessory minerals, viz., chlorite (Mg# 75), epidote, sulphides (chalcopyrite and millerite) and gold. The formation of noble metals in olivine gabbro is related to partitioning of Au into intercumulus sulphides and silicates in magma and their deposition along grain boundaries and fractures. It was carried out by hydrothermal fluids by transportation and concentration of immiscible sulphide phases, and depositing these in suitable locales during final stages of crystallization. Gold mineralization in layered gabbros of cumulate bodies opens a new avenue towards primary source of precious metals in NHO. Alternative secondary source of precious metals in NHO, e.g. (i) basal conglomerate of cover sediments derived from ophiolite (Jopi or Pokhphur Formation), and (ii) placers of arterial Tizu River in Nagaland, may be considered as favourable repository.