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When good ground control is indispensable: two case studies

by Graham Chandler on June 4, 2012 applied

Bathurst Mining Camp

As part of a broader federal initiative to breathe new life into abandoned and depleted mines in Canada, the Geological Survey of Canada (GSC) undertook a large geoscientific program including airborne gravity and magnetic surveys of the region around the Bathurst Mining Camp in New Brunswick. Producing from the 1950s, it’s one of Canada’s oldest VMS deposits.

Because the area has very limited outcrop, geophysics and remote sensing played a key role in the study. However the airborne data alone clearly wouldn’t tell the whole story with accuracy. First, modeling of the gravity and magnetic data in 2D sections was done. Magnetics provided the near surface geometrical control, and gravity the depth.

Bathurst Mining Camp. Location map, regional geology and modeled lines.
Geophysicist Hernan Ugalde—researching with McMaster University—worked closely with Cees van Staal, GSC’s structural geologist, to link the geophysical information with ground data. Van Staal’s intimate knowledge of the structure proved critical: he knew the way the faults were dipping and how many folded sequences were needed to put on. “So we combined his knowledge of the area with the resolving power of geophysics,” says Ugalde, “and ended up with a great 3D model based on geophysics, that also made geological sense.”

Example of GM-SYS modeled sections from the magnetic (top panel) and gravity data (middle panel)

3D data integration of the 2D modeled lines

 

NWT IronOre Exploration

This was a Fe-exploration project in Canada’s Northwest. The target was an iron formation within the Rapitan Group, interbedded with thin mixtite beds and extensive large exotic clasts.

Regional location map

The company flew magnetics on the assumption that because Fe is magnetic, large anomalies should be important targets and direct detection should work. However ground follow-up fieldwork recognized the Fe formation was principally hematite, which is non-magnetic. Instead of throwing away the data, 2D sections were modeled usingthe magnetic data and a geology map with the location of the known IF. Susceptibility measurements allowed determination of the range of values to use on each unit. Basic stratigraphic principles plus required susceptibilities allowed placing of the required units on the surface. Key again was working with a structural geologist to help separate what was geologically reasonable from what wasn’t.

Amplitude of the analytic signal from the magnetic data, used to map the initially interesting horizons)

Example of GM-SYS 2D modeled sections from the magnetic data. Blue: non magnetic carbonates; pink: high magnetic conglomerate; white: non-magnetic iron formation; yellow: moderately magnetic conglomerate unit.

“If the company had drilled directly on the airborne results, they most likely would have intersected areas where the IF was already eroded and the anomaly obeyed only to magnetic conglomerates,” says geophysicist Hernan Ugalde, who worked jointly with the company’s VP Exploration. “They might have got lucky with a few holes on areas where the IF was underneath, but without a thorough modeling analysis behind it they would not have known what was where.” It emphasizes the importance of fieldwork, mapping and measurement of rock properties—to recognize the different geological units that were the base of the model. In the end, drilling recommendations were defined.

Final product of the modeling exercise: depth to the top of the Iron Formation, obtained from the modeling and field mapping. It also shows the location of the modeled lines. 

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