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Gold

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Tweefo
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« on: September 04, 2010, 16:19:25 PM »

My understanding of the heavier elements (heavier than iron) is that they were formed in super nova explosions - stars that exploded. These atoms are in nebulae and when new stars and planets form out of the nebula they contain these elements. But now in the Sky Guide for South Africa I read that it had something (they don't say how) to do with the formation of the Vredefort dome. This is a meteor impact site and the main gold reef is in a arc with the dome right in the middle of it. the Sky guide is brought out by the Astronomical Society of South Africa so I assume there is more than two brain cells involved. So how does this work? Gold from a impact?
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Rigil Kent
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« Reply #1 on: September 04, 2010, 18:33:59 PM »

I doubt that the nuclear processes required to form significant quantities of gold can be triggered by the energy released by even a very large meteorite impact. Perhaps the impact caused a geological shift, such as the opening of a magma pipe, leaving any existing gold in a more exposed state. Or the gold particles already present in the earth and/ or the impacting body could have melted due to the heat generated in the collision, and finally condensed as nuggets.

As far as I know, any naturally occurring gold on earth must have been part of the accretion disc out of which our planet was reportedly formed.

Mintaka
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Mefiante
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« Reply #2 on: September 04, 2010, 20:28:53 PM »

Gold was definitely not formed from a meteorite impact.  As Mintaka points out, the energy is way too little to fuse heavier nuclei into gold.

The Vredefort dome is indeed a ±2 billion year old meteorite impact crater – the largest in the world that has been verified – with a diameter of around 300 km.

However, the Witwatersrand Basin, which was a very large inland lake about 3 billion years ago, is where the gold was collected.  Several rivers drained into the basin, principally from the north and east, carrying eroded rock and gold in suspension.  This load was deposited as the water velocity dropped when it entered the lake, resulting in what is called an alluvial deposition fan (a bit like a river delta).  This is how the Witwatersrand gold reefs were originally laid down and why they also contain rounded pebbles.  More sediment was deposited over time, eventually filling the basin and the lake gradually dried up.  Heat from the earth’s interior and pressure due to the load of the overlying strata metamorphosed the rock into the formations we find today, a process to which the meteorite impact no doubt contributed.

The gold itself very probably derives chiefly from the weathering and erosion by water of the Archaean greenstone belt around Barberton and possibly Zimbabwe too.  But don’t tell uncle Bob about this, otherwise he’ll start wanting his gold back… Roll Eyes

'Luthon64
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Hermes
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« Reply #3 on: September 05, 2010, 13:37:37 PM »

At a meeting of the Johannesburg branch of the Astronomical Society some three years ago I raised this issue during a discussion on the Vredefort dome.   My question did not relate to the chemical process by which gold is formed, but rather the fact that, in magma, the heavier metals gravitate to the core of the earth.   It would therefore require a cataclysmic event for such heavy metals to be propelled to the surface.   Considering that (a) the Vredefort meteorite strike is the biggest such event that we know of on earth and (b) that the world's richest gold fields lie in an arch around Vredefort, would this not point to a causal relation?
  The response corresponded to Mefiante's lucid explanation.   The decisive issue that precludes the meteorite strike from any possible causal effect on the gold fields is that the Witwatersrand Basin predates the Vredefort Dome.   I know: one should defer to specialists in their field; this case just presents such a seductive coincidence.
  The Astronomical Society makes the following claim in the 2010 Sky Guide: "Vredefort played a role in the concentration of the Free State and Transvaal gold deposits."  (p. 107)   I don't know what is meant by this vague claim and I can understand why Tweefo is puzzled by it.
« Last Edit: September 05, 2010, 17:44:20 PM by Hermes » Logged
Michael Meadon
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« Reply #4 on: September 07, 2010, 12:35:06 PM »

While I'm no expert on these topics, there does seem to be some causal connection between the gold-richness of Witwatersrand and the Vredefort dome. It's not that the impact created more gold (no nuclear fusion took place). The suggestion has been punted that the impact might have been sufficient to create the Bushveld Igneous Complex, one of the world's richest mineral deposits.
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Mefiante
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« Reply #5 on: September 07, 2010, 13:22:32 PM »

Except that the BIC contains very little in the way of gold deposits and has no known connection to our Wits gold reefs.  What the BIC does have is a wealth of PGMs and several base metals, principally chrome, in layers resulting from density differentiations that happened within the intruding magmas and hydrothermal fluids as they forced they way up through the ground and cooled.  Also, the BIC is igneous whereas the Wits type gold deposit is metamorphosed sedimentary rock, chiefly quartzite.

If a meteorite impact was responsible for the production of gold via nuclear fusion, one would expect to find other stable heavy elements, mainly lead and iron, occurring in large quantities associated with the gold.  We do not see that in the Wits gold fields.  The meteorite impact occurred around one billion years after the Wits gold was deposited, and one billion years is a long time, even by geological standards.  It is possible that the impact contributed to the development of the BIC, which formed at around the same time as the impact, but its connection to our gold deposits is very tenuous indeed in terms of our present geological and geophysical understanding.

'Luthon64
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« Reply #6 on: September 07, 2010, 14:09:37 PM »

I had an interresting chat with a geologist late last year, i posted an email on his site, and he actually phoned me!! sweet old guy.  cant for the life of me remember his name now.
he explained the vredefort situation to me as such:
(now, this is a basic timeline of events, and i wouldnt know where to find the years and such)
the vredefort meteorite hit.  this is way before there was life as we know it.

the actual vredefort dome, is not the crater, but the centre of the impact dropping back, there is a name for that, but i cant think what it is now.
its basically like this pic
 
where material escapes upwards, and creates 'a crater within a crater' . this illustrates the life of a meteor impact

like this meteor crater on some or other moon
.

anyhows

so, as you can see on the pic of the vredefort dome, the bottom end of it isnt visible, this is coz the great inland lake that everyone likes to talk about, covered the bottom edge of the crater with silt and deposits. 

the rim of the actual crater, have been obliterated by geothermic and geological and water related events since then.
allso consider, that unlike as many people would like to believe, that the magaliesberg is not part of the vredefort crater.
behind the berg, there was a huge underground lava lake. eventually the roof of this whole story collapsed in.
now if one can envision this, imagine that the crust that was 'floating' on top of this lava lake, consist of several kilometres worth of deposits, either from water, volcanoes, matter moved around by ice, you name it.
this crust tilted inwards, and basically broke off, and stuck out at an angle. sending these huge pieces of crust up into the air, where they pretty much stayed. now anyone that has seen the magalies, is struck by the sloping nature on the northern side, and the chopped off faces on the southern side. 
  this is not the best pic, but it shows the gentle slope on the left, and the allmost sheer drop on the right.
one must allso understand, that between the time that this happened, loads of stuff happened.  a good amount of water activity, volcanic activity (the mines in and around westonaria and rustenburg, is a result of hectic volcanic activity), a few ice-ages, tectonic activity, glaciers, etc etc.  so the original shape and form of the broken crust is long lost, the geologist actually told me in no uncertain terms, that galciers ground down the magalies berg, from a mountainrange that would make the andes look feeble.  and the pieces we see sticking up at the moment, are the really hard rock that had withstood the ravages of nature.
here you can see magalies from google earth.  you can see the ridges all over the place, where layers of hard rock had remained.  if one images a piece of crust turned on its side, you can see this easily.

point of all this?
the impact of the meteorite could very well have created gold deposits(and all sorts of other stuff), due to insane heat, pressure, and brining up elements from the depths of the earth. 
but the meteorite didnt create the goldfields.  there has just been to much activity since the impact.  the original geography is way long gone.
the idea of the deposits being waterbourne, is very likely.
there are plenty of volcanic avtivity since the impact too, bring loads of elements into play.
my opinion.  if the meteorite had any elements worth having around, has long since been re-distributed, and the debris from the meteorite, including itsself, has been most likely scattered far wider than joburg.  or even our country.
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StevoMuso
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« Reply #7 on: September 07, 2010, 14:14:03 PM »

Brilliant - stunning - post, thanks GCG  Smiley
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Brian
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« Reply #8 on: September 07, 2010, 14:23:43 PM »

Thanks GCG. I read/heard somewhere that when a big meteorite hits earth, the opposite side ofn the planet shows the force of the impact almost in a mirror image with earthquakes and mountains popping out of the surface. Any info on this?
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GCG
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« Reply #9 on: September 07, 2010, 15:30:01 PM »

as far as i've researched, this needs to be a hugely spectacular impact.  of such a manners that the entire planet is basically reduced to mush.
i think the most intense crater available for us to see, is the moon mimas

who got moered hard
now, in a planet with a solid state, and no liquid mantle, i should imagine the force of the impact travelling outwards from the empact site, so i would imagine the force to reach the opposite side of the body first, but then the consequent shocks would obliterate whatever significant changes the body would experience on the other side of the impact.

now, with a planet body that has a liquid, or semi liquid core, i would imagine that the core would absorb a lot of the shock, and the shocks would follow the path of least resistance, and follow the crust. 
all things considered, i wouldnt imagine much of the planet body being left, if the impact is bieg enough for the force to come popping out the other side.

now another moon of saturn, Tethys, got moered by a large object, and some people recon this impact crater

caused this great crack on the opposite end of the planet body



i think, one has to consider a lot of factors regarding the size of the meteor in relation to size of the body,  the geographical makeup of the body, and the makeup of the meteorite as well.

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Hermes
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« Reply #10 on: September 07, 2010, 16:04:32 PM »

We cannot be experts in everything, so when it comes to matters such as these we have to defer to such expertise.   The view among experts (geologists) is that the quartzite in which the Wits and Free State gold deposits occur predates the Vredefort impact by about a billion years - the gold was there long before the impact.
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« Reply #11 on: September 07, 2010, 16:18:48 PM »

i can very well imagine so.  as i imagine the elements were lain down when the earth was still being formed. 
i do, however, think, that even our best scientist and geologists are still guessing at some things, so it's educated guesses.
i, too, think, that we cannot fathom the immense changes our earth has gone through, the the huge geothermic and geological events that happened to shape the world. 
clever people guess at histories, based on evidence they interpret, according to their own line of thought.
if you believe a impact event killed off the dinosaurs, then you will have all evidence showing towards that.
if you believe that an evolutionary event did the same,.....
if you think a huge volcanic event.....

much like religion, actually.

you take whatever explanantion makes the most sense to you, and go with it.
i thought the magaliesberg was part of the vredefort cratere, untill the geologist explained it to me otherwise, and now, with a bit of reearch, a some common sense, and google earth, i am convinced otherwise.

i think the field of absolutes in proving or disproving a history where no human being was around to witness, and no event is around to show us how it happens, is wobbly at best.  we can only speculate, and make due with what we can figure out as we go along.
geologists can say that the gold deposits was lain down at this and this time, but they cant be sure what events followed to influence the strata wherein the deposits lie.
i think there are too many variables in science to make absolute facts just yet.
and its interresting as hell.  i wish i had studied to be a geologist  Cry
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Michael Meadon
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« Reply #12 on: September 07, 2010, 20:03:10 PM »

Except that the BIC contains very little in the way of gold deposits and has no known connection to our Wits gold reefs.  What the BIC does have is a wealth of PGMs and several base metals, principally chrome, in layers resulting from density differentiations that happened within the intruding magmas and hydrothermal fluids as they forced they way up through the ground and cooled.  Also, the BIC is igneous whereas the Wits type gold deposit is metamorphosed sedimentary rock, chiefly quartzite.

If a meteorite impact was responsible for the production of gold via nuclear fusion, one would expect to find other stable heavy elements, mainly lead and iron, occurring in large quantities associated with the gold.  We do not see that in the Wits gold fields.  The meteorite impact occurred around one billion years after the Wits gold was deposited, and one billion years is a long time, even by geological standards.  It is possible that the impact contributed to the development of the BIC, which formed at around the same time as the impact, but its connection to our gold deposits is very tenuous indeed in terms of our present geological and geophysical understanding.

'Luthon64

Right... That is a touch of a problem. Obviously I should rather shut up about geology...
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BoogieMonster
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« Reply #13 on: September 08, 2010, 21:55:03 PM »

Well, my mom used to tell me there's gold in my nose....
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cyghost
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« Reply #14 on: September 09, 2010, 07:09:36 AM »

eeeeeeeeeeeeeeuw  Undecided  Cheesy
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