Skarn gold mineralization at the Geodo Mine, South Korea

The Geodo Bi-Te-bearing skarn deposit is located in the Taebaeksan mining district, South Korea. Both endoskarn and exoskarn are associated with the Cretaceous Eopyeong granitoids, which consist of magnetitebearing oxidized (Fe₂O₃/FeO > 0.4) quartz monzodiorite to monzodiorite.

Gold in the Geodo Mine occurs in a retrograde alteration stage within a garnet-rich proximal zone. Skarn minerals associated with gold ore include Fe-rich garnet (Ad_57-95Gr_1-41) and a wide range of clinopyroxene (Hd 20-100Di 0-79) compositions, broadly similar to those of typical gold-bearing skarn deposits. The paragenetic stages of the Bi- and Te-bearing skarn in the Geodo Mine consist of early calc-silicate alteration followed by three mineralization stages: Fe oxide, Cu rich, and Au rich. The Fe oxide and Cu-rich stages were deposited at higher temperatures than the Au-rich stage, based on mineral assemblage stabilities. Gold in Au-rich ores from the Geodo Mine occurs as native gold, ranging from 5 to 400 ìm in diameter, and is closely related to the Bi- and Te-bearing minerals bismuthinite and tetradymite, which at Geodo tend to be strongly enriched in Bi and depleted in S compared to ideal compositions. This association of gold with bismuthinite and tetradymite suggests that gold was deposited at relatively low temperatures, consistent with fluid inclusion studies on calcite that indicate that the gold was deposited from fluids ranging from 181° to 281°C and 8.0 to 10.9 wt % NaCl equiv, and with chlorite geothermometry indicating a range of 198° to 211°C. The presence of significant amounts of Bi- and Te-bearing minerals associated with Au suggests that liquid bismuth may have acted as a collector, increasing the concentration of gold.