Trace element composition and significance of sulfides in the Koktokay No. 3 pegmatite

Abstract

The Koktokay pegmatite district in the Altay orogen of Xinjiang is a significant rare metal mineralized cluster in China, hosting the world-renowned Koktokay No. 3 pegmatite vein. This study focuses on sulfides, discovered for the first time in the saccharoidal albite zone (Zone II) of the No. 3 vein. We systematically investigated their mineral assemblages and geochemical characteristics using TIMA (TESCAN Integrated Mineral Analyzer) and LA-ICP-MS in situ trace element analysis to elucidate their formation conditions, origin, and implications. The results indicate that the sulfide assemblage in Zone II is dominated by sphalerite, accompanied by bismuthinite, chalcopyrite, and pyrite. This paragenesis suggests formation under moderate- to high-temperature hydrothermal conditions (~ 340 °C), high sulfur fugacity (log_f_(S_2) > – 11), and a reduced environment. The sphalerite is characterized by enrichment in Fe, Mn, and In, and depletion in Ga, Ge, and Cd. Both trace element discrimination and machine learning analysis consistently confirm a magmatic–hydrothermal origin, crystallizing during the magmatic–hydrothermal transition stage of pegmatite evolution. This study provides the first direct constraint on the thermal state (~ 340 °C) of the Koktokay pegmatite system during the magmatic–hydrothermal transition stage revealing the crucial role of these conditions in facilitating the migration and enrichment of critical metals like Be and the formation of economically significant mineralization. These findings enhance the understanding of the metallogenic processes in the pegmatite-type deposit.