Finding Gold in Ore: Micro-XRF and EDS on One SEM

Australia is one of the world’s largest gold producers, and much of the work behind each new discovery happens long before a mine is built: logging drill core, deciding which intervals to assay, and working out how the gold sits in the rock. That last question matters as much as the grade. Gold locked inside sulphides, or sitting next to arsenic, needs different processing from free gold, and the answer changes the economics of a deposit.

An SEM with energy-dispersive X-ray spectroscopy (EDS) is the standard tool for mineral chemistry at the micrometre scale. It has two blind spots for gold exploration, though. It looks at small areas, and its detection limit (typically around 0.1 wt% for most elements) is far above the parts-per-million levels at which gold is worth finding. Adding a micro-XRF source to the same SEM closes both gaps.

What micro-XRF adds to an SEM

Micro-XRF excites the sample with a focused X-ray beam instead of electrons. On an SEM, the X-ray source fits to a spare chamber port, and the existing EDS detector collects the fluorescence. Because X-rays produce very little background compared with an electron beam, trace elements stand out. Bruker quotes detection limits around 10 ppm for its QUANTAX Micro-XRF system with the XTrace 2 source.

Three other differences matter for geological samples:

• Higher-energy lines. The 50 kV source excites K-lines that a 20–30 kV electron beam cannot reach. That gives a second, cleaner line for heavy elements when the usual lines overlap.
• Large areas. With a fast stage, a whole polished slab or thin section, several centimetres across, can be mapped in one run.
• Little preparation. Non-conductive rock does not need a carbon coat, and a flat cut face is often enough.

A worked example: epithermal gold

Bruker’s application note on an Au-bearing epithermal sample from the Karangahake gold mine in New Zealand shows how the two techniques work together on one instrument.

The first step was a micro-XRF map of roughly 45 × 45 mm, covering the whole sample. It located gold-bearing zones across the slab, along with the base-metal sulphides around them. Here the high-energy lines proved their worth: the main gold line (Au Lα) overlaps a zinc line (Zn Kβ), which is a real problem in a sphalerite-rich vein. The software separated the overlapping peaks, and the gold Lβ line, which is clear of zinc, confirmed that the gold signal was real.

With the gold-bearing areas found, the analyst switched to the electron beam on the same stage and mapped those spots at high resolution with EDS. The EDS maps showed where the gold sat relative to galena, sphalerite, pyrite and chalcopyrite, and that the larger gold grains carry silver, as electrum. That is exactly the information a metallurgist needs to predict recovery.

The point is the workflow, not either technique alone: centimetre-scale screening with micro-XRF, then micrometre-scale mineralogy with EDS, without moving the sample to another instrument or re-registering coordinates.

Penalty elements and trace metals

Grade is only half the story in ore characterisation. Elements such as arsenic, mercury, cadmium and bismuth can attract smelter penalties or call for extra processing. A second Bruker note, on identifying trace elements in ore, mapped a cobalt-nickel ore and a lead-zinc ore from Saxony in Germany. The samples were simply cut flat, with no polishing or coating.

Micro-XRF picked out silver, copper, nickel and cobalt in one sample, alongside arsenic and mercury. In the lead-zinc sample it showed iron, cadmium and phosphorus concentrated in distinct bands within the sphalerite. The silver K-line, at around 22 keV, was clearly visible, well beyond what an electron beam normally excites. Mapping where those elements sit, and in which mineral, helps decide how an ore can be processed and what the concentrate will carry with it.

Where this fits in an Australian lab

For a university geology department, a geological survey or a mining services laboratory that already runs an SEM with EDS, micro-XRF is an addition rather than a new instrument. The same detector, software and stage are used for both, so analysts work in one familiar environment. Typical uses include:

• screening drill-core slabs and thin sections for gold, platinum-group elements and other trace metals before detailed work;
• finding rare grains (gold, tellurides, rare-earth minerals) across a whole section instead of hunting field by field;
• mapping penalty elements in concentrates and tailings;
• checking zoning in pyrite and other host minerals.

For finer phase identification once the targets are found, high-resolution EDS with Bruker’s QUANTAX EDS for SEM completes the picture. Where overlaps are severe, wavelength-dispersive spectroscopy (WDS) is another option.

If you are looking at micro-XRF for an existing SEM, we can help check compatibility with your chamber and ports and talk through the samples you work with. Contact us to start the conversation.

Application examples summarised from Bruker’s published notes, linked above. Figures are reproduced as Bruker published them, with their credits.