Fathom Geophysics Newsletter 2

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Analytical Methods News: Industry fields gadgets influx

DEVELOPERS of quantitative analytical methods have been working overtime to bring improved processes and gadgets to the extractive industries market. Here is a perusal of a few of them.

Gamma-ray drillcore density corrections improve

IMPROVED precision and accuracy of a hands-off drillcore density measurement method may help explorers achieve greater throughputs for their drillcore geotechnical work. [1]

The method, called gamma-ray attenuation, is non-destructive and faster than the conventional method that involves taking the weight of the drill core in air and in water.

It involves aiming a narrow gamma-ray beam through one side of then drillcore, and then detecting and analyzing the beam emerging from the other side.

By determining the gamma count rate of the emerging beam, and knowing the drillcore's diameter, the rock's density can be figured out.

Calibrations and corrections are required to do this reliably, though.

Based on work involving material — including sulfide-rich rocks — from the Matagami and Chibougamau districts of the Abitibi Greenstone Belt, Quebec-based researchers came up with an improved correction for whole NQ-sized drillcores with densities of up to about 5 grams per cubic centimeter.

This made corrections more widely available in mineral exploration applications, the researchers said.

"[B]ecause our denser samples were sulfide-rich but not heavily mineralized, it would be important in the future to do further testing on rocks containing several percent copper, zinc, lead or nickel to see if the same [calibration] relationship holds," they noted in their paper.

"However, uncut cores with high metal grades are difficult to find."

They also came up with an improved correction for whole BQ-sized drillcores. For that, they used material from the Lemoine area of Chibougamau.

And they improved the accuracy of empirically correcting gamma-ray density analyses done on partial core.

"Drill cores are commonly cut approximately in half by exploration companies, using a diamond blade, for assay purposes, so that in many core boxes, only cut cores are left," they said.

"The missing material needs to be taken into account when calculating the gamma density."

They said that the higher precision and better accuracy that came with improved corrections were particularly useful when the rocks being analyzed had fairly similar densities.

"[I]f gamma densities are ever to be used for mineral resource calculations, corrections such as those developed here will be essential."

New rapid gold-analysis facility in Oz by late 2017: Researchers

RESEARCHERS based in Australia have developed what they're calling the bromine reference foil method to improve gamma activation analysis as it's used to determine gold concentration in ore samples — and say they've been working to open a scaled-up high-throughput gold-analysis shop of their own. [2]

"We are currently designing a gamma activation analysis system to provide commercial gold analysis for the Australian mining industry," the researchers said in their paper.

"[I]t is anticipated that this facility will be operational by late 2017."

They said they anticipated being able to analyze 80 samples per hour at their planned optimized facility.

They said the projected 3-sigma detection limit was 30-35 parts per billion gold. The one-standard-deviation analytical accuracy was 3% at 1 part per million gold, and was 2% at 3 parts per million gold. At greater gold concentrations, accuracy was expected to be in the range of 1% to 2%, they said.

Gamma activation analysis involves directing high-energy photons at a gold-bearing sample. When a photon imparts its energy to a gold atom's nucleus, the atom is said to become activated. Gold atoms can undergo any one of a handful of different nuclear energy-level reconfigurations.

Activated atoms' nuclei are unstable. Upon radioactive decay, an activated gold atom emits a tiny packet of gamma radiation of a distinctive, measurable energy. The researchers selected the photonuclear reaction with a product isotope half-life of 7.73 seconds, and a gamma-ray emission signature of 279 keV.

As part of their efforts to make significant improvements in the method, the researchers needed a way to be able to accurately gauge fluctuations in the incident high-energy photon beam in real time.

They knew that a related analytical method (neutron activation analysis) had been successfully using for decades the technique of simultaneously irradiating both a sample and a reference material.

They chose bromine as their reference material. Bromine atoms had activation behavior and half-life that were similar to those for gold. Bromine's gamma emissions wouldn't muddy the picture for gamma emissions from gold atoms. And bromine wouldn't commonly be present at significant levels in the gold-bearing samples to be analyzed.

In practice, the researchers said, the incident beam stabilization approach was carried out by attaching the bromine-containing reference material to the outer surface of the gold ore-containing sample jar. The reference sample stayed in place during irradiation and measurement.

The currently most widely used commercial gold-analysis technique — fire assay — was a complex chemical process, the researchers said.

"The method is time consuming, highly labor intensive and requires very careful quality control procedures to maintain accuracy."

It involved carefully pulverizing an ore sample without losses, fusing a small representative portion of it in a very hot furnace with the help of lead oxide flux and other reagents, then remelting the resulting lead-gold alloy button to draw off the lead and leave behind a small ball of solid gold, which is then dissolved in an agressive acid solution in a standarized manner. This solution is analyzed for gold concentration with a carefully maintained atomic absorption spectroscope.

In situ Rb-Sr age dating now feasible

EXTREMELY localized single-spot determinations for the ages of thin-section and grain-mounted minerals are achievable now that rubidium-strontium age dating has been combined with laser-ablation ICPMS.

The Sweden-based researchers recently presented the first in situ geochronolgical results for this setup they've been developing. They looked at samples from the Laxemar and Aspo areas of the Transscandinavian Igneous Belt. [3]

"The results of this study demonstrate that the novel in situ rubidium-strontium technique can be used to date relict fluid systems with temperatures ranging from hot magmatic liquids and mineralized hydrothermal solutions to relatively low-temperature precipitation in fractures and shear zones," they said in their paper.

"The method made it possible to link greisen mineralization and far-field hydrothermal activity to granitic magmatism, in addition to detection of fracture reactivation in a mylonite shear zone and a validated Paleozoic timing of thin brine-precipitated veins."

Constraining the timing of ore bodies and related veins distributed throughout surrounding rocks helps explorers understand ore-forming mechanisms and relationships among different generations of mineralized veins, they said.

The use of laser-ablation ICPMS age-dating permits explorers to analyze minerals that would otherwise be off-limits to conventional modes of rubidium-strontium age-dating because of the latter's inability to avoid inclusions, to cope with very narrow veins, and to avoid 'smearing' of the age-date signal due to compositional changes, such as those caused by fine-scale zonations in minerals.

To establish their method, the researchers had to solve a major headache associated with mass spectrometry, which was that it was initially incapable of distinguishing between rubidium and strontium, because of their identical isotopic masses (of 87).

Their solution was to include a reaction cell in between two quadrupoles that were downstream of the laser-ablation equipment. When the cell is filled with a gaseous reagent, some strontium ions will react with it to produce a charged compound that can be discriminated easily by the second-stage quadrupole.

Rubidium ions, on the other hand, won't react with the gas because of their noble gas-like electron configurations. [4]

Reaction gas and other setup optimizations were still in progress, the researchers said.

They said that coming up with matrix-matched standard reference materials whose concentration data were accurately known would also help ensure future users of the method would be able to obtain high-quality analytical data.

Mineral maps realizable with laser-induced breakdown spectroscopy

Mapping of mineral distributions, grain sizes and other rock-texture information, such as the distribution of gangue minerals, can be reliably gleaned from drillcore samples using laser-induced breakdown spectroscopy, according to a Finland-based group of researchers. [5]

They looked at drillcore from the yttrium-rich Norra Karr rare earth element deposit in southern Sweden.

Many mining and beneficiation processes would also benefit from this sort of analysis, the researchers said.

Laser-induced breakdown spectroscopy involves creating a plasma out of the material to be studied. The precise chemical makeup of the plasma is then measured by analyzing emission line spectra. [6]

The researchers used singular value decomposition of spectroscopic data to map mineral distributions from samples they looked at. Singular value decomposition is analogous to the eigenvalue decomposition of a data-containing matrix, without requiring the matrix to be symmetric. [7]

Mineral mapping via singular value decomposition classification performed well, they said. The results allowed them to successfully estimate the mineral percentages of ore minerals versus gangue minerals in their samples.

They said their mineral-mapping method even coped with a finer-grained sample in which it was difficult for a human to visually recognize the location of ore minerals dispersed across the specimen's face.

The researchers said it would be possible to use the technique on drillcore in a continuous scanning fashion, as long as the spectral range was carefully selected — otherwise accumulating data volumes would become unmanageable.

References

[1] P.-S. Ross and A. Bourke (January 2017) "High-resolution gamma ray attenuation density measurements on mining exploration drill cores, including cut cores", Journal of Applied Geophysics, 136, 262-268.

[2] J. Tickner, B. Ganly, B. Lovric and J. O'Dwyer (April 2017) "Improving the sensitivity and accuracy of gamma activation analysis for the rapid determination of gold in mineral ores", Applied Radiation and Isotopes, 122, 28-36.

[3] M. Tillberg, H. Drake, T. Zack, J. Hogmalm and M. Astrom (2017) "In situ Rb-Sr dating of fine-grained vein mineralizations using LA-ICP-MS", Procedia Earth and Planetary Science, 15th Water-Rock Interaction International Symposium, 17, 464-467.

[4] T. Zack and K.J. Hogmalm (2016) "Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell", Chemical Geology, 437, 120-133.

[5] S. Romppanen, H. Hakkanen and S. Kaski (August 2017) "Singular value decomposition approach to the yttrium occurrence in mineral maps of rare earth element ores using laser-induced breakdown spectroscopy", Spectrochimica Acta Part B, 134, 69-74.

[6] R.S. Harmon, R.E. Russo and R.R. Hark (2013) "Applications of laser-induced breakdown spectroscopy for geochemical and environmental analysis: A comprehensive review", Spectrochimica Acta Part B, 87, 11-26.

[7] I.T. Joliffe (2002) "Principal Component Analysis", 2nd edition, Springer, 487 pages.

About Fathom Geophysics

In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.

Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.

We offer automated data processing, automated exploration targeting, and the ability to tailor-make data processing applications. Our automated processing is augmented by expert geoscience knowledge drawn from in-house staff and from details relayed to us by the project client. We also offer standard geophysical data filtering, manual geological interpretations, and a range of other exploration campaign-related services, such as arranging surveys and looking after survey-data quality control.