THE INFLUENCE OF INCREMENTAL MAGMA CHAMBER GROWTH ON ORE PRECIPITATION IN PORPHYRY COPPER DEPOSITS – A NUMERICAL MODELING APPROACH

Hydrous magmatic intrusions associated with the formation of porphyry copper deposits are inferred to have been incrementally built up by episodic sill injections. The magmatic injection rate has been shown to be a key factor for the magma chamber growth, but quantitative studies using thermal conduction models have so far neglected the effect of magmatic fluid production and hydrothermal convection. For this study, we developed new modeling functionalities that enable the representation of sill injections to augment our model for the hydrology of porphyry copper systems, which simulates heat transfer and release of Cu-bearing magmatic fluids in combination with the multi-phase flow of hydrothermal fluids and dynamic permeability evolution due to hydraulic fracturing and the brittle-ductile transition.

Our numerical simulations indicate that magma emplacement rates of at least 4 x 10^-4 km³/y are required to constantly maintain a small region of melt and continuously produce magmatic fluids. Higher injection rates (5 - 6.5 x 10^-4 km³/y) lead to growing magma chambers with sizes of up to 3 km in thickness. In comparison to an instantaneously emplaced magma chamber with the same dimension, the incremental growth scenario results in further stabilization of the fluid plume, leading to a narrower copper ore shell in a defined region and higher ore grades due to the more constant fluid production. In contrast, lower magmatic injection rates result in multiple overprinting pulses of hydrothermal mineralization and in an overall decrease of the maximum ore grade despite the same amount of total fluid production. The injection location of the magmatic fluid also has a direct effect on the ore shell, with fluid release at one side of the pluton leading to particularly high ore grades caused by a steeper temperature gradient within the fluid plume. The numerical simulations indicate that porphyry copper deposits are more likely to form in association with magma chambers which grow by rapid episodic injection of magma, whereas low injection rates cannot produce a magma chamber that is able to form a deposit.