Topic 24:  WEALTH UNDERGROUND  

Rocks of the crust have a small amount of valuable metals 
disseminated throughout their crystalline structure.  These 
small concentrations can not be extracted economically.  Before 
a mineral deposit can be worked profitably, the percentage of 
valuable metal in the deposit must be greatly enriched above its 
average percentage in igneous rocks of the Earth's crust.  

Ore - an aggregate of minerals from which one or more minerals 
can be extracted profitably.   In ore deposits the valuable metals 
have been enriched above their average percentage in igneous 
rocks of the Earth's crust.  

Gangue - non valuable minerals such as quartz and feldspar in 
which the ore is found.

Concentration of Ore Elements:

Element	        Concentration	          Concentration	     Concentration
	                 in Igneous rocks (%)	         in Ores (%)	         Factor

Aluminum	             8.0					                      35		                    4
Iron		                    5.0		                           50	                   	10
Copper	                0.007	                    	0.5 - 5		             70 - 700
Zinc	                    0.013		                    1.3-13		            100-1000
Lead	                    0.0016		                  1.6 - 16	        1000 - 10,000
Silver	                 0.00001		                   0.05		                 5000

Origin of Mineral Deposits:

1.)  hydrothermal mineral deposits - concentration by hot, aqueous 
solutions flowing through fractures and pore spaces.   -  (copper - 
Bisbee, Arizona;  tin - Cornwall, England)

2.)  magmatic mineral deposits -  concentration by partial melting 
and fractional crystallization.  -  (lithium, beryllium - pegmatites)  
(chromium - crystalline settling)

3.)  evaporate deposits - precipitation from evaporation of sea water.  
(sodium carbonate, halite, gypsum)

4.)  iron deposits - iron was deposited as a chemical sediment in sea 
water and later concentrated by metamorphism  (iron ore of Minnesota)

5.)  strata bound deposits - these ores are found in sedimentary 
layering when hydrothermal solution invades and reacts with a 
muddy sediment (lead, zinc, copper - Missouri)

6.)  placers - concentration by flowing surface water (gold - California)

7.)  residual mineral deposits - chemical weathering leads to 
mineral concentration through the removal of soluble materials in 
solution and the concentration of less soluble residue (aluminum 
[bauxite] - tropical climates).

Petroleum -  A broad term which includes both crude oil and natural 
gas 

Crude Oil - a liquid mixture of naturally occurring hydrocarbons 
(compounds containing carbon and oxygen).

Natural Gas -  A gaseous mixture of naturally occurring hydrocarbons.

The Origin of Oil and Gas -  Oil and natural gas originate from organic 
matter in marine sediment.  Microscopic organisms such as single-celled 
algae settle to the sea floor and accumulate in marine mud.  This organic 
matter may partially decompose, using up the dissolved oxygen in the 
sediment.  As soon as the oxygen is used up, decay stops and the 
remaining organic matter is preserved.  Continued burial subjects the 
organic matter to higher temperatures and pressures, which cause 
physical and chemical changes in the organic compounds.  These changes 
produce tiny droplets of oil and natural gas.  As muddy sediments 
compact, the oil and gas are squeezed out and move into more porous 
and permeable sandy layers.

Anaerobic - conditions under which oxygen is not present.  This is the 
condition necessary for the formation of oil and gas.  Such conditions are 
found in a highly restrictive environment such as in a closed basin like 
the bottom of the Black Sea.

Aerobic - conditions under which oxygen is present to oxidize organic
 material and sulfur.

Occurrence of Oil and Gas - four conditions or requirements are necessary 
in order for large accumulations of oil and gas to form.  The requirements are:

Source Rock - A shale must contain organic material which has not been 
oxidized and which is converted to petroleum by burial pressures and 
temperatures.

Reservoir Rock - A sandstone or other relatively porous rock is required 
to store and transmit the petroleum when wells are drilled into the reservoir.

A Trap -  a set of conditions to hold the petroleum in a reservoir and 
prevent its escape by migration.

Deep enough burial but not too deep - Temperatures and Pressures are 
required to "cook" the petroleum out of the organic matter.  Of course, 
if the rock gets too hot the petroleum is all "burned" off.