GEOCHEMISTRY AND GENESIS OF HYDROTHERMAL AU-AG-PB-ZN DEPOSITS IN THE HWANGGANGRI MINERALIZED DISTRICT, REPUBLIC-OF-KOREA

Abstract

The Hwanggangri mineralized district within the Ogcheon metamorphic belt of the southern Korean peninsula contains more than 60 W-Mo- and Cu-Pb-Zn(-Au-Ag)-bearing metallic ore occurrences and lesser fluorite and talc occurrences related to intrusion of Late Cretaceous (83-90 Ma), I-type and magnetite series granites. Fissure-filling veins in the district differ in mineralogy with distance from the granites: W-Mo vein deposits are located within or near (<500 m) the granites, whereas Cu-Pb-Zn vein deposits occur up to 5 km from the granites. The Geumsil and Daehyun Au-Ag-Pb-Zn mines are unique and represent the most distal deposits in the district. They consist of quartz-carbonate veins which contain economic concentrations of gold (3 g/t) and silver (250 g/t) in addition to abundant base metal sulfides (6 wt % Pb + Zn). Study of these Au-Ag-enriched mines provides a key to understanding ore-forming processes in the outer portions of a very large, batholith-driven hydrothermal circulation system. Phase equilibria, fluid inclusions, and stable isotopes indicate that ore deposition was the result of interaction of progressively less evolved meteoric waters with earlier, highly evolved meteoric waters. Early Fe sulfide mineralization occurred at temperatures of 380-degrees to 310-degrees-C from fluids (deltaO-18 = 10-12 part per thousand) whose chemistry was controlled dominantly by reactions with granite and carbonate-bearing host rocks. Main base metal-Au-Ag mineralization occurred at temperatures of 250-degrees to 210-degrees-C from fluids (deltaO-18 = 6-7 parts per thousand) whose chemistry changed rapidly as a result of fluid unmixing and cooling. Late, postore carbonates were deposited at temperatures of 250-degrees to 165-degrees-C from fluids whose stable isotope compositions (deltaO-18 = 1-4 parts per thousand; deltaD = -89 parts per thousand) and salinities (<3 wt % NaCl) reflect increasing meteoric water dominance. The most intriguing result of our studies is that gold deposition occurred from fluids whose chemistries were controlled dominantly by granites (and wall rocks) at temperatures well below magmatic conditions (250-degrees-C) at great distances from the intrusion. The occurrence of increased gold and silver grades in these distal areas bodes well for increased exploration for Au-Ag-Pb-Zn deposits at relatively great distance from granite batholiths.