Structurally controlled fluid flow associated with breccia vein formation

R. K. Valenta, I. Cartwright, N. H.S. Oliver

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A breccia vein sampled from a shear zone in greenschist facies metapelites at Mount Isa, Queensland, Australia, shows a systematic variation in vein geometry that is related to the geometry of folding and faulting within the sample. Calcite vein‐fill is coarse grained and equigranular, suggesting precipitation in a fluid‐filled space. Partially folded veins suggest that veining occurred during folding and faulting. The breccia vein contains a central zone in which dilation has occurred simultaneously in all directions in the plane of section, implying that this was a zone of high fluid pressure and nearly isostatic differential stress during folding and faulting. From these observations, it can be inferred that the breccia vein was a zone of high permeability and a likely fluid channel during deformation. This hypothesis was tested by stable isotope analysis of veins and host rocks. The calcite veins have δ13C values of ‐11.1 ± 0.1% and δ18O values of 6‐10%o, whereas the host metapelite has δ13C values of ‐10.62 and ‐10.11% and δ18O values of 14‐15%o. These values are consistent with an igneous‐derived, H2O‐dominated fluid that exchanged little oxygen with the host rocks, but derived much of its carbon from the wall rock. The isotopic disequilibrium between the veins and the wall rock confirms that the fluid was externally derived, and that the breccia vein acted as a channel for large‐volume fluid flow within the shear zone.

Original languageEnglish
Pages (from-to)197-206
Number of pages10
JournalJournal of Metamorphic Geology
Issue number2
Publication statusPublished - 1 Jan 1994


  • brecciation
  • dilatancy
  • fluid flow
  • stable isotopes
  • stress
  • veins

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