WALL-ROCK ALTERATION AT THE EMPEROR GOLD-SILVER TELLURIDE DEPOSIT, FIJI

In the Emperor mine area, widespread propylitic alteration was followed by limited K-silieate alteration and mineralization within and along vein edges. The propylitic assemblage is characterized by chlorite, ankerite, magnetite and pyrite. The K-silicate assemblage includes sericite, rare chlorite, quartz, K-feldspar and pyrite. The propylitic rocks are depleted in Si, Na, and Mg, have minor increases in K, and have substantial additions of CO2. The K-silicate rocks are depleted in Fe, Al, Na, Ca and Mg, and enriched in Si, K and CO2. Chlorite geothermometry suggests that the propylitic rocks were formed at temperatures below 170°C and the K-silicate rocks between 150 and 250°C. By comparison, fluid inclusion temperatures in the vein quartz vary, from 250-300°C in early quartz, down to 160-250°C in late stage quartz. The δ18O values of the fluids which caused the propylitic alteration are calculated to be near zero %o consistent with unmodified seawater. The δ13C values for wall rock carbonates are within 1.5%o of zero and consistent with this interpretation. The δ18C values of the fluids which caused the K-silicate alteration were 4.5-6%o and similar to the δ18O composition of fluids which formed the vein quartz. It is suggested that largely unmodified, downward moving, prograding seawater caused the low temperature propylitic alteration, dissolving silica and precipitating carbonates. The higher temperature K-silicate alteration and vein formation involved ascending hydrothermal solutions. This is in contrast with earlier suggestions that both types of alterations involved heated local waters and a magmatic component. © 1990, Taylor & Francis Group, LLC.