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

The Cu-W deposit of the Dalseong mine, Korea, occurs within a steeply plung ing, 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 tw o 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 matri x mineralization and late ore zone mineralization. The ore zone mineralizat ion of stage 1 consists of quartz, tungstates, base-metal sulfides and carb onates, and is further divided into three mineralization periods: early W-M o; 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 fluid s. The earliest recorded fluid in quartz of the matrix mineralization of st age 1 is found as hypersaline (31-42 wt.% equiv. NaCl) fluid inclusions tha t 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 zon e mineralization occur as lower salinity (<20 wt.% equiv. NaCl), lower temp erature (T-h-tot = 250-400 <degrees>C) fluid inclusions that consist of var ious types (aqueous vapor-rich, aqueous liquid-rich, liquid CO2-bearing, an d halite-bearing). These fluid inclusions suggest the presence of compositi onally heterogeneous fluids formed by fluid boiling and associated CO2 immi scibility. Fluid inclusions in quartz of the middle to late periods of stag e 1 ore zone mineralization are aqueous liquid-rich and very low in total h omogenization temperature (250-330 degreesC) and salinity (down to 12 wt.% equiv. NaCl), and record the history of mixing with cooler meteoric groundw ater. 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 i ncreasing amounts of meteoric groundwater into an early, W-Mo-depositing ma gmatic hydrothermal system. This increasing influx of meteoric water result ed in successive deposition of base-metal sulfides, Bi-bearing minerals, an d finally carbonates.