I agree entirely with Stuart Simmons' comments - there is abundant textural
evidence for fragmentation and hydrothermal transport of clasts in
low-sulfidation epithermal veins, though the distance transported is usually
difficult to estimate. I have seen apparently sedimentary structures and
crude bedding within veins, made up of material up to coarse sand grainsize.
Occasionally you also see larger clasts that appear to have had some degree
of rounding, though there is room to speculate on the cause (abrasion vs
dissolution). There are also reports of dark sludge in the water first
released when geothermal bores in New Zealand are opened - they have to let
the water run for a while to take a clean sample. I would quite like to have
a few tonnes of that sludge!

The opposite feature is also commonly seen in epithermal veins - during
periods of quiescence there is nucleation and settling of minerals,
resulting in a "snow on the roof" texture. I have seen quartz on quartz from
Las Torres, Guanajuato, and from Creede, smectite on calcite from a Japanese
deposit, stibnite needles on quartz from Hishikari, and quartz on adularia
from Mexico.

What this highlights is low-sulfidation epithermal deposits form in a very
dynamic environment. Quiet periods of slow vein growth and sedimentation of
fine particles are followed by explosive pressure release, fragmentation of
wall-rocks and vein material, and movement of these along structures - this
is entirely analogous to the rapid flow and particle movement in bores that
Stuart commented on in New Zealand. Rapid growth of minerals resulting from
vigorous boiling seals the conduits, re-establishing quiet conditions and
allowing the build-up of heat and pressure that leads to the next explosive
release. That is why the textures are so diverse and wonderful.

Best wishes

Noel

Dr Noel C. White
Consulting Economic Geologist

E-mail: [log in to unmask]

11 Fleming Road
Chapel Hill,  Qld 4069
Australia

Phone 	61-7-3878 3041
Fax	61-7-3878 7099
Mobile	0409058375


-----Original Message-----
From:	[log in to unmask]
[mailto:[log in to unmask]] On Behalf Of stuart f.
simmons
Sent:	14 April 2000 06:18
To:	[log in to unmask]
Subject:	Re: Flow velocities of hydrothermal fluids?

 << File: sf.simmons.vcf >> The simple answer is yes, hydrothermal fluids
can transport materials in a flow stream.  Geothermal fluids ascending deep
production wells (>1 km depth, >250 deg C)) in the Taupo-Rotorua region are
known to have transport altered pumice clasts (density ~2 g/cc) up to
a few centimeters diameter, along with sand sized particles that flushed out
from unconsolidated formations at >500 m depth.

Also, a few low sulfidation epithermal deposits show what appears to be
cross bedding of very fined grained quartz with rip up clasts.

Robert Willan wrote:

> A query from a colleague in southern Mississippi  !
>
> "I am having arguments online with someone about flow velocities of
hydrothermal fluids and whether they are
> able to entrain something like sand grains.  My opponent disagrees and
says
> that velocities are too slow.  Have you any knowledge of fluids that have
> entrained grains or fragments of the surrounding host AND moved them over
> some distance (metres or more)?"
>
> I can recall reading about small volumes of sediment in open spaces in
epithermal vein systems.  Given the explosive potential of porphyry and
epithermal systems (mill breccias etc) I am sure there must be so dramatic
examples that would settle the above argument.
>
> Can anyone help?
>
> Thanks - Rob Willan
>
> ----------------------------------------------
> Dr Robert CR Willan,
> Magmatic-Hydrothermal Processes
> Geological Sciences Division,
> British Antarctic Survey,
> Cambridge CB3 0ET, UK
> Tel: 01223 221420
> FAX: 01223 362616
> Email: [log in to unmask]
> -----------------------------------------



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