Fault and vein relationships in a reverse fault system at the Centenary orebody (Darlot gold deposit), Western Australia: Implications for gold mineralisation

The Centenary orebody within the Darlot gold deposit is located in the Yandal greenstone belt in the Yilgarn Craton, Western Australia. At Centenary, moderately (similar to 45 degrees) west dipping reverse faults and steeply dipping (>70 degrees) faults of variable strike failed during gold mineralisation in response to sub-horizontal east-west shortening and sub-vertical extension. Gently dipping veins are temporally, genetically and spatially related subsidiary structures to west dipping reverse and steeply dipping faults. Line analyses of subsidiary vein distributions in 23 drill cores around Centenary suggest that the gold-related subsidiary veins are localised within a 200-300 m wide tabular linking damage zone between three west dipping faults (Thompson, Lords and Walters). The damage zone is a laterally stepping relay zone between the Thompson and Lords-Walters faults and has a pull-apart geometry. Anomalous vein-related extensional strain (>0.005), vein density (>0.20) and power-law vein thickness population characteristics (D-t 0.58-1.84) distinguish this zone from the surrounding rock. Within the linking damage zone the highest number and volume of veins are observed at the tip of the Walters fault. At the fault tip, the exponent of the power-law distribution of vein thickness is highest (D-t 1.84) indicating that vein-related extensional strain is distributed on a high number of relatively small thickness veins. At approximately 300 m distance from the fault tip the densities of veins and the measured exponents of power-law vein thickness distributions are lower (<0.80 and D, <13, respectively). However, bulk vertical extensional strain remains high (>0.005), indicating that subsidiary vein material is concentrated on a greater number of anomalously thick veins. These systematic variations suggest that the fault tip imparted a strong control on vein localisation. Strain localisation within the linking damage zone is complex with coefficients of variation of vein spacing greater than one implying vein clustering. Gently dipping veins occur as wing crack arrays to the reverse faults and also in arrays comprising curviplanar, intersecting networks of subsidiary veins. The linking damage zone corresponds closely with the Centenary gold resource indicating that it has been an important locus for the focussed flux of gold-bearing hydrothermal fluids. However, within the damage zone, individual veins and vein arrays on the tens of metre scale do not always correlate with high gold grade indicating additional complexity within the system. (C) 2006 Elsevier Ltd. All rights reserved.