Timing of Volatile Degassing From Hydrous Upper-Crustal Magma Reservoirs With Implications for Porphyry Copper Deposits

The timing and duration of volatile generation from crystallizing magma reservoirs and fluid release across the magmatic-hydrothermal interface depend on complex coupled interactions controlled by non-linear, dynamic properties of magmas, rocks and fluids. Understanding these mechanisms is essential to explain the rare formation of economic porphyry copper deposits. For this study, we further developed a coupled numerical model that can simultaneously resolve magma and hydrothermal flow by introducing a description of fluid transport within the magma reservoir and volatile release to the host rock. Our simulations use realistic magma properties derived from published experimental and modeling studies and cover different magma compositions and water contents. We show that magma convection at melt-dominated states leads to homogenization, which delays fluid release and promotes a rapid evolution toward a mush state. The onset of magmatic volatile release can be near-explosive with a tube-flow outburst event lasting <100 years for high initial water contents of >3.5 wt% H2O that could result in the formation of hydrothermal breccias and vein stockworks or trigger eruptions. This event can be followed by sustained fluid release at moderate rates by volatile flushing caused by magma convection. Subsequent fluid release from concentric tube rings by radial cooling of non-convecting magma mush with a volume of similar to 100 km(3) at similar to 5 km depth is limited to remaining water contents of similar to 3.1 wt% H2O and lasts 50-100 kyr. Ore formation from hydrous magmas may thus involve distinct phases of volatile release.