Moraine-hosted Cu-Bi-mineralization from the Modi-Khola valley, southwestern flank of Annapurna-III, Central Himalaya

Abstract

The Cu-Bi-Ag-Te mineralization found in moraine blocks in the central part of the Modi-Khola valley, on the southwestern flank of Annapurna-III in central Nepal can be defined as a new type postcollisional hydrothermal system in the High Himalaya, related to a Miocene magmatic event of leucocratic granite intrusions or equigranular pegmatite bodies. The mineral assemblage consists dominantly of chalcopyrite and pyrite, with variable amounts of Bi-Ag sulphides, sulphosalts and tellurides (bismuthinite, tetradymite, hessite, aikinite-bismuthinite derivates). Ilmenite, magnetite, cubanite, garnet, rutile, chlorite and quartz complete the mineral assemblage. Common complex intergrowth textures, involving aikinte, hessite and tetradymite in highly variable proportions and forming equilibrium assemblages, resulted from segregation and crystallization of “droplets” of Ag-Bi-Te-(S) “melts” from the hydrothermal fluids at temperatures above the melting point of bismuth. Thus, the mineralization can be considered as mesothermal ore formation, most probably resulting from a magmatic-hydrothermal system that was active during the mid- Miocene in the Annapurna Himal