Numerical investigation of transient hydrothermal processes around intrusions: Heat-transfer and fluid-circulation controlled mineralization patterns

Abstract

New insights on the circulation of fluids around magmatic intrusions have been obtained through coupled hydrothermal numerical modelling that takes into account i) a continuous variation of permeability with depth, ii) the period of intrusion emplacement, iii) the physical likelihood of ore deposition using a restricted rock alteration index, and iv) the so-far unexplored pluton floor, and then comparing the results against well-constrained natural cases showing different emplacement depths, high permeability zones (cracked thermal aureoles), faults and plutonic apexes. We show that emplacement depth is a key physical parameter controlling the extent and geometries of advective heat dissipation zones, and that shallow apexes strongly modify the fluid-flow pattern by acting as a focus for convective fluids and mineralization zones. We also show that the cooling phase is not the main convective phase for large plutons commonly associated with long-lived magma emplacement; major advective heat dissipation and mineral deposition zones may also develop before and during the hottest phase of the emplacement, i.e. before magma crystallization. The comparison with natural cases shows that we successfully reproduce, in space and time, the physical conditions required for mineral deposition. In particular, extensional detachment is able to restrain and modify classical fluid-flow patterns induced by coeval intrusion. Finally, even though lacking chemical arguments, we conclude that convection induced by granite emplacement plays a major role in the genesis of granite-related Au deposits. Moreover, the formation of this type of deposit is favoured and controlled by the presence of a fractured thermal aureole around the intrusion.