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    Drillhole Compositing

The term "compositing", when used in the field to mineral resource evaluation, is applied to the procedure by which the values of adjacent samples from boreholes are combined so that the value of the longer down-hole intervals can be assessed. The grade of each new interval is calculated on the basis of the weighted average of the original sample grades. These are usually weighted by length and possibly by specific gravity and core recovery. It should be noted that compositing can also be performed on channel samples and also that the values of variables other than grade can be calculated for each composites.

Compositing is carried out in order to produce one of the following sets of results:

  • ore body intersections;
  • lithological or metallurgical composites;
  • regular length composites;
  • bench composites or section composites;
  • high grade composites;
  • minimum length & grade composites.

Each of these types of composite are produced for different purposes and in different situations.

These composites may be calculated for an ore body which is constrained by discrete geologically defined boundaries or specific assay cut-off grades and which has been sampled at regular intervals. Some of the samples may cross these boundaries.

The start and end points of each composite is known and the composite grade is calculated as a weighted average by combining the samples which fall within these limits. In the case of a sample which crosses these limits, only the portion of the sample which falls within the ore body is included in the calculation.

Decisions that must be taken in order to undertake this type of compositing include:e:

  • the values to use for portions of the hole which have not been sampled;
  • the type of weighting to use, i.e. length or length x specific gravity;
  • whether to cut high grade samples;

In some cases it may be deemed necessary to add dilution into the footwall and hangingwall, either as a fixed length or as a percentage of the composite length, to allow for expected stope overbreak.

Lithological or metallurgical composites

As well as determination of composite grades over potential ore zones defined by a specific cut-off grade or a range of such grades, it is possible to composite over a metallurgical zone (e.g. "oxide", sulphide or transition zone oxide-sulphide etc.) or over specific lithological intervals.

Regular length composites

These can be produced from samples of constant or variable length in order to reduce the overall variance of the database by effectively increasing the sample volume (the "volume-variance" relationship"). The method for creating regular length composites is straightforward. Following the decisions made above, it is necessary to chose the following parameters:

  • composite length (usually greater than the length of the samples);
  • start Point (usually the collar)

Additional parameters may also be chosen to control which composites are included in the results:

  • minimum composite grade;
  • minimum length for composites formed at the end or start of the hole.

A set of composites will then be created at predetermined, regular intervals down the hole by combining samples (and portions of samples) to give a length weighted grade. Statistical analysis of these composites can then be undertaken prior to their use for mineral resource estimation.

Bench Composites

The following parameters are used in place of the "composite length" and "start point" parameters listed above.

  • bench height or section width;
  • bench datum level.

A similar approach can be made for vertical section/panels of ore in which regular horizontal thicknesses are the basis for the compositing procedure.

In its simplest form this type of compositing only requires one parameter: the minimum grade. The results of this type of compositing will be a list of all of the intervals in a hole which can be created by combining adjacent samples which have grades equal to, or above, the minimum grade. The weighted average grade will be calculated for each composite.

Unlike the compositing methods described above, this method does not use pre-defined start and end points for the results. The number of composites created and the length of each composite is only dependant on the value of the minimum grade parameter and the grade of the samples in the hole. When additional parameters are included, such as, the minimum length of composites to include in the output results, this compositing method becomes similar to the methods described below.

Minimum length and grade composites

Economic compositing is carried out in order to create a set of intersections which meet certain pre-defined criteria. The two most important criteria are :

  • minimum grade (Gm)
  • minimum length (Lc).

The compositing can be carried out using these two criteria alone and will produce a list of intervals which are greater than, or equal, to Lc in length and have a weighted grade greater than, or equal, to Gm. A simple compositing algorithm using this method would perform the following steps:

  1. Start with the first sample in the hole.
  2. Compare the sample grade (Gs) with the minimum grade (Gm).
    If Gs > = Gm then go to step 3
    If Gs < Gm then move to the next sample and repeat step 2).
    3. Gc and Lc) are equal to the grade and length of the sample (Gs and Ls).
  3. Move to the next sample. Its grade and length are Gs and Ls.
  4. Compare the grade of the sample with the minimum grade
    If Gs > = Gm then combine the length and grade of the sample to the composite to give new values for Lc and Gc and then loop back to step 4) (unless at the last sample).
    If Gs < Gm then do not add the sample to the composite but move to step 6).
  5. Compare the length of composite with the minimum length
    If Lc > = LmLm then output this composite to the results.
  6. Search down the hole from the current sample until a sample with a grade greater than Gm is found or until the end of hole is reached.
  7. If a Gs > = Gm then loop back to step 3.

This algorithm can be adapted to include the following:

Internal dilution.
Using the method described above, no samples with a grade less than Gm can be included in the composite. Consequently, composites will only be output if the combined length the adjacent samples is > = Lm and ALL of the samples have a grade > = Gm. This can be resolved by allowing low grade samples to be added until the length of the composite is > = Lm. If the weighted grade of the composite is > = Gm then the composite is output. A maximum value can be set for the amount of low grade material that can be added to the composite.
Distance between composites.
A fixed distance can be set for the gap between composites formed in the same hole.
Minimum vertical thickness (Tm).
This criteria can be used instead of the minimum length to ensure that composites are only created if their vertical thickness > = Tm. For example, the value chosen could represent the smallest flitch height in an open pit gold mine.
Maximum composite length/thickness.
"Seed" Points
Instead of searching to find all possible composites the algorithm can be restricted to certain portions of each hole. This can be specified by entering either a fixed start or end-point for the composite or a point which must be included between the start and end of the composite.

The algorithm described above tends to produce composites which are longer than a geologist may produce manually. This is because the computer algorithm will continue to add samples to the composite until the grade is just above the minimum grade (unless the maximum length or maximum internal dilution criteria take effect). This can be modified by allowing the option of selecting composites which either have:

  • the greatest length
  • the highest grade
  • the greatest metal accumulation (grade x length)
  • economic composites

An algorithm which uses the first option (greatest length) will continue to add low grade samples (dilution) to the composite until its weighted grade reaches the minimum grade specified or until the maximum internal dilution is reached. The "highest grade" option produces composites which are unsatisfactory for use in an ore resource estimation while the results of the third option, which favours high values of metal accumulation, are less predictable and can also tend to include larger amounts of dilution than would appear necessary.

Economically optimized composites

The algorithm which produces these composites is similar to that for ordinary minimum length and grade compositing except that it attempts to optimise the net value of a composite. It does so by producing a composite with the highest possible result for the following formula:

Gc x Lc) - (Gm x Lc)

where Gc is the grade of the composite, Lc is the length (or thickness) of the composite and Gm is the minimum grade.

Compositing software

GeoBASys program is the most versatile and user friendly package available to date. This program produces all the composites described earlier including the economically optimised version described above. It has been successfully utilised for compositing lead and zinc deposits in the United States, Australia and Ireland and has been found to produce more satisfactory results than other methods. The compositing algorithm used by GeoBASys has been further modified to include the ability to vary the minimum grade criterion according to the length of the composite. This has been added to reflect the conditions in which mining costs are reduced when mining thicker portions of an ore body.

The above extract is from:

Mineral Resources Evaluation
A Practical Approach
(2nd Edition)

by Dr A. E. Annels Ph.D DIC FIMM CEng

 

    
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