ickel laterite deposits are formed by tropical weathering of ultramafic rocks. Nickel is leached from the rock resulting in an overburden that has a high (up to 2-3%) nickel content.
Problem
The overburden vertical profile in the deposit areas normally has an upper soft sediment-like (limonitic) zone over a saprolitic zone (nickel rich serpentonite boulders encased in limonite) over unweathered host rock. Deposit evaluation for mining potential needs definition of the thickness of the two zones.
GPR Contribution to Solution
GPR combined with exploration drilling were evaluated as a means to defining stratigraphy. At first assessment, the use of GPR in such an environment would be limited. Weathered rock in tropical settings is often clay rich and electrically conductive making the ground opaque to GPR signals.
Surprising exploration depths has been observed in what appears to be clay rich tropical areas. Frequent rainfall delivering large volumes of very fresh water appears to leach the soil to a fine clay-sized texture but without the normal clay mineralology.
Survey crew conducting common offset GPR reflection profiling. Lower frequency GPR is essential to successful deep sounding in environments with large scale heterogeneity.
While the detailed mineralogical explanation is complex, GPR penetration has been substantially greater than expected. Exploration depths of up to 50 m have been reported in some projects.
The original GPR success is illustrated in the data shown here. Data were acquired at the Loma Carribe deposit in the Dominican Republic. Common offset reflection profiling with a pulseEKKO IV provided GPR data that was interpreted by correlation with borehole control. Both 25 and 100 MHz center frequency antennas were tested with the 100 MHz data being limited to about 10m depth while the 25 MHz data reached the bedrock in many areas.
References:Watts, A., 1997, Exploring for nickel in the 90s, or "Til depth us do part", Proceedings of Exploration 97: Fourth Decennial International Conference on Mineral Exploration, edited by A.G. Gubins, 1997, p. 1003 - 1014
Results & Benefits
The value of GPR evaluating for lateritic nickel has been demonstrated many times since the study data were reported. Some key observations are:
- Low frequency GPR is needed to probe these complex heterogeneous environments
- Clay-like environments in areas of heavy rainfall can be surprisingly transparent to GPR
- GPR can sense the major stratigraphic units in lateritic deposits
- Light weight portable equipment is needed in these rugged often jungle-covered terrains
GPR responses vary greatly depending on the target being sought and the host material.
Download the case study: GPR for Lateritic Nickel
Learn more about the pulseEKKO system
When learning about GPR, the best practice is to review several similar case studies to develop an understanding of variability.
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