Geochemical evolution of hydrothermal fluids related to polymetallic mineralization in the Gyeongnam mineralized district, Korea

摘要

Polymetallic hydrothermal vein deposits in the Gyeongnam district are primarily copper deposits, with additional iron, tungsten, molybdenum, zinc, lead, gold and/or silver with some sulfosalts. Most of the polymetallic copper-bearing veins in the district occur in four mineralized areas - the Jisu, Hainan, Goseong and Masan areas. The mineralized veins in each area are hosted by different stratigraphic horizons as wall-rock in decreasing age as follows: the Jisu -> Haman -> Goseong -> Masan area, and are all related to the Cretaceous Chindong granite. Generally, successive polymetallic ore mineralization in the district shows a simplified mineralogy progressing through: 1) Fe-W-Mo, 2) Cu, 3) (Cu-)Zn-Pb with Pb-Bi-Ag-Sb sulfosalts, and/or 4) ferric (hematite) mineralization. The polymetallic veins in the Jisu and Haman areas are dominantly Fe-W-Mo and Cu mineralizations with tourmaline and actinolite as high-temperature gangue minerals. The deposits in the Goseong and Masan areas are dominantly Cu and Zn-Pb mineralization with ferric mineralization, respectively. The Jisu and Haman hydrothermal systems are characterized by high-salinity brine and/or CO_2-rich fluids. The vein mineralization initiated at high temperature (approx 550 deg C) from fluids with high salinity (up to about 60 equiv. wt. percent NaCl or NaCl + KCl) derived mainly from the granite source and/or CO_2-rich fluid by fluid unmixing coupled with boiling. The oxygen isotope data (delta~(18)O_(water) = 8.9 to 4.7 per thousand for the early mineralization of the main stage) suggest that early hydrothermal fluids in the Jisu and Haman hydrothermal systems likely represent magmatic and/or meteoric water whose isotopic composition was controlled by exchange with a large volume of igneous (and metamorphic or sedimentary) rocks at near-magmatic temperatures. In the waning portion of the main stage in the Jisu and Haman areas, the high-temperature, high-salinity fluids gave way to progressively cooler, more dilute fluids (down to approx = 150 deg C and approx = 2 equiv. wt. percent NaCl). These trends are interpreted to indicate progressive mixing of magmatic and/or equilibrated meteoric hydrothermal fluids with cooler and less saline meteoric groundwater. The early and main ore mineralization in the Goseong and Masan hydrothermal systems occurred by boiling at temperatures between about 400 deg C and 250 deg C from fluids with low to moderate salinity (up to about 13 equiv. wt. percent NaCl). In the waning portion of the main mineralization in the areas, the high-temperature, moderate-salinity fluids gave way to progressively cooler, more dilute fluids of the late ore mineralization (down to approx = 200 deg C and approx = 1 equiv. wt.percent NaCl). There is a systematic decrease in calculated delta_(18)O_(water) values with decreasing temperature in the Goseong and Masan hydrothermal systems (from 5.0 to -6.6 per thousand and 2.7 to -9.9 per thousand for the Goseong and Masan area, respectively). The results are interpreted to indicate the progressively cooler, more oxidizing meteoric water influx of the early Goseong and Masan hydrothermal systems formed by exchanged meteoric waters. The systematic ore mineralogy and evolution of hydrothermal systems in each area are thought to be due to their relative horizontal and/or vertical distance with respect to a magma source. The copper deposits containing Fe-oxides and W-Mo mineralization in the Jisu and Haman areas are proximal to a magmatic source, whereas copper dominant deposits in the Goseong area are transitional, and Zn-Pb-rich copper deposits in the Masan area are distal on the Gyeongnam district.