For a high definition mineralogical study of precious metals, more than one mineralogical equipment would be necessary: one is to search, identify and characterise the precious metal mineral(s), and the other method is needed for a quantitative analysis of target element for possible refractory appearance of the target element.
Prior to QEMSCAN and MLA era, for part one several steps should have been taken: concentration of the heavy mineral portion of the sample, magnetic separation and finally hand picking of the minerals of interest; beside to time-consuming and inconvenience, the risk of making mistakes in each step is not negligible; Thanks to the FEG high resolution automated SEMs, nowadays for part one all we need is a few gram representative aliquot of the sample in order to prepare the polished sections; the rest depends on the proper settings of the SEMs to detect the trace minerals; personally, I have detected precious metals with ppb levels in one polished section, which it had fairly a good reconciliation with the assays, assuming we are not dealing with the refractory presence of the precious metals.
The second part -checking the invisible precious metals- is generally done by either microprobe or laser ablation ICP-MS on handpicked target minerals, which it still necessitates long hours of hand picking under binocular microscope; also, we have to deal with loss of the hand picked grains during the polished section preparation.
By the following case study we show that how QEMSCAN could help on detection of the Pt and Pd minerals with abundances of 250-500 ppb in ultramafic rocks; and how we could skip hand picking by mapping the section and marking the target minerals for the EPMA lab.
The Project:
Four samples with elevated abundances of Pt and Pd were chosen for modal mineralogy analysis with main objection of characterisation of Pt and Pd minerals/carriers. The samples are amphibolite, pyroxenite and dunite. Samples were stage pulverised to 100% passing 200 µ; 3 grams representative aliquots was split from each sample using rotary micro plot riffle for preparing 30 mm polished section.
The modal mineralogy analysis was done using the default settings for BMA analysis (2.5 µ pixel size) and with line space of 200 µ (see the pie diagrams for modal mineralogy results).
For precious metal search, we used SMS mode (Specific Mineral Search); In these samples, Pt is appearing as Sperrylite (PtAs2) while Pd as fine native Pd grains associated with Sperrylite. Both Sperrylite and Pd appear as encapsulated grains in amphibole and quartz (see the representative BSE images).
So far we have detected and characterised the main carriers of the Pt and Pd, however based on some researches it was suspected that Pt and Pd could be present in other minerals as trace element. Regarding the mineral assembly of these samples, the only phase which can potentially accommodate Pt and Pd in their structures are Magnetite and Cr-Magnetite. In order to confirm such, a quantitative method like EPMA or laser ablation ICP-MS is necessary; regarding that the latter is a destructive method, we decided to proceed with EPMA.
We selected two of the samples (1 and 4: based on the reconciliation between the Pt and Pd from assay and what was detected by QEMSCAN) for EPMA analysis. Sample 1 had enough amount of Magnetite which locating them under EPMA was not hard; but this is not the case for sample 4 which has only 0.01% of magnetite; as mentioned above, conventionally in such cases hand picking of the target mineral from the HMC portion of the sample under binocular microscope would be the solution;
Yet enough magnetite grains have been detected by QEMSCAN in the polished section (sample 4) for an EPMA analysis, but the main problem is we do not have a software which we could export the co-ordinates of these grains to the EPMA system; the other option is just wondering around in section under EPMA and looking for target minerals, which is time consuming specially when looking for trace minerals and cannot be an option when dealing with volume number of samples. Therefore we decided to make a map from the section based on the scanned fields and mark the area of interest in the printed map of the polished section.
We stitched the scanned fields using the classified images layer and highlighted some of the large particles in the centre and corners of the section, which could be easily located under EPMA; based on these points, we marked a path to the target minerals in the printed maps. We could just skip hand picking step for saving time and money.
Now if you are interested in the EPMA results: Based on the EPMA analysis, it turned out that most of the analysed magnetites and Cr-Magnetites have up to 0.8% PtO (please see the table).
The recap for this post:
If the sampling and sample preparation are right, MLA and QEMSCAN are able to detect and characterise precious metals as low as ppb levels.
MLA and QEMSCAN can be great tools for skipping and substituting mineral concentrate and hand picking process; however user still need a software to export the coordinates in a way that can be usable by other tools such as EPMA or laser ablation; some companies have developed it but it is not a public access…
For a complete mineralogical studies, more than one method is needed; in this case as an example, the EPMA data revealed that Pt and Pd are present in the oxides as well
PS: Originally I had uploaded this post in another weblog, I just thought it wouldn't bother to drop here as well...
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