Portable XRF Analysis of Cu and Mo in Diamond drill core
USE OF PORTABLE XRF ON CORE
Anecdotal evidence suggests that many exploration companies are undertaking portable XRF analysis of diamond drill core using drag analyses, or continuous scanning of the drill core with only the beam for the element of interest active. However, there are few published examples of such analyses where the validity of the approach for various commodity elements or styles of mineralization have been assessed. This pilot study from several porphyry Cu and Mo projects was undertaken on diamond drill core kindly provided by Kiska Metals of Vancouver, Canada. The analyses were undertaken using a Delta Premium ECR 6000 with a Rh tube in soil mode (beam 2 only) with a 30 second count time. The XRF data were corrected based on a calibration using matrix-appropriate certified reference materials with a similar grade range as the material being analyzed.
Mineralisation Style
Three distinct styles of mineralisation were evaluated: finely disseminated chalcopyrite; coarsely disseminated chalcopyrite; and fracture-hosted molybdenite. The accuracy of the analysis is measured in terms of bias against the laboratory assays over the same measured interval and is expressed as a percentage. The precision of the analyses is measured as a relative standard deviation (or coefficient of variation) and is also expressed as a percentage.
Portable XRF Test Results
The results of these orientation tests indicate that while it is possible to obtain reasonably accurate Cu data for finely disseminated mineralisation using multiple spot analyses, the precision of the data is poor, even when as many as 16 spot analyses are used over a metre. Significant differences are also observed depending on whether the sawn (flat) side of the core or the rounded surface was analysed. By comparison, the average value from 3 drag analyses over the same core interval gives both accurate and precise results regardless as to which type of surface was analysed (sawn or rounded).
The results were less robust in the case of the coarsely disseminated style of mineralisation. At least 20 spot measurements are required in order to obtain reasonably accurate results, but the precision of the determinations remains poor. Conversely, reasonably accurate and precise results for Cu are obtained when at least three drag analyses are undertaken.
In the case of molybdenite mineralization hosted by narrow quartz veins orientated at a high angle to the drill core axis, multiple drag analyses give reasonably precise Mo results, but the accuracy of the drag analyses were as poor as the spot analyses.
Accuracy and Precision of pXRF in Drill Core
Summary of portable XRF spot and drag (scan) analyses of diamond drill core intersecting finely and coarsely disseminated chalcopyrite mineralization, as well as molybdenite is narrow quartz veins orientated at a high angle to the core axis. The sample mass calculations assume an escape depth for secondary X-rays of 50 microns, but this will be dependent upon the element of interest and the density of the rock (modified from Arne, 2018).
Conclusions
Such orientation work is recommended when analysing diamond drill core material in order to ensure that data of both sufficient accuracy and precision are obtained from portable XRF analyses. It must be kept in mind that sampling theory dictates that the sample size should increase where the grain size of the mineralization increases, or where the grade of the material decreases. Users of the equipment should be aware that multiple spot analyses, and in many cases drag analyses as well, may fail to generate data from a sufficient volume of material to provide a representative analysis.
Reference
This example is modified from: Arne, D., 2018, Practical application of portable XRF in mineral exploration. In Advances in X-ray Analysis, Volume 61, Proceedings of the 66th Annual Conference on Applications of X-ray Analysis (Denver X-ray Conference), in press.