To the previous point!  The metal must be present before it can be 
concentrated -- If it isn't present it won't form a concentrate.   This 
simple principal is often ignored.  

In nature there are several metallic crystals that don't fit the local 
chemistry.  Copper is one of these and I have seen native copper precipitated 
out on old mine timbers from Bisbee and Ray in Arizona and on a piece of 
trash wood in a copper deposit at Cuba, New Mexico.   There has never been a 
reasonable explanation of the chemistry of this occurrence other than carbon 
adsorption.  The process of adsorption is used in many process including 
recovering gold and silver from cyanide solutions.  The mechanisms and 
chemistry of this process are well documented but not explained.    

In the case of copper, although the entire solvent extraction electrowin 
process is based on organic chemistry it would be much cheaper to use a 
carload of sawdust or coal dust than those expensive lixivents.   The largest 
cost in the process is electricity in electrowinning.   Who ever finds a way 
around this will become rich and famous.

There was a story circulated some twenty years back about a reworked gold 
placer in the Yukon producing a  hobnail from the 1900's that had been 
partially replaced by gold.   Many old placer miners are convinced that gold 
in the placer deposit is often larger in grain size than the gold in the 
source rock and suggest that gold grains grow by some unknown chemical or 
physical mechanism in placer deposits.   Stuff like this makes the hair stand 
up on the back you neck.   Maybe all we need to do is seed some gold in our 
back yard dirt and we will all be rich.  (I'm joking.)

I have heard several theories about the South African Rand deposits being 
formed in shallow seas on mats of organic carbon.   Bacteria may have been 
involved.  

All of the Carlin type deposits in central Nevada are associated with trash 
carbon in old sedimentary formations.  This is a criteria for Carlin type 
deposits.  Microscopic analysis shows much of this gold to be in small 
metallic crystals.   There was no doubt some bacteria present with the carbon 
or maybe the bacteria was the source of the carbon.   

Among the other metals that have formed ore deposits on free carbon are 
uranium, vanadium and silver.  Carbon adsorption, as other writers on this 
thread have pointed out, is commonly used for the removal of heavy metals, 
odors, bacteria and many other chemicals from water, air and organic fluids.

Perhaps one of the other readers can explain the process of adsorption.  For 
an engineer from the old school there is a mystery in adsorption and 
catalytic chemistry that does not account for all the electrons swimming 
about in the soup.   Great possibilities and great potential for all kinds of 
things.   


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