Abstract

The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, the largest Pb-Zn producer in Korea, occurs as chimney-shaped carbonate-sulfide vein orebodies developed in Cambro-Ordovician limestones and shales. Compared with other deposits (the Bonsan and Dongjeom) of the Yeonhwa I mine, the Taebaek deposit is characteristically skarn free and more rich in lead and silver. These characteristics suggest that the Taebaek deposit formed farther from a deep, hidden, associated igneous body. A K-Ar date of alteration sericite indicates that the mineralization at Taebaek took place during Late Cretaceous times (75 Ma). Four stages of ore deposition have definite sulfide assemblages: stage I, hexagonal pyrrhotite + arsenopyrite + native bismuth; stage II, pyrite + sphalerite + arsenopyrite; stage III, monoclinic pyrrhotite + sphalerite + galena + electrum + Ag minerals; and stage IV, barren calcite. Later stages of mineralization occur at higher (shallower) portions of each orebody, indicating that the locus of mineralization migrated toward higher levels with increasing paragenetic time.Fluid inclusion data indicate that temperatures and salinities of ore fluids progressively decreased with time (380 degrees -250 degrees C and up to 8 wt % NaCl equiv during stage II, and 305 degrees - 180 degrees C and down to 0 wt % NaCl equiv during stage III), mainly due to progressive mixing with cooler and more dilute meteoric waters. Fluid inclusion data and geologic arguments indicate that pressures during the mineralization were in the range of 210 to 420 bars. Thermochemical considerations indicate the activity of H (super +) decreased when fluids became equilibrated with host limestones, which resulted in a pH change (from about 5 to 7) and decreases in f (sub S 2 ) and f (sub O 2 ) . The pH increase, combined with a decrease in temperature, was responsible for the spatial and temporal variations of iron sulfides and ore precipitation.The delta 34 S values of ore sulfides have a narrow range between 1.6 and 5.3 per mil. The calculated delta 34 S (sub Sigma S) values of ore fluids are constant at 3.5 to 4 per mil, indicating a deep-seated igneous source of sulfur. The delta 13 C values of carbonates in ores range between -7.2 and -5.5 per mil and are considerably more negative than those (-1.9 to +0.9ppm) of the host lime-stones. It is likely that carbon in the ore fluids was a mixture of deep-seated magmatic carbon and limestone-derived carbon. Oxygen and hydrogen isotope studies revealed that the delta 18 O and delta D values of waters in ore fluids decreased gradually with time from +9.8 and -68 per mil (stage II) to +1.9 and -95 per mil (stage IV), respectively. This indicates that magmatic water was important during the early zinc mineralization but was progressively replaced by meteoric water during the later lead and silver mineralization.

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