Portable XRF Data Under International Reporting Codes. Dennis Arne Principal Consultant Geochemistry

Transcription

1 Portable XRF Data Under International Reporting Codes Dennis Arne Principal Consultant Geochemistry Olympus Portable XRF Short Course Toronto March 9,

2 Introduction 2

3 Philosophy Under JORC 2012 The Joint Ore Reserves Commission (JORC) defines three guiding principles for the public reporting of exploration results, mineral resources and ore reserves. Last updated in 2012 and includes direct reference to portable XRF data. Diagram from Berry & Stoker, 2014 Olympus Portable XRF Short Course Toronto

4 Materiality Reporting pxrf data for a mid sized to major mining company would not be material, as it would have no impact on share price. By contrast, the share price of a one project junior company could be significantly impacted by the reporting of pxrf data. In the case of the latter, public releases should contain sufficient information to allow the reader to make an informed assessment regarding the material. The degree of detail may depend on the importance of pxrf to the release. 4

5 Case Study *Niton XRF The Niton XRF unit is a Company-owned portable analyser of various elements/metals which utilises an xray fluorescence tube to take rapid measurements over a pin-point area. It is used by employees of Company X to take readings of samples to evaluate the tenor but not absolute values of the contained mineralisation. The readings obtained have not been verified by an independent laboratory. Olympus Portable XRF Short Course Toronto

6 Whoops! Company retracts Au & Ag Company releases pxrf Cu, Au, Ag results 10% price jump Hole Depth Hole Depth Ag Ag Ag Au Au Au Cu (%) Cu (%) Cu (g/t) (%) (g/t) (g/t) (g/t) (g/t) (g/t) (m) pxrf (m) Lab pxrf pxrf pxrf Lab pxrf pxrf Lab < <0.01 Company releases lab results 20% price drop Share Price chart 6

7 Competence A Competent Person is a minerals industry professional who is a Member or Fellow of the Australasian Institute of Mining and Metallurgy, or of the Australian Institute of Geoscientists, or of a Recognised Professional Organisation (RPO) A Competent Person must have a minimum of five years relevant experience in the style of mineralisation or type of deposit under consideration and in the activity which that person is undertaking. 7

8 Transparency Transparency requires that the reader of a Public Report is provided with sufficient information, the presentation of which is clear and unambiguous, to understand the report and not be misled by this information or by omission of material information that is known to the Competent Person. In other words, all critical aspects of sampling and analysis by pxrf should be presented if the data are a significant component of the public release. eg. If the instrument was uncalibrated 8

9 JORC 2012 Table 1 References Nature and quality of sampling (eg cut channels, random chips, or specific specialised industry standard measurement tools appropriate to the minerals under investigation, such as down hole gamma sondes, or handheld XRF instruments, etc). For geophysical tools, spectrometers, handheld XRF instruments, etc, the parameters used in determining the analysis including instrument make and model, reading times, calibrations factors applied and their derivation, etc. 9

10 What Else Does JORC Say? For all sample types, the nature, quality and appropriateness of the sample preparation technique. Quality control procedures adopted for all sub sampling stages to maximise representivity of samples. Measures taken to ensure that the sampling is representative of the in situ material collected, including for instance results for field duplicate/second half sampling. Whether sample sizes are appropriate to the grain size of the material being sampled. Nature of quality control procedures adopted (eg standards, blanks, duplicates, external laboratory checks) and whether acceptable levels of accuracy (ie lack of bias) and precision have been established. 10

11 NI Item 11: Sample Preparation, Analyses, and Security Describe (a) sample preparation methods and quality control measures employed before dispatch of samples to an analytical or testing laboratory, the method or process of sample splitting and reduction, and the security measures taken to ensure the validity and integrity of samples taken; (b) relevant information regarding sample preparation, assaying and analytical procedures used,..; (c) a summary of the nature, extent, and results of quality control procedures employed and quality assurance actions taken or recommended to provide adequate confidence in the data collection and processing; and (d) the author's opinion on the adequacy of sample preparation, security, and analytical procedures. 11

12 Qualified Person qualified person means an individual who (a) is an engineer or geoscientist with a university degree, or equivalent accreditation, in an area of geoscience, or engineering, relating to mineral exploration or mining; (b) has at least five years of experience in mineral exploration, mine development or operation, or mineral project assessment, or any combination of these, that is relevant to his or her professional degree or area of practice; (c) has experience relevant to the subject matter of the mineral project and the technical report; (d) is in good standing with a professional association; 12

13 Sampling 13

14 Representative Sampling Sampling theory has long addressed the issues associated with the collection of representative samples in terms of minimum sample mass required. Why then do some practitioners think that sampling theory does not apply to pxrf data? No amount of instrumental advance, calibration or QAQC during measurement can compensation for a non representative sample to begin with. 14

15 How big is a pxrf sample? Single spot analysis 1 cm 2 by cm = cm 3 x 3.0 g/cm 3 = g Average of 10 spot analyses over 1 m g x 10 = 0.15 g Drag analysis over 1 m g x 100 = 1.5 g Approximate sample weight for acid digestion & ICP analysis, after homogenization Filet sample over 1 m 0.1 cm x 1 cm x 100 cm = 10 cm 3 * 3.0 g/cm 3 = 30 g Half core saw cuttings (NQ) 4.76 cm x 0.2 cm x 100 cm = 95.2 g Difficult to obtain a representative sample in low grade deposits Olympus Portable XRF Short Course Toronto

16 No Preparation/Calibration May be suitable for reporting relative differences between samples, rather absolute abundances. Need to determine the influence of sample bag material on elements of interest (attenuation and/or contamination). Should be stated to satisfy transparency. 16

17 Arsenic by ICP MS & pxrf From Arne, Mackie & Jones, 2014, The use of property scale portable x ray fluorescence data in gold exploration: advantages and disadvantages, Geochemistry: Exploration Environment Analysis, vol. 14, pp

18 Fine Chalcopyrite in Drill Core Increasing accuracy Increasing precision Multiple drag analyses produce the best data in terms of both accuracy and precision. Multiple spot analyses produce a reasonably accurate estimate, but the precision is poor. 18

19 Coarse Chalcopyrite in Drill Core Increasing accuracy Increasing precision The average of three multiple drag analyses again performed best on diamond drill core in terms of both precision and accuracy. However the precision is beginning to deteriorate for the drag analyses. 19

20 Molybdenite in High Angle Veins Precision (CV) Spot analyses Drag analyses Accuracy (% bias) No sampling approach gave good data in this case, with a significant negative bias, but the drag analyses had the best precision. 20

21 Analytical Aspects 21

22 Instrument Selection Have you got the correct anode (eg. Ta, Rh, Ag) for the elements of most interest to your project? Is the instrument set up correctly for the elements you are interested in? Are the detection limits suitable for the concentrations you will measure? Olympus Portable XRF Short Course Toronto

23 Lower Limits of Detection Know the likely detection limits of your instrument Work out actual lower limits of detection (LLDs) for your project if you need to what is your application? Will the data be Fit for Purpose? Good Okay Poor Good Good Poor Limits of Detection (ppm) Element LLD Value (ppm) % of Samples Above LLD Cu Mo Sb 6 30 Pb Zn W 2 4 Actual Delta Premium LLDs calculated for X act Count Technology on soil samples Olympus Portable XRF Short Course Toronto

24 Quality Time Standard LKSD 4 Analyzed in Soil Mode Handheld 1 Handheld 2 Handheld 3 Benchtop 1 Benchtop 2 Cr (ppm) Ca (pct) As (ppm) Good precision/good accuracy (from Hall et al., 2011) Good precision/poor accuracy Poor precision/poor accuracy L R: 5 pxrf Time (seconds) 10 replicates 8 beam times (20 120sec) 24

25 Accuracy Certified reference materials (CRM) should be matrixappropriate. CRM should span a range of expected values. CRM data should be collected on a regular basis to monitor drift. Calibrate with either matrixappropriate CRM or projectspecific samples analyzed by appropriate methods. 25

26 Summary Public reporting of pxrf data must be Material, Transparent and done by a Competent person Representative sampling is critical for reporting of absolute results Instrument must be suitable for the project with appropriate detection limits Calibration may be required if reporting absolute values Count time should be sufficient for precise measurement A QAQC program must be in place covering both accuracy and precision Olympus Portable XRF Short Course Toronto