Geological, mineralogical and ore fluid characteristics of the Tagun-Khin-Dan gold mineralization in Mogok-Mandalay-Mergui Belt, Central Myanmar

The Tagun-Khin-Dan gold deposit in the Mogok-Mandalay-Mergui Belt, Central Myanmar, is characterized by an array of quartz-veins hosted in mudstone of the Kogwe Formation of the Carboniferous Mergui Group. Two major deformational stages were recorded in the area; (1) N-S shortening and (2) uplifting and emplacement of various dykes and quartz veinlets. The N-S shortening within the area lead the development of km-scale faults, determined largely by the presence of a zone of major WNW-ESE trending dextral strike-slip faulting. Quartz veins in the deposit include: (1) type-A quartz veins, parallel to the dextral NW-SE trending major fault; and (2) type-B quartz veins which occur as isolated parallel veins. Gold in the type-A quartz vein is present as native gold and electrum locked within pyrite and associated with pyrite and galena and in the type-B quartz veins as electrum associated with sulfide minerals such as pyrite, chalcopyrite, galena and sphalerite. The mineralization stages can be classified into the type-A quartz vein stage and the type-B quartz vein stage. Two type of fluid inclusions; liquid-rich aqueous inclusions (L-type) and vapor-rich aqueous inclusions (V-type) are identified in the type-A quartz veins. The homogenization temperature of L-type fluid inclusions of the type-A quartz veins ranges from 203 to 321°C and salinity of the fluid inclusions varies from 0.4 to 1.6 wt% NaCl equiv. The homogenization temperature of V-type fluid inclusions of type-A quartz veins ranges from 290 to 340°C with a salinity ranging from 0.4 to 1.9 wt% NaCl equivalent. In the type-B quartz veins, only liquid-rich aqueous inclusions (L-type) are identified. The type-B quartz veins yielded low homogenization temperatures from 160 to 220°C, with low salinities from 0.2 to 1.9 wt% NaCl equiv. compared with those of the type-A veins. The depth range of ore formation is estimated to be a shallow depth of less than 0.2 km based on fluid inclusion microthermometry. Fluid boiling is evident during the type-A quartz vein stage, and fluid cooling and mixing in the later type-B quartz vein stage. Precipitation of pyrite in the ore zone occurred as four recognized types: arsenic-rich pyrite-1, 2, 3 in the type-A quartz veins and pyrite-4 in the type-B quartz veins. A positive relation between Au and As contents of pyrites suggests that the gold is present together with arsenic in the structure of pyrites of the type-A quartz veins as solid solution in addition to as nanoparticle inclusions. The high Co and Ni contents of pyrites of both the type-A and the type-B quartz veins, with no evidence of CO₂ in the system indicate that the ore-forming fluids were epizonal magmatic-hydrothermal fluids rather than metamorphic fluid. The hydrothermal fluids of the Tagun-Khin-Dan deposit were driven by faulting to form the mudstone-hosted epithermal gold mineralization and related to continuing northwards movement of the Indian Plate that initiated the displacement on the strike-slip Sagaing Fault.