THE USE OF QEMSCAN FOR ORE CHARACTERIZATION D. Connelly Director/Principal Consulting Engineer Mineral Engineering Technical Services Pty Ltd (METS) PO BOX 3211, Perth, 6832, WA ([email protected]) ABSTRACT The QEMSCAN system is the third generation of automated mineral analysis systems that began with the QEM*SEM at CSIRO 30 years ago (Butcher et al. 2000). It is now a successful commercial instrument with 39 instruments in operation around the world. Back Scattered Electrons (BSE) and Energy Dispersive (EDS) X-Ray spectra are used to create digital mineral images with mineral identification occurring online. Low-count EDS spectra are used preferentially to BSE brightness, allowing minerals to be accurately identified by chemical composition. Individual minerals or groups of similar compositions are identified by comparison to a comprehensive mineral database incorporated into the QEMSCAN software. Optical mineralogy and Scanning Electron Microscopy is a metallurgists’ tool for determining the ore process mineralogy and delivering a better flowsheet. These techniques have limitations, provide limited information and sometimes fail to provide representative information. In contrast, QEMSCAN provides information on the nature and occurrence of all mineral species present plus the liberation characteristics, looking at thousands of particles in the sample presented. Some of the data presented includes: • Particle and grain sizes • Dominant mineral species • Particle shape • Calculated metal content • Density • Deportment of elements of interest between the mineral phases • Quantitative mineralogy • Mineral associations, locking and liberation • Textural relationships (particle images) It can also be used on concentrates, polished sections or drill core. This is a significant improvement in ore characterisation providing lithology, mineralogy, liberation, ore grade and porosity from a sample. Theoretical grade-recovery curves can be created to be mineralogically limited from QEMSCAN analysis which, when compared with metallurgical results, are valuable in evaluating whether liberation or chemical selectivity is driving flotation performance. A number of case study examples are given where the QEMSCAN had a significant bearing on future project development at an early stage of exploration and mine development. INTRODUCTION Qemscan is a mineral analysis solution based on work conducted by the Commonwealth Scientific and industrial research Organisation (CSIRO). It uses electron beam measurement and sophisticated software to analyse mineral samples. QEMSCAN® uses advanced e-beam technology from Carl Zeiss; combines this with high resolution Backscatter (BSE) and Secondary Electron (SE) imaging and state-of-the-art liquid Nitrogen-free Energy Dispersive Spectrometers (EDS); and then integrates these using QEM*SEM® electronic control systems and the unique iDiscover™ software suite to provide quantitative information on the mineralogy in seconds. MAGNETITE IRON ORE EXAMPLE QEMSCAN is automated mineralogy solution providing quantitative analysis of minerals. QEMSCAN is an abbreviation standing for Quantitative Evaluation of Minerals by SCANning electron microscopy and can identify most rock and ore-forming minerals. It has been used in this testwork programme to identify the minerals and gangue associations in the selected YGDD001 and YGDD002 diamond drill core. The QEMSCAN investigation was performed on two polished sections from the YGDD001 drill core and five polished sections from drill core. In both the samples, magnetite and quartz were detected as the most abundant minerals. For YGDD001, amphibole and carbonates were found to be minor gangue minerals. Amphibole and olivine/pyroxene were found to be minor gangue minerals in YGDD002 polished sections. Table 1 provides a quantitative summary. Magnetite was the main iron bearing mineral in both samples. The iron deportment to magnetite was reported to be greater than 90% in all polished samples and is seen in 2. The QEMSCAN showed that magnetite existed in two forms, coarse grained and finely disseminated within the quartz host. The presence of coarse size grain suggested that the samples may be upgraded by magnetic separation at a coarse size. The silicon was predominantly present as quartz in YGDD001 and YGDD002 polished sections. Amphibole was found to be a minor source of silicon across all samples, while olivine/pyroxene was found to also occur in the YGDD002 mineralogy. Amphibole minerals were seen as lamella structures within the magnetite host. High Pressure Grinding Rolls (HPGR) may provide an attractive comminution tool. This technique tends to generate breakage along the lamella grain boundaries, which will unlock the amphibole and other gangue minerals from the magnetite host. This is shown in Figure 2. Some of the deleterious minerals were observed to be finely disseminated within the quartz host. This indicated that fine grinding followed by cleaner magnetic separation would most likely be required for the rejection of the contaminants. See figure 3. A majority of the carbonate material is seen in YGDD001 at a coarse size and is closely associated with the amphibole material. The presence of carbonate material in the sample may contribute to the positive LOI values seen in the head assays. Phosphorus was found to be mainly present as apatite. Despite the high association of the phosphorus with the magnetite grain boundaries in YGDD001 samples, the overall amount of apatite detected in the sample was minimal. The head assay results also showed this close association of the iron and phosphorus. The phosphorus deportment can be seen in Figure 4. Pyrrhotite was the main source of sulfur in the YGDD002 polished sections while negligible pyrrhotite was detected in the YGDD001 samples as shown in 5. Pyrrhotite is a ferromagnetic iron sulfide mineral; thus is recoverable by magnetic separation. Due to this, some upgrading in the sulfur content would be expected during magnetic separation. Flotation may be needed as a polishing step for the sulfur rejection (depending on the sulfur level in the concentrate). Minor traces of copper and lead sulfides were the main source of detectable sulfur in YGDD001 which are not recovered through magnetic separation. Figure 1 - Mass fractions of minerals in all polished sections Figure 2 - Iron deportment for YGDD001 and YGDD002 QEMSCAN Figure 3 - Silicon deportment for YGDD001 and YGDD002 QEMSCAN Figure 4 - Phosphorus deportment for YGDD001 and YGDD002 QEMSCAN Figure 5 - Sulfur deportment for YGDD001 and YGDD002 QEMSCAN EPITHERMAL GOLD EXAMPLE The master composite, or low Ag grade sample, was submitted for QEMSCAN to analyse the mineralogy of the sample. Both the gravity tail and gravity concentrate were analysed during the QEMSCAN. The samples tested contained a high percentage of quartz, muscovite and alkali feldspars (73.9% total). Minor species identified included biotite / chlorite, carbonates and pyrite (10.6, 6.4 and 5.2% respectively). Argentite, a gold and silver based mineral, was also detected in very small amounts. High levels of quartz and feldspars are common gangue material encountered during processing. The QEMSCAN identified 40 grains of gold and silver in the concentrate and 15 in the tail. The high number of grains identified allows more confidence to be applied to the interpretation of the results to a broader sample base. Only two gold containing grains were detected in the gravity concentrate as electrum. The gold electrum detected was exposed, associated with quartz and pyrite. The silver was present as argentite, electrum as above and native silver. The liberated Au-Ag minerals identified in Table are exclusively native silver. The argentite is associated with several species including native silver, pyrite, quartz and minor sulphides and is either exposed or encapsulated. The following tables taken from the provide QEMSCAN report (full report is in Appendix B) indicate the liberation breakdown of the various Au-Ag minerals, as well as pyrite. The pyrite liberation breakdown is included as this species is a target for sulphide flotation. Table 1 - Summary of mineral abundance Table 2 - Au-Ag mineral locking Locking Area % Liberated (>90% by vol) 94.07 Exposed (Au-Ag at surface of particle) 4.28 Encapsulated (Au-Ag is occluded) 1.66 Following from the liberation breakdown of Au-Ag species a majority of the argentite is seen to be contained within the range of 0< x <90 liberated ( Table 3 - Argentite liberation The native silver is contained within the 60< x = 100 range as shown in Table, with an additional 21.3% being located in the lowest 0< x <10 class. The detected major gold silver species are seen to be mostly liberated. This indicates that the leaching performance, at P80 106 micron as used in the QEMSCAN analysis, will provide good extractions. Table 4 - Native silver liberation The liberation of pyrite, Table 5, shows that the pyrite is mostly contained in the top two liberation classes in the gravity concentrate. Including the tail and viewing the combined breakdown it can be seen that a majority of the pyrite in the system is above 60% liberated. Flotation of this pyrite would allow for a clean concentrate with little pyrite being heavily encased in other species. Table 5 - Pyrite liberation LEAD ZINC EXAMPLE The analytical method used in the QEMSCAN was a Particle Mineralogical Analysis (PMA), which involved scanning the surface and analysing particles at random locations of the sample. The