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Jay_Bird
Carpal Tunnel
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Reged: 01/04/06

Loc: USA
Re: Where does gold REALLY come from? new [Re: Pess]
      #5979791 - 07/19/13 01:08 PM

That's not a question with a good short answer nor is it my earth science area of work since college, but I won't let that stop me:

'Veins' may form from the last portions of a magma (molten rock) mass to solidify or crystallize deep underground. These last fractions can be concentrated in volatile (water = link to 'hydrothermal veins') and in more aloof elements like nobel metals. The presence of volatiles helps make the vein fluids more mobile at a given temperature. This is one classic scenario for quartz+gold or quartz & other metals, some elements wind up in the veins because they resisted (for want of a better expression) crystallizing as long as possible.

There are plenty of other ways to concentrate typically scarce elements. In a large magma body, there can be fractional crystallization that 'rains' minerals with some elements to accumulate as a layer, because they are heavier or because they are first to crystallize and left behind as fluid portions are squeezed away; I think some USA Chromium deposits formed like this. Or, contact metamorphism can mobilize elements from the non-igneous 'host rock' and concentrate then in a zone or rind around the igneous intrusion, or in veins radiating out from the intrusion.

Sometimes there is second natural step in concentrating metals - the ore body may be further enriched by weathering that removes much of the rock mass and leaves more concentrated deposits of the less reactive elements in place.

Mantle hot spot plumes might(?)be an exception to lack of upward transport of heavier (iron-like) elements up to the crust from the deeper mantle. Maybe that's the other source besides asteroid/meteorite replenishment of heavy elements to the crust.


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llanitedave
Humble Megalomaniac
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Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: Where does gold REALLY come from? new [Re: Pess]
      #5979807 - 07/19/13 01:18 PM

Quote:

Quote:

Ore veins are the result of hydrothermal vents carrying mineral laden water. The ore crystallizes out of the hot water leaving behind veins of minerals.




I get that but why veins of 'pure' metal?

Why a vein of copper here, a vein of silver over there?

Pesse (And where is the nearest undiscovered vein of gold?) Mist




Pure metal veins are very rare. It's a function of the chemistry and temperature of the water, and the concentration of the metal. Gold doesn't combine easily with a lot of other elements, so it's more likely to be metallic (although even in an ore body, we're more likely to see parts per million concentrations of submicroscopic particles), but silver and copper are much more likely to be found as sulfides, oxides, carbonates, or any of a number of other combinations.


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llanitedave
Humble Megalomaniac
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Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: Where does gold REALLY come from? new [Re: llanitedave]
      #5979811 - 07/19/13 01:20 PM

Jay's answer was better.

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Widespread
professor emeritus


Reged: 05/11/11

Loc: Bowling Green, Kentucky
Re: Where does gold REALLY come from? new [Re: llanitedave]
      #5980001 - 07/19/13 03:17 PM

I haven't read the article, but they were talking about this on the radio on my way from lunch.

The astronomer (with help from the program host) made it sound like a certainty that only stuff like lead and some radioisotopes are made by SNs, whereas the "good stuff" like gold and platinum are made by neutron star collisions.

I, too, wondered how he could be so confident.

One cool datum mentioned was that the collision in question produced a weight of gold equal to that of Earth's moon.

Er, something just occurred to me. When two neutron stars collide to form gold, where do the protons come from?

Best,
David


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llanitedave
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Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: Where does gold REALLY come from? new [Re: Widespread]
      #5981360 - 07/20/13 12:23 PM

There's a process called Beta Decay that according to the link, occurs generally in "neutron-rich nuclei". I don't see how you can get more neutron rich than a neutron star.

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Ira
Carpal Tunnel
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Reged: 08/22/10

Loc: Mitzpe Ramon, Israel
Re: Where does gold REALLY come from? new [Re: llanitedave]
      #5981991 - 07/20/13 08:50 PM

Given how rare heavy elements are, it has to be surprising that so much of it ended up on earth. Indeed, when I pointed this out in a similar post a while ago, I was assured by all and sundry that it wasn't unusual at all and quite natural and normal.

A short while after that exchange I read another news piece where geologists had come to the conclusion that it was indeed puzzling how much gold there was readily available on earth and posited the bombardment of the planet by gold-rich asteroids at some remote period. Talk about your deus ex machina!

/Ira

Edited by Ira (07/20/13 08:51 PM)


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Jay_Bird
Carpal Tunnel
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Reged: 01/04/06

Loc: USA
Re: Where does gold REALLY come from? new [Re: Ira]
      #5982227 - 07/21/13 12:03 AM

A supernova (as I understand it) is an event with a minimum size for the precursor star but a considerable range of stars, maybe a range of 100 times in mass, above that minimum size for supernova?

So is a all one or the other, supernova vs. neutron star collision origin for heaviest metals a bit oversimplified? It seems to me that the size of the event, perhaps nearly as much as the type of event, also determine the extent of heavier element synthesis? Unless the collisions are more common than the largest SN and that is what the study is getting at...


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Ira
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Reged: 08/22/10

Loc: Mitzpe Ramon, Israel
Re: Where does gold REALLY come from? new [Re: Jay_Bird]
      #5982492 - 07/21/13 07:10 AM

Do we know how much gold there is on the moon or other planets?

/Ira


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Otto Piechowski
Pooh-Bah


Reged: 09/20/05

Loc: Lexington, KY
Re: Where does gold REALLY come from? new [Re: Ira]
      #6009362 - 08/06/13 12:34 PM

Let's get back into this, again.

The article Ira mentioned has now spawned an editorial in the New York Times.

Is all gold from kilanova (neutron/neutron supernovae)? Really?

Is gold also from ordinary supernova events?

Is gold from any other processes?

Otto


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Jarad
Postmaster
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Reged: 04/28/03

Loc: Atlanta, GA
Re: Where does gold REALLY come from? new [Re: Otto Piechowski]
      #6009669 - 08/06/13 03:06 PM

We don't know for certain.

It seems to be that normal supernovae would be far more common than the collision of two neutron stars, so I would think that most of the gold (and other heavy elements) in the universe is probably from normal supernovae. In order for most of it to come from these kilonovae, they would have to show not just that those collisions can produce gold, but that normal supernovae do not produce much gold.

I think the most likely answer is that gold comes from both types of supernovae. I don't think we have enough information to say exactly what percent comes from one type or the other. We would need to know the relative frequency and relative amount of gold produced by each type, and I don't think we know yet.

Jarad


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UND_astrophysics
sage


Reged: 01/19/13

Re: Where does gold REALLY come from? new [Re: Jarad]
      #6009836 - 08/06/13 04:26 PM

A Type II supernova.

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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Where does gold REALLY come from? [Re: UND_astrophysics]
      #6010428 - 08/06/13 10:08 PM

The problem may be that there are *too many* supernovae of the needed type to produce the observed amount of r-process isotopes (including gold, if we must). Supernovae have until now been where most folks thought the r-process takes place, but there are some (few) now who think the r-process takes place in neutron star collisions. Until now there has not been one actually observed to find this at work, but the paper claims it is likely that the authors have found one and that it shows signs of r-process products.

"r" stands for "rapid".

So people are pretty sure most gold, among other things, comes from the r-process (this going way back to a seminal paper I pointed to earlier, but they are not as sure where the r-process takes place, or if in more than one place, the relative productivity of the places.

One of the authors prattling on about gold is a way for a publicity-hungry person to get his name and face in the press, I think. Saying the same thing about lead or several other more mundane elements would be just as accurate but not as "newsworthy", perhaps?

Some simple info about s and r processes at a Univ. of Texas personal page

"s" stands for "slow"

I hope that's in the ballpark.


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