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We welcome the contribution of Matthäi et al. (2004), which emphasizes fluid convection as an important process for heat and mass transport in hydrothermal ore deposition and demonstrates the capacity of numerical modeling as a tool for understanding geological processes. Matthäi et al. (2004) investigate whether advection of hot material in the hanging wall of the steeply dipping Mount Isa fault drove hydrothermal convection to create the giant copper deposits. While we agree that hydrothermal convection may be a key process in the formation of the Mount Isa copper deposits, we find it difficult to accept the conclusion of Matthäi et al. (2004) that uplift is the driving mechanism. This is because significant components of the model are at odds with the current understanding of Mount Isa geology, particularly the architecture represented in the model, the magnitude and distribution of permeability, and the interaction of heat transport mechanisms.

There is ample evidence in the Mount Isa area for substantial fluid flow along faults, in particular along the Mount Isa and Paroo faults, which dilated in several tectonic events. While Matthäi et al. (2004) consider the permeability change due to deformation within the wall rock and the mineralized breccia, the faults are not considered as potential pathways. Ignoring faults as fluid pathways is problematic, particularly in view of evidence for large-scale free convection in faults (e.g., Bächler et al., 2003).

There are other aspects of the published cross section that differ from the actual geology. The Lena quartzite is a stratigraphic unit in the middle of the Eastern Creek Volcanics, not, as shown by Matthäi et al. (2004), a separate unit on top of it. Matthäi et al. (2004) have included a hitherto unknown stratigraphic unit, the “Ridewick” on top of the Lena quartzite and beneath the Myally Group. The depiction of a block of “uplifted metasedimentary rocks” west of the fault ignores the presence of metabasalt equivalent to the Eastern Creek Volcanics. Furthermore, it is unclear why the same stratigraphic unit on one side of the fault would be three orders of magnitude more permeable than on the other side, especially since these domains would have had identical metamorphic grade at the onset of faulting.

The hydraulic architecture of the Matthäi et al. (2004) model is characterized by an enormous range of permeability values for which no constraints are given. According to calculations in Turcotte and Schubert (2002, p. 395) the thick breccia zone (copper orebodies) with a permeability of 10−12 m2 should allow free convection at the inferred geotherm of 40 °C/km regardless of whether heat is advected due to movement on nearby faults or not. In this context it is not clear on what grounds a distinction is made between “self-organization of flow” and “forced convection” (Matthäi et al., 2004; their Fig. 2). The term “forced convection” is commonly used to describe pressure-driven, or pressure-head constrained hydraulic systems. No reference to this is given, leaving the impression that the process modeled by Matthäi et al. (2004) is temperature controlled and should correctly be referred to as “free convection.”

The modeling presented by Matthäi et al. (2004) is based on the assumption that 8 km of vertical movement on the Mount Isa fault occurred contemporaneously with copper ore formation. A widely accepted view however, is that copper formed after the main, peak-metamorphic E-W contraction during a minor NE-SW shortening event (e.g., Swager, 1985). In any case, a slip rate of 1 cm/yr (the only case for which flow patterns and temperatures are shown) is very high for a steeply dipping reverse fault like the Mount Isa fault and needs to be justified in terms of the geological record. The model also does not take into account any topography generated by such a rapidly slipping fault and its potential as a driving force for fluid flow.

Fault slip velocities can impact heat transport mechanisms, and Matthäi et al. (2004) claim that the uplift of the hanging wall of the Mount Isa fault, with rates between 1 cm/yr and 1 mm/yr (resulting in a 1.15 cm/yr and 1.15 mm/yr fault slip rate at 60° fault dip), drives the convection responsible for the Mount Isa copper deposit. The difference of one order of magnitude between slip rates can make the difference between a conduction-dominated versus an advection-dominated temperature structure, and therefore will also have significance as to whether a wave-like advective heat anomaly develops across the fault or not. Thermal transport across a moving fault is an “advection vs. diffusion problem” (e.g., McKenzie and Bickle, 1990), and can therefore be expressed by the dimensionless Peclet number:  
where l is the diffusion length of the system (in this case the height of the section affected by faulting), v is the velocity (here the fault slip velocity), and κ is the thermal diffusivity. This means that advection will only govern the thermal structure for Pe ≥ 1. Assuming a thermal diffusivity of 10−6 m2s−1, and taking the length of the system as 16 km (the vertical extent of the Matthäi et al. model), Pe has a value of ~5.8 for a fault slip velocity of 1.15 cm/yr, but only 0.58 for a velocity of 1.15 mm/yr. A velocity of ~1.7 mm/yr would therefore represent a threshold value (Pe = 1) for which both heat transport processes would be equally effective. It is therefore difficult to understand how these different velocities can produce a qualitatively similar pattern, as claimed by Matthäi et al. (2004).

Thermally driven flow patterns in hydrothermal systems are highly sensitive to the complex hydraulic architecture of the system, which requires that the architecture used in numerical simulations needs to be as accurate as possible with regard to the observed geology. The conclusions presented by Matthäi et al. (2004) must therefore be regarded as questionable, since the geological-hydraulic architecture of their model is not very well constrained, and the simulation of heat transport appears to be at odds with analytical heat transport calculations.