This study provides quantitative
data on vein distributions within seven selected epithermal systems, four
deposits hosted predominantly in rhyolitic rocks: Ohui, Wharekirauponga,
Neavesville and Komata, and three major deposits 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 per metre, the degree of vein
clustering, and vein thickness. An increase in any of these tends to increase
the fractal dimension.

It
is the relative relationship of these parameters that is important for the
characterisation of vein distributions, and the comparison of different
deposits. Within the same systems, the rhyolites are characterised by lower vein
thickness and vein density, but have a higher degree of vein clustering compared
to the andesites. Sections that are deeper and more centrally located within the
epithermal system and/or close to major structures tend to have a greater degree
of vein clustering, higher average vein thickness and density, and higher
thickness to density ratios. This depth control is most pronounced in epithermal
gold-silver systems because they have formed at shallow crustal levels where
physico-chemical conditions change drastically both vertically and laterally. In
addition to lithological and structural control at vein scale, deposit scale
structure is of great importance in the deposition of economic mineralisation.
Large open vein structures capable of sustaining high fluxes of hydrothermal
fluid, favourable for the formation of large, high-grade gold deposits, are best
developed in andesite sequences.