Geochemistry, mineralogy and mineral chemistry of the Welatam Cu-Mo-Bi-Au prospect, Northeast Myanmar

Abstract

The Welatam prospect, located in Kachin State, Northeast Myanmar, is the first discovered bismuth-gold locality within the Tengchong-Lianghe (Tengliang) metallogenic district. Two mineralization types, identified in this area, are related to three hydrothermal events. The massive sulfide mineralization was formed during the hydrothermal activities of stage I and II, producing mainly iron sulfides (e.g. pyrrhotite, chalcopyrite, pyrite, arsenopyrite). Whereas the activity of stage III resulted in quartz-molybdenite mineralization which is characterized by the deposition of molybdenite, galena, and Bi-Te-Au-bearing phases. The Bi-mineral assemblages suggest that at stage III, the prevailing temperatures were above 271 °C. Fluid-rock interaction that changed the pH, fO2, fS2, and, in particular, temperature in hydrothermal fluids is important in causing the precipitation of liquid bismuth from Bi3+. The high correlation between Bi and Au (R = 0.782) demonstrates that liquid bismuth may have played a crucial role in gold enrichment. Here, Bi-Au-bearing minerals commonly precipitate along the cleavages of molybdenite, forming a distinctive mineral assemblage. It may indicate that the cleavages, besides providing spaces for the precipitation of liquid bismuth, create local chemical conditions (e.g., reducing and catalytic surfaces) favorable for Bi-melt to scavenge Au from ambient fluid. The abundant Bi minerals, metal association (e.g., Cu, Bi, Mo, Au, Te, W), the restricted δ34S values (−7.3‰ to 1.9‰) of sulfides, and the proximity with the Welatam monzogranites suggest a magmatic contribution to the ore-forming fluids.