Case Study – Gold Mine in the United States

It is not uncommon to ask what the remaining resources in a heap leach pad are after a long period of leaching. Typically, this will be answered by conducting a characterization study to investigate the remaining gold and moisture distribution by drilling and assaying. The figure below is typical for a leach pad, where oven dried moisture and bottle-rolled soluble gold was measured from ore samples taken from a drilling campaign. More detail can be found here <>. The wells were not that far from each other and the data show very little spatial continuity in the gold and moisture distribution. It is difficult to get a good sense of perspective from these data other than bulk averages. Spatial interpolation of moisture or soluble gold would lead to deceptive results.

Gold and moisture distribution in a leach pad obtained from drilling

Similarly, a second example showing the moisture distribution from seven boreholes separated by 50ft is presented below. Loosely, there are some layers of high moisture that can be observed to cross through many of the boreholes, but the interpolation is tenuous. Both examples demonstrate that heaps can be highly heterogeneous at the scale of measurements that assaying provides. Additionally, it is difficult to make decisions on increasing recovery or amending the irrigation plans to keep metal production on schedule.

HGI has investigated the drilling results from many heap leach pads. The common theme is small scale variability in the gold and moisture distribution, making spatial interpolation difficult and misleading. Larger scale measurements, such as resistivity (ERT), can provide information at a more intuitive scale for decision making.

Moisture content from a gold heap showing loosely formed layers

HGI’s technology, on the other hand offers spatial information regarding the heap’s hydraulic structure at scales that are intuitive for decision making. In the geophysical case study below, we used mining geophysics (specifically electrical resistivity tomography, ERT ) to map the subsurface resistivity distribution, which has been shown to be related to the moisture distribution within the heap. The contour plots show highs and lows in the resistivity with targets that extend laterally for over 50 m (165ft). The section on top was taken through well 102 from the first graphic above. The heap is relatively uniform but slightly dry at this location. Approximately 120ft away in either direction, the heap changes radically and shows a lower resistivity layer, likely attributed to finer grained soils holding onto moisture, on top of a higher resistivity layer that will be dry. If our methodology was conducted prior to the drilling campaign, we would have recommended moving the drill rig to obtain a better understanding of the moisture distribution.

Adverse hydraulic processes can lead to underleaching of gold heap leach pads. HGI routinely deploys mining geophysics and electrical resistivity to investigate these processes.

The second, shorter resistivity section is from the same heap through well 10. Again, we see a strong qualitative correlation between the ERT data and moisture distribution measured from the well. The resistivity easily picked up the wet layer at about 5-13m (16-42 ft). Knowing the moisture distribution within a heap leach pad, whether during active leaching or draindown, can give great insights into the troubled areas that may need additional management for metal production and increasing recovery.

Two examples of electrical resistivity tomography (ERT) across a heap to demonstrate broader features for easier interpretation