Mineralogical, fluid inclusion, and stable isotope constraints on mechanisms of ore deposition at the Samgwang mine (Republic of Korea)-a mesothermal, vein-hosted gold-silver deposit

摘要

The Samgwang mine is located in the Cheon-gyang gold district (Cheonan Metallogenic Province) of the Republic of Korea. It consists of eight massive, gold-bearing quartz veins that filled NE- and NW-striking fractures along fault zones in Precambrian granitic gneiss of the Gyeonggi massif. Their mineralogy and paragenesis allow two separate vein-forming episodes to be recognized, temporally separated by a major faulting event. The ore minerals occur in quartz and calcite of stage I, associated with fracturing and healing of veins. Hydrothermal wall-rock alteration minerals of stage I include Fe-rich chlorite (Fe/(Fe+Mg) ratios 0.74-0.81), muscovite, illite, K-feldspar, and minor arsenopyrite, pyrite, and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, argentite, pyrargyrite, and argentian tetrahedrite. Only calcite was deposited during stage II. Fluid inclusions in quartz contain three main types of C-O-H fluids: CO_2-rich, CO_2-H_2O, and aqueous inclusions. Quartz veins related to early sulfides in stage I were deposited from H_2O-NaCl-CO_2 fluids (1,500-5,000 bar, average 3,200) with T_(htotal) values of 200 deg C to 383 deg C and salinities less than about 7 wt. percent NaCl equiv. Late sulfide deposition was related to H_2O-NaCl fluids (140-1,300 bar, average 700) with T_(htotal) values of 110 deg C to 385 deg C and salinities less than about 11 wt. percent NaCl equiv. These fluids either evolved through immiscibility of H_2O-NaCl-CO_2 fluids as a result of a decrease in fluid pressure, or through mixing with deeply circulated meteoric waters as a result of uplift or unloading during mineralization, or both. Measured and calculated sulfur isotope compositions (delta~(34)S_(H2S)=1.5 to 4.8 per thousand) of hydrothermal fluids from the stage I quartz veins indicate that ore sulfur was derived mainly from a magmatic source. The calculated and measured oxygen and hydrogen isotope compositions (delta~(18)O_(H2O)=-5.9 per thousand to 10.9 per thousand, delta D=-102 per thousand to -87 per thousand) of the ore-forming fluids indicate that the fluids were derived from magmatic sources and evolved by mixing with local meteoric water by limited water-rock exchange and by partly degassing in uplift zones during mineralization. While most features of the Samgwang mine are consistent with classification as an orogenic gold deposit, isotopic and fluid chemistry indicate that the veins were genetically related to intrusions emplaced during the Jurassic to Cretaceous Daebo orogeny.