Patterns of quartz veining in epithermal gold-silver deposits at
Wharekirauponga, Neavesville and Ohui in the Hauraki Goldfield, have been
analysed to determine the effects of different lithologies on vein
distributions. At Wharekirauponga, quartz veins are hosted in rhyolite and minor
andesite, with the andesite containing slightly higher gold grades. Local
basement andesite, rhyolitic tuff and silicified black shale host gold-bearing
quartz veins at Neavesville. At Ohui, quartz veins are hosted in rhyolite,
rhyolitic tuff and andesite.
Fractal analysis was used to quantitatively describe the distribution of
veining. The fractal dimension (D) of veining in silicified tuff is 0.25,
moderately silicified rhyolite has D = 0.23 - 0.26, clay-altered andesite has D
= 0.3, strongly silicified rhyolite has D = 0.3 - 0.39, silicified andesite has
D = 0.4, and silicified black shale has D = 0.43. The D values are related to
the composition of the host rock, and appear to be controlled by its competency.
Hydrothermal silicification results in increased competency and higher D values.
Vein thickness frequency plots for different lithologies show that andesite
at Ohui, and to a much lesser degree at Wharekirauponga, contains more thick
veins than rhyolite and tuff. Gold grades show a strong to fair correlation with
higher vein density in three drillholes at Wharekirauponga.
The fractal dimension of veining is an important control on fluid flow and
gold deposition within an active epithermal system. Vein systems with higher D
have a greater density of veining which would have favoured increased fluid-rock
interaction. Lower D vein systems have more clustered arrays. The increased
fluid-rock interaction induced by a high D vein system, when combined with a
reactive host rock, results in an environment which is highly favourable for
gold deposition. This situation is seen in the andesite at
Wharekirauponga.