Unit Two - Geochemical Methods
1. Geochemical Sampling
Lab Activity

Part 1:  Make a model of a secondary dispersion halo.

Part 2:  Plot stream sediment sample values and position a soil grid on the source area.

Part 3:  Plot soil geochemical values, create a geochemical contour map, and find the trend of mineralization.

-  Thick, flat, square, cellulose sponge

-  Food coloring (dark color like blue or green)

-  Clear plastic terrarium (approx. 13 X 9 X 9 inches)

-  White or light colored mortar sand, or fine silt

-  Water

-  Colored pencils (red, orange, green, blue)

-  Answer Sheet AS5:  Stream sediment survey map

-  Answer Sheet AS6:  Soil survey map

-  Teacher Supplement TS5:  Stream sediment survey map

-  Teacher Supplement TS6:  Soil survey map

Part 1:           

Do the following in advance:
Wet the sponge.  Soak it thoroughly with food coloring.  Let it dry out competely.
Prepare the model as follows:

1. Fill the terrarium with of sand a couple inches deep.
2. Position the dry, stained sponge with one edge flat against the inside wall of the terrarium so it can be seen.  It can be tilted, like an inclined bed of rock, or vertical. Use tape to hold the sponge in place against the wall of the terrarium if necessary.
3. Continue filling the terrarium with sand, until the sponge is buried about an inch deep.
4. Very slowly add water to the sand preferably away from the side with the sponge. Add water until the sand is saturated to the level of the top of the sponge.
5. The sponge represents a buried ore deposit.  The water represents groundwater.  The dye represents ions liberated from minerals in the ore deposit.  The migration of the dye is analagous to secondary dispersion of ions by groundwater.

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Part 2: 

Arsenic is a pathfinder element for gold-silver vein deposits.  A stream sediment survey was completed in the past, but the samples were only analyzed for arsenic.  A client wishes to

prospect for gold in this area by conducting a soil survey.  Plot the arsenic values of stream sediments and find the best location for a 2000 meter X 4000 meter soil grind as follows:

1. Plot the arsenic values for stream sediments (Table T4) at their respective locations on the stream sediment location map  (Answer Sheet AS5).
2. Choose 500 ppm as an arbitrary threshold value, ie, all values above 500 ppm are considered anomalous. Draw a red circle around the anomalous sample locations on the map.
   
3. Draw the outline of the soil grid in a position which encompasses the highest and furthest upstream arsenic anomalies.

Copper is a pathfinder element for platinum deposits in ultramafic rocks.  Contour the copper-in-soil values on the soil grid map (Answer Sheet A6) as follows:

1. Draw the isopleth (curve) for a copper value of 100 ppm.  For example, if the value at one location is 50 ppm and the value at an adjacent location is 150 ppm, then the 100 ppm isopleth will be halfway between these two locations. 
   
2. Draw the isopleths for copper values of 200 ppm, 300 ppm, 400 ppm, and 500 ppm.
   
3. Color the map with red for areas with >500 ppm, orange for areas with 400 – 499 pppm, green for areas with 300 - 399 ppm, and blue for areas with 200 – 299 ppm.

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