3.2.3. Gold Minerals

Next, the undetected Au-minerals in the Boliden AB analysis were targeted. In a first attempt, an entry in the SIP was created to identify any pixel with an Au-signal as Au. This entry was placed at the top of the SIP to guarantee that all pixels were filtered for Au. However, still, no pixels of Au were found. Manual inspection of some pixels that corresponded to Au-pixels in the CSM data revealed clear Au-signals, thus they should have been identified as Au based on the SIP. The problem was caused by the "boundary phase processor" used. Since Au-grains in the sample were dominantly below 5 μm in size, recorded Au-signals were mostly limited to single pixels. Consequently, processing of the data with the "boundary phase processor" reclassified all pixels that had been filtered as Au to their surrounding phase. Subsequent deactivation of the "boundary phase processor" for the Au-related SIP entries delivered a number of Au-pixels. However, evaluation by SEM-EDS revealed that many of the pixels were falsely classified, probably due to beam deflections. In order to exclude erroneous signals from being identified as Au, different threshold values for the Au-signal in the SIP were tested. Experimentally, the threshold was set to intensities of 20%, 30% and 40%, respectively, (compared to the Au elemental reference spectrum) and the results were verified by subsequent manual SEM analysis. The results are summarized in Table 3. At an intensity of 20%, 69 pixels were identified as Au, of which 20 were false. At an intensity of 30%, 30 pixels of Au were identified, with no errors. At an intensity of 40%, 20 pixels were identified as Au and no errors were found. Further tests with thresholds between 20% and 30% revealed that a threshold of 25% was the lowest possible intensity to avoid errors. At 25%, 39 pixels were classified as Au. A few pixels contained several <2 μm Au grains while some grains, >10 μm, were covered by more than one pixel.


**Table 3.** A range of Au signal intensity (SI) thresholds, defined in the SIP to identify pixels as Au, and their respective outcome after processing of the Boliden AB QEMSCAN® analysis. Pixels identified as Au include the errors.

Figure 8 shows the Au grains detected by the Boliden AB analysis using different Au intensity threshold values (Table 3). As expected, there is a clear correlation between detectability and Au grain size. Generally, decreasing the threshold value allows detection of smaller grains because their excitation volume creates a weaker Au signal. However, it also increases the risk for errors. The detection limit is, on average, approximately 5%–10% per analysis point at 1000 counts and depends strongly on the element and the matrix [18,20]. At an optimal threshold value of 25% of the signal intensity (Au), several grains below 2 μm were detected. Compared to the six Au grains detected by CSM, four were detected even at a threshold of 40% in the Boliden AB data (Figure 7a,c,d), one at a threshold of 21% (Figure 7b), and one was not detected at all despite a grain size of ~5 μm (Figure 7e). It is possible that the undetected Au grain lay just between two beam spots and thus was not measured. Another explanation could be its close spatial association with pilsenite and sphalerite, whose signals may have overlaid the Au signal. In contrast, two grains detected by Boliden AB were large enough that the 10 μm scan from CSM could have picked them up too, but missed them. Manual SEM inspection of the CSM mineral map showed that one grain was classified as pyrite and the other as rutile, despite none of these minerals being in the vicinity of the respective pixels.

**Figure 8.** Secondary electron images of false-colored gold grains detected by the QEMSCAN® analysis of Boliden AB for a range of signal intensity thresholds (20%–40%) of Au defined in the SIP-entry for the identification of Au minerals. A clear correlation between detectability and size exists. For the grains close to each other, they were marked by a single pixel in the mineral map and it was impossible to discern which had been hit by the electron beam. The grains with an asterisk were also detected by the QEMSCAN® analysis of CSM.

The experiments with the SIP-entry for Au showed that detection and correct differentiation were largely controlled by the SIP and that the same procedure used for the detection of Au could be applied to other trace metals. It could, therefore, be treated as a proxy for all trace metals. To test this methodology, a quick test confirmed that the detection of Ag-minerals was indeed possible by creating a SIP-entry specific for the detection Ag and placing it just below the entry for Au at the top of the SIP. Several tens of Ag-minerals were identified, despite none having been detected in the sample previously.
