Geochemical Sampling

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 in of any methods used in mineral exploration.  Sometimes mineralization can be extremely subtle, if not impossible to recognize, in hand specimen.  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 delineation 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 suites 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 associated in a given area.

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”.

Dispersion results in the transport of metallic ions 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.

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.  It is assumed that the values will decrease downstream from the source, so following the “path” of increasing values upstream. may lead to mineralization (Figure 12 – 2).

Figure 12 – 2.  Stream sediment anomaly pattern (SME Mining & Engineering Handbook).

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 gentler 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.

Studies have shown that the preferred material to collect for a stream sediment sample is the –100 mesh size fraction, which corresponds with silt size.  About ½ to 1 cup of this size material is sufficient in most cases.  If gravel or organic material is mixed with the silt, then a larger sample needs to be collected.  Steep areas may lack the hydrologic conditions which allow silt and fine grained sediments to settle, which can make sample collection very difficult.  The downstream sides of large boulders are sometimes the best place to look in these areas.  Moss growing on boulders within the stream can act as a filter, trapping finer grained sediments, and can be collected to provide samples from these more difficult areas.  The material needs to be collected from the active stream channel, not dried up side channels. 

A single sample taken at the mouth of a large drainage basin may be a good way to quickly evaluate potential of a large area, but it provides little detail of the location of a source of mineralization.  By sampling the entire stream network of an area, the location of mineralization can be narrowed down considerably.  This can be done by collecting samples at close spacings (approximately ¼-mile spacing is common) and by sampling 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 upstream towards the source and reach a maximum value in close proximity to the source, and then drop to background values further upstream from the source.

Another type of survey which relies on collection of alluvium is the “pan concentrate” survey.  In a pan concentrate survey, coarse materials (generally pebble-sized) are collected and screened to ¼ inch or smaller and placed in a gold pan.  The screened material is then panned using a standardized method, down to a volume size of approximately ½ cup.  This will be further processed in a laboratory setting and then analyzed.   Pan concentrate samples give an indication of the types of heavy minerals present in an area.  Due to inherent inconsistencies in sample collection and panning methods, results from these surveys are difficult to evaluate statistically.  To help remedy this problem, special methods are sometimes employed in the field which use screening and collection of specified volume of material, and minimize or eliminate the use of actual panning of the materials (ie, concentration 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.  Generally speaking 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.  In vegetated areas the “A” horizon consists largely of organic material.  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.

Soil surveys are typically situated to investigate target areas outlined by previous geophysical survey or stream sediment surveys, or they may be positioned to cover certain structural features or rock units which are known.  Generally close spacing (< 500 feet) is needed to detect subsurface mineralization, because large spacings may miss the target.  The pattern which usually emerges is one which shows highest values directly over the ore, and a broad area surrounding these with highly elevated values corresponding to alteration in the host rocks adjacent to the main ore zone (Figure 12 -  3).

Figure 12 - 3).  Soil anomaly profile (SME Mining & Engineering Handbook).

The strategy most often employed is to collect samples at set line or grid spacings.  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.  An anomaly discovered along a line survey gives no indication of trend, and usually must be followed up with a grid survey. 

Geostatistics

Geostatistics is the use of statistics to evaluate geochemical data.  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 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.  Geologists endeavor to select which values of a data set are truly significant and therefore worthy of follow-up geochemical sampling or other types of exploration.  Most element concentrations in geological materials follow a lognormal distribution.  This is demonstrated by plotting of histograms which show a skewed distribution of values towards either the high or low values.  Plotting the log values instead of the real values yields a typical “bell-shaped” distribution.  Plotting the of geochemical values using geostatistical methods helps define the following types of values:

Threshold values can be selected in several different ways.

• Arbitrary threshold – find the highest value, find the median value (the value at which half of the samples have higher values and half of the samples have lower values), and select a value in between, but closer to the highest value.

• Cumulative frequency diagrams – line up values in by rank; determine class intervals; determine frequency percent and cumulative frequency percent; plot a graph with class intervals on the X axis and concentration on the Y axis using log probability paper.  Then specify the percentile to use as the threshold value.  This often selected at the 97.5 percentile value (second standard deviation), however, lower cutoffs may be selected to highlight a greater number of anomalous values.  This method also highlights the presence of different “populations” of values which may be related to different geologic features or rock types.

The evaluation of results depends largely of the type of samples being studied.   For stream sediment, pan concentrate, and in some cases soil samples, the procedure is often to plot all the values on a map, determine an arbitrary or statistical threshold and highlight the anomalous values.  This will suffice to look for general mineralization trends.

For soil sample grids:  1) contour the data; look for trends  2) make a thematic map which color codes the samples according to specified class intervals; look for patterns and trends.

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 (Figure 12 – 4).  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 (Figure 12 – 5).  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 accentuates possible mineralization trends but is much more tedious to construct. 

 

Figure 12 – 4.  Thematic geochemistry map showing highest values in red and lowest values in blue.

Figure 12 – 5.   Geochemical contour map showing highest values in red and lowest values in gray.