The partitioning behavior of Mo during magmatic fluid exsolution and its implications for Mo mineralization

Abstract

Most of the global Mo resources are derived from porphyry-type ore deposits, the formation of which hinges critically on the ability of an exsolving magmatic fluid to efficiently extract the available Mo from the silicate melt. However, existing data on the extraction efficiency of Mo upon fluid exsolution is inconsistent. In this study, we determine the partition coefficient of Mo between aqueous fluids and granitic melts (DMof/m) at 800 °C, 150 MPa and the oxygen fugacity of the Ni-NiO buffer. To mitigate the problem of disequilibrium resulting from low diffusion of Mo in granitic melts, a new experimental method has been used, which allows the attainment of local equilibrium between silicate melt and microscopic-sized fluid bubbles on reasonable timescales. The results show that DMof/m increases from 2.2 ± 0.6 (1σ) to 33.3 ± 5.4 (1σ) as the total Cl concentration in the magmatic fluid (mCltotal) increases from 1 to 16 mol/kg H2O, with DMof/m = 3.1 ± 0.9 (1σ) for pure water. At a fixed mCltotal = 2 mol/kg H2O, DMof/m is constant at ∼ 3.2 at HCl/total Cl ratios varying between 0.09 and 0.33. The observed partitioning data indicates that Mo was dominantly present as (Na,K,H)MoO3Cl in the aqueous fluid at the experimental conditions. The addition of 4 wt% S to the fluid caused no change in DMof/m at 2 mol/kg H2O total chloride concentration, indicating that S2- does not complex with Mo at the employed experimental P-T conditions. Numerical modeling suggests that Mo concentration in the exsolved magmatic fluid and overall Mo extraction efficiency is favored by delayed fluid saturation during magma crystallization and high initial magmatic Cl concentration, respectively. However, the Mo extraction efficiency by the fluid is low enough to necessitate a large magma chamber to provide enough Mo to form a large-sized Mo deposit.