STRUCTURALLY CONTROLLED FLUID-FLOW ASSOCIATED WITH BRECCIA VEIN FORMATION

A breccia vein sampled from a shear zone in greenschist facies metapel ites at Mount Isa, Queensland, Australia, shows a systematic variation in vein geometry that is related to the geometry of folding and fault ing within the sample. Calcite vein-fill is coarse grained and equigra nular, suggesting precipitation in a fluid-filled space. Partially fol ded veins suggest that veining occurred during folding and faulting. T he breccia vein contains a central zone in which dilation has occurred simultaneously in all directions in the plane of section, implying th at this was a zone of high fluid pressure and nearly isostatic differe ntial 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 tes ted by stable isotope analysis of veins and host rocks. The calcite ve ins have deltaC-13 values of -11.1 +/- 0.1 parts per thousand deltaO-1 8 values of 6-10 parts per thousand, whereas the host metapelite has d eltaC-13 values of -10.62 and -10.11 parts per thousand. and deltaO-18 values of 14-15 parts per thousand. These values are consistent with an igneous-derived, H2O-dominated fluid that exchanged little oxygen w ith the host rocks, but derived much of its carbon from the wall rock. The isotopic disequilibrium between the veins and the wall rock confi rms that the fluid was externally derived, and that the breccia vein a cted as a channel for large-volume fluid flow within the shear zone.