Fluid inclusions of the Ilkwang Cu−W-bearing breccia-pipe deposit, Kyongsang Basin

Abstract

The copper-tungsten mineralization of Ilkwang mine, Kyongsang Basin was associated with crystallization of a small stock of granodiorite and quartz monzonite, explosive brecciation, and exsolution of magmatic hydrothermal fluids. Fluid inclusions in quartz phenocrysts of granitic rocks and hydrothermal quartz in breccia matrix trapped aqueous fluids that had caused brecciation and precipitation of coppertungsten ore mineals. Four groups of fluid inclusions are identified: Type I (vapor-rich and low salinity), Type II (liquidrich and low to moderate salinity), Types IIIa and IIIb (liquidrich and high salinity) and Type IV (CO2-bearing, vapor-rich, and low salinity). The earliest high-saline fluid (Type IIIa) and the following CO2-bearing fluid (Type IV) were exsolved from crystallizing melt and trapped at relatively high pressure (> 1 kbar). As water was partitioned from melt to aqueous fluid, the volume expansion caused overpressuring of the magmatic hydrothermal system and led to an explosive hydrothermal brecciation. The pressure quenching related to the brecciation was followed by crystallization of the matrix producing early-hydrothermal quartz and tourmaline. Additionally, a subsequent decrease in pressure produced boiling of aqueous fluid, generating Types I and IIIb inclusions. They were associated with sulfide-mineral precipitation at 493–343°C and 450–120 bars. The separation of vapor phase including acid volatile species CO2 from the liquid phase might have raised pH of the hydrothermal fluid, which was favorable for the formation of sulfides. The late-magmatic, high-salinity fluids (Type IIIb) were mixed with circulating meteoric fluids and then were cooled and diluted to form lower-temperature and lower-salinity hydrothermal fluids that were trapped as inclusions during the final stage of hydrothermal activity. The fluid-inclusion data are comparable with those from breccia-pipe deposits of porphyry-copper systems. It is suggested that hydrothermal activities and favorable structure factors in the Ilkwang mining district played a role for the generation of favorable P-T conditions and helped provide sufficient energy to form a breccia zone.