Matters of scale

31 July 2017 8 min read

Article from resourceful: Issue 12

FEATURE

So much of geoscience is about concentrating on a "global" all-encompassing picture. There are stories about breakthroughs in airborne electromagnetics, the coordination of large-scale state geoscientific maps and new stratigraphic drilling methods that improve our ability to look deeper into the architecture of the Earth.

Over the past few years Australian geoscience has been able to delve deeper into the world of nano- and micro-characterisation (or analysis) of mineral ore, as the technology around it has advanced. This analytical data provides insight into the chemical composition of orebodies and how it relates to the geological structure, texture and mineralogy of the rock.

It is an area with the potential to disrupt some previously held truths on geological formations, while benefiting industry with more informed decision making on whether to mine or not, as well as information that can make recovery and processing more efficient and productive.

Australian Resource Characterisation Facility expansion

At the very cutting edge of this science is the Australian Resource Characterisation Facility (ARCF), the infrastructure component of the National Resource Sciences Precinct headquartered in Perth, Western Australia. It has been funded by the Science and Industry Endowment Fund (SIEF) to the tune of $12.4 million over five years. The total project investment including partner contributions is $38 million.

Supported by its three founding partners CSIRO, Curtin University and the University of Western Australia (UWA), the ARCF acquired three advanced characterisation technologies, each separately managed and run by the partner institutes. In their own respective ways, each piece forms a key part of the workflow to analyse drill cores, rock and mineral specimens and was selected to address gaps in the sample scales used for data analysis. With this new offering, scientists can map samples from lengths of metres down to the atomic scale with unprecedented precision.

Multi-scale mineralogical analysis

Analysing samples at the largest scale is the CSIRO-developed Maia Mapper. With the world-first prototype now up and running, it can produce a detailed picture of a drill core of up to half a metre, looking at the sample's texture and chemical composition using an intense, focused x-ray beam.

A thousand times further down the scale, is the University of Western Australia's NanoSIMS (secondary ion mass spectrometer) for sub-micron scale imaging. It works by projecting a highly intense ion beam onto a sample less than the thickness of a human hair, releasing secondary ions that are carried into a mass spectrometer. A magnetic field is then able to sort the ions by their atomic mass to create a chemical profile of the sample.

Zooming in another thousand times smaller is Curtin University's Geoscience Atom Probe, the first in the world to be focused on geoscience applications. This technology uses high electric fields and rapid laser bursts to "evaporate" individual atoms from a tiny sample of rock. The time it takes for a particular atom to move from specimen to position-sensitive detector, indicates the type of atom present – and the order they hit the detector – allows an accurate 3D geochemical model of atom positions to be constructed.

The three partner organisations take a collaborative approach to sharing the equipment and building greater intelligence.

"Everything is available to everyone," Dr Louise Fisher, CSIRO's head of mineral characterisation research, says.

"Researchers at each institution can request access to the instruments at any time. We meet quarterly to discuss issues and results which are important to all of us. We want students to be able to come in and be part of the entire process."

New drill core laboratory

In a bid to be at the forefront of mineral characterisation and to fully understand the context of the samples, the ARCF is undergoing a further technological expansion to create a new drill core laboratory.

The expansion so far has seen CSIRO sign a research agreement with Swedish company Minalyse, to bring its line scanning x-ray fluorescence scanner into the picture, providing geologists with real-time measurements of element concentrations in drill core. The Minalyse XRF line scanning system allows data to be collected down the middle of the entire core sample, offering the chemistry of the core in increments.

The Hylogger infrared technology, initially developed by CSIRO and now licensed to mineral services company Corescan, is another key piece of equipment to form the new laboratory. It provides an objective, semi-quantified means of identifying minerals using infrared spectroscopy.

Corescan managing director, Neil Goodey, says the latest version of HyLogger is now able to detect a much wider range of wavelengths. It can now detect "more species" of minerals, such as quartz and feldspar.

"What we often see are changes in the chemistry of rocks that would be impossible for geologists to see with the core sample in hand," Mr Goodey says.

"Suddenly we will see mineral changes that are a proxy for giving you a distance to the hot part of the deposit. It will tell us whether the actual deposits are under pressure and their formation temperature. We can often get to the guts of an event."

Dr Fisher says the technology will provide "greater context" about what samples are selected for more detailed analysis by the ARCF – creating a more complete workflow for data collection and representative sampling at different scales.

Understanding ore formation

The work is challenging and Dr Fisher says that getting an elemental and contextual understanding of rock formation at a smaller scale often changes the assumptions made about how rocks are formed or what processes have occurred. If the ARCF can support more informed interpretation of regional geochemical datasets, companies will be enticed.

"It will help to weigh the relative importance of exploration targets and will also add a lot of value for those processing refractory ores," Dr Fisher says.

This message is echoed by Matt Kilburn UWA Associate Professor of the ion probe facility. What you see in the rock in hand isn’t necessarily the case when you get to the micro-scale, he says.

"Even the most skilled geologists can only see so much. You might identify that you have gold in a sample, but you might not be sure what mineral it’s in or what it's associated with," A/Prof Kilburn says.

The university's ARCF-funded NanoSIMS is proving to be one of the best available means to detect lithium, which is essential to the growing battery industry. The instrument can see what mineral the lithium is locked up in and what other elements are present.

Curtin University's atom probe is the final piece in the ARCF collaboration. Facility manager, Dr David Saxey, agrees that even at the atomic scale, larger "macro" puzzles can be solved. The atom probe's analysis of gold ores in West Africa has influenced the understanding of the formation of the ores and how best to process them.

Science leader for the geoscience atom probe, Professor Steve Reddy, emphasises the versatility of the technology in geoscience.

"As well as looking at ore deposits, we've been able to look at the composition of refractory metal nuggets in meteorites," Prof Reddy says.

"These are only a few 10s of nanometres in diameter and the atom probe data show that they must have rapidly migrated around the protoplanetary disk before they were trapped in the meteorite. Such results have major implications for the formation and migration of the first materials in the early solar system."

Greater resolution will lead to old assumptions being questioned and new opportunities to use natural variability to advantage.

"You may have cutting edge instruments but you still have to sell the concept to various communities," A/Prof Kilburn says.

Prof Reddy realises that potentially interested third parties need proof of value before they jump on the bandwagon.

"First we have to develop a track record and publish some high quality science that highlights the potential applications of these techniques.

"Our role is to develop the scientific potential and then convince industry that such analyses are a worthwhile investment," Prof Reddy says.

World-class capability for Australian industry and the research sector

They are already taking on clients from both the research and industry sectors, and agree that as the science improves and the mining and extraction industries comes to accept the processes, samples could easily move through all three technologies from the large-scale Maia mapping, down to the atomic probe.

Dr Fisher believes it will be intrinsic to solving some of the bigger geoscience problems, including the study of fluid-rock reactions and the geological processes which are driving metal deposition.

The end goal is having a full geological understanding of rock systems. We want to increase the base knowledge for industry and provide workflows which they can adopt.

—Dr Louise Fisher

In the interim, Dr Fisher says the ARCF has to build an infrastructure that will get maximum value from data collection that can then be fed back into "a bigger picture".

"We already have individual projects where we have demonstrated that multi-scale approach and have been able to wrap data analytics and machine learning around it. Demonstrating that kind of capability and applying it to industry requirements is what will drive uptake," she says.

Dr Fisher wants to see a migration of drill core logging and subsequent sample selection go from being a largely subjective skill, once determined by the keenness of a geologist’s eyes, to following more objective workflows, ruled by more scientific processes and underpinned by robust data analysis.