EVOLVING FLUID PRESSURES AND FRACTURE NETWORK CONNECTIVITY, REVEALED BY INTERACTING VEIN APERTURE PROFILES

Veins typically experience multiple episodes of fracture, fluid influx and mineral precipitation. Detailed outcrop observations of closely spaced veins within amphibolite outside of Cloncurry, Australia provide clues for vein formation and evolution of fluid pressure deep in the Earth’s crust. The 10 mm to > 2 m long quartz-filled veins have a spacing of ~25 cm, with significantly smaller spacing within overlap zones between veins. The aperture profiles of some overlapping veins are unusual with extremely gradual tapers and portions with no remnant vein fill within the overlap zones. Alteration adjacent to the portions with no vein fill reveal that these portions of the veins had, at some time, transmitted fluid. The similarity of the vein mineral assemblage between the veins with and without gradual taper suggests a common fluid source. Variations in vein profiles may result from mechanical interaction between adjacent veins with differing fluid pressure. Using 2D Boundary Element Method models, we simulate the development of this vein array. The base model includes the mapped vein distribution and uniform fluid pressure. As expected, in this model, all veins are open along their entire length. When we alter the model to consider variable fluid pressure within veins we see that veins with larger fluid pressure act to clamp nearby veins with lesser fluid pressure. Differences in fluid pressure as small as 1% can clamp portions of the lower pressure veins shut. From this we interpret that during vein opening there were coeval variations in fluid pressure between the veins, with the apertures of some veins within the network closing as their fluid pressures reduced relative to their neighbors. This suggests that despite the proximity of the veins to each other percolation was incomplete and that the veins were not interconnected in 3D as they evolved.