Mass transfer and preferred orientation development during extensional microcracking in slate‐belt folds, Elura Mine, Australia

Microstructures in slate belt rocks at the Elura Mine, near Cobar, south‐eastern Australia, indicate that volume loss by syntectonic dissolution is coupled with mass accretion by reprecipitation of the dissolved material in dilational sites. The mass accretion is sustained primarily by repetitive tensile microfracturing at high pore‐fluid pressures. Oriented growth in the inter‐ and intragranular microcracks is locally host‐controlled, creating lattice‐ and shape‐preferred orientations. The grain‐scale crack‐seal features throughout the rock reflect rhythmic fluid pressure fluctuations; a balance is achieved between the fracture‐induced permeability (and consequent flushing rates), and the rate of fluid build‐up in a relatively sealed environment. Instability in the balancing factors can lead to localization and intensification of tensile failure (and hence, tension vein formation) in the grain aggregate. Growth of veins by crack‐seal also reflects a steady state, but with more localized fluctuations of fluid flow on the aggregate scale. Still larger imbalances between flushing and fluid accumulation (i.e. pressure variations) induce breccia veining. The larger pressure gradients over greater distances, associated with dilation localization (from pervasive microfracturing to spaced breccia domains), allow fluid channelling with an increased potential for chemical fluid/rock disequilibrium. Therefore, large breccia vein systems tend to be sites of extensive fluid/rock interaction and replacement, as spectacularly illustrated by the syntectonic sulphide orebodies at Elura. The huge amounts of silicate, carbonate and sulphide accumulated during folding at Elura illustrate the large scale of source and sink couples possible in solute mass transfer.