We welcome that Wilde and Gessner's comment gives us the opportunity to elaborate further on several aspects of our simulation study (Matthäi et al., 2004). However, we object to all but one of their criticisms.
We accept their point about the stratigraphy in our Figure 1 and apologize to the Geology readers for our mislabeling: “Ridewick” should be the Pickwick Formation, which is the upper part of the Eastern Creek Volcanics above the Lena quartzite. It should have been shaded with the corresponding darker gray.
Wilde and Gessner state that flow patterns are sensitive to the “hydraulic architecture”of a system, a statement, which in this general form is correct but trivial. Our choice of stratigraphically controlled permeability variation is not arbitrary but based on actual field observations. This variation has a stronger influence on the flow patterns than the assignment of absolute values to individual beds. Regarding the permeability of the main fault and the western block, Wilde and Gessner should note that the core of transcrustal faults in a compressive setting is often the least permeable (Evans et al., 1997). Therefore, the results by Bächler et al. (2003) on a highly permeable fault in an extensional setting do not pertain to the Mount Isa reverse fault.
Nevertheless, if the western block is kept “impermeable” and the fault permeability is set “high,” there is no major change in the flow pattern. Simulations not reported in our paper show a moderate distortion of the westernmost upflow zone toward the fault zone. We did not carry out simulations with a permeable western block for lack of information about its upper part, which is now eroded. Rather than exploring all alternatives for which there is no geological evidence, our model is constructed on the basis of specific field observations indicating a high fracture permeability throughout the eastern low-grade-metamorphic block, including the mine area. Thus, we arrived at a specific model that is consistent with structural evidence and successfully explains all of the stringent mass-balance and isotopic evidence available on the regional and mine scale.
Wilde and Gessner also state that our model is based on an incorrect relative timing of fault movement relative to late-metamorphic ore formation. We used the observation that peak-metamorphic D2 folding predated the later fracture-accommodated D3 folding at mine (Swager et al., 1985) as well as district scale (Bain et al., 1992). D3 folding is contemporaneous with or possibly followed by copper mineralization and regional-scale iron-oxide carbonate alteration attending brine infiltration. A more recent attempt at absolute Ar-Ar dating (Perkins et al. 1999) yielded results that are consistent with this structural timing interpretation. One sample from the Mount Isa fault zone may indicate that part of the differential uplift postdated (but not predated, as Wilde and Gessner purport as a “widely accepted view”) the copper-mobilizing hydrothermal event.
With respect to the usage of “free convection” versus “forced convection,” Wilde and Gessner's objection actually serves to highlight why uplift of the western block is so important for copper ore genesis. Wilde and Gessner correctly identify that throughout our simulations convection occurs in the breccia body. It can be termed “free” because fluid cannot enter or leave this high permeability system (Freeze and Cherry, 1979, p. 508). Under “forced convection” fluid inflows and outflows are present and fluid motion in the flow volume is due to hydraulic forces acting on its boundaries (Freeze and Cherry, 1979). This is the flow regime in the breccia body that is conducive to the formation of copper ore. It only develops when there is uplift of the western block.
Wilde and Gessner's statement that “the simulation of heat transport appears to be at odds with analytical heat transport calculations” is a particularly serious critique and requires explicit rejection. Contrary to their claim, they provide no analytical calculations of heat transport at all and their simplistic analysis does not pertain to fluid circulation in the eastern block. Wilde and Gessner merely evaluate the dimensionless Peclet number, Pe, an estimator for the relative proportions of advective versus diffusive heat transport. To put the level of critique into perspective, we would first like to clarify a few misunderstandings about Pe in Wilde and Gessner's comment. Of the many equivalent definitions of Pe, Wilde and Gessner chose Pe = lv/κ, correctly assigning v to advection velocity and κ to thermal diffusivity. However, they incorrectly call l “the diffusion length of the system,” when l is in fact the opposite, an advection length given by l = vt (t denoting time). Using the correct definition for the characteristic diffusion length instead, another well-known formulation can be derived, i.e., Pe = 4l2/L2κ. Diffusion length now explicitly appears in the formula, but in the denominator rather than the numerator as stated by Wilde and Gessner. For Pe varying by a factor of 10, the ratio of these lengths will vary by a factor of , i.e., the effect on heat transport is far less drastic than implied by Wilde and Gessner.
In an attempt to substantiate their argument, Wilde and Gessner refer to a paper by McKenzie and Bickle (1990). However, that paper discusses neither Pe nor advective versus diffusive transport, but rather deals with a mathematical formulation of eutectic mantle melting. We could not decipher what Wilde and Gessner wanted to imply with this and can therefore only very generally reply to what we think that Wilde and Gessner meant to say.
The only sensible explanation is that they used Pe for a rough estimate of heat diffusion across the fault as a function of fault slip velocity. This alone, however, is insufficient for the eastern block after the onset of fluid convection. It is the very nature of this problem that precludes the meaningful use of any standard analytical heat transport solutions and requires numerical simulations instead. Therefore, these cannot be considered “at odds” with each other, as implied by Wilde and Gessner.