Abstract

The Gubong deposit consists of five massive, gold-bearing mesothermal quartz veins that fill fractures oriented northeast and northwest along fault shear zones over an area of 14 km2 in Precambrian metasedimentary rocks of the Gyeonggi massif. The veins are divided into three groups, based on their orientation and location. They have a ribbon texture that is interpreted to result from repeated hydraulic fracturing events. Mineral deposition was associated with hydrothermal fluid overpressuring within and below the active fault shear zones. The vein mineralogy and paragenesis of the veins allow two separate discrete mineralization episodes to be recognized, separated by a major faulting event. The ore minerals are contained within quartz and calcite associated with fracturing and healing of veins that occurred during both mineralization episodes. Wall-rock alteration minerals during stage I, the main ore stage, include sericite, chlorite, and minor pyrite and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, and argentite. Electrum also was deposited during stage II mineralization in the one group of veins, along with pyrite, sphalerite, chalcopyrite, and galena, but the second stage of deposition in other veins was barren.

Petrographic examination of textural relationships among sulfides, fluid inclusions, microfracturing, and quartz shows chronological and genetic relationships between gold deposition and fluid entrapment. Systematic studies of fluid inclusions in stage I vein indicate two contrasting events: a relatively high temperature (203°–432°C) and pressure (943–2,098 bars) event related to early sulfide deposition and associated with H2O-CO2-CH4-NaCl ± N2 fluids (less than about 13.4 wt % NaCl), and a lower temperature (202°–399°C) and pressure (670–850 bars) late sulfide depositional event involving H2O-NaCl fluids (3.9–17.3 wt % NaCl). The H2O-NaCl fluid involved in ore-related stage II mineralization had a salinity of 0.4 to 4.2 wt percent NaCl and a homogenization temperature of 201° to 378°C.

The calculated sulfur isotope compositions of hydrothermal fluids from the stage I veins (δ34SH2S = 3.5–10.5‰) indicate that ore sulfur was derived mainly from a magmatic source but also in part from sulfur contained in the host rocks. The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (stage I: δ18OH2O = 1.1–9.0‰, δD = −92 to −21‰; stage II: δ18OH2O = −0.1 to +0.3‰, δD = −95 to −93‰) indicate that the fluids were derived from magmatic and/or deep-seated metasedimentary rocks (stage I) and evolved by mixing with local meteoric water (stage II), by limited water-rock exchange and by degassing during mineralization in uplift zones.

The H2O-NaCl fluids involved in the stage I development of these veins represent fluids that evolved either through unmixing of H2O-CO2-CH4-NaCl ± N2 fluids following a decrease in fluid pressure or through mixing with deeply circulating meteoric waters, possibly as a result of uplift and/or unloading during mineralization. The H2O-NaCl fluid involved in stage II was derived from meteoric water.

Early deposition of gold in stage I was caused by a decrease in sulfur fugacity (H2S loss) that accompanied the immiscible separation of carbonic vapor from H2O-CO2-CH4-NaCl ± N2 fluids. Gold in late stage I and stage II veins was precipitated from H2O-NaCl fluids by cooling and by dilution caused by mixing with meteoric water.

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