In all instances of drilling, maximum recovery of a representative sample is the objective. The general notion is that the bigger the core size, the better/more representative the sample. As a result, samples collected from smaller drill core are often considered inferior/not representative. The goal of this article is to generate a debate by arguing that other than for issues to do with core recovery (in bad ground) and hole deflection (in deep drilling), bigger and smaller drill core can yield similar results for the same volume of pulp that is eventually analyzed in the laboratory. In other words: why get a big sample if you are only going to analyze a very insignificant part of it.
This argument is based on the findings from a recent assignment, in which Micon prepared a mineral resource estimate for a gold project whose database had a mixture of legacy AXT drill core size holes and recent NQ drill core size holes. The core produced from AXT diamond bits is 35.51 mm in diameter, while that from NQ bits is 47.6 mm in diameter. In order to determine whether the legacy drill hole data should be included in the mineral resource estimation, Micon conducted a statistical comparison of the composite assays within the deposit wireframe. The distribution of the drill holes is shown in Figure 1 below.
Distribution of Legacy and Recent Drill Holes
It should be noted that both sets of holes (i.e. legacy AXT core size holes and recent NQ core size holes) are roughly evenly distributed throughout the deposit. In fact, the NQ holes on this project are primarily infill drill holes. The deposit is orogenic with strong structural controls on both a mega- and a micro-scale.
The results of the statistics of the composites are summarized in Figure 2, from which it is evident that there is a close similarity between the medians, means, standard deviations and coefficients of variation for both sets of drill holes.
Box Plot Comparing the Statistics of the Composites from AXT Core Vs NQ Core
Based on the statistical comparison results, it is apparent that, for this deposit, drill core size is of no relevance to sample/assay quality, provided core recovery is good. In every case, what is important is to achieve a drilling density/drill hole coverage that is dense enough to be representative of the deposit. Many QPs and geostatisticians may disagree with me, but the argument open for debate is: what is the justification for a bigger core size when the volume ultimately analyzed by fire assay at the laboratory is either a 30-g or 50-g aliquot from the “mother sample”, described by Pierre Gy as the “lot”1. More likely than not, this is applicable to all analytical techniques which utilize a small portion of the mother sample, including general beneficiation metallurgical tests such as flotation and leach tests.
Geostatisticians (of which I am one) often argue that sample support must be the same to obtain meaningful variography. But surely while the “lot” may be different due to different drill core sizes, the fire assay result is based on the same support, i.e. a 30-g aliquot. Thus, again in this instance, drill core size is of no relevance for as long as the “lot” is not analyzed.
Bearing in mind that whilst several deposits can belong to the same class, each deposit is unique, and, by virtue of this, generalizations should be made with caution. However, it is reasonable to assume that the more homogeneous a deposit is, the less relevance the drill core size has, and vice versa.
In conclusion, at the scale of a mining block, a larger drill core sample only yields a more representative result than a smaller drill core sample if, and only if, a fairly large portion of it or the whole lot is analyzed. However, this is not the practice in routine analyses for resource estimates. There is more discussion regarding drill core size/sample size and far less discussion about the size of the sample analyzed, despite the efforts by researchers in this field.
A notion which can be of general application, particularly for heterogeneous deposits, is: the bigger the core sample actually analyzed, the more representative the result. It is for this very reason that, a pilot plant is designed to treat/analyze the whole bulk sample in order to obtain an accurate reflection of the quality of the deposit prior to commencing full-scale production.
1Pitard, F. F., Pierre Gy’s Sampling Theory and Sampling Practice, 2 Volumes, CRC Press, Inc., Boca Raton, Florida. 1989.
any other non geo-stats benefits of larger gauge core…core logging?
Not so much for core logging but for metallurgical test-work – yes
Better “exposure” for studying the rocks.
Clearer geology through reducing the spottiness of information. Structures are clearer to understand, easier to recognise.
Core photos more informative on meso rock and structures
Larger metallurgical samples for testing