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Combining geophysics and geochemistry with better quality control overcomes challenges in the field

Using organic chemistry to find orebodies

Though geochemistry has traditionally used the properties of inorganic chemicals to detect mineralization, exploration managers are increasingly embracing techniques that rely on organic chemistry as the search goes deeper.  Soil Gas Hydrocarbon (SGH) geochemistry is a prime example of the latter whose time has come, says Dale Sutherland of Actlabs.

SGH is an extractive procedure that releases organic (hydrocarbon) compounds metabolized by bacteria and adsorbed onto surficial sample particles. By identifying the particular hydrocarbon mixture, explorers can differentiate the type of metal deposit that lies below. 

Take Golden Band Resource’s Golden Heart property in the central metavolcanic belt of northern Saskatchewan. The area is underlain by deformed mafic volcanics, gabbros and debris flows intruded by granodiorite/diorite bodies and has been the site of several waves of exploration culminating in three areas of known gold mineralization.

Covered in moss and muskeg and blanketed by a layer of glacial till, the project’s rolling terrain is a field geologist’s nightmare. So instead of slogging it out with hand-held auger drills and other sampling tools needed to penetrate the overburden, Golden Band measured the hydrocarbons in near-surface samples, receiving Actlabs’ highest possible SGH rating in one location. Actlabs plotted the results using Geosoft’s geochemistry extension for Oasis Montaj to highlight anomalous areas.

Golden Band drilled on the hottest spot and intersected a 30-to-80-metre-wide vein of visible gold at 290 metres below the surface. Ongoing soil and bulk till sampling suggests the presence of numerous anomalies related to this new zone and other gold zones on the property.

Typical hand auger work in the James Bay Lowlands. Between 1999 and 2004, work crews  retrieved a few thousand samples using hand auger. A lot of this work was done during winter months. Photo by Randall Salo.

Golden Band Resources SGH anomaly at TKN Zone.

By Virginia Heffernan

Geochemical tools that can probe a little deeper are becoming increasingly important complements to geophysics in the search for buried ore deposits, but quality control remains the discipline’s biggest bugaboo.

“Exploration managers recognize that this is the way to go in future because just about everything that can be found by walking through the bush has been found already,” says Dale Sutherland, Director of Research Activation Laboratories Ltd. (Actlabs). “Now everything we discover is buried.”

The demand for new and improved techniques to help find the next generation of ore deposits has not escaped the attention of collaborative research groups such as the Canadian Mining Industry Research Organization (CAMIRO). Several of the projects sponsored by CAMIRO’s consortium of private companies are focused on deep exploration, including a 3-phase project investigating soil gas geochemistry and another multi-phase project looking at deep-penetrating geochemistry.

Still, quality control remains the largest barrier to the use of geochemistry in mineral exploration. Many companies are hesitant to rely on geochemical results because there is so much room for error in the sampling and analysis stages, and  again, when reporting and presenting results.

“Geochemistry can be very useful, but only if you rigorously follow procedures,” says Tom Lane, director of research development for CAMIRO’s exploration division. “You have to be careful about variability in the lab as well as in the field.”

These challenges are never far from the mind of geochemist Dermot Smyth, Northern Ireland Project Manager for Lonmin Plc, as he sorts through reams of public and private data from a vast, boggy region of Northern Ireland.

His job is to find PGE-bearing deposits associated with the Antrim basalts that underlie the northeast part of the country. As the world’s third-largest PGE producer, Lonmin was drawn to the area after the Geological Survey of Northern Ireland (GSNI) released results of the Tellus project, a comprehensive geophysical and geochemical survey over 14,000 sq. km of the country.

Completed in 2007, the Tellus project collected high-resolution airborne magnetic, electromagnetic and radiometric data as well as trace element datasets for soil, sediment and stream water samples. The airborne survey delineated new structures within the Antrim Lava Group, and the ground geochemical surveys identified elevated values of platinum group elements and base metals in the overlying soils and stream sediments.

When the results were released, the area of land licensed for prospecting in Northern Ireland increased from 15% to 70%. Lonmin secured nine separate licenses covering the Antrim basalts, with an additional license over a gravity anomaly in the southwest corner of Northern Ireland for a total of about 2,250 sq. km of prospective ground.

Now Smyth faces the daunting task of narrowing down this area enough to establish drill targets, by integrating the Tellus geochemical and geophysical datasets with incoming data from Lonmin’s own exploration program. He has at least one competitive advantage, though: he was geochemistry manager on the Tellus project before joining Lonmin in March 2008.

His first pass re-investigated the Tellus geochemical datasets to define discrete areas for high density soil sampling. Next, Smyth combined the Tellus geochemical and geophysical data to investigate source-anomaly relationships. Now he and his colleagues are inputting Lonmin’s own datasets into the exploration model to provide better direction for further exploration.

So far, Lonmin has completed a Full Tensor Gravity Gradiometry (Air-FTG®) survey over 3,579 line km and collected some 2,000 soil samples as part of an ongoing geochemical sampling program.

“We have licensed the Tellus datasets and have spent a lot of time working with them and finding the anomalies within them,” he says. “There are many lifetimes of interpretation in those datasets, particularly when they are combined with the datasets that we have collected as part of our exploration strategy.”

Though he was initially doing most of the data plotting and analysis himself, Smyth now shares the datasets with the junior geologists in his office, who can contribute to the gridding using Geosoft’s Geochemistry for ArcGIS extension.

“Now that we have the geochemical module, the junior geologists are involved in making preliminary maps to confirm that we are working in the right area and are on the right path in terms of our exploration strategy,” says Smyth. “At the moment, we are plotting up data to produce grids; then we add the topography and geology to produce maps that scale from 1:250,000 to 1:10,000. Part of the ethos of the office and Lonmin has been to train the junior geologists in new exploration techniques. This includes both in-the-field sampling methods and office tasks such as target generation.”

He is less prepared to abdicate the intricate task of quality control, even though Geosoft’s module comes with built-in QC standards.

He says the QC feature is useful for geoscientists who don’t have in-depth knowledge of geochemistry because it allows quick plotting of duplicates and replicates and produces plots of standards, showing how they vary over time.

“The Geosoft module is great for quickly evaluating if the laboratory data is within set limits, and by having quality control built into the module, it becomes a standard procedure,” he says. “But as a geochemist, I need more detail and more control over the integrity of the data.”

Smyth brought to Lonmin his own set of exploration standards that he developed as part of his PhD in exploration geochemistry and then refined during his time as geochemistry manager for the Geological Survey of Northern Ireland. The standards include methods of sampling, systems of recording samples, systems of quality control, and best practices for exploration.

The thick layer of glacial till and peat in Northern Ireland, for instance, creates challenges at the sampling stage. Boulder layers in the till can make collecting samples by auger difficult, and the high organic content of the peat promotes preferential absorption of specific trace elements (e.g., copper and palladium) to the organic matter. Smyth’s rigorous sampling and interpretation protocols prevent misleading information from distorting results. At the gridding stage, Smyth is working to establish a set of parameters that can be modified as data is added.

“The interaction with Geosoft has been very beneficial in understanding the intricate aspects of menus such as gridding in Geochemistry for ArcGIS,” says Smyth. “I’ve also suggested new features that I’d like to see implemented in future releases, such as interactive real-time visualization of geochemical data to aid geologists in target definition.”

Although the Lonmin exploration program is not yet advanced enough to justify using the powerful visualization tools that come with the geochemistry extension, Smyth is looking forward to producing 3-D maps and sections that can be used to illustrate the latest results in a meaningful way as data accumulates.

In the meantime, his team is producing preliminary maps quickly and easily. “You might have a call from headquarters about what the data looks like and within a few minutes you can produce a basic plot that shows the major trends in the data with overlays such as geology added.”

He is also keen to make more use of the geochemistry extension’s statistical tools such as the interactive histogram that can display selected field data and update data values whenever a value in the corresponding database changes.

And if and when Lonmin reaches the drilling stage, Smyth will be integrating his surface geochemical results with assay results from drill holes using the same tools in Target for ArcGIS, Geosoft’s subsurface geology extension, to produce 3-D maps that can lead to a better understanding of the results.

The combination of comprehensive geophysical and geochemical datasets, the technology to integrate them, and a quality control program that prevents errors and distortions will give Lonmin the best chance of finding buried PGE deposits underneath Northern Ireland.