MadSci Network: Earth Sciences

Re: why do gold miners like to search for gold near intrusions of rock?

Date: Thu Jan 21 11:37:11 1999
Posted By: Robert Chesson, Geologist (Certified Professional Geologist), Foothill Engineering Consultants
Area of science: Earth Sciences
ID: 915924711.Es

Dear Megan:

Your questions are:  Why do gold miners like to search for gold near intrusions 
of rock? And, Why diamonds, formed under ground pressure deep in the interior of 
the earth, can sometimes be found in rocks close to the surface?

Your first question concerns how gold comes to be concentrated near 
intrusive rocks.  This is actually a very scientifically complicated, and still 
hotly debated topic.  My answer to your question is, therefore, simplified and I
will concentrate on some of the more general ideas about gold ore 
deposits (concentrations of gold which are mineable) that you asked about.

Gold in a naturally occurring metallic mineral.  It is element number 79 on the 
Periodic Chart of the Elements (symbol Au).  Because is a rare mineral, 
it has been treasured since ancient times and our civilization has gone to 
great lengths to try to predict where gold ore deposits could be found.  Early 
on during man's study of gold deposits, miners noticed the common association 
of gold with intrusive rocks (such a granite).  Geologists and mineral 
scientists (called mineralogists) now call these types of gold 
deposits hydrothermal.
Hydrothermal is a fancy way of saying that the gold was concentrated by hot
fluids [mostly water]  produced by the intrusive rocks. Gold has a relatively
low melting point and this low melting point is probably a major reason 
that gold deposits can be concentrated by this hydrothermal process.  The heat 
from the intrusive rocks, along with other mineral fluids associated with these
intrusive rocks work together to concentrate these hydrothermal gold deposits.
Because gold is a rare mineral, not all intrusive rocks have enough gold to
make a mineable gold deposit.

Some famous hydrothermal gold deposits have been mined in the State of 
South Dakota (the Homestake Mine), in the Sierra Nevada Mountains of California, 
and in Canada.

There are other types of gold ore deposits that are not hydrothermal.
Probably the best know type are placer deposits.  Placer deposits ore
concentrations of gold from the weathering of gold bearing rocks.  Gold is
largely unaffected by weathering and is concentrated in these deposits 
because the gold is heavy.   Some famous placer deposits have been mined in 
California (your home state)in Alaska,  and in the Yukon of Canada.

Your other question concerns how diamond, which are formed deep within the
earth's crust, can be found at the earth's surface.

Diamond is a crystal form of carbon which can only be formed under 
high temperatures and pressures.  The conditions which form diamonds can only be
found at great depths in the earth's crust.   To get these diamonds up to 
the earth's surface requires a very rare type of process by what is called a
diatreme.   A diatreme is a general term for a type of volcanic vent or 
pipe formed by an explosive eruption of volcanic gasses.   The diamond bearing
diatremes are called Kimberlite Pipes, after the diamond bearing diatremes in
Kimberly South Africa.   Unusual rocks that exist deep in the earth's crust,
called ultrabasic rocks, are usually found in these kimberlite pipes. Ultrabasic 
is a fancy terms meaning the rocks contain no quartz and  feldspar
(these minerals are common in near earth surface and earth surface rocks).
Diamonds can also be found in river and beach deposits which have  
concentrated diamonds that have weathered-out of diamond bearing rocks.

Famous diamond producing areas include South Africa, India, and Australia.
The United States have also produced occasional diamonds.   There are
kimberlite pipes in both the states of Arkansas and Colorado that have
produced diamonds for both industrial and gem uses.

Your local library or university geology and/or geography departments are 
good sources for further studies about mineral and mineral deposits.

Robert Chesson

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