The Mugeuk mine was historically the largest gold-silver producer in South Korea, and yielded more than ten metric tons of gold from ores with an average grade of about 8 g/ton Au and Au/Ag ratios near 1:5. Ore deposits consist of >10 subparallel quartz-calcite veins (typically 0.6 to 1.0 m wide) that fill faults and fractures in Jurassic granodiorite and Cretaceous quartz porphyry. Veins formed during six successive hydrothermal events of Late Cretaceous age. Ore mineralogy of the veins is complex, consisting mainly of pyrite and base-metal sulfides with electrum (23-51 atom. % Au), native silver, argentite and silver sulfosalts (polybasite, pyrargyrite). Stage I veins contain no gold and silver (e.g., barren). During stages II to V, economic quantities of gold and silver were precipitated. Stages III and IV represent the culmination of gold precipitation in distinct sulfide bands, whereas stage V represents a shift to dominantly silver deposition. Stage VI veins are barren post-ore quartz-calcite-fluorite. A variety of types of geochemical data indicate that deposition of gold and silver resulted mainly from cooling of ore fluids, accompanying successive incursions of meteoric water into the hydrothermal system. Fluid inclusion data show general decreases of temperature and salinity within each stage. Ore mineral assemblages indicate decreases in the fugacity of sulfur with decreasing temperatures during the deposition of gold and silver. Measured and calculated δ18O values of hydrothermal fluids decrease generally from +3.0 to -7.4‰ from stages II through VI, and δD values range from -66 to -84‰. The Au-Ag deposits at Mugeuk, in particular their mineralogy, Au/Ag ratios and fluid geochemistry, are the result of episodic meteoric water events within the granite-hosted hydrothermal system. Early stages of the hydrothermal history involved successive introduction of new pulses of meteoric ore fluids, each of which equilibrated with the host granitic rocks at successively higher water-to-rock ratios and lower temperatures during deposition of gold-silver mineralization. Later stages were the result of inundation of the hydrothermal system by isotopically unevolved meteoric waters, resulting in a silver-rich overprint on the earlier gold-silver system.
Abstract: Gold mineralization of the Daerae mine represents the first recognized example of the Jurassic gold mineralization in the Sangju area, Korea. It occurs as a single stage of quartz veins that fill fault fractures in Precambrian gneiss of the central‐northern Sobaegsan Massif. The mineralogical characteristics of quartz veins, such as the simple mineralogy and relatively gold‐rich (65–72 atomic % Au) nature of electrum, as well as the CO 2 –rich and low salinity nature of fluid inclusions, are consistent with the ‘mesothermal‐type’ gold deposits previously recognized in the Youngdong area (about 50 km southwest of the Sangju area). Ore fluids were evolved mainly through CO 2 immiscibility at temperatures between about 250 and 325 C. Vein sulfides characteristically have negative sulfur isotopic values (–1.9 to +0.2 %), which have been very rarely reported in South Korea, and possibly indicate the derivation of sulfur from an ilmenite‐series granite melt. The calculated O and H isotopic compositions of hydrothermal fluids at Daerae (δ 18 O water = +5.2 to +5.9 %; δD water = –59 to –67 %) are very similar to those from the Youngdong area, and indicate the important role of magmatic water in gold mineralization. The 40 Ar– 39 Ar age dating of a pure alteration sericite sample yields a high‐temperature plateau age of 188.3 0.1 Ma, indicating an early Jurassic age for the gold mineralization at Daerae. The lower temperature Ar‐Ar plateau defines an age of 158.4 2.0 Ma (middle Jurassic), interpreted as reset by a subsequent thermal effect after quartz vein formation. The younger plateau age is the same as the previously reported K‐Ar ages (145–171 Ma) for the other ‘mesothermal–type’ gold deposits in the Youngdong and Jungwon areas, Korea, which are too young in view of the new Jurassic Ar‐Ar plateau age (around 188 Ma).
The Xiaoxinancha Cu-Au deposit in the Jilin province, located in NNE 800 km of Beijing, is hosted by diorite. The ore mineralization of Xiaoxinancha Cu-Au deposit show a stockwork occurrence that is concentrated on the potassic and phyllic alteration zones. The Xiaoxinancha Cu-Au deposit in the south is being mined with its reserves grading 0.8% Cu, 3.64 g/t Au and 16.8 g/t Ag and in the north, grading 0.63% Cu, 3.80 g/t Au and 6.8 glt Ag. The alteration assemblage occurs as a supergene blanket over deposit. Hydrothermal alteration at the Xiaoxinancha Cu-Au deposit is centered about the stock and was extensively related to the emplacement of the stock. Early hydrothermal alteration was dominantly potassic and followed by propylitic alteration. Chalcocite, often associated with hematite, account for the ore-grade copper, while chalcopyrite, bornite, quartz, epidote, chlorite and calcite constitute the typical gangue assemblage. Other minor opaque phases include pyrite, marcasite, native gold, electrum, hessite, hedleyite, volynskite, galenobismutite, covellite and goethite. Fluid inclusion data indicate that the formation of this porphyry copper deposit is thought to be a result of cooling followed by mixing with dilute and cooler meteoric water with time. In stage II vein, early boiling occurred at 497C was succeeded by the occurrence of halite-bearing type III fluid inclusion with homogenization temperature as much as 100C lower. The salinities of type 1II fluid inclusion in stage II vein are 54.3 to 66.9 wt.% NaCI + KCI equiv. at 383 to 495C, indicating the formation depth less than 1 km. Type I cupriferous fluids in stage III vein have the homogenization temperatures and salinity of 168 to 365C and 1.1 to 9.0 wt.% NaCI equiv. These fluid inclusions in stage III veins were trapped in quartz veins containing highly fractured breccia, indicating the predominance of boiling evidence. This corresponds to hydrostatic pressure of 50 to 80 bars. The value of sulfide minerals increase slightly with paragenetic time and yield calculated values of 0.8 to 3.7. There is no mineralogical evidence that fugacity of oxygen decreased, and it is thought that the oxygen fugacity of the mineralizing fluids have been buffered through reaction with magnetite. We interpreted the range of the calculated values for sulfides to represent the incorporation of sulfur from two sources into the Xiaoxinancha Cu-Au hydrothermal fluids: (1) an isotopically light source with a value of I to 2, probably a Mesozoic granitoid related to the ore mineralization. We can infer from the fact that diorite as the host rock in the Xiaoxinancha Cu-Au deposit area intruded plagiogranite; (2) an isotopically heavier source with a value of > 4.0, probably the local porphyry.
Electrum (79.67-88.68 atom.%Ag)-sulfide mineralization of the Byungjibang mine in the Hwoingsung area represents the first recognized example of mesothermal gold mineralization of late Cretaceous age in South Korea, and was deposited in three stages of mineralogically simple, base metal-rich quartz±calcite veins that filled the fractures in Cretaceous biotite granite and quartz porphyry. The K-Ar dating of hydrothermal vein muscovite indicates that mineralization took place during the late Cretaceous (81.9±1.8 Ma). Fluid inclusion data show that main ore mineralization (stage I) formed from high temperature (225°-370°C), H2O-NaCl(-CO2) fluids with low salinity (1.4-6.7 wt. % eq. NaCl). The deposition of abundant base-metal sulfides (sphalerite, chalcopyrite, galena, etc.) forming distinct sulfide bands was mainly a result of primary boiling and associated CO2 effervescence of the fluid with temperatures between about 320° and 370°C under considerable crustal depths (at least 1.3 to 2.2 km deep). Gold-silver deposition occurred paragenetically later than base-metal sulfide deposition, and was the result of cooling (down to 200°-250°C) and decreasing sulfur activity caused by mixing with meteoric groundwaters. Calculated sulfur isotope composition of ore fluids (δ34SΣS=3.4 to 5.6‰) indicates an igneous source of sulfur in hydrothermal fluids, probably the nearby Cretaceous quartz porphyry dikes. Measured and calculated, oxygen and hydrogen isotope compositions of stage I ore fluids (δ18Owater=3.1 to 7.9‰, δDwater=−72 to −87‰) indicate that the auriferous hydrothermal fluids at Byungjibang formed originally from either a magmatic water or an isotopically highly exchanged meteoric water under low water-rock ratio condition, and evolved through progressive mixing with an isotopically evolved meteoric groundwater (which is represented by stage III fluids; temperature=down to 130°C, δ18Owater=−6.4 to −7.2‰, δDwater=−86‰).
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