Explanation
Geochemical sampling methods are methods which involve collecting and analyzing various types of geological materials (such as soils, stream sediments, and rocks) or certain biological materials (such as plants). Historically these methods have been some of the most productive of any methods used in mineral exploration.  Sometimes mineralization can be extremely difficult to notice, if not impossible to recognize, by simply looking at a sample.  Without the use of geochemical sampling methods, many known ore deposits would probably not have been discovered. 

After discovery, geochemical sampling plays a key role in the description of mineralization.  For example, geochemical sampling of soils is often employed to outline the general distribution of mineralization at shallow depths where outcrops of bedrock are minimal or nonexistent.   

The procedure involves collection of materials in the field, laboratory (or field) analysis of the geochemistry of the materials, plotting of the geochemical values on maps, and interpretation of the results.  The materials may be analyzed for any number of elements.  Which elements are chosen for analysis depends on budget, the geology of the area, and the commodity which is being sought after.  Often there are specific elements or combinations of elements which are known to be associated with specific types of mineralization.  Therefore it is possible to evaluate the potential for the existence of certain types of mineralization by evaluating which elements are found in a given area.

Numerous samples of different types of rocks and other materials comprising the earth’s crust have been analyzed.  As a result, the average abundance of trace elements in these materials is fairly well known and established.  The average value for a specified rock is called the “background” value.  We are interested in values which are much greater than average or “anomalous” because these values may indicate the presence of an ore body.   A cutoff value, or  “threshold value”, is the value above which all values are considered anomalous.  The threshold value can be selected arbitrarily by simply viewing the data, or it can be selected by statistical methods.

The location of an anomalous value on the map is called an anomaly.  When values are plotted on a map, a pattern of increasing values may emerge which would give useful informationn as to the direction to the source.  For this reason, collection of location data is an extremely important aspect of geochemical sampling in the field.

Dispersion Halos

Dispersion is the process of dispersing elements outward from a source.  A dispersion halo is a zone around a mineral deposit where the metal values are less than those of the deposit but significantly higher than background values found in the country rocks around the deposit.  Geochemical sampling and testing can be used to outline the dispersion halo, and therefore help locate the source. 

Primary Dispersion Halos:   Primary dispersion refers to dispersion which occurs in rocks at or near the time of formation of a mineral deposit.  It is usually the result of “hydrothermal” (hot aqueous) fluids which are responsible for creating the deposit.  Fluid movements in rocks are so variable that the halo formed by primary dispersion may or may not reflect the shape of the ore deposit itself.  The extent of the primary dispersion halo can range from inches to hundreds of feet.   The extent of the primary halo is dependent on the nature of the rock.  Rocks which are extremely porous or highly fractured usually develop more extensive primary halos.

Secondary Dispersion Halos:   Secondary dispersion refers to dispersion which occurs in the secondary environment (soils, stream sediments or plants) long after the formation of a mineral deposit.  This type of dispersion is usually the result of mechanical and/or chemical weathering.  Mechanical weathering is caused by physical processes such as breakage due to freezing and thawing.  Chemical weathering is caused by chemical reactions between minerals and groundwater resulting in chemical decomposition of minerals.  Chemical decomposition can also be caused by bacteria. 

Halos caused by secondary dispersion are usually much more widespread than those caused by primary dispersion.  For this reason, sampling of soils, stream sediments or plants can detect the presence of a mineral deposit from a much further distance. 

Dispersion results in the transport of metallic ions (charged particles) away from a source.  Some of these ions are precisely the ones sought after, and others are called “pathfinder” metals or elements.  Pathfinder elements are those which are closely associated with the metal of interest.  High values of pathfinder elements may be more significant because they have better mobility, resulting in greater dispersion.  For example, arsenic and bismuth are good pathfinders for gold.

Another means of dispersion in the stream environment is caused by mechanical erosion.  Mechanical erosion leads to the breakdown of host rocks containing ore minerals.  Consequently, tiny grains of the minerals occur in the suspended load of the stream.  Turbulence of the water keeps the particles in suspension.  Turbulence is greatest in steeper areas where the stream water flows faster.  Downstream where the topography is more gentle, the stream waters move slower, thereby decreasing turbulence.  This causes the suspended load to drop out, resulting in deposition of the mineral grains in the stream sediments.  Heavy minerals, like ore minerals, tend to drop out first because less turbulence is needed to keep them in suspension.

Stream Sediment Sampling Surveys

Stream sediment surveys are very useful for mineral exploration because of greater dispersion in the stream environment.  Greater dispersion means greater ability to detect an ore body from a greater distance.  A drainage basin is an area with a network of streams like the branches of a tree:  smaller streams join together leading into larger and larger streams.  Geochemical values of sediments located downstream are an indication of rock geochemistry, or potential ore deposits, located upstream. 

Unfortunately, an anomaly discovered downstream in a large drainage basin gives only a general idea of location of the source.  The source of the anomaly could be anywhere within the drainage basin.  This is why it is a good idea to sample all the streams in a drainage area.  Typically a stream sediment survey will attempt to sample both sides of every stream fork.  In this manner, if an anomaly occurs on one side and not on the other, only the fork with the anomaly needs to be considered in locating the source.  The trail of anomalies forms a path upstream towards the source.  Generally the values will increase towards the source, and then drop to zero or background values upstream from the source.

Another variety of stream sediment survey is called a pan concentrate survey.  This is where sediment (usually gravel) is panned with a gold pan to collect a concentrate of heavy minerals. 

Soil Sampling Surveys

Soils are the product of weathering of bedrock, decomposition of organic material at the surface, and deposition of other materials which have been transported.  The soils tend to form certain layers called “horizons”.  The lowermost horizon consists largely of decomposed bedrock and is called the “C” horizon.  The uppermost horizon, called the “A” horizon, is variable in composition. The “B” horizon is between the “A” and “C” horizons, and is essentially a mixed zone.  Dispersion is generally greatest in the “A” and “B” horizons.  For this reason, soil samples collected from the “B”  horizon can detect a mineral deposit from a greater distance.   In arctic regions, the “B” horizon tends to be poorly developed (if present at all).   It is best to collect soil samples from the “C” horizon in these regions.

The depth of the horizons will vary from place to place. For more information visit http://generalhorticulture.tamu.edu/lectsupl/Soil/soil.html)

Soil surveys are most often conducted on specific targets at relatively close spacing (< 500 feet).  This is because dispersion in soils is much more confined, so it is much easier to miss a significant anomaly.  Stream sediment surveys are often done before soil surveys because they narrow down the size of the target area.  The geology of the area is also considered.  There may be specific rock types, or regional structural features like faults, which justify an area to be a better target. 

Soil surveys are generally conducted along a line or a grid.  The tighter the spacing, the more likely it will be to locate a soil anomaly over a buried ore deposit.  A grid survey has a big advantage over a line survey because the anomalies which are discovered may form a trend indicating the trend of the buried mineralization.  A anomaly discovered along a line survey gives no indication of trend, and usually must be followed up with a grid survey. 

Once a grid soil survey is completed the values can be plotted on a map and evaluated using several different methods.  One method is to assign a color code system or use symbols for specified ranges of values.  This type of map is called a “thematic” map.  The advantage of thematic maps is that they are simple to make and provide the reader with a quick understanding of the distribution of anomalies in an area.  Another method is to create a “geochemical contour” map.  Here the values are contoured:  lines of equal value (called isopleths) are extrapolated between every data point and the adjacent points.  This type of map points to possible mineralization trends but is much more tedious to construct.

Figure F3: Thematic Geochemistry Map

Figure F4: Geochemical Contour Map

Rock Sampling Surveys

Rock sampling reveals the true potential of an area for containing a mineral deposit. If the sample is collected within the area of primrary dispersion it will provide valuable information about the location of the mineral deposit. However, this applies only to rock samples collected from bedrock.  Rock samples or of float, rubble, talus, glacial material, etc... are much less reliable because they may have been transported some distance. 

Rock samples collected for mineral exploration purposes fall into many different categories.  Each category is based on method of collection and purpose.  The most common categories used in mineral exploration are:

Grab Sample:   A grab sample is a sample of rock material from a confined area (< 1 foot across).  It can be a single piece of rock.

Composite Sample:  A composite sample consists of small chips of uniform rock material collected over a large area (> 5 feet across). 

Chip Channel Sample:  A chip channel sample consists of small chips of rock collected over a specified interval.  Samples are collected systematically to provide a representative value for each interval.  Samples are usually collected in succession along a line laid out on a bedrock exposure.  The method is valuable because if mineralization is present, the width of the mineralized zone can be determined.

High Grade Sample:  A high grade sample consists of obviously mineralized rock material which has been “high graded” or separated from less mineralized or non-mineralized rock material.