Hi Ake
Are you the guy working with Rick Sibson I met at the STM workshop
just before AGU last year?
Hope things are going well.
I'll read your paper with interest.
A distal source for hydrothermal gold is probably not necessary. At
the end of the day all gold mineralisation associated with
hydrothermal fluids is "remobilised" unless there is a magmatic
component to the fluid. According to my understanding, the main
difference between the two models is that the modified placer model
seems to suggest all the gold is already in place specifically in the
conglomerate horizons, prior to remobilisation, and originated as
placer gold. In a hydrothermal model you can dissolve gold from small
background concentrations present in all the sedimentary units of the
Wits, or underlying Dominion rocks. Gold is then reprecipitated in
much greater concentrations in the narrow conglomerate horizons,
where the fluids become focused largely due to permeability
enhancement associated with thrusting and fracturing of the horizons.
>This is becoming an interesting debate, as it has been for many years
>already,
>
>please see
>
>Fagereng, A., Harris, C., La Grange, M., Stevens, G. (2008): Stable
>isotope study of the Archean rocks of the Vredefort impact structure,
>central Kaapvaal Craton, South Africa. Contributions to Mineralogy and
>Petrology, Volume 155, pp 63-78
>
>for recent stable isotope data demonstrating the large extent of syn- and
>postdeformational hydrothermal circulation which must have affected the
>Wits Basin.
>
>The sedimentary rocks in the Collar of the Vredefort Dome have whole rock
>d18O values among the lowest ever measured in such rocks. This is very
>different from rocks in the core of the dome structure, which have d18O
>ratios usual for granitoids globally, and the low values are therefore
>unlikely an effect of Archean sediments having inherently low d18O. A
>small sample set from Wits sedimentary rocks outside Vredefort Dome shows
>almost as low d18O, indicating that this abnormal isotope signature is a
>feature of the Wits Basin.
>
>Our conclusion is that the low d18O ratios result from water-rock
>interaction at high temperatures (>300degC) and high water-rock ratio. dD
>measurements indicate that this water was meteoric, so that the fluid-rock
>interaction was likely a result of fluids using secondary, structural
>permeability, caused by deformation of the sedimentary rocks of the
>Witwatersrand Basin, to circulate through the crust.
>
>This certainly creates a possibility for extensive remobilization of gold,
>in a modified placer model. Possibly, it could also cause hydrothermal
>precipitation of gold from a more distant source, but I cannot at the
>moment think of such a source, and if this hydrothermal system was
>constrained to the Wits, and I suspect it may have been, then local
>remobilisation is more likely.
>
>
>Ake
>
>--
>Ake Fagereng
>PhD Student
>Department of Geology
>University of Otago
>PO Box 56
>Dunedin 9001
>New Zealand
>Ph: +64 3 479 9088 (office)
> +64 21 251 5451 (mobile)
>Email: [log in to unmask]
>Webpage: http://www.otago.ac.nz/geology/students/ake.htm
--
Steven Micklethwaite
Postdoctoral Fellow
Research School of Earth Sciences,
The Australian National University,
ACT 0200.
Australia
T: +61 2 61255169
F: +61 2 61258253
M: 0428 231002
http://rses.anu.edu.au/em/rp/profiles/Micklethwaite/index.html l
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