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There are many geological settings in which fluid flow appears to have been focussed through a small rock volume over an extended period of time, resulting in the growth of extensive vein systems or other metasomatic features. In fact, this process is likely required to form the many different types of vein-hosted metal deposits that we mine today. Sometimes continued flow appears to result from continued deformation, thus recracking rocks so that fracture permeability is regularly renewed, and this is probably the most widely invoked cause of meta- somatic fluid infiltration. Often, fluid flow results in the precipitation of material from the fluid or the hydration of minerals in the wall rock; this leads to perme- ability reduction unless deformation and extension continues. However, in some instances, it appears that reactions themselves can create porosity and enhance fluid infiltration. This effect is seen most clearly in marbles and skarns that have undergone decarbonation reactions (Baumgartner et al., 1997).

For example, the reaction:

calcite + quartz = wollastonite + CO2

involves a reduction in solid volume of about 33%. If the rock remains rigid through the reaction, this volume change translates into new porosity, and hence enhanced permeability. If porosity collapses as the reaction proceeds, the increase in porosity will be less than this. An example of a wollastonite skarn developed from marble by focussed infiltration is the Valentine wollastonite deposit in the Adirondacks described by Gerdes and Valley (1994). The significance of this feedback between reaction progress and permeability is that the temperature for calcite + quartz to react is much lower if water is introduced to the fluid phase, lowering the CO2 partial pressure. When no reaction occurs, marbles have very

low porosity and permeability; water does not infiltrate and quartz survives to the temperature for reaction with a pure CO2 fluid. However once the reaction

has begun and caused a permeability increase, water from nearby schists may infiltrate the marble, and as it does so the partial pressure of CO2 is reduced, the

reaction becomes progressively overstepped and accelerates. This has the effect of increasing the permeability further, creating a positive feedback (Balashov and Yardley, 1998).

BY writes –The development of wollastonite bands from calcite marbles suggests that feedback does indeed occur when carbonates begin to break down at amphibo- lites facies temperatures, but is marble really so slow to compact under metamorphic conditions that porosity-creation by reaction can outstrip porosity destruction by

creep? I put this question to Victor Balashov, who I had met at Korzhinskii’s old institute

– the Institute for Experimental Mineralogy at Chernogolovka, outside Moscow, and he was able to bring together kinetic data for the reaction with information on the creep of calcite to generate a model showing how a quartz-bearing marble adjacent to a rock containing lithostatically-pressured water would develop once reaction began

developed at the marble edge which provided a focus for fluid flow parallel to the contact, but I was surprised to find that the rate at which the permeable layer migrated into the marble was diffusion controlled, limited by diffusion of water against the flow of carbon dioxide leaving the reaction front.

There is an analogous reaction in dolomitic marbles that produces coarse prismatic diopside. The chemical zonation that arises as infiltrating water intro- duces Fe means that this reaction results in the most effective petrographic images of reaction-enhanced porosity and permeability (Yardley, 2009; Fig. 3.9).

Figure 3.9 Back-Scattered Electron Image of diopside from the skarn illustrated in Fig.

1.2a. The central part of the grain is near end-member diopside (dark grey) and has been corroded and then overgrown by new diopside with a small amount of Fe present (modified from Yardley, 2009).

Yardley et al. (1991) argued that the formation of stratabound diopside skarn layers from dolomitic marble created a zone of high permeability, and therefore lowered fluid pressure. Further fluid migrated into the skarn layer from surrounding overpressured rocks and escaped from it at the structurally highest point, where the fluid pressure fractured overlying rocks and formed distinct garnet skarns replacing quartzofeldspathic metasediments (Fig. 3.10). This local flow cell may be a specific example of the type of behaviour that Connolly has

predicted should occur on a larger scale, although there is no evidence to suggest that the focussed fluid continued to travel as a discrete wave after it had equili- brated with the local psammitic metasediments. Possibly they responded elasti- cally to the surge of overpressured fluid, which was then dissipated through the rock mass. Irrespective of the detailed mechanism, it does seem likely that perme- ability is dynamic in the metamorphic environment (Ingebritsen and Manning, 2010), and the nature of the relationships was explored in an earlier volume in this series by Jamtveit and Hammer (2012).

Figure 3.10 Schematic representation of the focussing of metamorphic fluid flow by reac-

tions in marble, leading to skarn-formation. A marble-bearing unit outlines the refolded fold structure and reaction of dolomite + quartz to diopside in the marble has generated secondary porosity, drawing in quartz saturated water from overlying sillimanite schists, and leading to the development of bedded diopside skarn (Fig. 1.2a). Layer-parallel flow in the marble drains the fluid to a structural high point from where it breaks through the overlying schist. Andradite garnet skarn forms irregular crosscutting bodies in these schists and are inferred to complement the bedded skarn (modified from Yardley et al., 1991).

Reaction-enhanced porosity and permeability is particularly well-devel- oped in carbonate rocks, and explains why there is often a sharp transition from thoroughly reacted rocks (skarns) to more or less intact marble, but it seems likely that similar effects can occur in other rocks that experience metasomatism. For example, Tenthorey and Cox (2003) investigated the impact of serpentine- breakdown reactions on permeability.