Acid-base accounting (ABA) is a widely accepted method of classifying acid rock drainage (ARD) potential of geological materials. For remote projects that require classification and identification of potentially ARD generating (PAG) rock but lack onsite analytical capabilities, portable X-ray fluorescence (pXRF) is a field monitoring tool that is an option. Example applications include classifying PAG rock that needs special mitigation/ management after excavation, or classifying and segregating PAG waste rock from rock that is non-PAG and/or has a low risk of metal leaching (ML) for use as construction material.
Portable XRF is not an off-the-shelf product and its selection as a field tool for ML/ARD classifications must first be demonstrated. SRK has designed and implemented ML/ARD monitoring programs at proposed, operating and closed mines using pXRF at the operational scale. The first step in determining if pXRF is a suitable field tool is a review of geochemistry, geology and mineralogy. This desktop review will reveal if pXRF is potentially a viable field tool and identify proxies for characterization of ARD and/or ML potential. The next step is to establish threshold limits for material classification and design a pXRF program that will demonstrate proof of concept. The latter includes establishing sample collection methods that will be representative of the rock and sample preparation methods that will provide the fastest turnaround time. The program also needs to include a pair-wise sample analysis program using pXRF and laboratory methods to develop a site-specific pXRF calibration for parameters of interest and design an analytical program that includes a QA/QC program to ensure data integrity.
After the design of the pXRF program, the final step is to adapt the pXRF program to the operational requirements of the project. This includes designing a program that meets the overall geochemical objectives yet can be practically executed by the Contractor at the operational scale. Implementing a pXRF monitoring program can be an expensive undertaking; however, under the right circumstances, it can lead to overall savings for a project.
