Loading and Using Block Models in Deswik, and Common Problems Encountered

Block models are often touted at the "be all to end all" for grade estimation. I believe that block models should be carefully analysed before use, as they are often flawed, and there are often better options for short term planning and design.

In this article I will walk through how to load a block model in Deswik, as well as set up legends and annotations to match. I will also go through getting the tonnes and grades for a stope shape. Finally, I will discuss a number of issues I have come across with block models, and some thoughts on better methods to use instead of, or along side a block model.

Load a Block Model

1.??????Open the Deswik file you want to use.

2.??????Right click on a blank section of the toolbar. Turn on the “Block Model” toolbar.

3.???????Click on the first button on the block model toolbar.

4.??????Click on the “new” button near the bottom right in the dialogue box.

5.??????Click in the cell under the “Block Model” title. Click the … button that appears.

6.??????Navigate to where the block model is located. Depending on the speed of your network and the size of the file, you may be better copying the file to your computer hard drive to speed things up.

Create Legends for the Block Model

7.??????We are going to create a legend for each field of the block model that we are interested in. Usually this is the commodities that you are mining, which in my case are gold and antimony.

8.??????Click on the “Modify Legends” box in the bottom left hand corner of the dialogue box.

9.??????Click on the “new” button in the top left hand side of the legends dialogue box.

10.??Type in the name for the first legend. I am naming mine “Au” for gold.

11.??Click Create->Range Bins in the middle of the dialogue box.

12.??Click on the drop down arrow in the box next to the “Field”. Choose the field that matches your title. I have chosen Au.

13.??The bins will automatically populate using the information in the block model. You might want to change the minimum and maximum numbers to be whole numbers, so you can make the bin size an even number. A “bin” is the group of numbers that are going to be classified by the same colour. The “bin size” is the amount of numbers in each colour.

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14.??Deswik will automatically assign colours to each bin (range of values) in a rainbow. You can change the colours if you want.

15.??To change the colour of the bin, click on the colour in the middle, then click the colour on the right hand side, and choose the new colour.

16.??Repeat steps 8 to 14 to create legends for each field required. I have made a field called “Sb” for antimony.

17.??Click OK.

18.??Click in the cell below the “Legend” title. Click the drop down box in the right hand side of the box. Select the legend you want to show the colours for. I am using Au.

Add an Annotation Field

19.??The block model I am using does not contain AuEq (gold equivalent) in the block model, as the equivalently factor changes more often than the block model. We will add this as an annotation field.

20.??Scroll across to the right hand side of the dialogue box. Tick the box below “Draw Annotations”.

21.??Click in the box below the “Annotations” title. Click the “…” in the right hand side of the box.

22.??Click on the “new” button in the bottom right hand side of the dialogue box.

23.??Click in the box below the “Field” title, then click on the … in the right hand corner of the box.

24.??Expand the “+” arrow under the fields section on the left hand side of the dialogue box, next to the word “Standard”.

25.??My current formula for AuEq is AuEq = Au + 1.36* Sb. AuEq formulas can be much more complicated than this, especially if there are more elements mined. Consult with geology or long term planning to get your formula.

26.??You can double click on a field to add it to the equation. It will automatically add the square brackets around the name. Here I have double clicked “Au” in the fields list, then typed “+1.36*”, then double clicked on “Sb” in the fields list.

27.??Down the bottom of the box it will say whether the formula is valid. If it is not valid, check that the names of the fields are spelt correctly, that all fields have square brackets either side, that there are no singular brackets by themselves, and that you have no random letters or symbols in the formula.

28.??Click OK twice.

29.??It might take a while for the block model to load. Be patient.

Looking at the Block Model

30.??Click the second button on the block model toolbar to turn the block model on.

31.??Go into section view to see the block model. You can do this by typing “2”, then selecting two points to do a section along.

32.??This is a view of my block model with some development. Each block represents an area where the model thinks there is ore. This is based on what drill holes and other geological data is available in the area, with a formula to determine how far away to look for information. The colour of the blocks is based on the Au grade, and the number in the box is the AuEq value. This is what we set up in the previous steps. Any blocks that are black (such as on the left hand side of this picture) have no value.

33.??One of the most common issues with block models, is that because of their size and complexity, they are often updated less frequently than other sources of information. This includes ore body shapes and grade data, such as face grades. It can be seen that the ore shapes shown below do not match the block model. The first one is in a different spot, and in the second one, the ore shape is much thinner than in the block model. The ore shape was updated after the block model was last done. If you see these sorts of issues with your block model, contact your geology department.

Getting Stope Grades

34.??To get the grades of a stope shape, first create a 3D stope shape. Have this shape visible.

35.??Go to Tools -> Interrogate -> Interactive Interrogation

36.??Click the “Pick“ button, and select the stope shape.

37.??The information on the stope shape should auto-populate.

38.??Click on the “Settings” tab.

39.??Make sure that the block model location, default density, density field and legend are correct.

40.??Go back to the “Results” tab.

41.??Click the “Update” button.

?The “Total” column shows the average grades for the whole of the stope shape. The “Model” column shows the average for the area within the block model only.

As you can see in the picture below, because the stope shape is wider than the block model. This increases the volume/tonnes and dilutes the grades in the "Total" column compared to the "Model" This makes sense for my narrow vein orebody, but it may not make sense for your orebody. Analyse the numbers you get for the "Total" and "Model" columns, and ask yourself if they make sense.

Make sure you also compare your numbers to the numbers that geology get through their own calculation methods, to ensure that the numbers are realisitic and correct.

Common Block Model Issues

In addition to the common block model issues I have shown above, such as the block model being in a different location or the wrong size compared to the ore shape, other issues that I have come across with block models include:

• A block model update making almost an entire mine go below cut off grade due to a change in the estimation method. When you receive a new block model, do spot checks of previously designed stopes and areas to check that the new block model is at least similar to the previous model. If it is significantly different, ask why this is the case, and if this is realistic.
• Block models that reflect the overall average tonnes and grade of a large area (eg a whole orebody) well, but do not reflect the grades of smaller areas. I have previously had a block model where the tonnes and grades where modeled as pretty similar across the whole orebody, but when we mined it, the top half of the orebody was well below cut off grade, and the bottom half was a well above cut off grade. The overall tonnes and grades of the orebody averaged out to be correct on an orebody scale, but we wasted a lot of time and effort mining material that wasn't worth our while. If you have other sources of grade data available (eg face grades, previous stope grades, diamond drill hole data etc), these will usually provide a better estimate of local grades than a block model, and should not be overlooked.

In conclusion, block models can be powerful tools for grade estimation, but should be carefully analysed to ensure that the results they produce make sense, and that they meet the needs of the job you are trying do. In addition, using more local grade estimation methods such as face grades, mapping, stope sampling and reconciliation, and localized drill holes can produce much better results for short term planning and design.