[meteorite-list] Meteorites delivered Earth's gold

From: John Hendry <pict_at_meteoritecentral.com>
Date: Sat, 10 Sep 2011 19:33:24 -0500
Message-ID: <CA915652.1178C%pict_at_pict.co.uk>

Carl,

The earth's crust is under a continuous process of differentiation by
various processes. By differentiation I mean the separation and
concentration of the various elements. There are probably a multitude of
mechanisms that allow concentration of specific elements, and all three
rock types (sedimentary, igneous, and metamorphic) are formed by processes
that are capable of doing this. For instance..

Sedimentary processes (rivers, oceans, wind etc) can sort stuff out and
deposit it in different places due to density. Gold notably is
concentrated in this manner with placer deposits.

Igneous rocks are derived from molten magmas at depth in the earth's
crust. As the melt cools, depending on the pressure and temperature
regimes various minerals will crystallise out. At any given stage in the
cooling process the remaining melt will consist of the more volatile
constituents that still remain fluid (silica, water, CO2 etc), along with
the relatively unreactive or incompatible elements that don't easily
combine in the minerals that are precipitating. If the melt has a chance
to vent up a crack you get this siliceous solution migrating towards the
surface carrying incompatible stuff with it. As it gets closer to the
surface and cools the constituent elements are forced to precipitate at
some stage giving rise eventually to ore bearing quartz veins.

Gravity is also a big player in helping to physically differentiate a
cooling melt? Dunites (90% plus olivine) may be formed by olivine
precipitating out of a basaltic magma and falling to the bottom of the
magma chamber to form a thick deposit or 'cumulate'. Given a magma chamber
that doesn't vent or have fresh basaltic magma injected before it cools,
the very top layers of the cumulate body can get concentrated with all
sorts of rare stuff. I believe the south african ore body called the
Merensky Reef which is rich in the platinum group was formed along these
lines. Indeed the differentiation of the earth's interior into an
iron/(nickel?) core, outer dense mafic (silica poor) mantle, and felsic
(silica rich) granitic continental crust is driven in part by gravity.

Now on Earth, plate tectonics is a still active mechanism that is
continually recycling crust, bringing it from deep to the surface, or
melting and redifferentiating it.

Some of the larger asteroids presumably were molten long enough to undergo
a substantial degree of differentiation as evidenced by irons/pallasites
as analogues to the earth's inner/outer core material, but the mechanism
for exposing this material at the body's surface is probably catastrophic
impact, whereas the closest we get on Earth to sampling even the
moderately deep stuff is via ancient vulcanism like kimberlites.

But back to the original question which is an interesting one, whether
hydrothermal gold bearing quartz analogues exist on other bodies in the
solar system. Don't know but it wouldn't surprise me if Mars for instance
had them. It has patently had water and extensive volcanism. I think small
quantities of free quartz exist in some eucrites and basaltic shergotites
indicating sufficient differentiation to produce the mineral in some of
the parent bodies out there. Whether it has become further concentrated in
places with additional hydrothermal or magmatic processes is something I
don't know if there is any direct evidence for.

Maybe it's just very rare. Our planet has had a good 4 billion years of
active geology to push deep rocks to the surface, and take surface rocks
to the depths, and an active atmosphere to continually erode and expose
and redistribute material. We're still not exactly tripping over gold
bearing quartz, and you have to pick up an awful lot of random pebbles to
find a nugget.

Maybe our crust is gold poor relative to meteorites because we are
relatively overdifferentiated - maybe the bulk of it migrated to the core;
gold is dense and does alloy well with nickel. Is that a realistic
hypothesis?

Regards,
John






On 10/09/2011 16:36, "cdtucson at cox.net" <cdtucson at cox.net> wrote:

>Paul, List,
>It seems to me that much of the Gold found on Earth is accompanied by
>Quartz. In fact most of the finest Non-nugget specimens are usually
>found in quartz.
>That said; If this gold came from space then where did the quartz come
>from and for that matter why is gold not found buried in chonditic rock
>instead of quartz. . Quartz does not seem to be terribly abundant in
>meteorites.
>Just curious why we don't find gold / quartz meteorites. What changed
>meteorites? Do we have any witnessed falls of Gold meteorites?
>Do these researchers consider the Quartz issue here?
>Thanks.
>Carl
>
>--
>
>
>
>
>
>"Democracy is two wolves and a lamb voting on what to have for dinner.
>Liberty is a well-armed lamb contesting the vote".
>
>
>
>
>
>
>
>
>
>---- "Paul H." <oxytropidoceras at cox.net> wrote:
>> Young Earth was sprinkled with precious metals
>physicsworld.com, Sept. 7, 2011
>http://physicsworld.com/cws/article/news/47116
>
>Where does all the gold come from? University of
>Bristol, Sept 7, 2011
>http://www.bristol.ac.uk/news/2011/7885.html
>
>Meteorites delivered Earth's gold, by Leila Battison
>BBC News, Sept 8, 2011
>http://www.bbc.co.uk/news/science-environment-14827624
>
>The paper is:
>
>Willbold, M., T. Elliott, and S. Moorbath, 2011, The
>tungsten isotopic composition of the Earth?s mantle
>before the terminal bombardment. Nature. vol. 477,
>no. 7363, pp. 195-198. DOI: 10.1038/nature10399
>http://www.nature.com/nature/journal/v477/n7363/full/nature10399.html
>
>A related paper is:
>
>Marty, B., and A. Meibom, 2007, Noble gas signature
>of the Late Heavy Bombardment in the Earth?s
>atmosphere. eEarth. vol. 2, pp. 43?49.
>
>PDF file at:
>http://hal.archives-ouvertes.fr/docs/00/33/07/83/PDF/ee-2-43-2007.pdf
>
>Yours,
>
>Paul H.
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Received on Sat 10 Sep 2011 08:33:24 PM PDT