Lithological and spatial controls on the distribution of quartz veins in andesite- and rhyolite-hosted epithermal Au-Ag deposits of the Hauraki Goldfield, New Zealand

Vein distributions in line samples from four epithermal Au-AE deposits of t he Hauraki Goldfield were logged and quantified by vein spacing, vein densi ty, vein thickness and percentage of vein extension. One deposit is hosted in andesite lavas (Martha Hill), one in andesite lavas and dacite porphyry, dacitic tuffs and pyroclastic breccias (Golden Cross), and two in rhyolite lavas and rhyolitic tuffs with minor andesite lavas or andesite dikes (Ohu i and Wharekirauponga). The vein systems in these deposits form fault-contr olled arrays of extensional veins. Vein spacing distributions are non-fract al over two to three orders of magnitude (1 mm to 5 m), and therefore fract al dimension statistics are not applicable. The coefficient of variation (C -v) of vein spacing was used as a measure of the degree of vein clustering. Rock type has a marked influence on vein spacing distributions, with veini ng in rhyolite lava having lower average thickness and percentage extension ? but a generally higher degree of vein clustering compared with veining in andesite lava in the same deposit. Vein spacing distributions in well-join ted lithologies, mainly andesite lava, have C-v values (0.8-1.2) that are i ndicative of anticlustered to weakly clustered patterns, particularly in th e vein stockwork of the upper part of the Golden Cross deposit. These C-v v alues are consistent with field observations that joints are a major contro l on vein spacing. In the poorly jointed dacitic and rhyolitic rocks, the v eins are weakly to strongly clustered as shown by higher C-v values (1.2-2. 4), and are commonly associated with normal faults. Overall, andesite lava and dacite porphyry and pyroclastics host thicker and more persistent veins than rhyolite lava and tuff. These larger veins contain significant volume s of high-grade gold mineralisation. The higher chemical reactivity to hydr othermal fluids of andesite and dacite compared with rhyolite may have aide d propagation and thickening of the veins in andesite-hosted deposits. With in an individual epithermal deposit, location close to thick veins, represe nting major fluid conduits, commonly overrides the effect of different lith ologies. Sites that are deeper and located within or adjacent to major Vein structures have higher average vein thickness, percentage extension and de gree of vein clustering. Systematic collection and analysis of vein spacing , thickness and density data can be used to define trends that are useful i n the exploration of gold-bearing epithermal vein deposits.