A distinct feature of the world-class Mount Isa copper system is the presence of a massive silica body enveloping the chalcopyrite ore body in the Urquhart shale. Because silicification may be genetically linked to the copper mineralization, it is important to understand its formation. It can be shown by numerical simulations of reactive transport processes that both hydraulic head and buoyancy driven flow are capable of generating silicification patterns similar to the field example. Considering the physical conditions required to form the observed patterns we suggest, however, that head-driven flow rather than free thermal convection produced the silica body. Free convection requires very special conditions and only one of several possible scenarios reproduces the observed silicification whereas any of the tested forced flux scenario is capable of doing so. (C) 2006 Elsevier B.V. All rights reserved.