Fluid flow patterns due to free thermal convection in permeable rock masses have been investigated for the 3D geological architecture around the Mount Isa lead-zinc-silver and copper deposits. Key results for the range of parameters explored here are: (1) Flow pattern and temperature distribution in stratigraphic units with homogeneous permeability are similar to those in units with the same average permeability in a heterogeneous distribution. (2) The geometry of one highly permeable stratigraphic unit governs fluid flow to an extent where it spatially attracts a convection cell, irrespective of the volume of adjacent permeable units. (3) Even in a complex and heterogeneous system with patches of low permeability along the fluid pathways, the Rayleigh number can be used to determine whether free convection will occur or not.Numerical simulations in 3D Of Coupled fluid flow and heat transfer were carried out using the finite difference computer code SHEMAT. Two different levels of permeability heterogeneity were investigated. Firstly, heterogeneity introduced by stratigraphy and secondly, various heterogeneous permeability distributions within geological units. Further complexity was created by defining patches with very low permeability within those stratigraphic units. The results show that free convection in hydrothermal systems is highly sensitive to the 3D permeability distribution in the geological architecture. (c) 2006 Elsevier B.V. All rights reserved.