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Theory of Sampling and geostatistics for twin drill holes analysis in a niobium mine in Araxá, Brazil


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Author M T G C Marques, T M El Hajj, J M Braga Jr, A C Chieregati and H Delboni Jr


The estimate and control of sampling errors is essential for all mining
operations, especially when dealing with strategic materials, such as niobium.
This paper is based on a traditional quality assurance and quality control
technique used to confirm mineralised intersection zones, and proposes the
validation of sampling data using diamond drill twin holes, in order to evaluate
and improve the quality of the current data set. The geological model of the
niobium mine – located in Araxá, Minas Gerais, Brazil – was built including 77
diamond drill holes of a sampling campaign carried out during the 1970s. There
are few records about the collection and preparation procedures of these past
samples, making it difficult to estimate the errors associated with their
analytical results. Therefore, 11 twin drill holes were drilled to compare their
samples against the samples from the 1970s drilling campaign. The new hole
locations were chosen as near as possible to the old holes location. The JORC
Code, 2012 edition, relates the use of twin holes as a good sampling technique
to assess sampling and assay data. Three parameters were considered to validate
the 1970s drill hole campaign by the twin drill hole campaign:

  • the difference of the coordinates between the paired
  • the difference among the positions of the geological
    contacts to validate the geologic description
  • the difference between the paired samples grades was calculated to
    validate the analytical results.

The analysis of the grade range variation between sample pairs was based on
three techniques:

  • geostatistical interpretation of vertical
    semivariogram to obtain the nugget effect
  • calculation of the relative difference between
  • calculation of the error associated with the sampling protocol established
    according to Pierre Gy’s Theory of Sampling.

The results were satisfactory for hole coordinates; however, the ore content showed a high deviation, although this divergence can be explained by the nugget effect. For the lithological intervals, the new results contributed to an effective identification of variations among lithologies, ie brown weathered ore and saprolite. The twin drill holes campaign successfully validated the geological model data, allowing effective mine planning and higher reliability of the reconciliation results.


Marques, M T G C, El Hajj, T M, Braga Jr, J M, Chieregati,
A C and Delboni Jr, H, 2017. Theory of Sampling and geostatistics for twin drill
holes analysis in a niobium mine in Araxá, Brazil, Australia, in Proceedings
Eighth World Conference on Sampling and Blending
, pp 97–106 (The
Australasian Institute of Mining and Metallurgy: Melbourne).