Geochemical Studies of the Tongyoung Gold-Silver Deposits, Republic of Korea: Evidence of Meteoric Water Dominance

Epithermal gold-silver vein mineralization of the Tongyoung mine was deposited in five stages (I to V) of quartz-rhodochrosite-calcite-sulfide veins (0.3-7.0 m thick) which fill fault planes in Cretaceous volcanic rocks of the Gyeongsang basin. K-Ar dating of alteration sericite (72.9 _ 1.2 Ma) indicates a Late Cretaceous age for ore mineralization. Stages I and II are Au-Ag-bearing, whereas stages III, IV, and V are barren. Ore grades average 12 g/metric ton gold with a gold/silver atio of 1:11. Stage II is composed of five rhythmically banded substages which show a progressive change in ore mineralogy with increasing paragenetic time: electrum (68 wt % Au)-- • electrum (65 wt % Au) + argentitc-- • electrum (50 wt % Au) + Ag tellu-rides-- • Ag tellurides q- native silver. The shift in mineralogy reflects decreases in temperature and fugacity of sulfur, with a concomitant increase in fugacity of tellurium. Fluid inclusion data indicate a history of progressive cooling and dilution of ore-forming fluids. During stages I and II, higher temperature and salinity fluids (260øC, 6 equiv wt % NaCI) mixed with cooler, less saline waters, resulting in most ore deposition from fluids of intermediate temperature and composition (190ø-230øC, 2.5-4.5 equiv wt % NaC1). Con