[Aztlan] No: Biogeochemical prospecting & the 1848 CA Gold Rush

[Bruce Rogers]

Listeros,

While this is a bit far from the topic of using satellite photos to 
locate ruins in Mesoamerica, I would like to make a few short 
comments about using proxy plants to locate hidden deposits of useful 
earth materials.  Despite years of work, the field of biogeochemical 
prospecting for hidden metal deposits is still more of a "Black Art" 
rather than an ironclad science. 

BIOGEOCHEMICAL PROSPECTING
The following is from a classic in the field:
  Carlisle, Donald, and George Cleveland, 1958, Plants as a guide to 
mineralization, Special Report 50: San Francisco, CA Geol. Surv. 
(Formerly the CA Div. Mines & Geol.), 31 p.

"A buried ore deposit may provide to the soil above it an abnormal 
amount of metal or metals it contains; in turn, the soil may provide 
a large number of the same metals to the plant cover.  The ore 
deposit, there, under favorable conditions, be detected by the 
abnormally high concentration of these metals in the plants or by the 
distribution of plants that will or will not tolerate certain metals 
more readily than others.  If the plants are systematically collected 
and carefully analyzed, the results may indicate the possibility of a 
buried ore deposit.  This technique of sampling, analyzing, and 
interpreting the plant cover is called biogeochemical prospecting.

"Although the method has the advantage of rapidity and is relatively 
inexpensive, numerous factors (such as age and organ of the plant, 
soil pH, geochemistry of the soil and its exchange capacity) that 
bear on the concentration of metals within plants must be considered. 
Agronomists and other research workers have provided some data; 
biogeochemical fieldwork has added more.

SNIP (materials only of interest to CA earth scientists removed here.) 

"Several factors other than the metal content of the soil may 
influence the metal content of a plant.  If these factors vary within 
the area of the survey, and if they have a significant effect on the 
metal uptake, the biogeochemical results are misleading.  Soil 
moisture and drainage influence the availability of some metals 
although, in general, the influence of the physical properties of the 
soil is probably not great.  Even the amount of sunlight and shade 
has a slight effect for some plants.  The largest effects, however, 
are associated with the chemical properties of the soil both as a n 
influence on the metabolism of plants and as a control of the state 
of the metal in the soil, as most of the metal in the soil is present 
not in the minerals or in solution, but as ions sorbed or exchanged 
in the soil minerals, especially on clay minerals and organic matter, 
or else as complex ions.  Consequently, the ability of the plant to 
absorb metals is moderately to profoundly influenced by the kind and 
concentration of exchange materials, by the soil pH, by the 
concentration of other ions, and by organic materials and 
micro-organisms.  The exchange capacity of a soil also largely 
determines the amount of metal that will be taken and retained from 
various sources.  Available data suggest that the clay or organic 
content of a soil can be responsible for variations of a few fold in 
the metal content of that soil."

Given all that, still be best way to discover gold deposits in 
Mexico, copper in Panama, or even the blue jade deposits in the Maya 
Mountains of Guatemala is to use tried and true methods.  A simple 
panning of stream sediment will find precious metals.  Since they are 
typically heavier than most of the sand and silt coursing through the 
streams, they will settle into potholes and riffles in the stream bed 
rather than passing on.  Modern prospectors will still sample the 
streams by hand panning or collecting sediment samples for later lab 
analysis in areas outlined by high tech satellite photography that 
suggest valuable deposits of metals may be present.  Despite all the 
new technology present and being developed, nothing beats shoe 
leather for the ground truth.

CALIFORNIA GOLD DISCOVERY
Concerning W. Clement Smith's comments about the California Gold 
Rush, no plant indicators were involved with the discovery of gold in 
California in 1848.  Gen. Agustus "John" Sutter, a Swiss emigrant, 
employed James Marshall to build a water wheel-powered sawmill on the 
American River at his Sacramento, CA settlement called New Helvicia 
or Sutterville.  Marshall built the mill at the most inopportune 
location imaginable, but while doing so he accidentally found a small 
gold nugget in the tail race of the mill.  This started one of the 
largest migrations in the Western Hemisphere.  The impact on the 
growth of the US was incalculable.  Perhaps more importantly, the 
impact in the Native Americans in the West was disastrous. 

For further information about Gen. John Sutter, James Marshall, and 
the California Gold Rush of 1848, visit these websites:

http://www.sfmuseum.org/bio/sutter.html

and

http://www.sfmuseum.org/hist2/gold.html

Cheers,
Bruce Rogers, earth scientist on a good day