Evolution of the Mugeug gold-silver vein system in Korea
The gold-silver deposit around the Eumseong pull-apart basin in the Precambrian Gyeonggi massif, Korea shows a genetic relationship between two left-lateral sinistral strike-slip fault systems. The wing cracks on a tensile domain in the jog structure are formed by the movement of NNE-trending major faults, suggesting that fault margins are favorable environments for the formation of veins. The Au-Ag-Sb veins in the Mugeug area are the results of multistage mineralization showing well-developed structures such as crustiforms, drusy cavities, combs, breccias, feathery lattice and cockades, consistent with textures commonly found in shallow-level environments. The veins show spatially variable envelopes of sericitic, argillic, propylitic and silicic alteration. Alteration patterns of veins are characterized by a wide, proximal, sericite-dominated phyllic zone, and a distal chlorite or/and epidote-dominant propylitic zone. Vertical variation of the alteration zone in the northern vein system is from a wide phyllic zone at depth, to a subphyllic zone at relatively shallow levels. Our results show that ore-forming fluids (δ18OH2O; -1.2 ~ 3.8 ‰) in the northern area evolved by dilution and cooling from less-evolved meteoric water at high temperature and salinity (≈ 300℃, 1~9 eq. wt. % NaCl). In contrast, vertical alteration patterns in the southern area change from a narrow subphyllic/propylitic zone at depth to a propylitic/argillic zone at relatively shallow levels. Compared to the northern area, this finding suggests that the circulating ore-forming fluids (δ18OH2O; -5.5 ~ 4.0 ‰) at low temperature and salinity (< 230 ℃, < 3 eq. wt. % NaCl) evolved by compositional variation of meteoric fluids. These changes in alteration assemblages and mineralization between the northern and southern area can be accounted for by the variation of current systems in a paleogeothermal field.