REACTION PATH MODELING FOR TE-BEARING GOLD-SILVER VEIN MINERALIZATION OF THE TONGYEONG EPITHERMAL SYSTEM, KOREA

Epithermal Au-Ag-Te-(Se) mineralization associated with Late Cretaceous in Korea overlapping with the shallow magmatic activity. The epithermal Au-Ag-Te-(Se) deposits which indicate low sulfidation characteristics found in sediment-dominant basins in the central portion, and in volcanic-dominant basins in the south and southwest. Ore mineral assemblages and associated gangue phases in the Tongyeong deposit can be clearly divided into two general associations: an early cycle (band) that appeared with introduction of most of the sulfides, tellurides and electrum, and a later cycle in which base metal and carbonate-bearing assemblages with minor clay minerals (mostly rhodochrosite) became dominant. We have used computer modeling to simulate formation of four stages of vein genesis. The reaction of a single fluid with andesite host rock at 280°C, isobaric cooling of a single fluid from 260°C to 120°C, and boiling and mixing of a fluid with both decreasing pressure and temperature were studied using by the program CHILLER (Reeds, 1987). Calculations show that the precipitation of alteration minerals is due to fluid-andesite interaction as temperature drops. Speciation calculations confirm that the hydrothermal fluids with moderately high salinities and acid (pH=5.7), were capable of transporting significant quantities of base metals. The abundance of gold in fluid depends critically on the ratio of total base metals and iron to sulfide in the aqueous phase because gold is transported as an Au(HS)2- complex, which is sensitive to sulfide activity. Modeling results for Tongyeong mineralization show strong influence of shallow hydrogenic processes such as boiling and ground water mixing. The variable banding is best explained by multiple boiling of hydrothermal fluid followed by lateral mixing of the fluid with overlying diluted, steam-heated ground water. Thus boiling and mixing produce distinctly different mineralogy. Relatively large amounts of galena precipitate with a low degree of mixing, but sphalerite and acanthite contents increase with a high degree of mixing. Tongyeong Au-Ag-Te deposits indicates a natural consequence of repeated inundation of a hydrothermal system by progressively cooler meteoric waters.