[meteorite-list] Meteorites delivered Earth's gold - Then plate tectonics concnetrated it

From: Paul H. <oxytropidoceras_at_meteoritecentral.com>
Date: Sat, 10 Sep 2011 21:03:13 -0500
Message-ID: <20110910220313.619VM.1706185.imail_at_eastrmwml48>

In "[meteorite-list] Meteorites delivered Earth's gold" at:
http://six.pairlist.net/pipermail/meteorite-list/2011-September/079706.html
Carl Asked,

"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?"

Nothing has changed in the composition of meteorites.

The quartz came from a combination of primary
differentiation, by the crystallization and preferential
settling of mafic minerals, of the magma that intruded
into the Earth's crust and by melting and recycling of
older sedimentary rock to form silica-rich magmas. This
happened long after the gold was theoretically was
delivered to the Earth by meteorites and asteroids. There
was no quartz in the meteorites and asteroids, which
were proposed to have brought the gold to Earth.

During the last 3 or more billion years, the meteorites,
asteroids, impactites, and gold in them were completely
consumed by either erosion or plate tectonics and
recycled by plate tectonics along with large parts of the
Earth's crust. The crust included sediments in which quartz
has been concentrated by weathering. In a subduction
zone, some of these recycled materials were melted and
turned into magma, of which some intruded back into
the overlying crust. As these magma intrusions cooled,
some of it differentiated to enrich the magma in silica. As
these magmas cooled to form large bodies of plutonic rocks,
i.e. granitic batholiths, various processes concentrated the
gold and quartz from the magma together into gold-bearing
veins. Thus, the gold and quartz came together only in the
process of the gold-bearing deposits being created. Before
that time, they were separate from each other. For a more
detailed explanation, go see:

Macdonald, E. H., 2007, Handbook of gold
exploration and evaluation. Woodhead Publishing,
Cambridge, England. 647 pp. ISBN 1845692543

http://www.worldcat.org/title/handbook-of-gold-exploration-and-evaluation/oclc/77257822?referer=di&ht=edition

Yours,

Paul H.
Received on Sat 10 Sep 2011 10:03:13 PM PDT