Portable XRF for Gold (Au) and Au Pathfinders for Mineral Exploration and Ore Body Vectoring

by Philippine Resources - December 07, 2021

Olympus’ X-ray fluorescence (XRF) analyzers provide high-performance, real-time geochemical data for rapid multielement characterization of soil, rocks, and ores.

Recent advances in XRF technology have increased the number of elements measured, improved the limits of detection, and reduced analysis test times.

Vanta™ XRF analyzers provide convenient and rapid measurement of many types of samples associated with Au exploration, within Au mines, Au mine labs, and refined Au products.

Benefits

  • Rapid indication of potential Au mineralization through XRF analysis of pathfinder elements in soil, drill cuttings, and drill cores
  • Priority sample selection, maximization of analytical budgets, and improved drill target generation by prescreening samples with XRF
  • Improved ore deposit understanding, modelling, and vectoring with less dilution and better Au recovery via XRF mapping of structural features and identification of mineralized and altered zones
  • Inexpensive and rapid rock typing by using XRF for lithogeochemistry
Associated Geochemical Signature of Au Deposits
Source: Reflex Geochemistry

Pathfinder and Alteration Geochemistry

Most Au deposits have an associated geochemical signature (as shown in the table above). The XRF analyzer can detect these geochemical signatures, enabling geologists to better understand the geological system in which they are working. Typical Au pathfinder elements include As, Cu, Pb, Zn, Sb, Bi, Ag, and W.

Typical XRF LODs for Common Au Pathfinder Elements with Vanta VMR GeoChem Model

*120 s test time per beam, interference free SiO2 matrix; refer to Olympus LOD documents for further discussion on lower limits of detection (LODs)

XRF Analyzer for the Detection of Gold

It is common knowledge that handheld XRF analyzers do not support direct low level measurement of Au in geological samples (e.g., low ppm and ppb). The lab-based fire assay technique is commonly recognized as the method of choice for Au analysis. Au L-level X-ray lines are located in a very crowded area of the X-ray fluorescence energy spectrum. In this part of the spectrum, interference from other elements (e.g., As, Zn, W, and Se) can yield a false-positive Au determination.

Direct measurement of Au by XRF can be achieved in specific cases such as high-grade (> 5 ppm) quartz vein environments (relatively interference free) or within refined Au products (where Au is present in very high concentrations).

An increasing number of Au onsite mining laboratories are using XRF in place of, and as a supplement to, fire assays methods. Please refer to the Olympus Application Note “Use of Handheld XRF in Au Mine Laboratories.” Mines are also using Olympus handheld XRF analyzers for measuring Au in activated carbon. Please refer to the Olympus application note, "Measuring Gold in Activated Carbon."

Images taken from: Arne et al. (2014) – The use of property-scale pXRF data in gold exploration – advantages and limitations, Geochemistry: Exploration, Environment, Analysis.
Contor plots of As, Cu, and Pb in surface soil using field XRF compared with ICP analysis from Au exploration project in Canada.

 



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GERDA is used with a Vanta XRF analyzer to automate testing for geoexploration projects. Developing the GERDA System The GERDA system was developed by a team that has been using portable XRF analyzers on mineral exploration projects for over 15 years. Largely focused on the use of pathfinder elements as part of their gold exploration projects, the inventors have witnessed the development of portable XRF analyzers over time, particularly the improvements in speed and the reduction in the test times required to achieve the levels of precision needed. They have also observed Olympus’ developments in detector technology and signal processing and, in particular, the Vanta analyzer’s ability to achieve lower limits of detection (LODs) of elements, including gold (Au). As junior explorers on a limited budget, their focus has been on greenfield exploration, rapid identification of anomalies, generation of drill targets, and potential new discoveries, not resource definition. Key to this has been their ability to test a far greater number of samples using their portable XRF analyzers than would be possible if all samples were assayed in the lab. GERDA is critical to their ability to do this. GERDA is designed to: Increase sample throughput for existing portable XRF owners Help select samples for more expensive laboratory assay Provide faster access to meaningful results for real-time follow up/infill Reduce the overall cost of exploration analytical budgets Make exploration programs more efficient To see how GERDA works with portable XRF, watch this quick video: https://www.youtube.com/watch?v=3-Gesp0Gjpk What is Nugget Effect Gold Assay (NEFFA)? To understand this new technique, it is important to know how the “nugget effect” works. The nugget effect is a term used to describe the sampling problem that can occur when there are large gold particles (nuggets) in a sample. If a nugget is selected during a traditional sampling process, researchers can overestimate the amount of gold in the sample. But if the nugget is missed, the gold can be underestimated. For gold exploration and mining, this variability can lead to inaccurate results and costly mistakes. The Nugget Effect Gold Assay (NEFFA) technique aims to turn the traditional challenge associated with the nugget effect into an opportunity. NEFFA attempts to measure the nugget effect through multiple nondestructive, systematic analyses of a geological material (typically crushed rock or crushed drill core) to identify the presence of gold or other precious metals based upon its distribution within the sample. NEFFA completely redefines and reduces the traditional sample preparation procedure and aims to provide semi-quantitative gold results using portable XRF analyzers to identify samples of potential interest for further laboratory analysis. Here’s how it works: Representative samples of 20–30 g, depending on the nature of material, are isolated in a medium such as a clear sample bag or under plastic film to ensure that there’s no contamination from an external agent The sample is distributed evenly over a 10 cm × 10 cm square with the depth of the material being no thicker than 2 mm A grid or matrix is placed over the sample, with individual matrices measuring 1 cm × 1 cm Measurements are performed within each square cm, and the average of all results is reported The technique is most effective on crushed rock or crushed drill core samples with a grain size 3 mm, but not on homogenized pulp samples Rotary air blast and reverse circulation (RC) samples can be analyzed directly This method takes the throughput advantages of GERDA for general exploration projects and adds additional value for gold exploration by providing gold estimates using XRF. We acknowledge this method may be considered non-traditional. Portable XRF’s weaknesses in analyzing gold are well known (elemental spectral overlaps can cause false positives, relatively high LODs compared to the lab, and fire assay is the best technique to use), however the inventors’ initial work using GERDA and NEFFA suggests meaningful gold data and cost savings are possible with this system. Also, local radiation safety and licensing requirements must be followed with any proposed implementation of this technique. By Todd Houlahan - Director, International Mining Group Todd graduated from the University of Newcastle, Australia in 1993 with a degree in soils and hydrology. For the past 18 years, Todd has been a leader in the portable XRF revolution in the mineral exploration and mining industries, publishing numerous papers on the subject and presenting at international conferences. For the past 10 years, Todd has led Olympus’ International Mining Group helping explorers, miners, and their shareholders achieve cost efficiencies by using the best portable XRF and XRD technology solutions. To know more about Automated, Robotic Innovations for Analyzing Geological Samples with Portable XRF, visit https://trisco.com.ph/ or email at trisco@pldtdsl.net ​


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