Polymetallic Te-rich melts contribute to efficient enrichment and precipitation of Au in hydrothermal ore deposits

Abstract

Tellurides are abundant in a large variety of hydrothermal Au deposits. Previous studies have proposed that these phases crystallize from polymetallic Te-rich melts that may act as scavengers for Au to form high grade deposits. However, the conditions under which Te-rich melts form and whether such melts can act as agents to concentrate Au have not been directly assessed. Here, we present an experimental study using natural ore samples from a giant Te-rich Au deposit (Dongping, China) to show that tellurides in Au ores crystallized from polymetallic melts and these melts behaved as a critical agent for Au enrichment and precipitation. The samples investigated contain abundant polymetallic inclusions, which consist of various telluride-gold mineral assemblages as well as minor chalcopyrite. Individual inclusions typically comprise one micron-sized gold grain. Annealing experiments of these inclusions were carried out at 300, 350, 400, and 450 °C. The polymetallic inclusions had relatively subtle changes in morphology and texture at 300 and 350 °C, but changed markedly at 400 and 450 °C, accompanying with the formation of abundant droplet-like and/or vermiculate gold grains randomly dispersed in the telluride matrix. Three in-situ scanning electron microscopy heating–cooling experiments with different heating rates and durations revealed a eutectic temperature of 243 to 270 °C for polymetallic tellurides, at which native gold began to dissolve into molten tellurides. During the heating processes of the three experimental runs, changes in topographic contrast occurred, with the formation of transient holes in the tellurides and the appearance of tellurides on the surface of chalcopyrite. These observations are indicative of localized migration of the molten tellurides. Upon cooling in run #1 and #2, native gold recrystallized at ∼ 340 °C from the molten tellurides and subsequently regrew in a gradual manner. After these two runs, many newly-formed telluride-gold globules were observed occurring as fracture- and/or cavity-infillings within the host pyrite, further confirming molten state of the polymetallic minerals. Collectively, our new data demonstrate that polymetallic Te-rich melts with a complete melting temperature of ∼ 340 to 400 °C are responsible for the formation of telluride-gold mineral assemblages in natural Te-rich Au ores. Such melts can act as an important Au scavenger and facilitate Au precipitation. Our findings highlight the significance of polymetallic Te-rich melts as a key agent for concentrating Au to form high-grade Au ores worldwide.