Porphyry magma cooling and crystallization control of mineralization: Insights from the dynamic numerical modeling

The ore-forming metals (such as Cu, Mo, and Au) and hydrothermal fluids necessary for the formation of porphyry copper deposits (PCDs) are separated and precipitated from the felsic magma during its cooling and crystallization process, due to changes in physical and chemical conditions triggered by magma ascending and emplacement. Magma cooling and crystallization process is influenced by various factors, mainly including the initial cooling temperature of magma, the startup speed of magma chamber circulation inherited from the pulsations of deep-seated magma, and the cooling rate caused by the contact between magma chamber and surrounding rocks. However, the precise mechanisms of these factors triggering the evolution of porphyry magma in the shallow crust and their role in controlling ore-forming during this process are still not clearly understood. Therefore, this study proposes to use multiphysics numerical modeling in conjunction with thermodynamic calculations to investigate porphyry magma cooling and crystallization control of mineralization. Firstly, based on the whole-rock geochemical data of porphyry intrusions in the world's famous Bingham Canyon cooper deposit, we utilized MELTS to calculate the thermodynamic properties including density, melt fraction, thermal conductivity and heat capacity of porphyry magma. Then, we established the finite element numerical model coupled heat transfer and fluid flow that employed these four thermodynamic properties to simulate the dynamic processes of porphyry magma cooling and crystallization. Finally, we conducted numerical modeling experiments by varying the three parameters, including the initial temperature and startup cycling speed of magma, as well as heat transfer coefficient at the up boundary of magma chamber, to investigate their controls on magma evolution and mineralization of PCDs via calculating and analyzing temporal and spatial variations of the magma temperature, crystallinity, velocity, and metal (i.e. copper) concentration. The results show that the evolution of a porphyry magma chamber is primarily controlled by its initial temperature and the heat transfer coefficient of its upper boundary. 900 C is the most favorable initial cooling temperature for forming both porphyry magma plumes with high metal endowment, and connected tunnels for volatile to rapidly migrate upward and concentrate to apical parts of the magma chamber. A lower heat transfer coefficient, indicating a magma chamber in a relatively closed tectonic environment, is more beneficial for forming metal migration tunnels within the magma chamber. The startup cycling speed significantly influences the early circulation and convective patterns within the magma chamber, may leading to heterogeneous mixing of magmas with different properties inside the chamber.