Compositional variation of arsenopyrite and fluid evolution at the Ulsan deposit, southeastern Korea: A low-sulfidation porphyry system

The Ulsan Fe-W mine is located within the Cretaceous Gyeongsang volcano-sedimentary basin at the southeastern edge of the Korean Peninsula. Distinct hydrothermal events resulted in calcic skarn and vein deposits in recrystallized limestone near a Tertiary epizonal granite stock. The deposits of the Ulsan mine present a unique opportunity to document geochemically the complex evolution of a skarn-vein system that is related genetically to a low-sulfidation system. Isochemical contact metamorphism of an early skarn stage (stage I) is displayed by the presence of anhydrous Ca-Al-Mg skarn minerals at the contact between granite and recrystallized limestone. Following magnetite deposition in the main prograde skarn (stage II), the first deposition of arsenopyrite occurs intergrown with rammelsbergite - niccolite - gersdorffite - lollingite - native bismuth - bismuthinite - hexagonal pyrrhotite. These common sulfide assemblages are characterized by an overall low-sulfidation state during the main skarn stage. Retrograde skarn (stage III) is characterized by minor impregnations of scheelite in calcite and quartz, with actinolite and chlorite. During the latest part of stage III, Cu-Zn and polymetallic sulfide mineralization was introduced. The latest episode in the hydrothermal system (stage IV) is characterized by Zn-Pb-Ag mineralization in siderite-quartz veins. Decreasing As contents in arsenopyrite from stages II to IV indicate a decrease in temperature or sulfur fugacity (or both) with time. The various skarn-forming events and ore minerals from various stages are interpreted to have resulted from an evolutionary trend from hypersaline magmatic fluids during prograde skarn formation associated with Fe-As(-Ni) mineralization to low-salinity and low-temperature fluids during the retrograde skarn formation, associated with W-Cu-Zn mineralization. As the influence of magma-derived fluids waned, surficial fluids descended to deeper levels along fractures, resulting in siderite-quartz deposition associated with Zn-Pb-Ag mineralization. These results demonstrate that the Ulsan deposit is likely a skarn deposit that is genetically related to a low-sulfidation porphyry system.