Comparison of vein styles and distributions between rhyolite - and andesite - hosted gold / silver epithermal deposits of the Hauraki Goldfield

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SR_1995-028-pdf
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Cargill, H.J.; Christie, A.B.; Brathwaite, R.L.; Swain, A. 1995 Comparison of vein styles and distributions between rhyolite - and andesite - hosted gold / silver epithermal deposits of the Hauraki Goldfield . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 95/28 110 p.


Abstract: Vein distributions were logged in selected sections of seven epithermal deposits of the Hauraki Goldfield. Four of the deposits are hosted predominantly in rhyolitic rocks: Ohou, Wharekirauponga, Neavesville and Komata, and three major deposits are mainly hosted in andesite: Martha Hill, Golden Cross and Karangahake. The technique of fractal analysis was used to help quantify vein distribution patterns. The fractal dimensions (D) obtained are related to the number of veins/m, the degree of vein clustering, and vein thickness. An increase in any of these tends to increase the fractal characterisation of vein distributions, and for the comparison of different deposits. Within the same epithermal systems, the rhyolites are characterised by lower vein thickness and vein density, but have a higher degree of vein clustering compared to the andesites. Well jointed lithologies tend to have higher vein densities, distinct vein size distributions, and lover degrees of vein clustering compared to poorly jointed lithologies. Sections that are deeper and more centrally located within an epithermal system, and/or close to major structures, tend to have a greater degree of vein clustering, higher average vein thickness and vein density, and larger thickness to density ratios. The influence of location within the system on vein distribution patterns is significant in epithermal systems, since at the shallow crustal levels where epithermal systems form, physico-chemical and stress conditions can change drastically over short horizontal and lateral distances. In addition to vein distribution at vein scale, deposit scale structure is of great importance in the development of economic mineralisation. of particular importance is the ability of andesite sequences to host large open structures capable of sustaining hydrothermal systems with the high fluid fluxes needed to produce an ore deposit. (auth)