"steven micklethwaite" <[log in to unmask]> schrieb:
Steve, you picked the wrong email address for this one.
Falk
> 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
>
--
.___________________________________________________________________
| Dr. Falk H. Koenemann Aachen, Germany |
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