By Seequent Senior Resource Geologist Carrie Nicholls
In this blog, I will teach you how to use Leapfrog EDGE to report using open pit shells.
You have completed the resource model and now you need to compile the table of resource figures. If there has been mining activity, either historic or present, then the deposit will have to be depleted. In addition, if appropriate and available, you will most likely need to constrain the resources with a resource pit shell and/or underground stope optimisation.
Whether it is an open pit shell or closed volumes, as would be the case with underground mining, the basic procedure will be the same. A ‘Geological Model’ will need to be made to represent the reporting volumes. The estimated grade model as completed in Leapfrog EDGE and the ‘pit’ model will be evaluated onto a block model. This will then be used to report the resources at the cut-offs required. Creating the ‘pit’ model itself is straight forward. Some open pit surfaces may be a little trickier to incorporate, for example if the pit edge daylights it may not be enough to extend to the geological model extents. If it’s not be possible to go back to the mining engineer who made it to extend the pit shell, I’ll show you how to deal with this.
The procedure to make the pit model in Leapfrog Geo is as follows:
- Import the pit as a Mesh.
- Create a new Geological Model – selecting the Base Lithology Column as <none>.
- Add to the Lithologies appropriate rock codes e.g. “resource pit” and “outside pit”.
- Surface Chronology > New intrusion > From Surface – select the imported pit mesh.
If you receive the following message in Figure 1, it’s telling us that the mesh used does not intersect the geological boundary and in this case, it likely means that the pit does not intersect the geological model extents at the surface. Of course, the pit may extend to the topographical surface locally, but as the extents of the whole model is greater in elevation than at the point of the pit, the pit is deemed ‘too small’.
Figure 1 Geological Model warning that the pit is too small and does not intersect the surface boundary
You will need to extend the pit edges at the surface beyond the model extents – which will be determined by the maximum elevation of the geological model you are adding it to (or could be maximum topographic elevation).
- Right click on the pit mesh and Extract Vertices.
- Create a new polyline around the outline of the pit, ensuring it is above the model extents to be safe. Also make it larger than the footprint of the pit, so it would extend the pit at an angle (this gives a better result than making a vertical extension).
Figure 2 Create polyline to extend pit
- Create a New Triangulated Mesh with both the pit vertices and the polyline.
- Use this new extended pit mesh to create the pit model as above, remembering to activate the surfaces and check the Surface Chronology, Figure 2.
When you have created the pit model, which is sitting in Geological Models, evaluate it onto the Block Model which you would have created for the estimate. In the Calculations and Filters tab, create Filters to select the appropriate areas of interest e.g. within a resource pit and/or outside the mined pit (Figure 3).
Figure 3 Use the pit model to create a filter in ‘Calculations and Filters’
To create a Grade Tonnage graph:
- Right click on the block model in the project tree.
- Select New Grade Tonnage Graph.
- Use the relevant Block Filter, Figure 4.
- Select the appropriate density variable and unit to calculate the tonnage.
- Press ‘Add’ for new grade tonnage curve and select the appropriate grade variable and metal unit.
Figure 4 Grade Tonnage Graph using filter
To get the table of tonnage, grade and metal select Copy>Copy Data at the top of the tab and paste into an Excel spreadsheet. Adjust the step size to an appropriate interval (Figure 4).
- Create a ‘pit’ geological model, extending mesh/es at surface if required.
- Evaluate onto block model that contains the estimates.
- Create filters for the material inside/outside/between pits.
- Use the filter in the grade tonnage graphs to extract the tonnes and grade.
- Copy the graph data into a spreadsheet to get exact figures for the report.
Using Leapfrog EDGE you can do this last stage of depleting and constraining your resource model without having to take it out into another package.
To see what else Leapfrog Edge is capable of, watch this quick 6 minute demonstration.