Contamination, Precision and Accuracy of pXRF Geochemistry: Case Study-Polymetals Resources Ltd., Mansala Gold Project, Siguiri Basin, Northeast Guinea

Abstract

Quality Assurance and Quality Control (QA/QC) is a critical component of all pXRF geochemistry processes. A properly constructed pXRF QA/QC programme identifies possible instrumental errors and provides a means of securing fit for purpose data from the pXRF programme. pXRF QA/QC programmes involve daily contamination, precision, and accuracy checks to en sure the generation of fit for purpose data. In the exploration field or mine-site , pXRF is capable of producing extremely valuable data that is fit for purpose if calibrated properly. However, it should not be used as a replacement for acquiring data from an accredited laboratory using established analytical techniques that produce high quality data. Contamination is the checking of the cleanliness of the analyser window or the presence of dust in the measuring environment. At Polymetals, using Olympus Vanta C-Series pXRF analyser with silver anode, contamination is assessed by measuring an instrumental blank (SiO 2 ), to identify any foreign matter on the analyser window. Assuming that the window film is new, and the fused silica disc is dust free, only Si should be detected. If any other significant element is detected, the film is replaced, and the test is re-run. Accuracy is a measure of how close the measured value is to the true value and is assessed by measuring the abundance of selected elements contained within a Certified Reference Material (CRM) or the NIST check standard sample supplied with the pXRF analyser. Elements of interest must report within ±20% of the standard value. Precision is a measure of how close repeat measurements are to one another and is assessed by taking multiple readings on a particular sample to determine the stability of the analyser. The Relative Standard Deviation (RSD) of the replicate measurements is then calculated. The RSD values should be less than 20% for most analytes, except chromium, for which the value should be less than 30%. Once contamination, accuracy and precision are within accepted limits, the batch/daily measurements are considered to have passed the QA/QC protocol. The data is thus fit for purpose and transferred to the data file. Any batch/daily measurement reported to have failed due to instrumental errors is re-analysed. QA/QC protocols should be applied to each project. The QQ/QC protocols instituted after the pXRF samples meeting the quality sample conditions thus pulverised dry samples in pXRF sample cup covered with thin pXRF films, are used to generate fit for purpose data from soils samples at Mansala which is used to generate pathfinder element(s) to delineate anomalous pathfinder trends for further exploration works.