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=101.47±0.73exp[(387±25)/RT],DW,anhy=101.28±1.05exp[(396±35)/RT],DMo, 2.7wt%H2O=105.37±0.52exp[(211±18)/RT],DMo, 5.1wt%H2O=106.87±0.69exp[(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:

logDMo=(1.94±1.58)(0.87±0.36)w[(19341±2784)(23 12±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.

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