I've been reading with interest the material pertaining to flow in
hydrothermal systems, particularly that of Noel White. Find below and
extract of a message from one of our exploration personnel (Peter Grieve)
in New Zealand, which is relevent to this discussion.
"The Ohakuri (NZ) epithermal system would provide the examples to satisfy
the
argument. Delta drilled this area last year. As mentioned by Stuart
Simmons
in your attached note the epithermal systems of the Taupo Volcanic Zone
have
examples of sedimentary infill of open fractures, some of which are
associated
with gold mineralisation. Previous companies have called these clastic
dykes.
"The host rock is a quartz-adularia-chlorite rhyolitic ignimbrite. The
fractures are be infilled with clay to sand sized particles of host rock
and
even some fine carbonaceous material derived from wood burnt at the time of
emplacement. The sediments display bedding on a mm to cm scale, with grain
size fining upwards, and occur in near vertical structures (with vertical
continuity of at least 100 m in at least one case) which I believe to be
both
due to extensional faulting and the result of shrinkage of the ignimbrite
forming cooling columns and resultant vertical open spaces. Some infill
sediments may have been formed by meteoric waters washing surface ash in to
the pre-existing fractures, but others appear to be related to hydrothermal
breccias and the flow of hot fluids from depth.
"At the other end of the flow velocity scale, hydrothermal breccias are
present
which contain finely milled host rock and larger clasts up to 20 - 30 cm."
Dr Brett Davis
Group Structural Geologist
Delta Gold NL
PO Box 152
Kalgoorlie
Western Australia 6430
Australia
Ph 61-8-9021 7622
Fax 61-8-9021 3200
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