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Dual Source Applications for Exploration and Mining: Au-bearing Epithermal Samples

The combination of micro-XRF with SEM enables the potential to analyze samples at multiple scales, from centimeters (cm) to millimeters (mm) to micrometers (µm) and below within a solitary system. Thus, by adding the micro-XRF to an SEM is converted to a dual source system, meaning that there are two excitations sources, the e-beam and photon beam. Either source can be used individually, or simultaneously, to generate sample X-rays that will be measured using the same EDS detector. In addition, the benefits of each analytical technique can be utilized: (i) the XRF source has very low background, which means that it is possible to observe element concentrations down to 10 ppm (element and matrix dependent), as well as much greater information depth, which means that it is possible to see structures or elements underneath the sample surface. For example, inclusions that are below the surface can be detected, even at very low concentrations; (ii) the electron beam can be focussed to an extremely small area and yield extremely high resolution information. 

Such a combination can now create new workflows within a single system.  For example, it is possible to quickly scan a large rock sample, in this case an Au-bearing spcimen from Karangahake Epithermal deposit, using the micro-XRF . This enables the identification of areas of interest that include Au-bearing grains (Fig. 1 and 2).  Subsequently, it is possible to analyse these "areas of interest" at much higher resolution using the electron beam (Fig. 3).  Therefore, this dual-beam system can simultaneously identify relevant information on a large scale (cm to mm) to enable detailed small scale (mm to µm) can be performed efficently and accurately.

Fig. 1: SEM-XRF HyperMap of total X-ray intensity overlaid by Au. The sample is from the Karangahake gold mine in New Zealand. The analytical area is approximately 45 x 45 mm².
Fig. 2: SEM-XRF elemental intensity maps for the Au-Lα lines on the left. The Au-Lα and Zn-Kβ lines overlap, however the deconvolution of these overlapping peaks is correct as shown by the spectra images on the right, which confirm the presence of the Au-Lβ lines verifying that these grains are gold. The middle map is 45 x 45 mm².
Fig. 3: SEM-XRF elemental intensity maps for the Au-Lα lines on the left. Selected areas are then mapped by SEM-EDS which has an improved resolution and highlights the relationship between the gold grains and the surrounding sulphides, e.g. galena (PbS), sphalerite (ZnS), pyrite (FeS2) and chalcopyrite (CuFeS2). The map on the left is 45 x 45 mm².