Let us discuss the advantages and limitations of Handheld XRF analysis in situ for gold and silver analysis vs results you get from say ICP and Fire Assay in the lab.
In both ICP and Fire Assay you destroy the matrix of the sample which liberates the Gold and Silver from the matrix and allows a chemical process of collecting the precious metals using collectors like Bi or Pb, which alloy with the gold. A second step which takes the metal slug off the bottom of the fused flux from the fire assay and cupels it on a magnesium refractory which allows the collector and other base metals to permeate the cupel and leave the gold/silver behind, The resulting alloy can then be treated in nitric acid to dissolve the silver and leave the Gold. That gives you a mass of gold per gram of ore and allows you to ascertain the gold content of the ore.
Using ICP we chemically dissolve the matrix into a solution of strong acid, very often we use a complexing agent like an amine to concentrate the Gold and or Silver before reading the resulting solutions on the ICP.
In XRF we are analyzing whatever volume of sample is being excited by the XRF beam. Then we generate secondary X-rays in the ore based upon its elemental profile. The larger concentrations of elements will have the strongest x-ray response and that translates into higher counts. Unfortunately, the process is not linear in response as elements with higher atomic numbers will absorb radiation emitted by lower atomic number elements giving erroneous responses unless you correct for this absorption and enhancement effect.
Fig 1. Schematic of a Handheld XRF showing the locaion of the x-rayt ube and detector and the barriers between the x-ray source and detector and the sample. The close proximty allows for relatively low powered instruments to produce reasonable x-ray flux volumes
Fig 2 -Results are typically available in a few seconds . Counting for longer improves the precision of the analysis and reduces the uncertainty (+/-) figure. A result is deemed to be significant at 3x the uncertainty value
Fig 3 The volume of sample penetrated that produces secondary x-rays that can be measured by the detector and the escape depth for x-rays from different matrices gives a good indication of whether your measurement parameters are meeting x-ray physics constraints especially for Gold and Silver analysis
For most ores Au can occur in several forms – free gold (not chemically bound)
– Gold associated with other complex chemical formations like Arsenopyrites and thus not readily accessible to be measured.
The limitation of XRF is we are measuring a very small sub sample of the ore and the Au distribution may not be homogeneous throughout the ore which means XRF results may be meaningless even if they are positive because of the disparity of gold distribution. If you hit a gold vein – really high results, if you measure ore with 400micron distribution of Au we may not see it, depending upon what other elements are present that could mask the Au x-rays – (elements like Fe, Cu, Zn, As, Pb and Bi may make it impossible for the Au x-rays to escape from the sample to the detector).
Using a handheld XRF is only going to work well with prior knowledge of the gold distribution in the samples and the possibility of measuring placer elements which are easier to detect thereby allowing you to create an elemental map of the physical area you are quarrying.
The chemistry/geology must be understood to interpret the results obtained meaningfully. Multiple readings of samples and using averaging may give a more meaningful idea of distribution in a defined area. The good news is that programs exist to feed multiple data points into for a long list of elements to be able to do discriminant analysis in which one can easily isolate the areas of greatest mineralization and thus economic promise.
Measuring Placer Elements
XRF comes into its own by reliably measuring placer elements – Cu, Zn S as indicators for Ag. As, Fe S for possible Au mineralization. Quartz veins with Carbon inserts can have Au in it. You obviously are crushing the ore that you are feeding into your air tubes to a certain mesh size to get consistent separation based on gravity. On your gravity air table – your ability to selectively drop out the Au either in free form or in simple mineralization can easily be measured by XRF and give positive indications of amount for optimization purposes.
The more concentrated the treated material is in Gold / Silver / PGMs the more reliable the XRF results are.
The method is powerful but it’s not a magic wand and must be used with a good understanding of the physics involved. It is largely a surface measurement with x-rays penetrating the top few mm of material ( depending on the kV applied to the Xray tube – the higher the kV the heavier the atomic umber element that can be excited , the deeper the penetration into the sample). By regulating the kV, we can optimize which elements are excited and selectively measure elements of interest. The intensity of the x-rays generated will depend upon the amount of primary x-rays ( from the tube) you pump into the system – the mA setting. In XRF it’s a balance between exciting the sample and being able to read all of the secondary x-rays generated since the detector has a finite counting capacity. We optimize excitation by balancing the count rate into the detector by the count rate output. The higher number of counts you can process the better the sensitivity of the analysis (ability to discern concentration differences within a sample)and the lower your limits of detection are ( the smallest amount you can reliably detect).
This is where the Prospector 3 comes into its own. Dynamic adaptive electronic manipulation of the detector output allows it to process much higher count rates than competitive instruments out there. (3 to 5 times) – this translates into increased speed of analysis and lower limits of detectability for certain crucial elements of interest.
Learn more about the Prospector 3 here – https://xrfsco.com/handheld-xrf
Using Fundamental Parameter Processes to measure ores
XRF is the only analytical technique that will allow us to simulate theoretical Xray intensities once we know the complete matrix constitution. All other techniques are purely comparative requiring reference materials. Modeling the x-ray response and matching it to the measured sample allows meaningful iterative processes to estimate the actual concentration in the measured sample. The accuracy of the Fundamental Parameters approach is based on how close the sample is modelled (all elements present in the sample).
As you can imagine the mathematical computation behind our Fundamental Parameter approach is very sophisticated and really comes into its own when we can match the actual measured sample spectrum against the theoretically generated spectrum. The closer the match the better the simulation works for unknown samples.
Therefore, using a hybrid approach with some known reference materials allows one to create a regression calibration based on FP constants. The method is extremely versatile and quite robust.
Having said that the FP method cannot compensate for the mineralization effect or the particle size disparity. Samples need to be ground to an 80 – 100 mesh size to give reasonable XRF results. Measurement of Ca element in mixtures of compounds like CaCO3
will lead to erroneous results for Ca. With mixed oxides it is best to calcine or fuse the sample to convert the compounds to oxides.
In conclusion while measuring samples in situ in a sample bag is very appealing and is often the driving force behind using Handheld XRF in the field , the accuracy of the analysis is highly dubious unless the samples have been reduced to a meaningful particle size and the mineralization is known, so that data produced can be interpreted correctly. XRF works well on homogenized samples of uniform particle size and known mineralization.
Method of Analysis
The majority of natural samples are heterogeneous. So, sample preparation before measurement is strongly recommended, especially for light element analysis. Samples must be crushed, ground, mixed and placed in 32mm XRF cups. Six different types of ores and concentrates were analyzed with ElvaX ProSpector LE: iron, copper, chrome, lead, tin, silver, and gold ores.
ProSpector 3 has 2 main geology calibration modes: «Mining» mode and «Mining 2pass» mode.
In «Mining» mode only one pass with one beam settings (35 kV voltage) is used. It allows to determine elements from Cl to U in low concentrations (until ~15%). This mode is best for geo exploration and analysis of low-grade ore. Advantage of this mode is absence of influence of LOI (loss of ignition) to measurement results. «Mining 2pass» mode consists from two passes (35 kV and 12 kV voltages) and allows to
analyze light elements (Mg, Al, Si, P, S). Mining 2pass mode is used for analysis of high grade ores, minerals and silicate materials. It is assumed that LOI is low in this mode.
For more deep analysis, LOI can be calculated using any other technique and then added to measurement result as undetectable element.
Results of measurements of multiple ores showing correlation of Measured vs Certified Values
Examples of Measurements in different Ore types showing the versatility of the Handheld XRF especially for Gold and Silver ores
From the above results it can be clealy seen that Handheld XRF is a huge advantage when measurements are made on samples that meet XRF constraints.
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