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

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 ensure 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


Introduction
Generating fit for purpose data from a pXRF programme requires quality sample preparation, optimal sample irradiation interval, appropriate sample containers, films and QA/QC protocols [1].Energy calibration check is done at the prior to the first analytical batch of the day and repeated after 4 hours of continuous measurements to ensure the analyser is still operating within resolution and stability tolerances (i.e.no shifts in energy line positions, regions of interest, or shift in gain control due to temperature changes) [2].This should be carried out each day with the manufacturer's check ("coupon"), often an alloy or pure metal, or if drift is suspected.
Assay irradiation time has great impact on precision and limit of detection, with precision improving with increased testing time.This is particularly important when light elements are being measured [2].For the Polymetals Mansala Auger pXRF programme, 45 seconds per beam was used as the optimum time in order to undertake enough measurement daily.This assay interval was based on test work which showed no significant variation in precision and accuracy for the standard suite of analytes when irradiated for 30, 45 and 60 seconds per beam.Refer to Figure 1.
Quality sample condition is necessary for all pXRF measurements.This means that: • Samples are properly dried; • Samples should be pulverised to 75% passing 150 µ mesh to ensure homogeneity; and • Aliquots of sample transferred to assay cups covered with thin pXRF films.
Pulverised samples minimise effects of heterogeneity associated with varying grain-size and nuggety element distribution, and the precision of repeat analyses is significantly improved [1].The level of sample preparation normally depends  Samples for the Mansala auger pXRF programme are pulp samples (grain size −75 µ) from the laboratory, prepared in pXRF sample cups covered with thin polypropylene film.Sample condition is thus optimal.Note that polypropylene films have the best transmittance of fluorescence signal [2].It is very important to always ensure that samples for pXRF measurement are always dry as moisture in the sample can absorb some of the X-rays, making some elements to be under reported and, the pXRF machine is also factory calibrated on dry samples and will always under report results on moist samples.The standard operating procedure used comprises: • optimum sample preparation; • energy calibration check; • optimum test time; • contamination checks-first sample of the day should be a blank; • accuracy checks; • precision checks.
When these procedures are put in place, fit for purpose pXRF data will be generated and can be used profitably for sample screening and selection tasks.QA/QC monitoring of pXRF results should be done systematically especially if the pXRF field measurements are to be used as data by the project or if operational decisions are based on them.QA/QC protocols are the same as in the laboratory, with blanks, CRMs and standards, systematic and random duplicates, and careful performance monitoring of each instrument [2].A good rule of thumb is to run QA/QC 1 out of every 20 samples in a pXRF programme.The first sample of the day should be the blank to ensure that there is no contamination on the detector window [3].Monitoring the quality of analysis after good sample preparation, testing time, appropriate sample cups and films, is fundamental to ensuring the generation of fit for purpose data and providing the necessary confidence to make informed decisions when interpreting all the available information.A comprehensive quality control system is necessary to monitor the sampling-preparation-analysis sequence in an effort to ensure the generation of fit for purpose data [4].Companies are strongly encouraged to designate someone with technical acumen within their group to be their pXRF specialist, to undergo training, be responsible for the maintenance and calibrations of the pXRF instruments, and to train others in the operation of pXRF analysers.• Good filtering of X-rays emissions by the polypropylene film.

Accuracy
Accuracy is a measure of how close the measured value is to the true value and is influenced by the quality of reference standard used in the calibration, the calibration procedure, and the duration of the measurement [5].• No values fell below the minimum acceptable Cu value by 20%.

Precision
Precision is a measure of how close repeat measurements are to one another and is influenced by statistical nature of the x-ray tube emission process, statistical nature of the sample's x-ray absorption/emission process, statistical nature of detection process and unpredictable variations in substrate or matrix effects [5].A total of 20 repeated analyses were conducted on 4 samples, 5 analyses per sample, for the entire Polymetals Mansala Auger pXRF programme on four (4) different days.A plot of the readings for Arsenic (As), Copper (Cu) and Lead (Pb) as in Figure 5 indicates a very good repeatability signifying a good stability of the pXRF analyser.

Pathfinder Analysis
Pathfinder elements provide a means to evaluate large areas for their mineral potential by eliminating likely barren areas from the areas under consideration during exploration studies [6].Nude et al., in their gold exploration studies in the Wa-Lawra Belt, Northwest Ghana, using a multivariate statistical approach, found that Fe and Mn are strongly associated with gold, and alongside Pb, Ag, As, and Cu, these elements could be used as pathfinders for gold in the area, with ferruginous zones as targets [7].
With all QA/QC protocols being satisfied and generating fit for purpose data, the Mansala auger pXRF data is used to determine gold (Au) pathfinder elements(s) on the Mansala permit.Since the Mansala property lies in the Siguiri basin with expectation of a typical Orogenic gold deposit, Orogenic gold deposits

Delineation of Pathfinder Trends
Correlation analysis from the initial 130 auger samples identified arsenic as good gold (Au) pathfinder on the Mansala property.With this in mind, a soil sampling programme consisting of 500 soil samples on 12 crosslines on a 100 × 25 m grid system was initiated to delineate arsenic (As) trends at the most prospective area of the Mansala property for further exploration works.

Conclusion
This case report suggests that, with the right QA/QC protocols (contamination, precision, and accuracy) put in place after pXRF samples meeting the quality sample conditions thus pulverised dry samples in pXRF sample cup covered with thin pXRF films, fit for purpose data is expected to be generated.This data can then be used to make further decisions.

Figure 1 .
Figure 1.Optimum testing of assay intervals analysis using As, Cu, Fe and Pb.

Figure 2 .
Figure 2 shows constant values measured during the day signifying.• No loss in beam voltage

Figure 3
Figure 3 shows: • All values plotted above the As value in the standard, NIST 2711A; • No value fell above the maximum acceptable As value by 20%; • No values fell below the minimum acceptable As value by 20%.

Figure 4
Figure 4 shows: • All values plotted above the Cu value in the standard, NIST 2711A; • No value fell above the maximum acceptable Cu value by 20%; pXRF precision is assessed by calculating the Relative Standard Deviation (RSD) of the replicate measurements for the analytes.The RSD values should be less than 20% for most analytes, except chromium, for which the value should be less than 30% (Anon, No date).The smaller the RSD, the better the precision.pXRF precision determines the stability of the analyser in terms of measurement re-N.Y. Asante International Journal of Geosciences peatability by doing multiple readings on a particular sample and looking at the variability of the data.Precision check is conducted on a daily basis with a different sample.

Figure 5 .
Figure 5. Repeatability plot of As, Cu and Pb.

Figure 6
Figure 6 shows delineated Arsenic (As) trend from the soil sampling programme superimposed on auger and pit samples and geophysical structures delineated from the aeromagnetic survey.Interpretation of the pXRF soils sampling results, delineated Arsenic (As) trend coincident with the NNE trend from the pits and auger samples.Discrete Arsenic (As) anomalies also occur at the west of the main NNE trend.The delineated Arsenic (As) trends are open to the North and South.The Arsenic (As) trends also confirm the high prospectivity of the area sampled that is already delineated by the pit sampling, auger drilling and aeromagnetic survey.

Table 1 .
Summary statistics as in Table1, shows As, Cu, Pb, and Zn are likely to be abundantly available on the Mansala property as these elements were detected in almost all the 130 auger samples.S and Mo are likely to be moderately present on the Mansala property whiles W, Hg and Sb are likely to be less present on the Mansala property as these elements were not detected in most of the 130 samples Summary statistics of auger samples.

Table 2 .
Correlation analysis using log 10 transformation.

Table 3 .
Correlation analysis using power transformation.

Table 4 .
Summary statistics of Arsenic (As).