Abstract

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 + or - 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 ratio 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 tellurides --> Ag tellurides + 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 (260 degrees C, 6 equiv wt % NaCl) mixed with cooler, less saline waters, resulting in most ore deposition from fluids of intermediate temperature and composition (190 degrees -230 degrees C, 2.5-4.5 equiv wt % NaCl). Continued cooling and dilution (coupled with decreasing fugacity of sulfur) resulted in deposition of Ag-Te mineralization and ultimately in barren stage III and IV quartz and stage V calcite. Stable isotope compositions of mineralizing fluids from Tongyoung (delta D = -44 to -61ppm; delta 18 O = -6.8 to -9.4ppm) are similar to modern meteoric and thermal spring waters in the area and indicate meteoric water dominance during all stages of the Au-Ag-Te mineralizing system. Comparison of Tongyoung to other Au-Ag-(Te) deposits indicates that its complex sequence of mineralization is a natural consequence of repeated inundation of a hydrothermal system by progressively cooler meteoric waters.

First Page Preview

First page PDF preview
You do not currently have access to this article.