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Volume rendering of an arsenopyrite crystal (blue), showing pyrite (yellow) and rutile (red) inclusions. The image, generated through X-ray nanotomography, provides a top-down view of the arsenopyrite crystal from the Suurikuusikko gold deposit in Finnish Lapland. GTK’s Research Professor Ferenc Molnár leads the MinSysPro project.

The importance of setting

Text: Greg Moore

Backed by a mineral system approach, lasers, electron microscopes, mass spectrometers, nanotomography, neural networks and big data computing, geologists are exploring geological settings in surprising new ways. Geofoorumi talked with Research Professor Ferenc Molnár, structural geologist Muhammad Sayab and Research Professor Vesa Nykänen about the Mineral Systems and Prospectivity Mapping (MinSysPro) project.

Understanding critical processes

The MinSysPro research consortium, including GTK, the University of Oulu, private industry partners and international scientists now aims at exploration within the geological setting to predict under-cover mineral resources in Finnish Lapland through application of novel research techniques and computer-based mineral prospectivity mapping based on mineral system models. The MinSysPro project is supported by the Academy of Finland’s Mineral Resources and Material Substitution Programme.

Ferenc Molnár, who leads the MinSysPro project, encountered the “holistic” mineral systems approach in the 1990s while working in Canada.

– Back then it was more of an intuition as to how we might improve our exploration approach, but as the power of analytical tools grew, geologists began to make huge strides in evaluation of ore exploration areas through improved knowledge of behavior of metals and their transporting media in large-scale geological systems. Today, we don’t jump to an analogous deposit until we have systematically examined the context of ore formation broadly to identify the processes critical in its creation. We convert this understanding into mappable parameters to establish vectors useful for fi nding ore deposits, and more specifi cally, to establish the probability of fi nding related ore deposits.

Finland offers ideal conditions for developing the mineral systems approach. GTK’s excellent database of discovered deposits is suitable for combining with mineral prospectivity mapping and probability calculations, notes Molnár,

– We validate and calibrate our approach by seeing if our models can ‘discover’ or reveal known deposits. If they also predict unknown deposits, well, even better.

Through selection of relevant data based on an improved understanding of the geological evolution processes involved in ore formation, researchers not only characterize the probability of an occurrence, but identify the critical parameters and incorporate them into mineral prospectivity mapping.

Sulfur surprise

Most of us associate sulfur with volcanoes, acid rain or sewer smells, but a quick look at the periodic table of the elements indicates why it is so important in ore exploration. Sulfur, because of its versatile atomic structure, seems to be a critical factor in formation of many, but certainly not all, ore deposits. It binds readily, for example, with copper, nickel and iron to form commercially important sulfide ores.

– The capacity for sulfidic ore formation in magmatic processes requires understanding the source of the parent melt such as a mantle plume or some melting process in the lower crust. But you also need sulfur, lots of sulfur. The intruding magma is not necessarily rich in sulfur and must encounter sulfur-rich rocks during its emplacement. Sulfur is important both in making valuable minerals and transporting certain metals. Under appropriate conditions, gold is transported by sulfur-bearing complexes in hydrothermal solutions.

Sulfur, the ninth most common element in the universe, forms deep in large stars and groups with the building blocks of life: hydrogen, carbon, oxygen, nitrogen, and phosphorous. Indeed, when life was starting out in the time before our oxygen atmosphere, bacteria relied on sulfate-reduction to “breathe.” The formation of Finland’s Paleoproterozoic basins, which took about 400 million years, coincided with an explosion in bacterial life on earth. These sediments are rich in sulfides from microbial activity that concentrated metals such as nickel, cobalt, gold, copper, molybdenum and vanadium, says Molnár,

– One aspect of our interest in sulfur, which has several stable isotope forms, are the specific isotopic signatures of these systems that allow us to date mobilizing events. The sea bottom where these metal-bearing sediments collected may have been anoxic for millions of years, we then see a sudden high temperature gradient and mobilization of hydrocarbons. Finland’s black schist deposits, which were great for metal deposit formation, display this feature.

Synchrotron time

GTK micro-structural geologist, Senior Scientist Muhammad Sayab notes that geologists are enlisting powerful non-traditional tools in their work.

– Microscale studies allow a closeup examination of the transportation channels in the rock, mineral texture and composition. Nature has fractal properties. What we see at microscale manifests most of the time at the macroscale.

The revolution in microtomography, the 3D imaging of tiny objects, is leading developments in microstructural geology.

– Traditionally, structural geologists relied on cutting thin slices to see inside the rock. Thin-section preparation is both time-consuming and destroys sample integrity. Microtomography is a non-destructive technique and that gives us the inside story of the rock sample without cutting it! Last year we performed our first ultra-high-resolution X-ray nanotomography experiment at the European Synchrotron Radiation Facility in Grenoble, France, reports Sayab.

Microscale understanding can be easily connected to larger structures, the fabric of the rock, the distribution of metals and minerals in the rock. It also gives valuable clues about the channels of mineral formation.

– We can today take a single mineral grain and collect tremendous amounts of information, including compositional data, trace elements, isotopic data, and 3D mineral quantification down to the nanoscale to determine shape, volume and orientation of individual minerals.

With limited availability of synchrotron beam time, competition is intense and only peer-reviewed projects get approved.

– When we applied for European Synchrotron beam time, we turned out to be rather odd birds. You don’t see too many geologists using synchrotrons! We thought we had rather mundane samples, some arsenopyrite grains from the well-studied Suurikuusikko deposit in northern Finland, site of Europe’s biggest gold mine operated by Agnico-Eagle Finland Oy. We had already scanned the prepared drill core samples to establish the size, shape, spatial distribution and geometrical orientation of the
mineralsin situ before we removed the crystals for the nanotomography scans.

– The 3D nanotomography showed the distribution of gold grains and other mineral inclusions within the crystals. The gold was not free, but locked and oriented in the arsenopyrite crystal. The microstructure was astounding! You could easily see a rule of distribution at ten micrometers that correlated back up to the visual level of around one centimeter.

The experiment also showed the effects of geological processes in microscopic fractures within the crystal. However, as the Synchrotron beam could not capture the antimony-nickel-cobalt inclusions, the sample was further examined at GTK with a laser ablation inductively coupled mass spectrometer (LA-ICP-MS), which enables highly sensitive elemental and isotopic analysis to be performed directly on solid samples. Summarizes Sayab,

– Our 3D micro- and nanoscale analysis provided insights into the microstructural processes affecting gold formation. It comports well with our broader goal of obtaining information useful exploration, and provides a new multi-modal approach for ore geologists in analyzing ore textures through the combining of micro- and nanotomography with the trace element geochemistry.

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3D distribution of gold grains and mineral inclusions in three grains of arsenopyrite from the Suurikuusikko deposit imaged with Synchrotron radiation-based nanotomography carried out at the European Synchrotron Radiation Facility, Grenoble, France:

A: Rutile inclusions rendered in red and pyrite inclusions in yellow.

B: Individual minerals and crystal rendered in different colors for better visibility and contrast in 3D. Gold is rendered in vivid yellow, rutile in red, what is probably chromium oxide (CrO) in green, and pyrite in brown.

C: The gold grain is rendered yellow and rutile in red. The preferential alignment of rutile inclusions within the crystal is quite clear here.

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1. Analysis of the compositional zoning in pyrite, a technique for reconstructing the ore-forming process, is applied here to a pyrite sample from the Suurikuusikko deposit. A: False-color scanning electron microscope image of zoning in arsenic distribution (warmer = more As). B: Zoned pyrite showing outer zone lacks gold and subtle changes in S-isotope composition that suggest the Au-bearing and Au-absent fluids have different origins. The Au-absent rim of pyrite was formed during the superimposing deformation of the gold-bearing pyrite. Microscale analyses were performed with laser ablation ICP-MS technique.

2. Mappable criteria selected according to the mineral system model are integrated into a prospectivity map with known deposits (circles) to highlight areas favorable for orogenic gold deposits in Central Lapland.

Targeting the work

Prospectivity mapping is the final piece of the ore discovery puzzle. With the relevant data based on the mineral system model established, the patterns found in the data are imposed on the data, which may come from physical geology, geochemistry, or airborne and ground geophysics measurements. The results are integrated with the model and subject to weights of evidence, fuzzy logic, neural networks and logistic regression. The result is a prospectivity map that can be tested against known deposits.

Vesa Nykänen, who heads up MinSysPro mineral prospectivity mapping projects, says,

– Once our data are integrated with the model to provide a basis for the prospectivity mapping workflow, we move to the crucial step of validation. Here, we first perform statistical tests on what we think we know about mineral deposits to see how well the prospectivity model classifies known deposits or occurrences. The ultimate test, of course, is to drill predicted targets not known earlier. Exploration costs can skyrocket at this point. But if we have done the science right and we have asked the right questions, we should find new deposits.