MOLYBDENUM BEHAVIOR IN RHYOLITIC MELTS: EFFECTS OF OXYGEN FUGACITY, MOLYBDENITE SATURATION AND TECTONIC SETTING

We found that molybdenum abundances in evolved rhyolites are often buffered by saturation with molybdenite (MoS₂), especially in reduced systems found in continental interiors. In a survey of quartz-bearing rhyolites from around the world, we identified molybdenite microphenocrysts in 13 out of 27 systems. The molybdenite, uniquely identified by its raman spectra, occurs as small (<20 μm) triangular or hexagonal platelets, typically included in quartz phenocrysts but also evident in other phases. Laser-ablation ICP MS analyses of melt inclusions in molybdenite-saturated samples reveal 1-13 ppm Mo in the melt and geochemical signatures that imply a strong link to within-plate felsic magmas. Indeed, our survey of felsic volcanic rocks from arcs demonstrate they are only rarely molybdenite-saturated. This systematic dependence on tectonic setting seems to reflect the higher oxidation state of arc magmas compared to within-plate magmas. To explore the effect of T, fO₂ and fS₂ on molybdenite solubility, we used thermodynamic data to model the stability of molybdenite in rhyolitic melt. The model reliably predicts measured Mo concentrations in molybdenite-saturated samples if the magmas are assumed to have been saturated also in pyrrhotite, a phase commonly observed to regulate S solubility in rhyolites. The model allows independent calculation of system fO₂ and fS₂ if molybdenum concentration and temperature are known.

Interestingly, all four rhyolitic systems associated with Climax-type Mo mineralization were found to be molybdenite-saturated during crystallization of quartz at magmatic temperatures. This is partly due to the low oxidation state of these systems, compared with Mo-bearing Cu porphyries, which are more oxidized. The relative ease of froth flotation of molybdenite in industrial processes may hint at possible degassing processes whereby molybdenite microphenocrysts could ascent in bubble slurries to become concentrated at the top of an ore-forming cupola.