Complex geochemical evolution of hydrothermal fluids related to breccia pipe Cu-W mineralization of the Dalseong mine, Korea

Abstract

The Cu-W deposit of the Dalseong mine, Korea, occurs within a steeply plunging, pipe-like breccia zone (40-90 m by 120-240 m in horizontal dimension) of intensely altered andesitic rocks and lies about 1.5 km of a Cretaceous quartz monzonite stock. The hydrothermal mineralization occurs mainly as two stages (stages 1 and 2) of open space filling in breccia blocks (mainly, <1 to 30 cm in size). Stage 1 mineralization is classified into early matrix mineralization and late ore zone mineralization. The ore zone mineralization of stage 1 consists of quartz, tungstates, base-metal sulfides and carbonates, and is further divided into three mineralization periods: early W-Mo; middle base-metal sulfides; and late Bi-bearing sulfides and sulfosalts. Stage 2 mineralization occurs as barren carbonate veins. Fluid inclusion data indicate a complex evolution of the hydrothermal fluids. The earliest recorded fluid in quartz of the matrix mineralization of stage 1 is found as hypersaline (31-42 wt.% equiv. NaCl) fluid inclusions that are homogenized totally at high temperatures (440-550<degrees>C), and was likely formed by condensation of an immiscible magmatic vapor phase. Later fluids trapped in quartz of the early to middle periods of stage 1 ore zone mineralization occur as lower salinity (<20 wt.% equiv. NaCl), lower temperature (T-h-tot = 250-400 <degrees>C) fluid inclusions that consist of various types (aqueous vapor-rich, aqueous liquid-rich, liquid CO2-bearing, and halite-bearing). These fluid inclusions suggest the presence of compositionally heterogeneous fluids formed by fluid boiling and associated CO2 immiscibility. Fluid inclusions in quartz of the middle to late periods of stage 1 ore zone mineralization are aqueous liquid-rich and very low in total homogenization temperature (250-330 degreesC) and salinity (down to 12 wt.% equiv. NaCl), and record the history of mixing with cooler meteoric groundwater. Measured and calculated O and H isotope compositions of hydrothermal fluids gradually decrease with time (and decreasing temperature): delta O-18(H2O) values, from 6.3 to -1.0 parts per thousand; and deltaD(H2O) values, from -73 to -99 parts per thousand. This indicates the influx of progressively increasing amounts of meteoric groundwater into an early, W-Mo-depositing magmatic hydrothermal system. This increasing influx of meteoric water resulted in successive deposition of base-metal sulfides, Bi-bearing minerals, and finally carbonates.