At Ohio Creek, 3 km northwest of the Thames bonanza goldfield, porphyry
Cu-Au mineralisation is hosted in a dacite porphyry associated with porphyry
intrusion breccias and overlying andesites and dacites. The geological setting
is a Jurassic greywacke basement overlain by Late Miocene andesite-dacite
volcanics which are extensively altered to propylitic and argillic assemblages.
Diamond drilling at Ohio Creek in 1978-1981 by Amoco Minerals intersected low
grade Cu-Au (0.12-0.20% C~ 0.23-0.4 glt Au) mineralisation An advanced argillic
'alteration cap containing quartz-alunitedickite or
pyrophyllite-dickitediaspore is centred on the adjacent Lookout Rocks and has
potential for high sulfidation Au-Cu mineralisation. There is a zonation from
potassic K-feldspar-magnetitealteration at depth in the dacite porphyry, through
an overlying phyllic quartzsericite- pyrite zone which is overprinted by
pyrophyllite in the overlying advanced argillic cap. Early quartz stockwork
veinlets associated with the potassic alteration contain fluid inclusions that
trapped a highly saline (50-80 wt% NaCI equiv) fluid at high temperatures (450
to >600C) indicative of a magmatic fluid. Some chalcopyrite is associated
with slightly later quartz-magnetite veinlets, but the bulk of the copper occurs
as disseminated chalcopyrite associated with the phyllic alteration. The
potassic alteration is surrounded and overprinted by propylitic chloriteepidote
alteration. Late polymetallic sulfide-quartz-anhydrite veins occur within and
around the margins of the porphyry system as in the Kaiser Reef. Fluid
inclusions associated with these late veins were formed at temperatures of about
250-320C from low salinity (1-3 wfl~ NaCI equiv) fluids. The spatial
association between porphyry eu-Au mineralisation, advanced argillic alteration
and polymetallic sulfide veins suggests that they are genetically part of the
same hydrothennal system. This is consistent with reconnaissanceK-Ar dates of
11.6-10.7 Ma for the intrusive porphyry at Ohio Creek, alunite in the advanced
argillic alteration zone at Lookout Rocks and sericite selvedges from the
Waiotahi gold-quartz veins at Thames.