Diffusion of molybdenum and tungsten in anhydrous and hydrous granitic melts

• Published in: The American mineralogist Vol. 103; no. 12; pp. 1966 - 1974
• Main Authors: Zhang Peipei, Zhang Peipei, Zhang Li, Zhang Li, Wang Zhongping, Wang Zhongping, Li Wancai, Li Wancai, Guo Xuan, Guo Xuan, Ni Huaiwei, Ni Huaiwei
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.12.2018; Walter de Gruyter GmbH
• Subjects: Activation energy; Cations; Coefficients; Diffusion; Diffusion coefficients; Diffusion rate; Diffusivity; Economic geology; Equilibrium; experimental studies; Experiments; Geochemistry; granitic composition; granitic melt; high pressure; Melts; metal ores; Metals; Mineral deposits; mineral deposits, genesis; Molybdenum; molybdenum ores; partitioning; Porphyry deposits; porphyry molybdenum; pressure; Profiles; Saturation; silicate melts; Tungsten; tungsten ores; water vapor
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2018-6569

To better understand the transport of Mo and W in granitic melts and the formation mechanism of porphyry ore deposits, we have investigated the diffusivities of Mo and W in granitic melts with 0.04-5.1 wt% H2O at 1000-1600°C and 1 GPa using a diffusion couple approach and a Mo saturation approach with Mo sheet serving as the source. The Mo and W diffusivities obtained from diffusion profiles measured by LA-ICP-MS can be described as: DMo,anhy=10-1.47±0.73 exp[-(387±25)/RT], DW,anhy=10-1.28±1.05 exp[-(396±35)/RT], DMo,2.7wt%H2O=10-5.37±0.52 exp [-(211±18)/RT], DMo,5.1wt%H2O= 10-6.87±0.69 exp[-133±20)/RT], where D is diffusivity in m2/s (with the subscripts denoting water contents and "anhy" representing nominally anhydrous melt), R is the gas constant, T is the temperature in K, and the activation energies in the exponential are in kJ/mol. When the influence of H2O is incorporated, Mo diffusivity in granitic melts with <5.1 wt% H2O can be modeled as: log DMo=-(1.94±1.58) - (0.87±0.36)ω - [(19341±2784)-(2312±620)w]/T where w is H2O content in the melt in wt%. The diffusion behavior (low diffusivities, high activation energies, and strong H2O effects) of Mo and W indicates that they exist and diffuse in the melt in the form of hexavalent cations. Their low diffusivities imply that the bulk concentrations of Mo and W in exsolved hydrothermal fluid and those in the melt are probably not in equilibrium. However, because of the large fluid-melt partition coefficients of Mo and W, they can still be enriched in the hydrothermal fluid, although to a lesser extent than equilibrium partitioning would allow. Slow Mo and W diffusion can be a significant rate-limiting step for the formation of porphyry Mo/W deposits.